Sherrell Greene on his Accomplishments and on Nuclear Safety

Sherrell Greene recently retired from ORNL after 33 years employment. Sherrell was ORNL's Director for Nuclear Technology Programs from 2004 through 2010, and Director of ORNL's Research Reactor Development Programs from 2010 through his retirement in 2011. A "relatively-complete" list of Dr Greene's professional publications can be found in his blog. A few weeks after Sherrell Greene's retirement from ORNL in September I meet with him for a preinterview. Subsequently I sent him a number of interview questions for his written responses. This is the first section of Sherrell Greene's responses to my questions. Greene has made conspicuous contributions to nuclear safety, and my first series of questions dealt with nuclear safety.

1. What do you view as your greatest accomplishment at ORNL?

I don’t think in terms of “greatest accomplishments”. Frankly, I’ve not given any consideration to my “greatest accomplishments” (smile)

Overall, I’m most gratified to have played a leadership role in maintaining and growing ORNL’s nuclear energy R&D programs during the past decade. When I left the Lab, ORNL’s nuclear energy R&D portfolio had grown to roughly $100M / yr for four federal agencies and the commercial nuclear industry.

Technically, several things come to mind: (1) the pioneering work we did in the early 1980’s in BWR severe accident analysis, (2) our work in the 90s on reactor-based weapons plutonium disposition that laid the technical foundation in Russia and in the US for disposition of excess weapons-grade plutonium, (3) the successful execution of the Coupled End-to-End Demonstration at the Radiochemical Engineering Development Center a few years ago in which we demonstrated the first reprocessing of LWR fuel and fabrication of mixed oxide fuel without separating pure plutonium, (4) the development of the Department of Energy’s Office of Nuclear Energy R&D Roadmap in 2008, (5) development of the OR-SAGE national electrical generation siting model, and (6) development of the Small advanced High Temperature Reactor (SmAHTR) fluoride salt cooled reactor concept. I found each of these challenging and rewarding, and feel blessed to have had an opportunity to engage in such a wide variety of endeavors. National laboratories are special places in terms of the breadth of opportunities afforded one with an entrepreneurial spirit.

Ultimately though, my greatest satisfaction came from the fantastic people I worked with, and building multi-disciplinary teams to tackle really tough challenges.
I don’t think in terms of “greatest accomplishments”. Frankly, I’ve not given any consideration to my “greatest accomplishments” (smile)

Ultimately though, my greatest satisfaction came from the fantastic people I worked with, and building multi-disciplinary teams to tackle really tough challenges.

2. What was your biggest disappointment at ORNL?

Probably my inability in recent years to convince Lab management to aggressively re- embrace its high temperature advanced reactor development legacy and aggressively pursue fluoride-salt cooled (FHR) and molten salt reactor (MSR) system concept development.

3. What do you believe is your most important contribution to Nuclear Safety?

Probably the work I did in BWR severe accident simulation and analysis in the 1980’s through the early 1990s. I was part of the team that conducted the first detailed analysis of unmitigated BWR station blackout and loss of decay heat removal sequences. The former work has been widely reference during the past few months since the Fukushima Dai-ichi events.

4. Would your ORNL peers agree with you, or would they point to other accomplishments?
I don’t know.

5. What other contributions have you made to nuclear safety?

I authored a report in 1984 (NUREG/CR-­2940) that had some impact on the direction of LWR severe accident code development (particularly MELCOR). To my knowledge, I was the first one to identify and characterize the potential role of BWR reactor buildings in severe accident mitigation. I also conducted the first port of reactor safety code (CONTAIN) to a supercomputer (ORNL’s CRAY-I at that time). This opened our eyes to the potential for advances in computing technology to revolutionize our understanding of reactor safety.

6. How do you view the performance of the Fukushima reactors from a nuclear safety perspective?

Well, I think the primary containments of Units 1-3 have held together remarkably well given the challenges presented to them. It’s a testament to their fundamental design.

7. What safety lessons can be learned from the Fukushima reactor events?

We will be learning lessons from the Fukushima event for years to come. As you know, the NRC has taken a first cut at identifying lessons learned and creating an action plan. However, there are a few observations that standout to me. I’ve discussed the on my blog, www.SustainableEnergyToday.blogspot.com. First, from a risk based design/analysis perspective – get your top event frequencies/probabilities right. In hindsight, it appears the probabilities of the grand earthquake and coupled tsunami were underestimated. Second, understand the potential for events at one unit (such as explosions) to damage another unit. Third, understand the implications of events that not only damage one or more units on a multi-unit site, but also render the surrounding area incapable of providing assistance to the site. Few, if any, probabilistic risk assessments to date have fully explored the last two issues. Fourth, there’s no substitute for defense- in-depth in plant design. BWRs have a remarkably diverse set of options for injecting water into the reactor and containment. Despite all of the system failures at Fukushima, it was this diversity that has allowed the plant operators to manage as well as they have. Lastly, train and think as if these accidents can happen. They do. I could go on and on, but the point is, there’s much to be learned – and much will be learned from Fukushima.

8. How can the Fukushima lessons be best applied to existing reactors?

We need to take a second look at how we develop and validate the basic event data for our PRA models. We need to take a second look at our fundamental assumptions regarding the independence of events in multi-unit sites. We need to re-evaluate the implicit assumption in almost all PRA’s, that the outside world can gain rapid access to the site to provide assistance. Potential plant backfits, operational procedures changes, and emergency response procedures changes need to be examined and high-value modification should be made. We need to re-evaluate, from the systems perspective, our requirements with regard to station battery lifetime, backup diesel-generator availability, and off-site power feed to the plants. I feel the value of dedicated BWR containment flooding systems that our team identified in the 1980s should be revisited.

9. How can the Fukushima lessons be applied in future reactor designs?

All of the observations I made earlier also apply to the design of future reactors. All reactor technologies (water cooled, or otherwise), have inherent vulnerabilities that must be addressed by a combination of system design features, operational protocols, and emergency planning.

10. Do you believe that current LWR designs, for example the Westinghouse AP- 1000 solve safety problems revealed by the Fukushima accidents?

There’s no doubt in my mind that more recent designs are more robust than older designs. This is the case both for BWRs and PWRs. That’s not a condemnation of older designs. It’s just evolutionary progress. The fact remains, of course, that water and over-heated zirconium-clad fuel do not peacefully coexist. The newer plants do incorporate a variety of design features that reduce the probability this situation will arise.

11. What other safety features would you like to see in future Light Water Reactor designs that are not included in current designs?

As you know, the key issue for reactor safety is extraction and rejection of the core’s decay heat so that the fuel and its containment boundaries are not damaged. A more robust fuel design would be wonderful. But it’s a long path (probably 10-20 years) for introduction of a new fuel in the LWR business. Most of the new LWR designs already provide passive mechanisms for flooding the reactor and containment in the event of a station blackout event. Though we’re learning that many of the fears we originally had about massive damage in the spent fuel pools at Fukushima were not realized, I think it’s a good idea to remove spent fuel from close proximity to the reactor as soon as practical after the fuel is discharged from the reactor.

I’m an advocate of a design philosophy called, “Irreducible Complexity”. It is a line of reasoning that states there is an optimal design point – in terms of system complexity – at which removal of a design feature or artifact will result in an unacceptable reduction in system functionality; while addition of a design artifact will not significantly improve system functionality. Old-fashioned mousetraps and clothespins are good examples of irreducibly complex systems. We’ve been trying without success to improve upon their design from the moment they were created. While Irreducible Complexity may not be achievable in the real world, I’ve found the thought process to be an invaluable Gedanken Experiment countless time during my career. So my hope is that future plants can pursue simplified design architectures. Alvin Weinberg told me one time he felt one of the two major contributors to the demise of the MSR was that it looked too much like a chemical plant to the traditional reactor community. They just couldn’t get comfortable with it. That’s a lesson worth remembering...

Another area I would like to see explored in future plants is the concept I call “Centurion Reactors”. A few years before Alvin Weinberg died, I had the privilege of spending the better part of an afternoon with him at his home in Oak Ridge. We discussed his career, his dreams, and his failures. During the later years of his life, he was passionate about the concept of “Immortal Reactors”. His idea was that commercial power plants are capitally-intensive investments that actually yield inter-generational benefits. Once their capital cost is amortized, today’s LWRs produce electricity at less than 5 cents / kilowatt- hr. That’s a huge benefit that lasts as long as the plant operates and accrues to its customers – the children and grandchildren of its builders. Before Dr. Weinberg died, he sent me his file on Immortal Reactors. After some thought, I concluded a realistic near- term goal would be to design nuclear power systems for an operating lifetime of 100 years. Thus the term “Centurion Reactors”. The entire plant would be designed by integrating design-for-life, design-for-maintenance, design-for-monitoring, and design- for-replacement technologies and architectures, with the goal of enabling the plants to be licensed for and operator for 100 years.

(Sherrell's web site can be found at, www.SherrellGreene.com.)

Underground Reactor Advantages

Underground sited. This scale was chosen so that the physical size allowed it to be factory manufactured and transported to the site,which is a significant potential cost reducer. . . .

- John Rawls, Chief Scientist at General Atomics

From the dawn of the Nuclear era, Edward Teller was deeply concerned about reactor safety. Teller favored underground placement of civilian nuclear power plants primarily for safety reasons. Indeed in his last paper Teller advocated underground placement of Molten Salt Reactors, although arguably MSRs could be designed to be safe enough to make further safety measures unnecessary. A further justification of underground placement of MSRs, would be that it would be consistent with low manufacturing costs and rapid reactor deployment.

Underground deployment of nuclear tractors offers a number of advantages including,

Higher Resistance to...

– Terrorist attack

– Aircraft impacts

– Proliferation

– Sabotage and vandalism

– Conventional warfare effects

Underground sites offer superior protection against the effects of severe weather events and some potentially protection even from the effects of earthquakes. Underground sites also offer superior protection against fission product release in the event of a serious reactor accidents. Studies of underground siting conducted during the 1970's reported that underground siting would cost more than traditional reactor siting, but these studies assumed the use of conventional nuclear technology and that the entire nuclear facility would be located underground. From the viewpoint of safety and security it is only necessary to house reactors underground. Turbines and generators, as well as other Nuclear Power Plant related facilities can be located above ground without any disadvantages if the cost of underground facilities placement become a matter of concern. In addition Generation IV reactors are generally more compact than conventional reactors. There are other ways to limit underground housing costs. For example salt formations offer unique advantages for nuclear reactor housing, with low cost excavation. Existing underground salt mines offer unique placement advantages. In addition to existing salt mines, many old mines and natural caverns offer potential underground siting for reactors. Studies of underground placement of nuclear facilities made during the 1970's assumed that reactors would be placed 300 feet or more beneath the surface, but reactor manufacturer Babcock and Willcox intend to place their small mPower Reactor just below the surface.

Underground placement of small, compact Generation IV nuclear power plants would be inexpensive, and underground placement is often featured in many small Modular nuclear designs including the B&W mPower Reactor. A recent report to the American Nuclear Society by Mark S. Campagn and Walter Sawruk and titled, "PHYSICAL SECURITY FOR SMALL MODULAR REACTORS" states,

Rely on government response for SMR facilities with vital assets underground or otherwise well protected. Shallow burial or a hardened structural design provides excellent protection against large explosive weapons and aircraft impact as well as an excellent means of enhancing security system effectiveness against sabotage. Application of the traditional multilayered defensive approach of detection, deterrence, delay, and defeat can be used effectively for physical protection of SMRs. Detection, deterrence, and delay concepts must be integrated into the early design phase of the facility in order to provide sufficient lead time for government response.

A few years ago three University of Tennessee Nuclear Engineering Graduate students, William A Casino, Kirk Sorensen, and Christopher A Whitener wrote a paper titled "A Small Mobile Molten Salt Reactor (SM-MSR) For Underdeveloped Countries and Remote Locations." The paper won first prize in an American Nuclear Society reactor design contest. This design exercise focused on a reactor small enough to be transportable by truck, yet large enough to be transportable by truck. The design is highly suggestive although it turns out to be a little big to be truck transported. The reactor was designed to produce 100 MWe, with an active core region that weighed 216 tons (about 200 metric tons). This is too heavy to be easily transported by truck, but it might be possible to shave that weight down significantly. More than half of the core weight is contributed by core graphite (about 147 of 215 tons). Thus a method of inserting core graphite into the core at the destination site, would offer considerable advantage if this could be accomplished quickly and at low cost. The use of graphite pebbles would be consistent with these goals. This would lower of the weight of the core moduel to 68 US tons, which would certainly be manageable by either truck or train. Further the primary heat exchange and connecting pipes are included in the core module, and this might be considered a flaw in the design.

There were a number of flaws in the SM-MSR design, which was after all a student design exercise. Although a core dump tank was included in the SM-MSR design, no colling system was included. However, a passive cooling system for the disposal of fission product decay heat is possible with an underground MSR. Air can be drawn into the underground chamber and heated by the dump tank exterior, and then the heated air could rise through a chimney. The rising heated air, would, of course, draw more air into the underground chamber by lowering its air pressure, thus creating a passive decay heat cooling system.

Casino, Sorensen, and Whitener noted that,

One site specific limitation is that the primary containment module as proposed is to be placed into a silo to be trenched into the earth. This silo needs to be approximately 28 meters in depth and be approximately 25 square meters in area. The water table in most locations will likely occur above this level, and the SMMSR containment module shall be constructed to withstand moisture impingement on the outer surface. Other corrosive elements in the water need to be checked for.

The reactor silo would not requite a significant amount of excavating, and thus could be dug quickly. As has already pointed our, building a silo from scratch might not always be required. Silos built for cold war guided missles, and well as a variety of underground mines might be useful, although preexisting underground structures would not be the only solution to small reactor siting the problem. Rapid drilling of a silo could advance at rates of as much as 10 meters a day. With prefacricated silo liners, site preperation might require no more than a week. Thus the re-use of coal fired power plants sits to house clusters of small baseload reactors could easily include underground housing of a number of reactors.

Underground housing of small reactors appears to be practical and it is credible to argue that Underground reactor housing can lower nuclear costs, and dramatically shorten reactor construction times. In addition underground housing can increase nuclear safety and offer significant protection of reactors from aircraft and other forms of terrorist attacks.

The oh so slightly revised history of nuclear safety

This post dates back to the early history of Nuclear Green. Periodically I revise and update old posts and repost them. Because this post is related to the history of the Windscale fire, and I wanted to refer to it in my discussion of the Windscale incident. What I regard as important about this post is the notion of nuclear critics that nuclear power lies out of the scope of history. Thus my 2007 Harry's Place critic seems to believe that if something happened once in the history of nuclear power, it will happen again over and over. Critics of nuclear power seem to believe that it is imposible for nuclear safety related knowledge and practice to evolve and change.

One of the follies of my youth was to spend a couple of years being trained to be a Historian of Ideas. This gives me an unusual perspective as a nuclear blogger. I had a couple of guest posts on Harry's Place in 2007. In my second post, alas now lost, I discussed the positive secondary benefits of using nuclear power as an energy source. My post attracted an inordinate number of anti-nuclear responses. One of my most vociferous critics was an English woman who was chemist.

My Harry's Place critic focused on a number of events in the history of the British nuclear adventure. It is clear that everyone who was doing nuclear science in the 1950's cut corners, and covered up problems, and no one more so than the British. The Windscale fire was a major nuclear accident, and the British covered up quite a lot of the problems. My critic however, chose to attribute to something she called "the nuclear industry" all of the characteristics of what was the British quasi-military nuclear production establishment of the cold war 1950's.

In the 196'ss my father research fission particle release during the Windscale accident, because he was researching the movement of radioisotopes in the environment during and after reactor accidents. Lots of radioisotopes had escaped into the environment because of Windscale, so studying the Windscale accident was high on nuclear safety researchers interest list in the early 1960's. Several things about the Windscale reactors, and the 1957 Windscale fire caught researchers attention. First the design io the Windscale piles was primitive by American standards. They were graphite piles designed to produce bomb grade plutonium. unlike the Hanford Reactors, which were water cooled, the Windscale reactors were air cooled. The X-10 reactor was the only American large air cooled graphite reactor. Eugene Wigner had rejected the use of air or gas cooling in the Hanford reactors. The British did not have a Wigner, and ended up up with an unsafe reactor design. In addition to being badly designed, the Windscale reactors were poorly instrumented, and the British were having increasing problems managing the reactors graphite moderator. Those problems had significant reactor safety implications, which the British failed to identify and analyze.

Thus the history of the Windscale fire must include questions about why the British had in the late 1940's chosen a production reactor design that was already considered obsolete in the United States by the time it went into operation, and why they chose to manage it the way they did. The combination of the reactor design and the management style adopted by the British made an accident in the Windscale reactors quite probable. The Windscale reactors were being opushed beyond their designed capacity and operated om unsafe conditions. The British nuclear program was being directed to fulfill the military ambitions of the UK Government that had nothing to do with the national security of the UK. The purpose of the United Kingdom nuclear program was to convince the political leadership of the United States, that the UK was still a great military power. Of course there was a Windscale coverup because culpability for the accident ran to the top of the British Government and military.

Windscale delivered a message to the American nuclear research community, that nuclear safety had to be attended too. The British Nuclear Research community had not payed enough attention to nuclear safety issues, and had allowed the operation of the Windscale reactors under unsafe conditions without protest. There was a lesson for American scientists and it significantly impacted what American nuclear scientists thought they should be doing, and what they thought their responsobilities were.

Critics of nuclear power ignore the history of nuclear safety. The history of nuclear safety is both a history of ideas and of a technology and a socio-political history. The two are intertwined. My British critic on Harry's Place, however, took an ahistorical viewpoint. She refused to place the Windscale reactor fire into the historic context of the British states management of cold war related technology. It was her view that if something was true of nuclear technology at one time, and in one place, it was true everywhere and always. Thus what was characteristic of the Windscale reactors was true of every reactor. And thus she argued that the management of the consequences of the Windscale fire by the British government is characteristic of all aspects of nuclear safety at any time and in any place. Critics of nuclear power similarly ignore the history of nuclear safety.

An ahistoric views of the development of any technology is profoundly unsophisticated. Technologies evolve in socio-economic and historical contexts, and attitudes towards technological issues like safety, are in no small measure related to the context in which the technology evolves. Knowledge evolves and with that evolution comes a greater appreciation for risk and understanding of methods of controlling risk,. As knowledge evolves it can begin to change the social and political context, thus altering public attitudes and beliefs.

A historian would, of course,

* note changes in attitude toward nuclear safety,
* developing research on safety,
* the introduction of new safety concepts, conflicts within the research community,
* conflicts over safety involving scientists, interest groups, self styled experts, research funders, policy makers, and policy implementation establishments.

Partisans in a disagreement about the significance of a historic event might well take a less nuanced view. My British critic from Harry's Place surely took and extremely unsophisticated view that reduced the history of nuclear safety to a simple narrative of good verses evil, With "the nuclear industry" embodying evil, and the critic fantasizing herself to be a warrior on the side of good. This fantasy, this myth, has characterized the anti-nuclear movement since the 1970's. At its heart then the anti-nuclear movement, to the extent it rejects a historical view of nuclear safety, is wedded to a mythological politics of identity.

In fact scientists like Alvin Weinberg, George Parker and my father, C.J. Barton, Sr., set out to do something about nuclear safety.

I have pointed out in Nuclear Green, that nuclear critic Ralph Nader's sister Claire had a professional association with with Alvin Weinberg and had discussed nuclear safety issues with Weinberg. Claire Nader undoubtedly passed on the substance of her discussions with Weinberg to her brother, who was later to talk directly with Weinberg about safety issues. The Nader's were both treated with respect by Weinberg. In turn Ralph Nader should have known of Weinberg's expertise both on reactor design and on nuclear safety issues. Nader also know of Weinberg's struggle over nuclear safety issues with Chet Hollifeld and Milton Shaw, a struggle that eventually lead to Weinberg's termination at ORNL. Thus Nader had no reason to doubt Weinberg commitment to nuclear safety. Nader could have undoubtedly used Weinberg's knowledge in a fight for nuclear safety. Instead Nader made his cause the fight against nuclear power.

Nader posed for the public as a good little guy, who fought against evil incarnate, represented by such evil forces as "the nuclear industry". Unfortunately this absurd story was bought be an increasingly simple minded media, that wanted to interpret every story for the public as a matter of good verses evil. Good verses evil was easy to sell to ther public, and drew eyes and ears to the media that told the stories. Stories with shades of gray were complicated. They required a lot of thinking and a lot of information. Thinking and information lost readers and viewers. In order to understand the history of nuclear safety, we must understand the increasing incompetence and corruption of what past for the mainstream news media during the last third of the 20th century.

There were some bad actors in the nuclear safety story, and Ralph Nader turned out to be one of them. The television networks, and the press were simply too lazy to get the whole story, so the media was content to sell the Saint Ralph line.

Nader tells stories about himself, in which he claims to be a saint of knowledge. For example, Nader claims that in 1964 he visited Oak Ridge National Laboratory. Over lunch Nader claims that he began asking nuclear engineers some questions.

They couldn't answer them, or the answers weren't satisfactory.

Nader claims.

'What could happen if a system goes wrong?

Nader asked. According to Nader the engineers avoided any such descriptions or said, '

we've got defense in depth' -- and other jargon.

"Defense in Depth" is, of course, a fundamental nuclear safety concept, that was proven to be effective during the Three Mile Island accident. At the time of Nader's visit to Oak Ridge George Parker, my father and other Oak Ridge scientists were working to understand nuclear accidents. By describing a discussion of things things that he did not understand as jargon, Nader revealed his lack of understanding of nuclear safety. As Gomer Pile use to say, "surprise, surprise surprise." There were of course, other people at ORNL who could have the answered Nader's 1964 questions, or at least would have known the answers within the state of knowledge. If Ralph Nader wanted to talk with peoplewho could answer his questions about nuclear safety he could have talked tp George Parker, or he could have talked to my father. Needless to say, Nader did not seek out nuclear safety experts to answers to his questions. Certainly his sister Claire's friend, Alvin Weinberg, would and could have answered Nader's questions about nuclear safety, and would have made himself available to Ralph and his sister Claire. It is quite possible that Nader talked to someone in Oak Ridge who did not answer his question, or alternatively gave Nader an answer that Nader did not understand. Had Nader sought out answers about nuclear safety in 1964, he would have found them, but Nader wanted answers that made nuclear scientist look bad, not in truth.

Nader was not interested in truth, he was looking for witnesses for his drama which would feature Saint Ralph fighnting an evil dragon "the nuclear industry." People, like Alvin Weinberg, George Parker, and my father were much to dangerous to rely on as witnesses. George Parker might start talking about how improbable it would be for most radioisotopes to escape from Light Water Reactors. My father might have started talking about how coal fired power plants and natural gas furnaces were delivering more radioisotopes to the environment than reactors were. Such people might blow Nader's cover, night reveal that Nader was only concerned about radiation coming from reactors. If natural gas delivered radioactive gases to American homes, the Saint Ralph and the nuclear dragon myth might fall apart. People might start asking why does Saint Ralph ignore the Natural Gas Dragon, that is brining radioactive gas to the lungs of so many Americans. If people knew that Alvin Weinberg had been fired over nuclear safety, he might steal attention from Saint Ralph. Weinberg was so dangerous to Nader's because he actually understood reactors, and safety, and his integrity was unquestionable unlike Saint Ralph's. Thus Nader's account of the history of nuclear safety, is self serving and dishonest.

Thus in the case of my British Harry's Place critic, as for other critics of nuclear power, the history of nuclear safety was something to be ignored. In their myths nuclear safety was simply impossible, therefore it could have no history. Nuclear power is a manifestation of something called "the nuclear industry", an evil despicable entity that transends time and space. "The Nuclear Industry" is always and everywhere the same, thus it cannot evolve, it cannot change, and has no history. Thus it is impossible to speak of something called the history of nuclear safety.

For nuclear critics, such as Ralph Nader, the topic nuclear safety exists to promote their own reputations. Nader, as well as Amory Lovins has maintained for over a generation that reactors are always and everywhere unsafe. Thus Nader and Lovins also believes in a mythic "nuclear industry" which also exists outside of time and space. There is for Nader or Lovins the history of nuclear safety is nothing more than accounts of their own struggle to slay the nuclear dragon, that is to outlaw nuclear power.

Are safer reactors possible?

Critics of nuclear power argue that all reactors are inherently dangerous, and point to nuclear accidents such as Three Mile Island, Chernobyl and Fukushima as evidence of the danger. The primary worry about nuclear power stems from the release of radioactive fission products, and other radioactive materials that are produced inside reactor cores. Among those materials Plutonium which is produced by a nuclear process which occurs when Uranium-235 and U-238 fail to fission following the absorption of neutrons inside reactor cores. Tritium a hydrogen isotope is also viewed with concern, although in practice tritium is viewed as so safe that it is used to illuminate the hands and dials of some wrist watches. The Nuclear Regulatory Commission (NRC) explains,

Tritium (H-3) is a weakly radioactive isotope of the element hydrogen that occurs both naturally and during the operation of nuclear power plants. Tritium has a half-life of 12.3 years and emits a weak beta particle. The most common form of tritium is in water, since tritium and normal hydrogen react with oxygen in the same way to form water. Tritium replaces one of the stable hydrogens in the water molecule, H2O, and creates tritiated water, which is colorless and odorless.

Tritium can be found in self-luminescent devices, such as exit signs in buildings, aircraft dials, gauges, luminous paints, and wristwatches. It is also used in life science research and in studies investigating the safety of potential new drugs.

In fact, if there were any significant danger from tritium, the NRC would outlaw using it in wrist watches. Just how dangerous is tritium? Recently a very small tritium leak from the Vermont Yankee power and was the subject of a big todo in Vermont. Nuclear critics insisted that the tritium represented a huge danger to the people of Vermont. The blog Minor Heresies suggested,

Tritium causes all the usual radiological effects: cancer, genetic defects, cell death, birth defects, and loss of fertility. . . .

My conclusion from all this is that the present tritium leak at Vermont Yankee is no small thing. The material is dangerous at low concentrations, persistent in the human body, impossible to filter, and hard to contain. The leak is limited to the area in and around the plant for now, but I can’t imagine the isolation and cleanup is going to be easy.

Well not exactly. Not in the amount we are talking about.

The silliness of the Vermont Yankee tritium scare was captured by a former Naval Officer who served in nuclear powered submarines, Rod Adams, who is currently a well regarded blogger on the uses of nuclear energy. Adams noted,

Based on reading a number of different articles and checking through the tables provided by the Vermont Department of Health, the fluid that was leaking into the ground contained tritium at a concentration of approximately 2.5 million picocuries per liter. That is equal to 2.5 x 10^-6 curies per liter. The rate that it was leaving the pipe was roughly 100 gallons (370 liters) per day. If the leak had been going on for a year before being detected and stopped, the total quantity of fluid that left the pipe would equal 138,000 liters. The total activity released would be 0.35 curies.

If a single person consumed every drop of that water, their whole body radiation dose would equal roughly 30 rem. According to a 1977 UNSCEAR study, the LD-50 (lethal dose for 50% of the population receiving the exposure) for tritium in adult rats was determined to be 1000 Rad. For the kind of low energy beta emissions that are produced by tritium, a rem is equal to a Rad. A dose of 30 rem received over a 1 year period would be unlikely to cause any immediate health effects, though it might add an additional risk of developing cancer sometime during the person's life. The magnitude of that risk could be computed using the conservative linear, no-threshold dose assumption.

Of course, a person who tried to drink 378 liters per day for a year would have problems more immediate the possibility of increasing their lifetime risk of cancer.

I also was asked to put this discharge into some kind of perspective, so I decided to compare it to the allowable and measured releases from a well operated and safe CANDU reactor in Ontario. Pickering B has a Derived Release Limit (DRL) for tritium of 490,000 terabecquerels each year. That is 4.9 x 10^17 Bq or 13 million curies.

Critics of nuclear power would insist that if we had a release of 13 million curies of tritium in the United States, we would have a crop of two headed babies. However, in Canada where 13 million curies annual releases of tritium are the norm, two headed babies are not being born.

On the other hand no one doubts that the escape of plutonium from a reactor can be a dangerous matter. Yet unlike tritium which is less dangerous than salt, but likely to escape from a reactor, Plutonium os very dangerous, but very unlikely to escape from a reactor. Nuclear critics love to recited how dangerous plutonium is. For example, journalist David McNeill stated

We might also cite the example of MOX fuel and plutonium, a substance so toxic “that a teaspoon-sized cube of it would suffice to kill 10 million people,”

In fact the Guardian reported,

In a possible sign that the contamination is more widespread than previously thought, a university researcher said at the weekend a small amount of plutonium had been identified a mile from the front gate of the Fukushima plant.

It is the first time plutonium thought to have originated from the complex has been detected in soil outside its grounds.

But was the escaped plutonium dangerous? The Guardian reportsed,

Masayoshi Yamamoto, a professor at Kanazawa University, said the level of plutonium in the sample was lower than average levels observed in Japan after nuclear weapons tests conducted overseas.

In conventional Light Water Reactors plutonium is produced inside fuel pellets. The fuel pellet is a ceramic and in almost every case the fuel plutonium will remain there. The only way the plutonium might escape the fuel pellet, would require that the reactor core overheat to such an extent that the ceramic fuel pellets start to melt. Once the plutonium escapes the fuel pellet, it faces a further barrier, the pressure vessel, which contains the reactor core inside a thick steel wall. If the plutonium managed to get past the wall of the pressure vessel, it would face one or more cement barriers, and then the forces of gravity as it pulled the plutonium to the outside grounds of the nuclear power plant. So how did the plutonium manage to travel a mile away from the Fukushima reactors? The answer is probably because it was ejected from the reactor building by a hydrogen explosion. More likely the plutonium was contained inside the spent fuel pellets that were housed in a pool above the Fukushima reactors. It is far from satisfactory that any plutonium managed to escape from the beyond the grounds of the Fukushima reactors, but in fact the amount that escaped was so tiny that it could do no harm.

So is there anyway, to insure that no plutonium ever escapes from a reactor core? Yes there is, in fact no plutonium can escape from a reactor if plutonium is not produced inside the core. But how is that possible? First while a lot of plutonium is produced in uranium fuel cycle reactors, less than 10% of that amount is produced in the thorium fuel cycle. A 1 GW LFTR would produce about 40 Pounds of Plutonium a year. If the goal is to minimize plutonium production this can be easily done. If the goal is to destroy plutonium, the presence of thorium in a reactor core facilitates the burning of plutonium. Finally if the goal is to produce no plutonium, then the use of fluid fuel thorium breeders (LFTRs) is highly recommended, because Neptunium-237, a plutonium predecessor isotope can be cleaned from a molten salt coolant before it can be converted from neptunium into plutonium by absorbing a neutron. Cleaning NP-237 from molten salt fluid is a relatively easy and low cost procedure. Once out of the LFTR core the neptunium can be destroyed in a burner reactor.

Thus if preventing the escape of plutonium from a reactor core is a major nuclear safety goal, designing molten salt thorium fuel cycle reactors that feature neptunium cleaning from core salts, would prevent the production of plutonium. If there is no plutonium production there can be no escape of plutonium.

Thus we have a choice of safety approaches to plutonium management, with the possibility of complete elimination of plutonium from waste stream a real possibility if it was desirable to do so. Total burn of plutonium would be yet another option.

A further approach to Plutonium safety issues would involve the use of underground reactor placement. If the reactor core and all radioactive fluids were kept underground. In Thorium fueled underground power plant based on molten salt technology, Ralph Moir and Edward Teller, Nuclear Technology 151 334-339 (2005), the authors explain,

An important feature of our proposal is to locate every- thing that is radioactive at least 10 m underground—where all fissions occur—while the electric generators are located in the open, being fed by hot, nonradioactive liquids. The reactor’s heat-producing core is constructed to operate with a minimum of human interaction and limited fuel additions for decades. . . .

Under- grounding will preclude the possibility of radioactive contam- ination in case of airplane disasters. A combination of 10 m of concrete and soil is enough mass to stop most objects. It would eliminate tornado hazards and, most particularly, contribute to defense against terrorist activities. In case of accidents, under- grounding, in addition to the usual containment structures, en- hances containment of radioactive material. The 10-m figure is a compromise between safety and plant construction ex- pense. We anticipate the cost to construct underground with only 10 m of overburden using the berm technique will add ,10% to the cost.

Moir and Teller note the safety advantage of underground placement,

A fourth safety measure is locating the reactor underground, which itself is one extra “gravity barrier” aiding confinement. A leakage of material would have to move against gravity for 10 m before reaching the atmosphere.

Plutonium is very heavy. In order for plutonium to overcome the "gravity barrier" some force would have to transport it to the surface. That force cannot be an explosion, because there is nothing in the reactor or its fluid salts that cannot explode. Nor can it be a fire, because no fire is possible. Thus the plutonium is trapped underground. In "Migration Paths for Oklo Reactor Products and Applications to the Problem of Geological Storage of Nuclear Wastes," G. A. Gowan repotted that plutonium along with many fission products including Zr, Nb, Ru, Pd, Ag, Te, Bi, and the rare earths, were immobile the the Oklo natural reactors core areas over a period of time of well over a billion years.

D.G Brookins found,

The actinides . . . were all retained in the host pitchblende.

In fact it appears that less than 10% of the actinides present when the original Oklo deposit was laid down had been lost to natural causes over a nearly two billion year period of time. Thus we can have a high degree of certainty that plutonium would be contained in underground reactor chambers, following the very unlikely event of the release of plutonium carrying salt inside the underground reactor chamber.

As noted what holds true for Plutonium also holds true for many fission products. The Oklo Reactors "natural experiment," that an underground reactor accident would not lead the the release of most fission products and actinides in a reactor or in a nuclear cool down storage. Neptunium would be a potential exception and for that reason it should be removed from long term storage of nuclear wast and disposed of by nuclear burning.

The fission products that are likely to escape in the event of a nuclear accident are well known and their behavior is well understood. They are noble radioactive gases, and volatile fission products. In fact the noble gases appear to pose little danger, and while volatile fission products are more dangerous, that danger can easily be mitigated. However, in Molten Salt Reactors it is easy to prevent the escape of fission products from the core fluid simply be removing them by simple and well understood processes. Reactor researcher, David LeBlanc states,

The volatile fission products such as the noble gases and noble metals come out of the salt as produced. Noble gases simply bubble out and are stored outside the reactor loop. Noble and semi noble metals will plate out on metal surfaces and can be collected by replaceable high surface area metal sponges within the loop.

Ralph Moir and Edward Teller note,

The molten salt reactor that operated in the 1960s had a big advantage in the removal of many fission products without much effort. Gases ~Kr and Xe! simply bubble off aided by helium gas bubbling, where these gases are separated from the helium and stored in sealed tanks to decay. Noble and semi noble metals precipitated. In the planned reactor, the old method of removing the gases may be repeated.

And what of Tritium? Tritium can be completely eliminated from MSRs by elimination of Tritium predecessor isotopes from the salt formula, or if that is not considered desirable for other reasons, it can simply be trapped in coolant salts, or by venting it in the off gas system and then trapping it in sodium fluoroborate.

Clearly then if the public wishes to be assured that it will never be exposed to radioactive isotopes from reactor cores that can be accomplished at a relatively trivial cost without sacrificing any of the advantages offered by nuclear power. I can not assess how much safety the public will want or view necessary in order to be comfortable with nuclear safety, but the technology to provide the public with safety that would assure no human deaths due to accidental releases of radioactive isotopes in a reactor core accident. This standard may not be rational, but the Oklo Natural reactors demonstrate that such a standard is obtainable, and with Molten Salt Reactor technology it is obtainable without paying a high financial cost.

Efficient Cars and Nuclear Power Efficiency

I must confess that I am of two different minds on the topic of efficiency. On one hand I am critical of the notion that efficiency can replace post carbon energy generation, yet I also have devoted a considerable effort to increasing the efficiency of the process of building nuclear power plants. In addition, I am conducting a personal case study of how to make my personal energy use more efficient. The contradiction involves the the fact that I some times consider adopting personal efficiency approaches that I reject when consider the proposals of the efficiency crowd. Lets consider the case of the 300 kg car. Julian Allwood and his Cambridge University associates have proposed radical steps such as the use of 650 Pound cars.
My inclination is to reject this notion out of hand, and I would were it not for the fact that I have considered doing just that for some time, and am presently considering an even lighter vehicle.

The whole idea strikes me as mega cool. So cool that we could name the electrical three wheeler the Apple! Needless to say there are some draw backs. The first is that the electrical three wheeler might not be safe, or at least safe enough, Can you imagine Ralph Nader writing an expose of the 650 pound car titled "Unsafe at any Voltage."

My concern rests on an analysis of a shopping trip in my spiffy three wheeler.
Now I drive to the nearest shopping center on Clinton Highway via Pleasant Ridge Road. a winding 40 MPH two lane artery, with less than great visibility.

Pleasant Ridge Road does not look safe for such a tiny, flimsy electric vehicle, does it? Not with cars that are more than five times more massive, not to mention the pick up trucks. So I face this quandary, I can get an electric three wheeler for under $2000. It is sort of cool, and will get me to the shopping center, where I usually shop. But I cannot convince myself that it is safe.

In addition I have other concerns. My wife and I travel in our area. Our latest trip was to Norris Dam. Making such trips are quality of life issues for my wife and myself. Norris Dam is not far from our home, but we would probably need a vehicle with a 50 miles range to be comfortable driving there. A hundred mile range might be needed if we wanted to spend the day in the Smokies. Greater range means more batteries and greater weight. Only the first vehicle on this page might be remotely practical for such trips. It is a hybrid, and has a cost of perhaps $18,000 to $23,000. You can buy a lot more car for that sort of money, although a lot a lot better gas millage will come with the three wheeler. Bigger all-electric cares are possible, but they will require bigger batteries, and bigger batteries cost money and add to auto weight.

Thus the argument for the 650 pound car breaks down once safety and transportation range requirements are considered. A 650 pound or even lighter car might be possible for in town shopping trips, but will consumers be comfortable with their safety characteristics?

The energy efficiency question can be looked at from another perspective, applying efficiency to the production of energy. My argument has always been that efficiency is the solution posed by most nuclear related problems. One reason for adopting Molten Salt nuclear technology, is its potential for efficient manufacture and efficient use. The Molten Salt Reactor is the reactor equivalent of the 650 pound car, but with out the safety hazards posed by Pleasant Ridge Road, and without the 650 pound car's trip distance limitations. In fact the Molten Salt Reactor can manufacture its own fuel, so potentially you can keep it running for periods of time up to 30 years.

I have pointed out several times on Nuclear Green that the factory manufacture process makes more efficient use of labor, and that the limitation of factory manufactured large reactor is that they have to be manufactured as kits, with final assembly taking place on site. By shifting to smaller reactors, the number of pieces in the kit can be limited, so on site assembly does not require a lot of labor or time.

The cores of Molten Salt Reactors can be very simple and easy to manufacture, Molten Salt Reactor parts need not be built form exotic and expensive materials. It may be possible to build Molten Salt Reactors from composite materials similar to the composite materials that go into aircraft. Even when comity type materials such as steel are used in MSR parts, MSRs can be designed to operate at higher temperatures than conventional reactors, and thus will produce electricity with greater thermal efficiency.

One of my Nuclear Green readers "Engineering" recently commented on an old Nuclear Green Post,

Writing in the wake of the Fukushima events, I find it striking that the report speaks of “safety” without distinguishing between radiation exposure hazards inside the plant and massive release of radioactive materials into the environment. Avoiding massive external release is (a) more important, (b) the greatest MSR safety advantage, and (c) an unmentionable advantage in the already-risk-free Milton Shaw world.

"Engineering" is arguing that in MSRs greater nuclear safety is consistent with more efficient, low cost nuclear technology. Now that is quite a trick, but it is part of the beauty of the Molten Salt Reactor concept.

In conclusion, it would appear that efficient 650 pound cars face a rocky road, with safety and range problems, but more efficient nuclear technology via the Molten Salt Reactor concept is possible, and is consistent with improved nuclear safety.

Did Graphite in the Chernobyl Reactor Burn?

In two previous posts, " Does Nuclear Grade Graphite Burn?," and "Did the Graphite in the Windscale Reactor Burn?" I reviewed a number of reports and other information sources on Nuclear Graphite Flamibility. Although I did not come to a firm conclusion, i did find strong evidence that Nuclear Graphite does not burn under many conditions in which one would expect fire. There is also startling evidence that at least one of the the two reactor fires which are attributed to graphite, the Windscale accident, appears to have not involved a graphite fire. I concluded my Windscale review with the statement,

Given these facts, the assertion that there was a core graphite fire at Chernobyl ought also to be revisited.

This post considers several reports that are relivant to an evaluation of the role of graphite in te Chernobyl fire.

In the wake of the Chernobyl Reactor fire, the United States Department of Energy had a serious concern. The DoE operated a reactor that was similar to the Chernobyl reactor, the N reactor at Hanford, Washington. The N reactor, like the Soviet RBMK-1000, had graphite in its core. The DoE wanted to know if a Chernobyl type accident would be possible at Hanford. The DoE commissioned a review of N Reactor safety in light of the Chernobyl accident. The researchers asked

What is the potential for obtaining conditions conducive to a graphite fire in N Reactor?

And answered,

The graphite stack is protected by a helium cover gas contained within the shield structure. Combustion cannot occur unless the shield structure is sufficiently damaged to leak inert gas faster than available makeup supply. Should that occur, the rate of oxidation would be very slow because graphite temperatures would remain below the threshod for rapid oxidation because of heat removal from the stack by the ECCS [Emergency Core Cooling System] or the GSCS [Graphite and Shield Cooling System], The GSCS alone is capable of removing both decay heat and any heat load from graphite oxidation, stabilizing temperatures in a range which ensures control.

In the Chernobyl accident sequence, the plant was effectively destroyed and conditions for exothermic chemical reactions involving a number of core materials were present before graphite fire made any contribution. It is likely that the major contribution from graphite was to serve as a refractory container for decay heat buildup, zirconium oxidation along with carbothermic reduction of the UO2, and complex gas producing redox reactions. For any N Reactor accident where the GSCS and biological shield are intact, there is no way to achieve ignition of the graphite. It has been demonstrated experimentally that oxidation nuclear grade graphite takes very high temperatures to initiate, and the contribution to total heat load is only a small fraction of the decay heat.

They also reported finding that

Detailed reaction rate models have been developed to analyze graphite oxidation. These models tend to show that graphite oxidation in N Reactor would be limited both by available oxygen and the requirement that a high-temperature source (>1100°C) be available to drive a significant reaction. The analyses have effectively shown that graphite will not con- tribute significant accident heat loads.

Why then did the Chernobyl reactor graphite burn? According to the N Reactor review,

The Chernobyl release must be viewed as resulting from both very high temperatures in the core rubble, extensive mechanical disruption and dispersal of core material and the large draft "chimney effect" that followed the total disruption of that particular reactor configuration. There is no accident sequence that could produce an equivalent disruption of N Reactor; there would be some confinement even in the lowest probability event sequences. Because of the horizontal arrangement of pressure tubes, Chernobyl fission product release rates and magnitude are not pertinent to N Reactor accident scenarios with mechanistic initiators.

In 1987 the NRC did its own safety assessment of the Graphite Reactors it licensed. The NRC report described the limitations of graphite fires,

For reasons that are well understood, graphite is considerably more difficult to burn than is coal, coke, or charcoal. Graphite has a much higher thermal conductivity than have coals, cokes or charcoals, making it easier to dissipate the heat produced by the burning and consequently making it more difficult to keep the graphite hot. Concomitantly, coals, cokes and charcoals develop a porous white ash on the burning surfaces which greatly reduces radiation heat losses while simultaneously allowing air to reach the carbon surfaces and maintain the burning. In addition, coals, cokes and charcoals are heavily loaded with impurities which catalyze the oxidation processes. Nuclear graphite is one of the purest substances produced In massive quantities.

The literature on the oxidation of graphite under a very wide range of conditions is extensive. Effects of temperature, radiation, impurities, porosity, etc., have been studied in great detail for many different types of graphites and carbons [Nightingale, 1962]. This information served as a foundation for the full scale detailed studies on graphite burning accidents In air-cooled reactors initiated and completed at Brookhaven National Laboratory [Schweitzer, 1962a-f]. After British experimenters at Harwell confirmed the results obtained at BNL [Lewis, 1963] there appeared to be no new conclusions from additional work in this field. The aspects of the work pertinent to evaluating the potential for graphite burning accidents are described here In some detail.

Burning, as used here, is defined as self-sustained combustion of graphite. Combustion is defined as rapid oxidation of graphite at high temperatures. Self-sustained combustion produces enough heat to maintain the react- ing species at a fixed temperature or is sufficient to increase the temperature under actual conditions where heat can be lost by conduction, convection, and radiation. In the case where the temperature of the reaction Increases, the temperature will continue to rise until the rate of heat loss Is just equal to the rate of heat production. Sustained combustion is distinguished from self-sustained combustion when, in the first case, the combustion is sustained by a heat source other than the graphite oxygen reactions (e.g., decay heat from reactor fuel).

Early attempts to model the events at Windscale [Robinson, 1961; Nairn, 1961] were followed by the BNL work described here.

Some 50 experiments on graphite burning and oxidation were carried out in 10-foot long graphite channels at temperatures from 600°C to above 800°C. To obtain a lower bound on the minimum temperature at which burning could occur, the experiments were specifically designed to minimize heat losses from radiation, conduction, and convection.

The objectives of the full scale channel experiments were to determine under what conditions burning might initiate in the Brookhaven Graphite Research Reactor (BGRR) and how it could be controlled if it did start. Channels 10-feet long were machined from the standard 4 in. x 4 in, blocks of AGOT graphite used in the original construction. The internal diameter of the BGRR channel was 2.63 Inches. Experiments were also carried out on channel diameters of one to three Inches on 10-foot long test channels In order to obtain generic Information. The full length of the channels was heated by a temperature controlled furnace and was Insulated from conductive heat losses. At intervals along the length there were penetrations in the furnace through which thermocouples used to read the temperature of the graphite and air were introduced, and from which air and air combustion products were sampled. A preheater at the inlet of the graphite channel was used to adjust the air to the desired temperature. The volume of air was controlled and monitored by flow meters to allow flow measurements in both laminar and turbulent flow conditions.

In a typical experimental run the graphite was first heated to a preselected temperature. The external heaters were kept on to minimize heat losses by conduction and radiation. The temperature changes along the graphite channel were then measured for each flow rate as a function of time with the heaters kept on. It was observed that below 675°C it was not possible to obtain temperature rises along the channel if the heat transfer coefficient (h) was greater than 10~ cal/cm-sec-°C. Below 650°C it was not possible to get large temperature rises along the channel with 30°C inlet air temperatures at any flow rate. For h values lower than 10~ cal/cm-sec-''C maximum temperature rises were 0-50"C and remained essentially constant for long periods of time (five hours). For h values greater than 10~ cal/cm-sec-°C the full length of the channel was cooled rapidly.

There were two chemical reactions occurring along channels. At low temperatures the reaction C + O2 to form CO2 predominated. As the temperature Increased along the channel CO formed either directly at the surface of the channel or by the reaction CO2 + C. At temperatures above 700"C, CO reacts in the gaseous phase to form CO2 with accompaniment of a visible flame. It was observed that the unstable conditions which were accompanied by large and rapid Increases in temperature Involved the gas phase reaction CO + O2 and occurred only for h values below 10~ cal/cm-sec-°C below 750"C. Temperature rises associated with the formation of CO2 from C + O2 were smaller than those due to CO + O2 and decreased with time. They too occurred at h values below 10" cal/cm-sec-°C.

In a channel which was held above 650°C there was an entrance region running some distance down the channel which was always cooled. A position was reached where the heat lost to the flowing gas and the heat lost by radial conduction through the graphite was exactly equal to the heat generated by the oxidation of the graphite and of the CO. This position remained essentially constant with time. Beyond this point rapid oxidation of graphite occurred with the accompaniment of a flame (due to the CO-0 gas phase reaction). Under conditions of burning, the phenomena were essentially Independent of the bulk graphite chemical reactivity. Rate controlling reactions during burning were determined by surface mass transport of reactants and products.

The experiments were used to develop an equation which expressed the length of channel that can be cooled as a function of temperature, flow rate (heat transfer coefficient), diameter and reactivity of the graphite. It was found that the maximum temperature at which thermal equilibrium (between heat generated by graphite oxidation and heat removed by the air stream) will occur in a channel can be predicted from the heat transfer coefficient, the energy of activation and a single value of the graphite reactivity at any temperature. Above this maximum temperature the total length of channel Is unstable and graphite will burn. The studies show that the bounding conditions needed to initiate burning are:
1. Graphite must be heated to at least 650°C.
2. This temperature must be maintained either by the heat of combustion or some outside energy source.
3. There imist be an adequate supply of oxidant (air or oxygen).
4. The gaseous source of oxidant must flow at a rate capable of removing gaseous reaction products without excessive cooling of the graphite surface.
5. In the case of a channel cooled by air these conditions can be met. However, where such a configuration is not built into the structure it is necessary for a geometry to develop to maintain an adequate flow of oxidant and removal of the combustion products from the reacting surface. Otherwise, the reaction ceases.

The report went on to discuss the potential contribution of Wignarian energy to a graphite reactor fire, and found that if a reactor operated at a high enough temperature to preform Wignerian annealing its graphite would not accumulate Wignerian energy. The report also stated that,

The factors needed to determine whether or not graphite can burn in air are the graphite temperature, the air temperature, the air flow rates, and the ratio of heat lost by all possible mechanisms to the heat produced by the burning reactions [Schweitzer, 1962a-f]. In the absence of adequate air flow, graphite will not burn at any temperature. Rapid graphite oxidation in air removes oxygen and produces CO2 and CO which, along with the residual nitrogen, suffocate the reaction causing the graphite to cool through unavoidable heat loss mechanisms. Self-sustained rapid graphite oxidation cannot occur unless a geometry is maintained that allows the gaseous reaction products to be removed from the surface of the graphite and be replaced by fresh reactant. This necessary gas flow of Incoming reactant and outgoing products is Intrinsically associated with a heat transfer mechanism. When the incoming air is lower in temperature than the reacting graphite, the flow rate is a deciding factor in determining whether the graphite cools or continues to heat. Experimental studies on graphite burning have shown that for all the geometries tested which Involved the conditions of small radiation and conduction heat losses, it was not possible to develop self-sustained rapid oxidation for graphite temperatures below about 650*'C when the air temperatures were below the graphite temperature. At both high and low flow rates, the graphite was cooled by heat losses to the gas stream even under conditions where other heat loss mechanisms such as radiation and conduction were negligible.

At temperatures above about 650°C, in realistic geometries where radiation is a major heat loss mechanism, graphite will burn only in a limited range of flow rates of air and only when the air temperatures are high. At low flow rates, inadequate ingress of air restricts burning. At high flow rates, the rate of cooling by the flowing gas can exceed the rate of heat produced by oxidation.

Studies have shown that burning will not occur when there is no mechanism to raise the graphite temperature to about 650°C [Schweitzer, 1962a-f]. If the temperature is raised above 650°C, burning will not occur unless a flow pattern is maintained that provides enough air to sustain combustion but not enough to cause cooling. Since the experiments were designed to minimize all heat losses other than those associated with the air flow, 650°C can be considered a lower bound for burning.

Thus the NRC's answer to the original question which I asked at the beginning of this series is "yes, graphite does burn" but only under a very limited set of conditions.

The NRC report simply assumed that those conditions had been meet at Windscale and Chernobyl. We now know what the NRC did not know in 1987, that the Windscale fire was not a graphite fire. Neither report reviewed here offers conclusive evidence that the Chernobyl fire was a graphite fire. A major conclusion of the report draws a big question mark over the Chernobyl graphite fire hypothesis,

in order to have self-sustained rapid graphite oxidation in any of these reactors certain necessary conditions of geometry, temperature, oxygen supply, reaction product removal and favorable heat balance must exist.

Yet the Soviets claimed and American nuclear safety experts like H.J.C Kouts accepted the notion that Graphite could burn like charcoal.

The emission of radionuclide continued for about nine days, aided by burning of the graphite. It is estimated that upwards of ten percent of the graphite in the core burned, in a manner similar to the rapid oxidation of charcoal.

We know that Kouts view cannot be correct, nuclear graphite does not burn like charcoal, and the assertion that only 10% of the Chernobyl core graphite burned does not suggest graphite was the major source of the Chernobyl fire. The question is were the conditions conditions that are conducive to a graphite fire present at Chernobyl, and if so how? In answers to these questions, and without other evidence we must consider the claim of a graphite fire at Chernobyl to be unconfirmed.

As we have seen, the use of graphite in a reactor core is consistent with safe reactor operations. The danger of a core fire due to graphite burning is quite limited. The time has now arrived to ask the question, is it dangerous to use graphite in the core of a Molten Salt Reactor.

We have already noted that the possibility of graphite fires in a reactor core can be eliminated by core design. In the case of Molten Salt Reactors, the possibility of a core fire is eliminated by the two modes of MSR operation. A MSR is only active if liquid salt is present in the core of the reactor. But if liquid salt is present then air cannot be. In the case of the presence of molten salt in the core, the presence of salt would prevent air from reaching the graphite. If the salt is drained, either deliberately, by accident or by operation of the freeze valve safety system, then the heat producing fission products will be drained from the core as well. The absence of fission products in the core would mean that a high enough temperature required to trigger a graphite fire would not be possible. Thus the use of graphite in a Molten Salt Reactor core would be inherently safe.

Nuclear Accidents and Public Perception of Nuclear Safety

Nuclear safety is both about public perception, the viewpoint of the enemies of nuclear power, and about actual industrial design and practice. Relative to other industries the safety practices of the nuclear industry are very good. This assessment can be made even though the nuclear industry has just gone through its second worst accident. An accident which involved not one but 4 reactors. There were significant releases of highly radioactive fission products, although the total public exposure was small. Workers at the Fukoshima Dai-ichi nuclear plant were exposed to higher levels of radiation, although not enough to toast them. Several reactors were destroyed, and explosions destroyed several containment buildings.

The Dai-ichi accident was due to a planning failure. The reactor site plan did not allow for a 10 + meter high tsunami, ands important reactor safety equipment was overwhelmed and taken out of service by a 10 + meter tsunami. Beyond the failure of the emergency back up generators, the Dai-Ichi reactors were were designed utilizing the nuclear safety science of the day, and while reasonably safe, they were not the safest reactors possible. Indeed the term "safest reactor possible" is ambiguous, because there is a history of nuclear safety, and the history of nuclear safety demonstrates that not every choice that was made regarding nuclear safety was made with the idea of developing the safest possible nuclear technology in mind.

Unfortunately the goal of the United States Atomic Energy Commission in the 1960's was not to create the safest possible nuclear technology, it was to promote the expansion of still very weak nuclear manufacturing and energy production industries to a position of dominance in electrical production. This can be illustrated by a document which Kirk Sorensen has recently drawn attention too. A 1962 report by the AEC to President Kennedy titled, "Civilian Nuclear Power."

This report was signed by a Nobel Prize winning scientist, who was also the Chairman of the Atomic Energy Commission, Glenn T. Seaborg. The word safety appeared only once in the report. One page 60 the report contained the suggestion that future licensing reviews should concentrate

on those features which have an effect on the health and safety of the general public.

the report added,

This will be easier to accomplish as reactors become more standardized.

Thus the attitude of the AEC and of Seaborg appears to have been to let nuclear safety take care of itself without further research. Nor did the AEC consider the safety potential of various nuclear technologies important enough to note in its Report to President Kennedy. This neglect was not by accident. Rather it reflected a fundamental attitude of the leadership of the Washington nuclear establishment, which included Seaborg, fellow AEC Commissioner James T. Ramsey, Congressman Chet Hollifield, and AEC bureaucrat Milton Shaw. Within a few years this neglect of nuclear safety would serve as a back drop for the development of a powerful anti-nuclear movement, and a split within the AEC's own research establishment, that would see research scientists testifying against the AEC before Congressional committees.

The Washington nuclear establishment appears to have jointly held a broad set of beliefs about nuclear technology which included:

* The safety of Light Water Reactor (LWR) technology had been established by the United States Navy
* Reactor safety could be assured by adhering to United States Navy nuclear safety practices
* Of all advanced nuclear technologies, Liquid Metal Fast Breeder (LMFBR) technology was the most promising
*Like LWR technology, LMFBR technology was mature
* Other nuclear technologies were less promising, and there for future AEC programs should focus on LWR and LMFBR technologies
* LWR technology simply needed to be implemented, and obstacles should be moved out of that path
* The next step in the development of nuclear technology was the construction of a LMFBR prototype

This set of beliefs was to have an extremely unfortunate effect on the development of nuclear power in the United States, and globally.

It should be noted that scientists within the AEC's own research establishments did not accept the Washington Nuclear Establishment's consensus. Scientists at the AEC's national Laboratories were by no means satisfied with the safety of Light Water Reactors. In particular scientists at the AEC's reactor research facility in Idaho, as well as at Oak Ridge National Laboratory, were concerned that not enough was known about reactor safety, to judge the safety of Light Water Reactors. In addition a continuing series of accidents involving LMFBR prototypes, suggested that the maturity of LMFBR technology had not reached to level of safety that would justify a description of that technology as mature.

One particular problem troubled early nuclear safety researchers,

Because of the scarcity of useful information on fission-product release from fuels, it was necessary, in order to evaluate the safety of early nuclear reactors, to assume that 100% or a large percentage of the fission products would be released to the containment systems in nuclear reactor accidents.

Thus early on conceptual evaluations of nuclear accidents began to paint dark pictures of huge numbers of civilian casualties. Unfortunately, these dark pictures. though not justified by research, still influence public concerns over nuclear safety. The Washington nuclear establishment, focused as it was on the development of a nuclear industry, did not understand the extent to which the public perception of nuclear power would be influenced by the concerns of reactor scientists. Thus by the late 1960's as the nuclear establishment's project was taking shape, the public's perception of the danger of that project was also growing. The nuclear establishment's opposition to further nuclear safety research, which had emerged during the 1960's, became item one in the case against nuclear power presented by a powerful and growing anti-nuclear movement.

In addition to its mistaken beliefe that the safety of light water reactors was established beyond reasonable doubt, the nuclear establishment had concluded that the liquid metal fast breeder reactor wasw by far the prefered line of development for the future of nuclear power. Yet scientists at Oak Ridge National Laboratory had been able to demonstrate that reactors cooled by liquid salts had the potential to offer numerous advantages over water or liquid metal cooled reactors. Not the least of those advantages lay in the relm of nuclear safety. Molten Salt nuclear technology has superior safety potential, but since the Washington nuclear establishment underestimated the importance of the nuclear safety problem, it did not considered MSR safety potential to be an important attribute.

I personally have no doubt that in most situations that reactors are extremely safe when judged by conventional industrial safety standards. Those standards, however, have not penetrated public perception of nuclear power, and we still face both a public and political leadership, which still believes that the consequences of a nuclear accident may be far worse, than is rationally possible, and hens reactors are far less safe, than experience suggests they are.

It is clear that LWRs are not 100% safe. The Fukushima Dai-ichi accident (or accidents) has demonstrated that at least some safety features of older reactors can be overwealmed by natural disasters. To date the consequences of the Dai-ichi accident have fallen far short of a catastrophy. But whether the public is aware of the distinction between an accident and a catastrophy is open to question. For the enemies of nuclear power, acident and catastrophy are the same thing.

It is clear however, that reactors that could have withstood the natural events that brought about the Dai-ichi accidents are possible. It is clear that better nuclear safety is possible. Better public information on nuclear safety is also possible. It is urgently important to move forwards with the development of safe, low cost and scaliable nuclear technology will be of vital importance for the future of sociate. We now have lss than 40 years to accomplish this. The nuclear safety issue must be resolved, and the public reassured that a nuclear future wqill be a safew future.

How the LFTR would have survived the Japanese Earthquake/Tsunami

Future nuclear safety tests should include capacity to survive the events which lead to the Fukushima Dai-ichi nuclear plant crisis. The real survival test would require the same flawed backup generator system that was destroyed by the tsunami that struck the plant. Of course, we are not going to talk about every conceivable tsunami. For example, it is highly likely that the island of Oahu will rupture someday, dropping a large part of it into the Pacific Ocean, creating a huge tsunami. That would be a megatsunami. Since megatsunami can be up to 1000 meters high, we need not worry about a coastal reactor surviving all tsunami. No one will be around in the vicinity of a 1000 meter megatsunami to worry about a subsequent nuclear accident.

This leads us to the question of acceptable nuclear risks, a fit topic for another post. At any rate we are looking, right now, at how well the Liquid Fluoride Thorium Reactor (LFTR) or any otherMolten Salt Reactor (MSR) would have survive the natural disaster that overwhelmed the Fukushima Dai-ichi nuclear plant.

First it should be noted that Molten Salt Reactors do not require water cooling at all. Hence the loss of the emergency cooling generator system would have not been a serious problem by itself. Lets explore an accident scenarios that is at worst remote possibility, a key pipe ruptures triggering a loss of coolant accident. Say the entire content of the core coolant system - a liquid salt mixture - drains on to the floor of the reactor chamber. and forms a puddle. Since the nuclear fuel is dissolved in the coolant salts it will be deposited into the puddle. Now the interesting thing about a puddle is that its geometry is not at all conducive to a chain reaction, so the loss of coolant in turn triggers a withdrawal of nuclear fuel from the core, which in turn triggers a termination of the chain reaction, so the reactor automatically stops functioning.

Now the puddle, even though we can expect it to be short lived, might be a problem because radioactive gaseous fission products, dissolved in the fuel salt, are likely to bubble out along with volatile fission products.

Our puddle will not stay on the floor - it quickly drains into a pipe leading into a set of emergency coolant tanks, which are intended to hold the fuel mixture until the reactor can be repaired and restarted. The geometry of the tanks would be intended to prevent a chain reaction from occurring, but the fuel salt would include some radioactive fission products capable of generating the sort of post reactor shutdown heat that created so many problems in the Japanese reactors. Is there any way to insure that the liquid salt in the emergency coolant tanks does not start boiling and releasing a lot of nasty stuff? There turnout to be several passive solutions to this problem.

One: Draw air over a simple heat exchange system designed to dissipate some of the heat in the emergency tanks. Not too much heat, since we want the coolant liquid to remain hot and well, liquid. The heated air can be directed to a chimney, through which it flows into the atmosphere. The system thus requires no power, no controls and no operators. It works automatically, relying on the laws of nature to function.

Two: Rely on a thermal sink, most likely a molten or solid salt, such as the salt that carries the fuel. The inner emergency coolant tanks could be surrounded by an outer thermal sink tank. Or the coolant tank could be shaped like a donut, with inner and outer thermal sink tanks removing heat. A large enough thermal sink would probably be sufficient to dissipate heat without requiring any further heat transfer system. There is a cost for a large thermal sink salt tank system. The Integral Fast Reactor (IFR), for example, relies on this thermal inertia to prevent the reactor from overheating during an emergency shutdown, but this approach is likely to add considerably to IFR costs. However, a thermal salt vault approach combining energy storage with the safety function of decay hear dispassion, would probably would add minimally to reactor cost, while offering a source of reserve electricity.

Three: One proactive LFTR safety approach would be to remove some or even most of the fission products from the reactor. Removal of radioactive gases would be very desirable for a number of reasons. For example. as Uri Gat, and H.L. Dodds pointed out,

The source term, which is the inventory of radioisotopes in the reactor available for dispersion to the environment, contributes two-fold to an accident. The source term is the measure of the radiation which needs to be contained from reaching any sensitive location or target. The energy contained in the source term also provides the driving force for the dispersion of the source term as it is also a measure of the after heat, or the energy, to damage a reactor in the event of heat-removal failure or loss-of-coolant accident (LOCA). For an MSR, as for any fluid fuel reactor, on-line fuel processing can be applied. The on-line processing, at the least, removes the gaseous and volatile part of the source term. This part is the most likely to be dispersed when there is a breach of containment. Fuel processing also reduces the inventory of longer and long-lived isotopes as their accumulation is time dependent. The MSRs processing can be adjusted to have a small source term. The safety advantages of this small source term are many fold: The driving force for dispersion is reduced; the gaseous and volatile components, which are the most likely to disperse, are essentially all but eliminated; the long half-life isotopes (elements) are reduced such that the long-term effect of even the most unlikely accident is not severe; and, the short-lived isotopes require a proportionately short-term protection time till they decay. Thus, even a hypothetical severe accident is ameliorated a priori.

A properly designed processing facility quickly removes the separated radioisotopes from the purview of the reactor. This makes them totally unavailable to the reactor source term even under the most extreme hypothesized circumstances.

In addition to removing fission product gases, and volatile fission products, removal of nobel metals would be highly desirable from an operational point of view. Gat and Dodds state,

The source term, which is the inventory of radioisotopes in the reactor available for dispersion to the environment, contributes two-fold to an accident. The source term is the measure of the radiation which needs to be contained from reaching any sensitive location or target. The energy contained in the source term also provides the driving force for the dispersion of the source term as it is also a measure of the after heat, or the energy, to damage a reactor in the event of heat-removal failure or loss-of-coolant accident (LOCA). For an MSR, as for any fluid fuel reactor, on-line fuel processing can be applied. The on-line processing, at the least, removes the gaseous and volatile part of the source term. This part is the most likely to be dispersed when there is a breach of containment. Fuel processing also reduces the inventory of longer and long-lived isotopes as their accumulation is time dependent. The MSRs processing can be adjusted to have a small source term. The safety advantages of this small source term are many fold: The driving force for dispersion is reduced; the gaseous and volatile components, which are the most likely to disperse, are essentially all but eliminated; the long half-life isotopes (elements) are reduced such that the long-term effect of even the most unlikely accident is not severe; and, the short-lived isotopes require a proportionately short-term protection time till they decay. Thus, even a hypothetical severe accident is ameliorated a priori.

A properly designed processing facility quickly removes the separated radioisotopes from the purview of the reactor. This makes them totally unavailable to the reactor source term even under the most extreme hypothesized circumstances.

Removing all radioisotopes from a Molten Salt Reactor removes the protection that those isotopes afford. As long as the salt contains radioactive fission products, it will be far too dangerous to handle for nefarious purposes, such as the eternal bogeyman of nuclear proliferation. Salt processing can be conducted by automatic equipment inside the reactor core hot cell. The heat and radiation inside the hot cell would prevent anyone having near real-time access to a MSR.

One way of managing a reactor situation that is likely to lead to an accident, is to design a built in failure point, analogous to an electrical fuse or other weak link, which will fail before anything else. One such deliberate failure point in the MSR is the freeze valve; if a LFTR or other MSR begins to overheat, the freeze valve is designed to melt as Gat and Dodds explained,

The MSR can utilize freeze valves in critical locations or where desired. Freeze valves can be ordinary sections of pipe which are exposed to a cooling stream of environmental gas to the extent that it creates a frozen plug that blocks the flow and acts as a valve. Where such a valve has a safety function, as in draining the fuel to the storage tanks, it is prudent to design it such that the required flow is
gravity-driven. The frozen valve itself can be designed such that when the salt rises above a certain predetermined temperature the heat overrides the cooling, melts the frozen plug and opens the valve. Such an arrangement is passive, inherent and non-tamperable (PINT-safe).

Furthermore, the properly sized external cooling of the freeze valve cooling drive, such as an electric driven fan, will cease with any failure of the power and release the valve to melt and perform its safety function. This mode of operation is again PINT-safe.

Once the freeze melts, a MSR will simulate a total loss of coolant accident, with fuel/coolant salts dumped into a tank or tanks that are designed with a criticality inhibiting geometry. In his paper 2006 paper, Molten-Salt-Reactor Technology Gaps (Proceedings of ICAPP ‘06, Reno, NV USA, June 4–8, 2006, Paper 6295), MIT nuclear scientist Charles Forsberg stated,

Under emergency conditions, the liquid fuel is drained to passively cooled critically safe dump tanks. By the use of freeze valves (cooled sections of piping) and other techniques, this safety system can be passively initiated upon overheating of the coolant salt. MSRs operate at steady-state conditions, with no change in the nuclear reactivity of the fuel as a function of time. Last, the option exists to remove fission products online and then solidify those radionuclides into a stable waste form. This minimizes the radioactive inventory (accident source term) in the reactor core and potential accident consequences.

In addition to the very useful freeze plug, the capacity of molten salt to freeze at a still relatively high temperature is directly responsible for another MSR, its automatic leak control. As hot salt leaks from reactor piping, it begins to cool on contact with hot cooler air, and as it cools, it freezes, blocking further escape of coolant salt. Reportedly this mechanism is very effective in stopping leaks if they occur.

At this point I have established the case which I have sought to prove, the tsunami that destroyed the back up generators of the Fukushima Dai-ichi nuclear plant, would have left the safety systems of the LFTR in tact.

This is not the only MSR advantage. According to a group pf French nuclear scientist from the University of Grenoble, the MSR does not simply offer a high probability of safety, it offers an

excellent level of deterministic safety,

That is, safety depending only on the laws of nature and thus safety that is beyond doubt. The MSR is uniquely stable. It can be designed to safely operate without any human intervention, until such time as repairs or parts replacement is required. Thus, MSRs do not require on site operators, and indeed the stability and load following ability of the MSR are such that operators would have quite literally nothing to do. The absence of human operators would probably add to MSR safety, rather than inhibit it. that is safety that depends on the laws of nature and thus safety that is beyond doubt.

It is probably true that the safety systems of the AP-1000 and the ESBWR would have survived the Dai-ichi tsunami. But compared to the simple safety features of MSRs, the safety systems of even advanced LWRs are complex and expensive. Molten salt nuclear technology offers many potential cost saving advantages, and if all of them are employed, MSR costs could be substantially lower than the costs of LWRs. Part of the MSR advantage is higher safety at lower cost.