Sherrell Greene on Liquid Chloride Reactors, "Business as Usual," and a second Manhattan Project

The third part of my interview Q&A with Sherrell Green focused on questions concerning the future of nuclear technology. My father had been a pioneer in research on Liquid Chloride Reactor technology in the 1950's. Sherrell Greene mention LCR development during out preinterview, so I wanted to ask him some follow up questions. The LCR is a potential competitor to the Liquid Metal Fast Breeder Reactor, but it is not clear if itsadvantages would out wight its potential costs. Sherrell notes that,

The environment in today’s nuclear energy enterprise is hostile to innovation.

This is undoubtedly the case, and the consequence of our unwillingness to take risks on new concepts in nuclear power are ominous. for our country are ominous. Continuing a business as usual pattern is likely to bring national economic and environmental failure, but it is not clear what it is we should be doing.

30. You view the development of a Liquid Chloride Reactor (LCR) as an important path to Generation V nuclear technology. What do you view as the most important contribution of LCR development to improving nuclear technology?
No I don’t. I’m actually very much on the fence with regard to LCRs. They are potentially attractive fast-spectrum systems and could offer some innovative options for burn/breed fuel cycles. But they also face much more daunting chemical compatibility challenges than fluoride salt-cooled concepts. I don’t consider LCRs to be in the same category of feasibility as liquid fluoride reactors.

31. Would a LCR have significant advantages over Liquid Metal Fast Breeders (LMFBR).
I don’t think we know enough about LCRs to answer the question.

32. Would a LCR have safety advantages in comparison to LMFBRs?
Theoretically, they should have because the coolant wouldn’t interact as energetically with air and water. But again, we know more about LMFBRs than LCRs.

33. Would a LCR have any notable safety problems, in comparison to LMFBRs?
I don’t think we know enough to answer the question. They two reactors have very different shutdown mechanisms. Again, until one has a specific mechanical design in front of them, it’s mostly speculation.

34. Do any Liquid Chloride Salt formulas hold potential advantages in comparison to Flibe (The lithium fluoride (LiF) and beryllium fluoride (BeF2) mixture, often referred to as the preferred carrier/coolant salt formula for MSRs?
Most of the chloride-based salts don’t perform well in the thermal spectrum. Corrosion management is more demanding than the fluoride salts. LiF-BeF2 (FLiBe) mixtures are very attractive from the nuclear, thermal, chemical, and thermo-mechanical standpoint. But with you have a tritium production issue, a lithium enrichment cost issue, and a beryllium occupational exposure issue to deal with. All of that drives up the cost. George Flanagan and David Holcomb at ORNL have recently evaluated some of these issues. They conducted an initial screening of LCR salt options, and developed a pre-pre- conceptual concept for a LCR. MIT and a few others have also done work in this area.

35. Do any Liquid Chloride Salt formulas have the potential of being technically competitive with Flibe, but at a lower cost?
Again, I think you will want to consider chloride salts for harder-spectrum systems. So it’s a different application. That’s said, I don’t think we know enough to answer the question. I am pessimistic there is a chloride salt that can match FLiBe from an integrated performance perspective.

36. What changes would you view as desirable in the current business-as-usual pattern of the nuclear technology industry?
This is probably the most important question we’ve discussed! I don’t pretend to have many answers. But I’m thinking a lot about this issue.

I’ve been privileged to work in one of the world’s premier energy research laboratories for over three decades. I’ve seen the ins and outs of the Department of Energy, and its national laboratories. I’ve been involved in major long-term international collaborations. I’ve supported the NRC. I worked with the nuclear industry. All of these venues are populated with extraordinarily bright, committed people with noble motives. Yet “business-as-usual” doesn’t seem to be working very well in the US.

The environment in today’s nuclear energy enterprise is hostile to innovation. Not by intent, but in reality nevertheless. The industry is highly regulated. It is very costly to do research, development, and demonstration. It’s a very capital-intensive business. The barriers to entry are incredibly high. The down-side risks of innovation are more easily rendered in practical terms than the upside gains. Often it seems everyone in the enterprise (federal and private sectors) are so risk-averse that innovation is the last thing on anyone’s mind. In this environment, “good-enough” is the enemy of “better”. Humans learn by failing. It’s the way we learn to walk, talk, and ride a bicycle. Our environment today has little tolerance for failures at any level. There’s no room for Thomas Edison’s approach to innovation in today’s world. On top of all of this, or perhaps because of it, the nuclear industry invests less on R&D, as a percentage of gross revenues, than practically every other major industry you might name.

We can and must change this paradigm if the 3-4 billion people on this earth who are in dire need of electricity are to ever have it. I view this as both a moral imperative and a practical necessity if global peace and stability are to be sustained throughout this century.

I’m an advocate of “Design Thinking”, which is a multi-disciplinary, human-centered approach to innovation pioneered by Tim Brown and others. One of the things I hope to do in the post-ORNL phase of my career is to integrate the Design Thinking paradigm with my experience in the nuclear energy enterprise to find ways to accelerate the rate of innovation and deployment of improved nuclear energy technologies and systems. This basically begins with questioning (notice I did not say attacking) everything about the status quo - assumptions, approaches, frameworks, paradigms, systems, etc. at all levels. It means re-visiting all constraints and asking a lot of “What if” and “How might” questions. All of this while maintaining a zealous adherence to scientific and technical integrity and engineering discipline. It means looking to both natural and man-made analogs for inspiration. (As an example, I believe the nuclear industry can learn a lot by emulating the petroleum industry’s approach technology deployment.) A fresh look at government – private sector partneships for research, development, and demonstration (RD&D) is in order. Realistic and practical methods for risk / reward sharing must be developed.
I’m a dedicated free-market guy. But it is clear to me the free market is not currently serving the long-range interests of our nation and our globe in terms of strategic energy resource development. There are policy, regulatory, business system, and other issues, but it’s clear to our current processes for policy and regulatory framework synthesis, and energy technology development and deployment do not capture all of the essential feedback loops required to drive us to good long-term decisions.

37. Would the Anthropogenic Global Warming problem justify a new Manhattan Project?
Well, this may surprise you, but I’m not sure I know what a “new” Manhattan Project looks like. If your definition is simply a focus of enormous federal resources on the problem, my answer is that’s a necessary, but not sufficient action.

In my mind, the achievement of the atomic bomb, the moon landing, and the Interstate Highway system are three of the greatest achievements of the US federal government. But the context and circumstances surrounding each of these accomplishments were unique to each. R&D investment in the Manhattan Project spawned a second generation of scientists and engineers who achieved the moon landing. Investment in the space program inspired and enabled a third generation of scientists and engineers whose talents and creations have benefited society in a plethora of ways during the past few decades. The interstate highway system is a trophy of perseverance. The Manhattan Project was done in response to an imminent threat to our existence. The lunar landing was motivated out of national pride and our desire to outshine the Russians during the cold war era. The interstate highway system was created out of recognition of the need for America to be “one nation” in culture and commerce.

The problem with global climate change is that it’s everybody’s problem. Which too often in this world means it’s no one’s responsibility to address. I’m reminded of both anecdotal and experimental studies showing that a large group of witnesses are less inclined to intercede to aid someone being attacked on the street, than is a individual who witnesses the event. When everyone owns a problem, no one owns the solution. It’s also a perfect “frog in the kettle” problem - big change in small steps over a long (in human lifespan terms) timeframe. Another issue is that the challenge is made so complex and so enormous by many who discuss it, that the average person surrenders in confusion or hopelessness. Climate change in many ways is the ultimate test of the “think globally, act locally” mentality. Finally, the “dirty little secret” of climate change is that if it occurs as many predict, there will be winners and losers in terms of countries, cultures, and populations. Massive population shifts would occur, but the story is not all bad. Large regions of land that are currently un-inhabitable could become prime real estate.

I feel we need to approach the global climate change problem with a hefty dose of humility. This beautiful blue planet we inhabit is a wonder of complexity and balance. We need a revolution in understanding of natural systems, their interactions, and their feedback loops. Understanding the natural system leads to predictive capability. Predictive capability enables us to examine the value of various interventions. This, in turn, allows us to gauge the value of various technologies. When we understand the value of various intervention technologies, we can prioritize our technology RD&D options. One thing is pretty clear: more carbon in the atmosphere is not a good thing. So reducing our carbon emissions and exploring ways to terraform or remove carbon from the atmosphere would be a critical element of a major attack on the problem. But we are talking about massive amounts of carbon. Nuclear energy could play an important enabling role in both of these endeavors.

Lastly, I believe our great scientific and technical institutions would have to be refocused in many ways to be successful in a “new Manhattan Project.” One challenge we face is that for too long the federal sector has tended to focus on “basic science” to the detriment of use-focused R&D. We’ve avoided the what Donald Stokes called “Pasteur’s Quadrant” of RD&D. After all, how many people can point to a single innovation from our national laboratory system in the past twenty years that has had a major impact on the lives of ordinary people? What grand problem have we actually solved? I’m not disrespecting the national labs. I love them. But in fact, there’s been an aversion in many quarters of the federal government to pursing research that has near-term payback or impact. I think it is time for us to rethink the value propositions for federally-funded R&D. Often, many of the challenges are in technology development and systems integration phases of development – not in the very fundamental basic research. This (technology development and systems integration) is the so-called “valley of death” between discovery and impact. It is precisely this type of RD&D that has been out of favor for too long in the federal sector, and where the interface between the federal and private sectors is broken.

38. Should the AHTR and/or the SmAHTR be targeted for development by a second Manhattan Project?
I feel that FHRs (SmATHR, AHTR, PB-AHTR) and MSRs warrant consideration for future development and deployment. I do feel the nation would be well served by a balanced program of FHR and MSR system concept development and enabling technology development. Such work is necessary to enable us to sufficiently mature system concepts and technologies to the point required to inform downselect decisons. We’re not there yet.

39. What other nuclear technologies would you see as candidates for rapid development by a Second Manhattan project? (If you view that a second Manhattan Project is desirable.)
Let me broaden your question.... I feel there are cluster of key technologies, working in harmony, that could revolutionize our planet: high temperature nuclear energy systems; carbon capture and sequestration to enable use of fossil fuels; ultra-high density electrical energy storage for individuals, vehicles, homes, and the grid; and “retrofitable” residential and commercial “super insulation” to reduce building energy consumption in existing buildings. These four technologies, along with a fortified and modernized electrical transmission and distribution grid, could revolutionize our future.

Sherrell Greene on AHTRs, SmAHTRs, and MSRs

Sherrell Greene's research at ORNL included exploration of Fluoride Salt Cooled High Temperature Reactors (FHRs). Molten Salts can be used as both reactor coolants and as reactor coolants/fuel carriers. Advanced High Temperature Reactors (AHTR) are hybrid reactors which combine features of Molten Salt Reactors with features of Gas Cooled Graphite Structured or of Pebble Bed Reactors. The nuclear fuel for AHTRs is solid and embedded in graphite structures rather than a liquid salt dissolved in liquid salt coolant/carriers. SmAHTRs are Small Advanced High Temperature Reactors. (Sherrell Greene's web site can be found at, www.SherrellGreene.com.)
(Questions 12 and 13 have been omitted.)

14. What do you view as the advantages of Molten Salt cooled Advanced High Temperature Reactors (AHTR)?
First, to avoid confusion, at ORNL we developed some terminology I would like to see adopted more widely. We liked to call liquid salt-cooled reactors, “FHRs” – Fluoride salt cooled High temperature Reactors. These are salt-cooled, but not salt fueled. Then, of course, there’s the molten salt reactors or “MSRs”, with are both cooled and fueled with fluoride (or possibly chloride) salts.

The first FHR concept ORNL developed in conjunction with SNL and others, was the Advanced High Temperature Reactor (AHTR) in the early 2000’s. The AHTR in a large GW+ class central-station electricity generator. The second concept we developed during the past year or so was the Small modular Advanced High Temperature Reactor (SmAHTR). SmAHTR is a 125 MWt / 50+ MWe modular FHR for both process heat and electricity production.

In my view, FHRs integrate the best attributes of liquid metal-cooled reactors (LMRs), gas-cooled reactors (GCRs), and molten salt-cooled reactors (MSRs). They are high-to- very-high temperature, low pressure systems. They employ fluoride salt coolants, TRISO particle graphite fuels, and Brayton power conversion systems. Due to their low pressure, salt coolants, and graphite fuels, they inherit the best safety attributes of LMRs, GCRs, and MSRs. They inherit the economic advantages of low pressure systems – which means thinner-walled vessels and piping.

However, FHRs have their own issues. The favored fluoride salt (FLiBe) is very expensive. The high-temperature salt-tolerant structural materials are expensive. There are tritium control issues. And since you have to make the jump from a low pressure reactor to a high-pressure power conversion system in the electricity production application, there are a number of component design and reliability challenges – particularly with regard to heat exchangers.

Having said all of that, I’m big on the promise of FHRs.

15. How might the development of the AHTR contribute to the development of the MSR?
In many ways. In order to successfully develop and deploy an FHR, one must solve many of the fundamental technology challenges required for MSRs: the fluoride salt supply chain, the structural materials supply chain, fluoride salt pumps and heat exchangers, tritium control, instrumentation and control technologies... just to name the more important technologies. The development of an FHR would leave the country with a robust development infrastructure for MSRs. And the successful deployment of an FHR would be a path-finder for the licensing and regulatory framework environment required for successful MSR deployment. However, the fact that MSRs have a liquid, mobile fuel will be a large, unaddressed hurdle in the regulatory arena.

16. What do you view as the potential uses of the AHTR?
High-to-very high temperature process heat and high-efficiency electricity production for central station and remote applications.

17. Does the AHTR have a potential cost advantage compared to the LWR?
Yes. Their low pressure character translates to less metal in their construction. The coolant doesn’t interact energetically with air/water. This means one is not driven to massive containments. Less concrete. SmATHR could benefit enormously from factory fabrication. However, there are off-setting issues. I’ve already mentioned that fluoride salt coolant is really expensive. The nickel alloy structural materials aren’t cheap either.

ORNL and UC-Berkeley have both published analyses that indicate the FHRs should be competitive or superior in cost performance to modern LWR technologies. However, I would caution that due to the relative immaturity of the AHTR and SmAHTR concepts, it isn’t feasible to do the detailed bottoms-up cost estimates that support a truly compelling case at this point. We need to bring the concepts along to a higher degree of fidelity before that is really possible.

18. What are the AHTR potential safety advantages?
TRISO graphite fuel is a very robust high-temperature fuel form and operates with large thermal margins in FHRs.

Fluoride salt doesn’t react energetically with air or water.

Fluoride salt is an extremely attractive heat transport medium. This means less coolant to be pumped during operation, and the core can be cooled effectively with passive decay heat removal loops after shutdown.

Low pressure systems remove a major mechanical driving force for expulsion of radioactivity from the reactor system in the event of an accident.

19. Does the AHTR have potential safety advantages in comparison with advanced LWRs?
I believe FHRs do have advantages due to the attributes I mentioned above.

20. What are the useful characteristics of the Small Modular Advanced High Temperature Reactor (SmAHTR)?

SmAHTR can be factory fabricated, transported by a large semi-tractor trailer rig, and “installed” on site. It is a dual function high-temperature process heat and electricity producer. When coupled with the “salt vault” energy storage system, they can be clustered to meet a variety of power and energy demands.

21. What contributions would you foresee the SmAHTR making to Post-carbon energy technology?

An entry-level or prototype SmATHR would probably operate at ~600oC to 700 oC. Later systems would probably move up to 800 oC to 850 oC as materials performance limits are resolved. In theory, there’s no reason a SmAHTR or an AHTR couldn’t operate as high as 1000 oC – but that’s a long-term goal. In its early manifestations, SmAHTR and AHTR could provide high quality process heat required for production of a variety of fossil-based synthetic liquid fuels, hydrogen, and ammonia. Charles Forsberg, formerly at ORNL and now at MIT, is exploring such “hybrid energy system” concepts.

(There is no question 22.)

23. What would the SmAHTR do better than any other post-carbon energy technology currently being considered?
Well, it has the potential to be a game-changing high-temperature process heat system – particularly when coupled with the “salt vault” energy storage system I described in the SmATHR pre-conceptual design report (ORNL/TM-2010/199). I’m tempted to say, “Anything a gas-cooled reactor (GCR) can do, an FHR can do better”. But the fact is we know more about GCRs than we do FHRs at this point. So it’s a bit premature to make that claim.

24. What would be the safety advantages of the SmAHTR?
It has most of the advantages of an MSR and it’s small. The differences are almost entirely a function of utilizing solid TRISO fuel in the FHR rather than the liquid fuel of the MSR: Discrete fuel that can’t leak, high thermal margins, low pressure, relatively inert coolant in terms of interaction with air and water, the ability to remove decay heat passively, etc. It’s also an integral primary system concept, with no reactor vessel penetrations so traditional pipe-break loss of coolant accidents are precluded by design.

25. Would there be any serious safety problems for the SmAHTR?
None that I am aware of. However, I would want to really understand air-ingress accidents in FHRs due to their use of graphite fuel and other graphite structures in the reactor vessel. Refueling operations are also due serious scrutiny.

26. Do you believe that SmAHTRs can serve as the basis of a new energy related enterprise?
On paper, SmAHTR is excellent choice for distributed hybrid energy systems such as those Charles Forsberg likes to discuss. In our SmAHTR pre-conceptual design report (ORNL/TM-2010/199), I described an integrated “salt vault” energy storage system that could be a key enabler of multi-functional FHRs.

27. How much would it cost to develop an SMAHTR prototype?
A lot. That’s the case for any new reactor. If by prototype, you mean an NRC-certified design and functioning prototype, simply look at the investment today’s commercial rector vendors are making to secure a design certification for a new evolutionary LWR. It’s in the neighborhood of $0.5–1B dollars. But we are not in a position to estimate SmAHTR’s development cost until the concept is more mature. Reactor development is a “deep pockets” endeavor.

28. What could an SmAHTR do better than a Molten Salt Reactor (MSR)?
I haven’t looked specifically at the question. So my answers are fairly speculative. Given my view that FHRs would probably be easier to commercialize than MSRs, I would turn your question around and ask, “What can an MSR do better than an FHR?” My instincts are that in terms of the basic functions of high-temperature process heat production and high-efficiency electricity production, FHRs such as SmAHTR and the MSRs are equals. Due to the differences in fuel, they will have different waste streams. I don’t think we understand that issue well. SmAHTR is a uranium-based, thermal spectrum system. Right now, the reference SmAHTR concept employs a cartridge core comprised of hexagonal fuel assemblies employing graphite fuel plates. There’s more trade study work to be done to optimize the core configuration. Both pebble-bed cores and cores comprised of discrete fuel assemblies are possibilities. Per Peterson’s team at UC-B have looked at pebble bed FHRs and seed-blanket thermal breeding strategies. We’ve not looked at a SmATHR breeder. But I think the MSR has the potential to be a better thermal breeder than the FHR. The MSR probably would also have an advantage in terms of assured shutdown mechanisms due to the ability to use freeze plugs in the fuel loop. The flip side is that MSRs have the potential for fuel leaks and FHRs don’t. Finally, since there is, at least on paper, the potential for a fast-spectrum MSR, the MSRs almost certainly have an advantage for actinide burning.

One thing I would add... SmAHTR is an integral primary system design. Previously MSRs concepts have been loop designs. In my view, they’ve been plumber’s nightmares. That’s one of the reasons ORNL abandoned the two-fluid Molten Salt Breeder Reactor (MSBR) concept. If you look in detail at the Molten Salt Reactor Experiment (MSRE) at ORNL, they had many, many side streams of hot liquid salt. That was appropriate for an experiment and necessary at the time for a variety of reasons. But, today one would never design a commercial reactor that way. My friend Jess Gehin at ORNL and I have been discussing the idea of an integral-design MSR for some time. We believe integral concepts could potentially improve the reliability and operability of the MSR concept.

29. Might the SmAHTR be viewed as a stepping-stone to MSR development?
Absolutely. I believe a small FHR would be easier to design and license from the overall plant perspective. One doesn’t face all the salt processing and and flowing-fuel challenges posed by MSRs. However, one must again proceed with a hefty dose of humility here. FHRs are paper reactors. And there is no modern MSR design. So we do not have FHR or MSR design criteria. We do not have the FHR and MSR design details required for development of a PIRT (phenomena identification and ranking table) – much less the detail needed to conduct PRAs. There’s no licensing roadmap for either concept. That said, the MSR employs a flowing fuel that must be kept contained under all circumstances. This creates a plethora of design challenges that have never been faced in a modern regulatory environment. From the plant design perspective the flowing mobile fuel impacts system complexity and redundancy, plant reliability, plant cost, and licensing difficulty. The FHR does not have to face those challenges.

Finally (though many in the FHR and MSR community would not agree with me) I am concerned about the overall cost (supply chain, operations, and disposal) of FLiBe. I believe it could be an impediment to deployment of FHRs and MSRs. In my view, FLiBe is the worst possible FHR/MSR salt – other than all the other salts (laugh). It’s wonderful in terms of its integrated nuclear, thermal, and thermalhydraulic performance. However, the cost of removing the Lithium-6 will be high. An you still have to worry about tritium production after you do it. Beryllium is also a human occupational risk and environmental disposal issue. So I wish we could develop an affordable alternate coolant salt that performs as well as FLiBe. There’s been little serious coolant salt design work since ORNL’s original efforts in the 1950’s and 1960’s. I think we have a shot at developing a good alternative if we really applied ourselves. In the mean time, FLiBe’s overall performance is compelling.

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.)