Sunday, November 13, 2011

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, 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,


Jim Baerg said...

I would be most interested in a comparison of the Fukushima I set of reactors with the Fukushima II set. Fukushima II is only a few km along the coast from Fukushima I & was built a few years later, but they didn't hit the news since they didn't have anywhere near as many problems as Fukushima I.

So *why* did Fukushima II handle the quake & tsunami so much better?

Anon said...

Probably a lot to do with the generators at II not being taken out by the tsunami.


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