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.