Progress Toward an American Gas Cooled Reactor and Beyond

I am no longer a big fan of pebble bed reactors. At one time, I was excited about there more advanced features, but then I got a better understanding of their draw backs. The Chinese have a project to build PBRs, quite a lot of them. But as it turns out they are not going to be any cheaper than Chinese built light water reactors. The only advantage of the PBR then would be its ability to operate at a far higher temperature than light water reactors. That heat potentially makes PBRs useful as a source of industrial heat.

Industrial heat is important because there are no renewable post-carbon energy sources that are particularly good at producing the sort of heat that is required by many industrial processes. Very high temperature nuclear reactors turn out to be about the only source of industrial process heat that does not produce CO2. If any nation wants a chemical industry, or even manufacture cement in the post carbon era, it will need access to a very high temperature nuclear technology.

Somewhere in the bowels of the United States Department of Energy there are people who know this. They have reviewed the Generation IV options, and in their somewhat less than great wisdom, have determined that the United States should spend $4,000,000,000 between now and 2021 to produce an American gas cooled pebble bed reactor. Now four billion is not a lot to spend to build a new reactor prototype, but then pebble bed technology has had and continues to have a lot of money spent on its research. So the design of the very high temperature reactor will not exactly start from scratch. There will still be some questions to be asked and answered, however, before the prototype goes critical.

Among the attractive features of the pebble bed reactor design are some nifty passive or natural safety features. Some of the very bad things that might happen with a conventional water cooled reactor just are not going to happen to a pebble bed reactor, because PBR design does not introduce water into its core. Furthermore, as the core of the PBR heats up, the chain reaction will slow down. Make the core hot enough, and the chain reaction will slow down. The core of a well designed PBR is also meltdown proof. In fact you an heat the PBR, turn its heat up, and then turn off the fans that circulate its coolant gas. In a conventional reactor turning off the coolant pump is a sure formula for disaster. Reactor operators at Chernobyl did that, just before their famous reactor disaster. But PBR researchers have already tried turning the coolant gas circulation fans off, and the reactor shut down and cooled off on its own, without operator intervention. So a PBR has some impressively safety features.

Never-the-less, Rainer Moormann has argued that the PBR is not without safety problems and reports that

the primary circuit of the [experimental] AVR is heavily contaminated with dust-bound metallic fission products (strontium-90, caesium-137), which create major problems in the current dismantling effort.

Moormann claims,

If temperature limits for specific metallic fission products are exceeded, they diffuse through fuel kernels, coatings and graphite over the long term.

Not good! Not good at all. Even worse,

Had AVR core temperatures been known from the beginning of its operation, the AVR hot gas temperatures would have been limited to values far below 950°C. Its main advantage, its apparent capability for process heat generation, would not have been available.

Steve Thomas argues that Moormann was wrong,

the high temperatures experienced at AVR were known about 20 years ago.

Dr Albert Koster, responded to the allegations by Moorman and Thomas,

of hiding facts and supposed safety problems in pebble bed reactor . . . that PBMR knew about safety problems all the time and opted to keep quiet about it.

Koster suggests

Moormann makes a few statements on the closure of both the AVR and THTR that are not supported by the literature and personal recollections of the people involved. In effect, the AVR had come to a natural end of life for a research reactor. . . . If safety problems existed at the AVR, continued operation would not have been allowed, nor would significant experiments be approved by a regulator. There is no known document that cites safety reasons for the shutdown, . . .

Koster quotes Prof. Theenhaus, a member of the FZJ board

In more than 20 years of operation the advantages and positive characteristics of this type of reactor have convincingly been demonstrated. Many experiments have been performed, with particular emphasis on safety research…This demonstration reactor and in a certain sense research reactor completely fulfilled its mission.

Koster also pointed to a statement by Dr. Marnett, technical director of the AVR GmbH,

he AVR-Experimental Power Station has operated for 21 years...and was taken out of operation in 1988 for reasons unrelated to the plant itself.

Koster notes,

the technical problems experienced by the THTR were teething troubles that had been anticipated and budgeted for in the risk-sharing agreement between the utilities and the government. The unplanned increased cost in 1989 was due to the updated estimates of the decommissioning costs and the potential delays caused by having to re-license the fuel plant or have no fuel for more than two years

He also points to a paper by Prof. Knizia, the then chairman of the board of VEW (whose subsidiary HKG operated the THTR) and Dr. Baumer, who at the time was station manager for the THTR:

…It was not technical and especially not safety related technical problems in the plant, but external economical factors that caused risks that were outside the influence of the operator, together with a lack of commitment from the political sides to further support the project that caused the eventual early closure of the project THTR...

Koster complained that Moormann

unfortunately . . . has chosen to include so many unsubstantiated (by references) statements

In other words Moormann offered no proof for his charges. Brian Wang has previously covered the controversy, and is due a word of appreciation.

So basically the Very high temperature reactor prototype that Idaho National Laboratory intends to build during the next ten years, will be based on a previous German design, that has been worked over by South African and Chinese researchers. It might be added that the South Africans recently decided against further development of their PBMR design, and reportedly the Chinese have found that their PBR design has no cost advantage over conventional reactors.

We have to ask if the Department of Energy knows what it is doing, by choosing to concentrate on what amounts to a third hand pass me down German reactor design, that has previously rejected by both Germany and South Africa. Is Pebble Bed Technology the best possible use of the $4 billion the DoE is considering spending on advanced nuclear technology over the next 10 years. Several other potential claimants for that money are waiting in the wings. These include, The Pebble Bed Advanced High Temperature Reactor (PB-AHTR), the IFR and the thorium fuel cycle Molten Salt Reactor, the LFTR. In addition a chloride-salt cooled fast reactor is possible, and has attractive features, when compared to sodium-cooled fast reactors. The Pebble Bed Advanced High Temperature Reactor is a hybrid cross between a gas cooled Pebble Bed Reactor and a Molten Salt Reactor.

The most significant advantage of this hybrid is its small core. The use of gas coolants in PBMRs requires large cores proportional to power outputs. The large cores have a negative impact of PBMR costs. In fact the Chinese are expecting that their PBMRs will be at least as expensive as their LWRs. A molten salt cooled Pebble Bed Reactor would have a much smaller core. This would in turn lower construction costs. A Pebble Bed Advanced High Temperature Reactor would take advantage of both research and development histories of both the Pebble Bed Reactor and the Molten Salt Reactor, allowing the rapid development of a new type of very useful, safe and practical reactors. The hybrid reactor would be inexpensive to build in factories, easily transportable at power outputs that would be useful to industries, could be designed to operate in cogeneration modes, with the further use of residual heat for district heat, and fresh water production through desalinization.

The major down sides of the hybrid would be its limitations as a breeder, and the difficulty reprocessing fuel imbedded in pebble bed pebbles. For Molten Salt Reactor advocates, the hybrid offers a route to full MSR development. The Hybrid is in effect a MSR that does not carry fuel and fission products in its liquid salts. It would be a more than half way step to full MSR development.

The hybrid might be more attractive to regulators than a MSR. It looks more like a conventional reactor, and it relies on pebble bed technology, which most have read about in their textbooks. In addition, the use of liquid salt as a core coolant is something that regulators might feel comfortable with. Liquid salts in the hybrid core would function like water in a LWR core, but without the safety problems of LWRs. It would certainly be a safer coolant than liquid sodium. At he same time the liquid salt coolant is compatible with the very high temperature operations required to produce industrial heat. Again once regulators became familiar with the role of liquid salt in hybrid technology, convincing them of the viability and safety of MSR technology would be easier.

For anyone who is interested in learning more about hybrid reactor technology, ORNL is offering a workshop on Fluoride Salt-Cooled High-Temperature Reactor technology on September 20–21, 2010. A meeting of The GIF MSR Provisional Steering Committee will follow on September 22.

The hybrid technology thus offers a very attractive alternative to the gas cooled pebble bed concept, with a potential of producing up to 1200 C heat directly, and even higher heat indirectly through hydrogen production.

The readers of Nuclear Green are aware of the case I have laid out for the potential value of a thorium breeding Molten Salt Reactor - the LFTR. I have argued the LFTR could potentially supply all human energy demands for a billion years without running out of fuel, that it would be safe, potentially could be built at a low cost, and would largely and perhaps completely solve the problem of nuclear waste.

The Integral Fast Reactor (IFR) is a sodium cooled breeder reactor. Advocates claim that it offers a very high breeding ratio, but my review of IFR plans on the The United States Department of Energy information Website, the Energy Bridge, does not substantuate this claim. Statements by IFR designers indicate that the IFR is designed to be a burner of plutonium and other actinides, that can be operated at a low conversion ratio. Current IFR designs are not intended to be high ratio breeders, and in order to performe this role, the IFR would have to be substantually redesigned. Never-the-less, commercial sodium cooled breeders are expected to come into commercial use during the next 10 years in India.

The IFR has considerable built in passive safety features, but the use of sodium coolant will always be a safety worry. Protection against a sodium related accident will probably add to IFR expenses, and it remains open to question whether the IFR with emerge with the sort of cost advantages, which currently seem plausible for the LFTR. In addition the IFR would not operate at a high enough temperature to provide industrial heat for many processes. This is not to say that the IFR reactor is a bad reactor but to say that it potential may be limited, and perhaps far more limited than the LFTR.