By Charles Barton
The "Keys" series has pointed to two Generation 4 reactor designs as having potential for lowering reactor costs. They are the Pebble Bed Reactor and the Liquid Fluoride Thorium Reactor. Of the two the LFTR has far superior potential for eliminating the problem of nuclear waste. The LFTR also has a potential safety feature would revolutionize nuclear safety, because fission products (FP) are either bonded to fluoride or readily extracted from the liquid fluoride salt mixture. It is possible to extract the fission products from the liquid fluoride salt mixture either by a continuous processing of the salts, or by a periodic batch processing. From the viewpoint of nuclear safety, continuous processing holds a decided advantage. If fission products were to be stripped out of the reactor on an ongoing basis, there would be a number of advantages. This possibility was actually explored at ORNL in the MSBR days. (see ORNL-TM-3579: Design and Cost Study of a Fluorination-Reductive Extraction-Metal Transfer Processing Plant for the MSBR) The ORNL study estimated the cost of of a 3 day turn around processing plant for a 1000 MW LFTR would be $25M. This facility would have similar processing capacity to that required for a 6 hour turn around on a 100 MWe LFTR. In 2008 terms the cost would be something over $100M. However it appears that extracting FPs were only a small part of the 1972 fuel processing system's aim, and the extraction of many FPs was expected to take place within a 2.4 hour time frame. Hence a processing plant designed to remove FPs from a 100 MW reactor might cost far less than $100M.
The first event was basically preventable by use of fluid fuel, As the chain reaction increased in the reactor core, heat levels would rise, fluid fluoride salts expand as they are heated, the expanded salts are pushed out of the reactor core carrying fissionable materials with them. The decrease in fissionable materials means that the nuclear reactor will slow and its heat will drop. Thus a runaway chain reaction is impossible because of the design of the reactor and the physical properties of its fluid salt core.
"There are two major possible events that can lead to a dispersal of radioactivity from a nuclear reactor and become a health hazard. The first is an uncontrolled reactivity increase that will yield a power burst which would damage the reactor and disburse the radioactive fission products inventory. The other event could be failure to remove the decay heat of the fission products resulting in overheating and dispersal of inventory. The so-called source term - the likelihood for a quantified release of radioactivity -is the product of the inventory of fission products and the driving force or the energy to disperse this inventory."
Gat believed that it was possible to build ultimate safety into fluid-fueled reactors, and the key was the continuous extraction of fission products:
"The source term in the U[ltimately] S[afe] Reactor is controlled by continuous removal of fission products at the rate they are produced. Fission products are allowed to accumulate only to a level of 1 to 6 hours of full power operation equivalent. That is an equilibrium level as if the reactor had operated for the equivalent time without any removal of fission products ; then any additional fission products are removed as they are produced. . . . It has been determined 2] that at the 1 to 6 hours equivalent build-up time the fused fuel salt of The U .S . Reactor will not reach boiling due to after-heat even without any heat removal . Thus decay heat cannot provide sufficient energy to disperse the fission products, and there is no source term associated with the decay heat."
Ralph Moir is a retired Lawrence-Livermore National Lab scientist. His professional career was spent developing the concept of a Fusion-fission hybrid reactor, but in his retirement Moir's interest has shifted to the development of the LFTR. Moir's co-authored Edward Teller's last paper, which made concrete proposals on the subject. In a previous"Keys" posting, I briefly described the Teller-Moir views on underground siting of LFTRs.
It should be noted that Teller and Moir (Ralph Moir and Edward Teller, Nuclear Technology 151 334-339 (2005), http://www.geocities.com/rmoir2003/2mlt_slt.htm) also advocated stripping FPs from the fluoride salt mixture. Not withstanding the often-stated notion that stripping
FPs meant that containment was unnecessary, Teller and Moir advocated a 4 barrier containment system. Their containment system structure was, however, not massive. Indeed gravity provided one of the 4 protective barriers to FP release.
Would continuous stripping of FPs from LFTR fuel/coolant salts be justified from the viewpoint of cost? This is a topic for research. Stripping FPs would make LFTRs not just safer, but safe, and this would boost public confidence in the safety of nuclear power. In addition, some construction savings would result from this feature. The reactor would not require a primary emergency cooling system, nor would it require a cooling system for core drain tanks. Although a system of multiple barriers to FP escape would still be in place, one of the 4 barriers would be gravity which has no cost, and two of the 4 barriers would be the reactor vessel, and the reactor housing, that is their serving as safety barriers is only a secondary function. One further barrier is required by the Teller-Moir system, but that outer barrier would need not be massive or expensive.
Massive protection from suicide aircraft attack would be provided by the under ground setting.
Would there actually be construction savings from continuous stripping of FP from the the LFTR fluoride salts? That is a matter for research! Research would lead to one of three conclusions:
1. The stripping FP's from the LF salts, would low construction costs.
2. The cost of stripping FP's from the LF salts would exceed other construction savings, but the added safety benefit would be worth the added expense.
3. Stripping FP's from the LF salts would not be cost effective.
Ralph Moir has provided a list of proposed LFTR research topics here. Moir's research projects would lead to the creation of a first generation of LFTRs commercial power reactors. In a paper titled "Recommendations for a restart of molten salt reactor development," (Ralph Moir, Energy Conversion & Management) Moir justifies the LFTR research on the grounds that it could lower nuclear generated power prices by as much as 20%. In fact the potential savings entailed in my "Keys" series could lower the price of nuclear power to the point where it would be significantly less expensive than wind, and quite possibly competitive with CSP in the Southwestern United States, and less expensive than CSP: in the rest of the country. In an interview which I posted on Nuclear Green, Moir stated that a crash research program to develop LFTR technology could be brought to completion for a billion dollars. This is just a guess, and suggestions I have advanced in the "Keys" series. could well cost more than a billion dollars to research.
Moir's suggested research program in his "Recommendations" is a business as usual program. The time for business as usual has past, and in my interview with Moir, he agreed that a Manhattan Projects style research program for the development of the LFTR was warranted.
It is not as if the LFTR is a newly invented idea. My father started research on it in the summer of 1950. One of the research projects to look at solubility enhancement, corrosion, neutron loss of NaF, ZrF4, UF4 salts, is actually a return to a fuel salt formula which my father patented. (I must add that I would in no respect profit from my father's research, patent rights are held by the U.S government.) Two liquid-fluoride reactors were built in the 1950's and '60's, and both were quite successful. I have demonstrated with a detailed analysis of WASH-1222 and a description of the destructive career of AEC bureaucrat Milton Shaw, who made many serious mistakes including the decision to terminate liquid-fluoride reactor research (see here, here and here).
ORNL generated a large body of research data on liquid-fluoride reactors from about 1950 into the 1970's. These research reports, many of which can be found in the "Energy from Thorium" document repository, constitute the starting point for any LFTR research and development program. The LFTR has been the focus of ongoing research programs in France, the Netherlands, Russia, Japan and the United States. In addition to the Teller-Moir paper, numerous other papers and reports have recommended expanded programs of LFTR research and development.