[I]t is incumbent on those in high positions to reach wise decisions, and it is reasonable and important that the public be correctly informed. It is incumbent on all of us to state the facts as forthrightly as possible. – Hyman Rickover testifying before Congress in 1953,

QUESTION FROM SENATOR SHAHEEN

Q3. Of the six Gen IV nuclear power technologies proposed by the US in 2000, DOE Idaho National Labs have been pursuing two – (1) high temperature gas-cooled reactors for hydrogen production, and (2) sodium-cooled fast reactors for waste burning. Separately, liquid-fluoride thorium reactor research is ongoing at UC Berkeley, MIT, Redstone Arsenal, and in other countries including France, Japan, and Canada.

As the Department analyzes advanced reactor designs, can you tell me if the liquid-fluoride thorium reactors are under consideration? What are the benefits of liquid-fluoride thorium reactors? What are the drawbacks or downsides of liquid-fluoride thorium reactors? How does power generated from liquid-fluoride thorium reactors compare, on a price per kilowatt hour, with power generated from the current coal generation fleet in the United States? As we confront our nation’s energy and climate challenges, what role might these types of reactors play?

Secretary Chu:

A3: The “liquid-fluoride thorium reactor,” otherwise known as a molten salt reactor (MSR), where molten salts containing fissile material circulate through the reactor core, is not part of the Office of Nuclear Energy’s research program at this time. Some potential features of a MSR include smaller reactor size relative to light water reactors due to the higher heat removal capabilities of the molten salts and the ability to simplify the fuel manufacturing process, since the fuel would be dissolved in the molten salt. One significant drawback of the MSR technology is the corrosive effect of the molten salts on the structural materials used in the reactor vessel and heat exchangers; this issue results in the need to develop advanced orrosion-resistant structural materials and enhanced reactor coolant chemistry control systems. In addition, operational practices would have to address the fact that the liquid salts solidify between temperatures of 300 C to 500 C, thereby requiring the use of special heating systems when the reactor is not operating. From a non-proliferation standpoint, thorium-fueled reactors present a unique set of challenges because they convert thorium-232 into uranium-233 which is nearly as efficient as plutonium-239 as a weapons material. A cost per kilowatt hour estimate has not been developed.

From ORNL/TM-6002 (J. R. Keiser, 1977):

As a result of these studies, we have found that Hastelloy N exposed in salt containing metal tellurides such as Li Te and Cr Te undergoes grain boundary embrittlement like that observed in the MSRE. The embrittlement is a function of the chemical activity of tellurium associated with the telluride. The degree of embrittlement can be reduced by alloying additions to the Hastelloy N. The addition of 1 to 2 % Nb significantly reduces embrittlement, but small additions of titanium or additions of up to 15% Cr do not affect embrittlement. We have found that if the U(IV)/U(III) ratio in fuel salt is kept below about 60, embrittlement is essentially prevented when CrTel.266 is used as the source of tellurium.

From ORNL/TM-6415 (1979):

The nickel-based alloy Hastelloy N, which was specifically developed for use in molten-salt systems, was used in construction of the MSRE. The material generally performed very well, but two deficiencies became apparent: (1) the alloy was embrittled at elevated temperatures by exposure to thermal neutrons and (2) it was subject to intergranular surface cracking when exposed to fuel salt containing fission products.

Recent development work indicates that solutions are available for both these problems. Details of this work are given by McCoy; a summary of the results follows

Irradiation experiments early in the MSR development program showed that Hastelloy N was subject to high-temperature embrittlement by thermal neutrons. The MSRE was designed around this limitation (stresses were low and strain limits were not exceeded), but the development of an improved alloy became a prime objective of the materials program. It was found that a modified Hastelloy N containing 2% titanium had much improved post-irradiation ductility, and extensive testing of the new alloy was under way at the close of MSRE operations.

The second problem, intergranular surface cracking, was discovered at the close of the MSRE operation when surface cracks were observed after strain testing of Hastelloy N specimens that had been exposed to fuel salt. Research since that time has shown that this phenomenon is the result of attack by tellurium, a fission product in irradiated fuel salt, on the grain boundaries.

As a result of research from 1974 to 1976, two likely solutions to the problem of tellurium attack have been developed. The first involves the development of an alloy that is resistant to tellurium attack but still retains the other required properties. This development has proceeded sufficiently to show that a modified Hastelloy N containing about 1% niobium has good resistance to tellurium attack and adequate resistance to thermal-neutron embrittlement at temperatures up to 650°C. It was also found that alloys containing titanium, with or without niobium, exhibited superior neutron resistance but were not resistant to tellurium attack.

The second likely solution involves the chemistry of the fuel salt. Recent experiments indicate that intergranular attack on Hastelloy N is much less severe when the fuel-salt oxidation potential, as measured by the ratio of U4+ to U3+, is less than 60, the possibility that the superior titanium-modified Hastelloy N could This discovery opens up be used for MSRs through careful control of the oxidation state of the fuel salt.

Bath of the above solutions appear promising, but extensive testing under reactor conditions would be required before either could be used in the design of a future MSR.

For those of you who have read through all of the text reported here, no further comment is necessarily.