Thorium Utilization in Fast Breeder Reactors and in Cross-progeny Fuel Cycles

Abstract

Utilization of thorium fuel in fast breeder reactors has a distinct advantage, namely the reduction in the positive sodium void coefficient, which has very favorable implications for the licensing of the liquid metal fast breeder reactors (LMFBRs). However, the breeding ratio obtained is lower than that obtained when the U-238-based fuels are employed. Thorium-fueled fast reactors breed large quantities of U-233, which can be employed in advanced reactors and thermal reactors moderated by water and heavy water to increase fuel utilization by ≈20%; thereby extracting a greater amount of energy per mined ton of uranium ore. The cross-progeny fuel cycles indeed provide excellent fuel utilization performance. Thorium-based metal alloy fuel in LMFBR applications would eliminate the breeding gain penalty incurred by employing the thorium-based oxide fuel. However, there is not an extensive database in the world for the lengthy irradiations of thorium metal fuel in LMFBRs. Thorium metal and oxide fuels, in general, should have better properties and stability than the uranium metal and oxide fuels. Thorium fuel cycles have to be closed since the benefit is obtained only when the U-233 is used. India is the only country in the world, which has extensive facilities for reprocessing of irradiated uranium- and thorium-based fuels, in thermal reactors moderated by light and heavy water and in 500 MWe LMFBRs. The cross-progeny fuel cycles would be a natural vision to pursue India. This paper was written in 1982 and presented at the US–Japan Seminar on thorium fuel cycle held in October 1982. The calculations performed and the results quoted in this paper are of that vintage. However, the cross-sectional data for Th and other materials has not changed significantly since that time. The same holds for the methodologies in computer codes, diffusion theory, and the other methodologies employed in this paper, versus those in computer codes currently in use. This is a review paper incorporating results from several papers from different authors, and it is being submitted to remind the community that with the introduction of GEN IV LMFBRs, other possibilities for thorium utilization could spring forth and should be studied further and in more depth.