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APPENDIX (file A13)

by Ludwik Kowalski, Sylvie Leray and David Whittal

The main fuel of contemporary reactors is 235U, the isotope whose natural abundance is 0.7%. The supplies of this fuel are limited, they are likely to be exhausted before the end of the next century unless a substitute can be found. Thorium is the most desirable substitute. It is more common than uranium and it can be transformed into highly fissionable 233U. That transformation, and a similar transformation of common 238U into highly fissionable 239Pu, are explained in Figure 10. Neither 233U nor 239Pu exist in our natural environment; manufacturing of these human-made isotopes is commonly referred to as breeding. The amount of nuclear fuel is practically unlimited because we no longer have to depend only on 235U whose supplies are limited. Large deposits of natural uranium and thorium can be turned into fuels by breeding.

click to see Figure 10 (use the back button to return later).

The negative aspect of breeding should also be mentioned. It has to do with the possibility that bomb-making materials; 239Pu and 233U, produced in civilian reactors, could be secretly extracted and used by terrorists. Common uranium reactors are not specifically built to optimize breeding but byproducts of their operation always contain 239Pu. The typical output is 170 kg per year; enough to construct several weapons. A chemical extraction of plutonium (a mixture of various isotopes) is not a very difficult operation while a physical separation of 239Pu from the much less fissionable 240Pu is a formidable task. For the sake of safety all civilian reactors must be designed to make production of bomb-grade materials as difficult as possible.

According to (15) the risk of nuclear proliferation associated with thorium-based reactors "is negligible because the potentially strategic material, namely, 233U is present in the fuel as an isotopic mixture of 233U (43.7%), 234U (30%), 235U (4%), 236U (22.1%) and 238U (0.068%)". Furthermore, an illegal transportation of 233U would be difficult to hide because highly penetrating gamma rays are emitted when this substance is present. The above percentages refer to a breeder based on slow neutrons. Fast neutrons often result in the 233U(n,2n) reaction which leads to the accumulation of 232U. The presence of that isotope makes "it very hard, albeit impossible, for any military diversion of the material".

Similar considerations apply to plutonium. According to (7) the mixture of isotopes of this element in spent fuel is 238 Pu (1%), 239Pu (58%), 240Pu (27%), 241Pu (9%) and 242Pu (5%). The half-lives of the isotopes, however, are such that the percentage of 239Pu increases with time. This aspect of plutonium wastes is significant in the context of long-term geological storage of the unprocessed spent fuel. The deposits may possibly be used as mines for the bomb-grade plutonium in 2000 years or so. The concentration of 235U also increases with time (the half-life of 236U is shorter than that of 235U) but the process takes place on a much longer time scale. The geological storage of radioactive wastes is objectionable on the basis of such considerations.

Thorium breeding is more desirable than uranium breeding because the unprocessed waste from the 233U fuel contains considerably fewer actinides than the waste from the 239Pu fuel (27). This argument would be particularely significant if spent fuels were to be stored unprocessed, as planned for the Yucca Mountain. The presence of actinides in geological deposits is highly undesirable in terms of long-term risks. The actinides, as previously indicated, are dominant producers of heat and radiation after several hundreds of years.

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