Largely because of political concerns over nuclear proliferation in some countries, there has been some interest in cutting the total inventory of plutonium: first by reducing the amount of plutonium that is produced in nuclear fuel and, secondly, by utilizing the existing stockpile of separated plutonium, including ‘military’ plutonium. In the first case, thorium offers useful possibilities because it can be used as fertile material without producing plutonium; of course, it produces instead another fissile material, U-233, which can also raise proliferation concerns but mitigating measures are available, as described in Section 8.4.1. In fact, if thorium is used with medium enriched uranium (< 20%), the reactor will still produce plutonium but in lesser amounts than the standard uranium-plutonium cycle. If it is used with HEU (which would itself raise proliferation concerns), plutonium production would be very low. But if only pure U-233 is used as a fissile material in combination with thorium, then plutonium production is essentially zero. Thus, extensive use of a thorium cycle would provide a significant reduction in the rate of plutonium accumulation.

With respect to the utilization of existing plutonium stockpiles, the standard solution is the manufacture of MOX fuel for LWRs, many of which can be operated with 100% MOX. The same may also be true for HTRs. Of course, while these cycles will reduce the stockpile of separated plutonium, they will also produce fresh, if unseparated, plutonium. Thorium-plutonium fuels, on the other hand, could be deployed in various reactor types (see Section 8.2.3) and would very effectively reduce the stockpile without, in an ideal Th/Pu-fuel option, creating fresh plutonium. In the case of ‘military’ plutonium (i. e. weapon-grade plutonium now being declared excess to military needs in the United States and Russia), it has been proposed by the US that HTRs would allow plutonium to be burned in a very efficient manner. This solution is now under study in Russia, in cooperation with several countries. Thorium matrix fuel is a possibility and, although it is not strictly required in this case, the elimination of uranium from the HTR fuel would prevent the formation of fresh plutonium.