If medical applications were the only use of 233U, consideration should be given to converting it to non-weapons-usable 233U to reduce security costs. Studies are underway to understand the relative production costs of some 213Bi production routes. These studies may provide definitive answers whether 233U is needed just in the short-term or both the short- and long-term as a source of 213Bi. Such studies may also determine whether the CEUSP material is worth saving or whether processing costs make it uneconomical under any scenario. These studies should be expanded to include all major production options and be completed at the earliest date and receive peer review.
4.2 REACTORS FOR DEEP-SPACE AND OTHER SPECIAL MISSIONS Reactors fueled with 233U can be designed with a smaller mass than either 235U or 239Pu reactors, thus there has been an interest in using 233U as a nuclear reactor fuel for deep-space missions, for which a premium is placed on minimizing mass. For this application, only high-quality 233U would be used to minimize the launch weight of spacecraft. A space reactor is first put into earth orbit and then is started. This procedure avoids the need for massive shielding of the reactor before and during launch operations. Table 4.3 summarizes this use of 233U. The preferred type of nuclear power source to provide electricity or heat for a deep-space mission depends upon the power, energy, and safety requirements. •
Power. For power production levels up to many kilowatts, the minimum-mass nuclear power source is a radioisotope generator (Fig. 4.4). The currently preferred radioisotope is 238Pu. Nuclear reactors provide minimum-mass, steady-state power generation at higher power levels. For steady-state power levels of a few kilowatts to several megawatts, nuclear power reactors fueled with 233U may provide the minimum mass (MacFarlane 1963; Lantz and Mayo 1972). For each fissile material, a minimum mass of that fissile material is required for a nuclear reactor to operate. This minimum mass is substantially smaller for 233U than for 235U. Uranium-233 and plutonium have similar critical masses; however, the mass of a 233U reactor, including the nuclear moderator and other required components, is less than that for a plutonium reactor. Furthermore, the physical properties of uranium in high-temperature space reactors are substantially better than those of plutonium, and there may be fewer launch safety issues. These features may make 233 U the preferred material for such applications. At higher-power levels, the reactor must have large, internal heat-transfer surfaces to transfer heat from the reactor to the electric generator. To obtain the heat transfer, the reactor fuel assemblies require a significant amount of fissile material. In a large nuclear system, the choice of fissile material does not significantly impact weight because the amount of fissile material needed for heat transfer far exceeds the minimum critical mass needed for a reactor.
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