10.78. Fertile thorium-232, which can be converted to fissile uranium — 233 (§1.35), is an important energy resource as an alternative or supplement to natural uranium. Thorium mineral reserves are believed to be of the same order as those of uranium, but there has been little demand for thorium since reactor fuel cycles presently use uranium and are likely to continue to do so for some years. This preference is based primarily on economic considerations for LWRs [25]. However, as uranium supplies are depleted its price will increase, and there will be a greater incentive to utilize thorium instead. An additional incentive for thorium utilization is to reduce the risk of nuclear weapons proliferation (§10.80). Thorium resources appear ample for many years of projected use.

10.79. As shown in Table 10.1, the conversion (or breeding) potential (ті — 1) for uranium-233 at thermal-neutron energies is significantly higher than for either uranium-235 or plutonium-239. In fact, a system in which the uranium-233, formed from thorium-232 in a low neutron loss reactor, would be recycled has the potential for breeding or self sufficiency. For
reactors with poorer neutron economy, fissile atom makeup, proportional in amount to 1 — CR, would be required, but they would still have a favorable resource utilization. With the thorium-uranium-233 cycle, con­version ratios greater than in LWRs are readily achievable.

10.80. Some complications arise in the thorium-uranium-233 cycle that do not occur in the uranium-plutonium-239 cycle. For example, an inter­mediate stage in the production of uranium-233 from thorium-232 is protactinium-233 with a half-life of 27 days; the latter has relatively large cross sections for neutron absorption in both the thermal and resonance energy ranges. This absorption, which depends on the power density and the neutron spectrum, can have an important effect on the reactivity, although only a minor one on the thorium-to-uranium-233 conversion.

10.81. Another problem results from the presence of uranium-232, formed by (n, 2n) reactions with uranium-233 and with thorium-232 by way of thorium-231 and protactinium-231 and protactinium-232 (see Fig. 11.3). Some of the members of the uranium-232 decay chain are strong gamma — ray emitters, and hence remote handling and shielding are required for fabrication of uranium-233 fuels.