The possibility of breeding 233U from thorium was demonstrated by the MSRE experiment [49]. The MSBR [50] project has produced a rather detailed design for a large molten salt reactor, with interesting breeding possibilities. As an alternative to the solid fuel U-Pu breeders we have studied the potential of the Th-U cycle with MSR reactors. The first fissile loads of the 1 GWe MSR are made of industrial plutonium obtained from spent PWR fuel reprocessing. Due to the mediocre neutronic properties of this plutonium, our simulations show that 4tons/GWe are needed to ensure criticality.[11] The initial plutonium load is replaced by 233U. Every year, all 5 year old available plutonium is used for new MSRs. This is not sufficient to ensure the required rate of increase. The complement is obtained from the excess 233U produced in the operating MSRs, used to start new Th — U reactors. Only 1 ton/GWe of U is needed to ensure criticality of an MSR. The doubling time is 25 years with a 10 day cycling time of the salt. The chemical treatment amounts to extracting fission products and protactinium. 233U is re-injected into the salt after protactinium decay.

Figure 2.9 is similar to figure 2.7 for the U-Pu cycle, and shows the evolution of the reactor park. We have distinguished Th-Pu and Th-233U

reactors according to their initial loads. The lifetime of the reactors was assumed to be 40 years, which explains the decrease of the ‘Th-Pu’ reactors after 2070.

Figure 2.10 shows the evolution of the 233U stockpile outside the reactors. The plutonium stockpile is not displayed since all the plutonium produced is, after 5 years of cooling, used for new Th-Pu reactors. As for the U-Pu cycle, the amount of available 233U measures the flexibility of the system which could be used for fission product transmutation and/or smaller production units. It is interesting to note that the final stockpile of 233U is only 16000 tons, to be compared with the much larger stockpile of 80 000 tons of Pu dis­played in figure 2.4. However, due to the difference of inventories (1 ton versus 4 tons), the number of new reactors which could be fed with these stockpiles is the same, namely 16 000 GWe. This illustrates the fact that the value of q (2.9 for Pu versus 2.3 for 233U) is not the only relevant quantity to evaluate breeding potentials. In the case of MSRs, the ability to remove the fission products continuously is another determining factor.