As already stressed, the neutron balance of 239Pu is deteriorated in a thermal spectrum, due to a large parasitic capture cross-section. Moreover, the isotopes of Pu, Am and Cm with an even number of neutrons will reach large inventories because of their small absorption cross-section. Even if the asymptotic Th/U cycle has large advantages in an epithermal neutron spectrum, the Th/Pu —- Th/U transition will be more difficult to optimize in such a spectrum. Moreover, the disappearance of 241Pu by decay plays an important role in the plutonium inventory needed to start a Th/Pu reactor. The isotopic composition is also an important parameter, and a plutonium coming from a MOx fuel, where the 239Pu proportion is smaller than for a plutonium coming from a UOx fuel, will be less advantageous. Figure 11.10 emphasizes this phenomenon.

The thermal spectrum, largely favourable to the asymptotic Th/U cycle compared with the fast spectrum, seems not to be the best way to start the transition towards the thorium cycle, starting from the plutonium coming from the PWR spent fuel. An optimized way to operate the transition could thus be a coupling between fast and epithermal reactors. The role of the fast-neutron reactors would be the management of plutonium and minor actinides produced by the present PWRs, and the production of 233U which could be used to start Th/U MSR very close to equilibrium. Different possibilities can be considered for the fast-neutron systems, depending on the development target. For example, fast-neutron reactors could have a U/Pu core which breeds the required quantity of plutonium to maintain the reactivity, surrounded by thorium blankets, in which 233U is produced. This coupling offers the possibility of fast deployments, as can be seen in figure 11.11.