Since hybrid reactors should not require control rods, it is of course very important to check that, in time, the reactor cannot become critical. We address this question in the present chapter, having in mind, especially, the possible evolution of the fuel.

8.1 Long-term evolutions

More complete calculations than those presented in section 3.6 are needed in order to characterize the behaviour of specific fuels which might be used in hybrid reactors. As examples we show the results of three such calculations [129] in figure 8.1 where we show the evolution of кю for a plutonium mixture originating from PWR spent fuel, for the Th — U system, and for a fuel made of a mixture of minor actinides. Figure 8.1 confirms that hybrid reactors with solid fuels are not fit for plutonium incineration. On the other hand, minor actinide fuels behave like a mixture of fissile and fertile nuclei. Figure 8.2 shows the evolution of the fission rates due to the various nuclei as a function of time. It appears that the stabilization of the variation of кж is chiefly due to the formation of 238Pu, which has a high fast-neutron fission probability. It is formed by neutron capture by 237Np, which behaves like a fertile species. To a lesser extent the rise of 244Cm fissions counteracts the decrease of the 241Am and 243Am fissions. Figure

8.2 also gives an idea of the long times required for a significant decrease of the fission rate of the fuel and, correspondingly, of the total number of transplutonium nuclei.