One of the problems for hydride used in a fast spectrum core is the thermal spike wherein a large power peak occurs in the fuel pins near the hydride used zones. Figure 16.6 shows the intra-assembly radial power distribution in the outermost assembly of the core, that is, the fuel assembly adjacent to the MA-hydride assembly. If a Zr-hydride assembly without MA was used instead of the MA-hydride assembly, a large power peak appeared at the No. 15 pins adjacent to the hydride assembly (shown by pink line of ZrH16 case in Fig. 16.6). In our proposed case, MA-hydride works as an absorber of thermal neutrons, and thus a thermal spike is suppressed. As a result, the radial power distribution of the core has an ordinary profile in the core zones (Fig. 16.7). The power of the first row in the blanket region is, however, a little larger than that of ordinary fast reactors because of the fission reactions of MAs or daughter nuclides, although this power increase is considered to be controllable by adjusting the assembly flow distribution.

Figure 16.8 shows the mass balance of MA for the system of about three 1GWe — class LWRs and one FBR with MA-hydride target as previously described. The LWR annually produces spent fuel with burn-up of 45 GWd/t containing 23 kg MA.. The mass of transmuted MA per year is almost equivalent to that produced annually in about three LWRs, which means that most of the produced MA is

Fig. 16.6 Intra-assembly power distribution of the assembly adjacent to hydride assemblies

transmuted in the system and only a small amount of MA is transferred to the waste stream. As seen in Fig. 16.8, MA recycling is necessary for higher transmutation efficiency. It takes ten times the effective half-life to reduce the mass of MA to 1/1,000 of the initial mass. When the irradiation time of one cycle is 2.19 year, ten fuel cycles are necessary to reduce the mass of MA to 1/1,000.