Fission-rate distributions in lithium and hybrid fusion blanket assemblies

Absolute fission-rate distributions in four types of spherical blanket assemblies have been measured by micro-fission chambers of 232Th, 238U, 237Np and 235U. The four types, i.e., the lithium-metal and hybrid blanket assemblies with and without a graphite reflector were constructed by stacking lithium, graphite and natural uranium blocks. They are named Li, Li-C, U-Li and U-Li-C Assemblies corresponding to their respective configurations. In the case of U-Li-C Assembly, the effective outer radii of the central cavity, natural uranium, lithium metal and graphite regions were 3.3, 10.0, 34.1 and 55.3 cm, respectively. It is necessary for the absolute measurement to know the number of 14-MeV neutrons produced by an accelerator and the number of fissionable-atoms coated on the chambers. The neutron yield was estimated by the counts of an α-monitor which was set up on the accelerator. The fission chambers were calibrated by use of the same accelerator. The results of the measurement were compared with those of one-dimensional transport calculations employing 100-group neutron cross-sections obtained from ENDF/B-IV data file