Measurement of fission-rate distribution on a standard uranium shell

Makrofol KG solid-state track recorders (SSTRs) are used to measure the absolute fission-rate distribution in the NISUS natural-uranium shell in the three orthogonal planes. The results show that the fission-rate ratios of the outer surface to the inner surface of the shell at different positions are not constant, contrary to the prediction from the ANISN one-dimensional transport code. The minimum and maximum fission-rate ratios in the NISUS uranium shell are 7.64±0.24 and 11.58±0.27 in the horizontal midplane at θ = 30° and θ = 180°, respectively (θ = 0° is the out-of-pile direction). These ratios are compared with that of the ANISN calculation which gives the value of 10.86. The discrepancy is because of the non-uniformity of thermal neutron source distribution in the graphite cavity block.