DISTINGUISHABL VOID SIZE AND NEUTRON DOSE-EQUIVALENTRATE INCREASING DUE TO A VOID IN A CASK SIDELD

As a neutron shielding material of a high-burnup spent fuel shipping cask, a kind of epoxy resin, NS-4-FR, has been employed in Japan. The NS-4-FR shield is solidified by mixing the base and the hardener in the normal temperature, and or rare occasions, small voids might be produced in it during the solidifying process. The void is a defect of a shield, and as an inevitable consequence, the defect makes some deterioration of the neutron shielding ability of the shield. In order to simulate the void produced in the NS-4-FR shield, seven plugs in which each plug has an artificially made void were prepared. The plugs with the void were set at the center of the NS-4-FR slab, alternately. The neutron dose-equivalent rates penetrated through the shielding systems were measured with a moderator-type neutron survey meter. The neutron source was a 252Cf and the intensity was 5.02X 107 nls at the experiments. The void sizes prepared in the experiments were from the minimum size of 0.3 • X 0.3 em to the maximum size of 5 • X 5 em, and the neutron survey meter employed in the experiments has relatively large volume of 20 • X 23 em. Accordingly, the measured dose-equivalent rates obtained with the survey meter were the values averaged over the volume of it. Therefore, the volume of the survey meter was taken into account in the analysis by the continuous energy Monte Carlo code MCNP 4A with the NESXE (Next Event Surface Crossing Estimator). The shielding experiments were carried out to evaluate the void effects with three-types of arrangements. Arrangement 1 : only the NS-4-FR slab with a void was set, Arrangement 2 : the slab with a void was sandwiched by the slabs without a void, and Arrangement 3: Type 304 SS (stainless steel) slabs of 25 em thick were located on the neutron source side of the NS-4-FR slab with a void. The Arrangement 3 is simulating an actual structure of a spent fuel shipping cask. The experimental results and the Monte Carlo analyzed data were indicated as the increasing ratio of neuron dose-equivalent rate as a function of the void size.