Measurement and analysis of cadmium ratios of activation foils in thermal critical assembly (KUCA B 3/8” P36EU-NU-EU)

To examine the applicability of foil activation technique for the estimation of neutron spectrum in a thermal reactor, Cd ratios of 8 activation foils (Au, Th, Dy, In, Mn, W, D. U. and E. U.) were measured in the void at the core center of KUCA B3/8”P36 EU-NU-EU assembly. The Cd ratios were analyzed with SRAC code system using 107 group cross sections based on ENDF/B4. To make the correction for polyethylene plates facing to the void to the calculated spectrum with 2-dimensional (r–z) diffusion model, softening factor calculated with 1-dimensional infinite slab model was introduced. This model gave almost same neutron spectrum as that without this correction. For the model which distributes atoms of Al sheath and support cylinder homogeneously into simulating materials, and using pointwise (fine group) cross sections for Au, Th, W and D. U., the calculated values except for W and D.U. almost agreed with the experimental ones. For W and D. U. C/E values were -1.1. Since Cd ratios are sensitive to the change in neutron spectrum except for D. U., this method is useful to judge the appropriateness of calculated neutron spectrum.