Integral Experiments on Critical Irradiation of 237Np and 241Am Foils at Kyoto University Critical Assembly

At the Kyoto University Critical Assembly (KUCA), critical irradiation experiments on Np-237 and Am-241 foils are carried out in the neutron hard spectrum core. For nuclear transmutation of minor actinides, special attention is paid to determining Np-237 and Am-241 fission reaction rates and to Np-237 capture reaction rates in the KUCA hard spectrum core. In the back-to-back (BTB) fission chamber, two nuclide foils (test: Np-237 or Am-241; reference: U-235) are set closely to each other to measure the aforementioned fission reaction rates. Interestingly, the experimental Np-237 and Am-241 fission and capture reaction rates are attained by critical irradiation at low W reactor power for 1 h in the core and are successfully deduced through signals from the BTB fission chamber (fission) and the gamma-ray detection (capture) after the irradiation, respectively, together with MCNP calculations.