Simplified method for calculation of neutron capture transformation effects of fission products on decay power

Abstract

A simplified method is proposed for the calculation of the effects of neutron capture transformations of fission products (FPs) on the decay power of FPs. The decay power of FPs after shutdown changes by the neutron capture transformations of FP nuclides during reactor operation. It is proposed to calculate the neutron capture transformation effects considering the production of the following 7 nuclides103Ru,134Cs,136Cs,148mPm,148Pm,154Eu and156Eu by the neutron capture reaction of the direct mother nuclide alone giving a cumulative fission yield for the mother nuclide. The present method was assessed by comparing the calculation results with the rigorous calculation results for the thermal-neutron fission of235U irradiated between 1 and 5 yr in a light water reactor with thermal-nentron flux between 3 × 1013 and 6 × 1013 n/cm2·s and for the fast-neutron fission of239Pu irradiated between 1 and 5 yr in a fast breeder reactor with total neutron flux between 3 ×l015 and 6 × 1015 n/cm2·s. It has been clarified that the present method can calculate the neutron capture transformation effects within the accuracy of ±1% of the decay power for the irradiation of 1 yr and cooling time less than 109 s irrespective of fission type and neutron flux. The accuracy varies little with neutron flux but considerably with irradiation time. For a irradiation of 5 yr the present method can calculate the capture effect within the accuracy of +1% and –5% of the decay power. The accuracy can be improved to ±1% of the decay power with the simple correction factors. © 1986, Taylor & Francis Group, LLC.