Absolute quantification of 137Cs activity in spent nuclear fuel with calculated detector response function

Abstract

ABSTRACT A new non-destructive method for evaluating 137Cs activity in spent nuclear fuels was proposed and experimentally demonstrated for physical measurements in burnup credit implementation. 137Cs activities were quantified using gamma ray measurements and numerical detector response simulations without reference fuels, in which 137Cs activities are well known. Fuel samples were obtained from a lead use assembly (LUA) irradiated in a commercial pressurized water reactor (PWR) up to 53 GWd/t. Gamma rays emitted from the samples were measured using a bismuth germinate (BGO) scintillation detector through a collimator attached to a hot cell. The detection efficiency of gamma rays with the detector was calculated using the PHITS particle transport calculation code considering the measurement geometry. The relative activities of 134Cs, 137Cs, and 154Eu in the sample were measured with a high-purity germanium (HPGe) detector for more accurate simulations of the detector response for the samples. The absolute efficiency of the detector was calibrated by measuring a standard gamma ray source in another geometry. 137Cs activity in the fuel samples was quantified using the measured count rate and detection efficiency. The quantified 137Cs activities agreed well with those estimated using the MVP-BURN depletion calculation code.