Measuring the Ratio of 242Cm to 244Cm in Operating BWR Cores Using Start-Up Range Neutron Monitors

The purpose of this study is to compare the measured and calculated ratios of neutrons originating from Cm-242 and Cm-244 in commercial BWRs in order to evaluate the prediction error for a weakening start-up range neutron monitor (SRNM) signal due to the decay of in-core neutron sources over a ten year period. Measurements were carried out over a period of several months, using the least-squares method to fit the time series of SRNM signals with the theoretical model. Neutron sources of each fuel assembly were calculated by the macroscopic depletion model of a plant process computer. The ratios of neutrons originating from Cm-142 and Cm-244 detected by each SRNM were calculated by whole core fixed source calculation using the nodal expansion method. One BWR core measurement was taken at the beginning of the cycle and three BWR core measurements were taken in the middle of the cycle. A comparison of the measured and calculated ratios of Cm-242 and Cm-244 number densities showed reasonable agreement. The slight difference observed between the measured and calculated ratios will be discussed.