Delayed neutron measurement in high frequency mode and its application to the determination of fissile content in spent fuel assembly

The delayed neutron measurement has been used for the fresh fuel inspection and ²³⁵U enrichment measurement of uranium materials for many years. The origin of delayed neutron is the beta decay of fission fragments and the population of fission fragments of right after a fission event decreases as time goes by. In the process of optimization of delayed neutron measurement system using a DT neutron generator to determine the fissile content of spent fuel assembly, we could find out that the utilization of delayed neutron can be maximized by performing the data acquisition of delayed neutron in an earlier time after the fission event. Monte Carlo code simulation results show the delayed neutron count rate obtained in a high frequency mode (100 Hz) is 3.25 times bigger than that acquired in a low frequency mode (0.5 Hz). The influence of delayed neutrons by ²³⁸U fission on delayed neutron counts is not negligible. Thus the delayed neutron count rate for 64 cases of spent fuel assembly is corrected by considering the ²³⁸U influence. To obtain sufficiently large ratios of signal to background for many cases of spent fuel assembly, 10¹⁰ ~ 10¹¹ n/s strength of DT neutron generator is needed.