Differential Die-Away Technique for Determination of the Fissile Contents in Spent Fuel Assembly

Monte Carlo simulations were performed for the differential die-away (DDA) technique to quantify its capability to measure the fissile contents in spent fuel assemblies of 64 different cases in terms of initial enrichment, burnup, and cooling time. The DDA count rate varies according to the contents of fissile isotopes such as 235U, 239Pu, and 241Pu contained in the spent fuel assembly. The effective 239 Pu concept was introduced to quantify the total fissile mass of spent fuel by weighting the relative signal contributions of 235U and 241Pu compared to that of 239Pu. The Monte Carlo simulation results show that the count rate of the DDA instrument for a spent fuel assembly of 4% initial enrichment, 45 GWD/MTU burnup, and 5 year cooling time is ~ 9.8×104 counts per second (c/s) with the 100-Hz repeated interrogation pattern of 0 to 10 µs interrogation, 0.2 ms to 1 ms counting time, and 1×109 n/s neutron source. The 244Cm neutron background count rate for this counting time scheme is ~ 1×104 c/s, and thus the signal to background ratio is ~ 10.