Improved analysis for matrix effect correction in LLW neutronic assay

The matrix effect correction for the differential die-away (DDA) measurement is an improvement in the fissile material content determination. In low-level radioactive waste (LLW) packages examination, the most widely used methods are based on neutron flux monitoring with 3He tubes, associated to a “matrix interrogation source” (MIS) originally developed for passive neutron measurement and which determine an experimental detection efficiency. This paper describes two new approaches developed with the goal of increasing the accuracy of the matrix effect correction and reducing the measurement time, which is a major objective in the non destructive assay (NDA) of large number of waste packages. The first method is based on an active prompt neutron coincidence measurement using a new generation list mode data card, which is an alternative to the MIS. Monte Carlo simulations have been performed to determine the correction function parameters. An experimental agreement within 20% is obtained with a fissile sample localized at the centre of different matrices provided that the positioning effect remains negligible. Homogeneous distributions of the fissile material have also been simulated and lead to a deviation less than 15% for most of the cases. The second method exploits the effect of matrices on the total active signal. A simulated annealing algorithm, using a reference data base of multi-channel scaling (MCS) spectra, is performed to fit the raw signal. The construction of the MCS library involves a learning phase to define and acquire the DDA signals as representative as possible of the real measurement conditions. Most of the cases are within a 4% agreement interval with the expected experimental value.