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Improvement of the matrix effect compensation in active neutron measurement by simulated annealing algorithm (June 2009)

To answer safety authority requirements and to optimise the management of radioactive waste produced in retrieval and decommissioning activities, which contains a large variety of matrix materials, the accuracy of neutron measurement techniques has to be continuously improved. Active neutron measurements such as the Differential Die-Away (DDA) technique involving pulsed neutron generator as the neutron source, are widely applied to determine the fissile content of waste packages. Unfortunately, the main drawback of such techniques is coming from the lack of knowledge of the waste matrix composition. Thus, the matrix effect correction for the DDA measurement is an essential improvement in the field of fissile material content determination. Different solutions have been developed to compensate the effect of the matrix on the neutron measurement interpretation for a long time. In Low-Level radioactive Waste (LLW) packages examination, the most widely used methods are based on neutron flux monitoring using small 3He proportional counters added inside the detection device and associated to the “Matrix Interrogation Source” (MIS) measurement. This technique was originally developed for passive neutron measurement. It needs a specific measurement step which can be operated with the neutron generator or, most of the time, with an external isotopic neutron source such as 252Cf located as closed as possible to the waste drum. This step represents a limiting factor for the examination management and duration. In this context, this paper describes a new approach developed with the goal of increasing the accuracy of the matrix effect correction and reducing the measurement time. This is a major objective in the Non Destructive Assay (NDA) especially to enhance industrial process efficiency of large number of waste packages inspection. It deals with an innovative matrix correction method for radioactive waste embedded in a large variety of matrices regarding the density range (0.07 – 0.9 g.cm−3) as well as the composition (wood representative of hydrogenized matrix, PVC, iron, etc.). The implementation of this method is based on the analysis of the raw signal with an optimisation algorithm called the simulated annealing algorithm. This algorithm needs a reference data base of Multi-Channel Scaling (MCS) spectra, 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. This database has been provided by a set of active signals from experimental matrices (mock-up waste drums of 118 litres) recorded in a specific device dedicated to neutron measurement research and development of the Nuclear Measurement Laboratory of CEA-Cadarache, called PROMETHEE 6. This equipment has been designed to reach an empty cavity detection efficiency of 25%. It is equipped with a pulsed (D-T) neutron generator which can reach an average neutron emission rate up to 2.4 109 ns−1 with a pulse duration of 200 µs. This high technology performance allows achieving very low detection limits with the classical DDA measurement of fissile matter located in light waste matrices (close to 30 µg of 239Pu with an active total measurement time of 900 s). The simulated annealing algorithm is applied to make use of the effect of the matrices on the total active signal of DDA measurement. Furthermore, as this algorithm is directly applied to the raw active signal, it is very useful when active background contributions can not be easily estimated and removed. Most of the cases tested during this work which represents the feasibility phase of the method, are within a 4% agreement interval with the expected experimental value. Moreover, one can notice that without any compensation of the matrix effect, the classical DDA prompt neutron signal analysis may induce an underestimation of more than a factor of 200 on the fissile mass determination for the cases tested in this study. The unexpected so good agreement is a very promising result for the method knowing that the compositions of the mock-up drums are quite representative of the most frequently encountered matrices in LLW packages. This work is the first step of a more general thought carried out to increase the relevance of the whole treatment of DDA measurements from innovative electronic tools (specific fast charge amplifiers, list mode data card system…) up to optimised home made algorithms developed for the post-treatment of the measurements recorded by the list mode data card system.