Comparative analysis of nondestructive ASSAY techniques for 235U and 239Pu in structural materials at a high gamma background level

Abstract Potential techniques to identify small quantities of fissionable materials (FM) (0.001% wt.) in conditions of a high gamma background level have been reviewed and compared, and the optimal possibility for the nuclear material (NM) control in spent fuel assemblies (SFA) has been selected. It was found through numerical simulation that a system based on a passive neutron control method could be used to detect FMs indirectly when the spent nuclear fuel (SNF) burn-up and cooling time are known. Two types of detectors have been compared: 3 He counters and 235 U-based fission chambers. Better application prospects of 3 He counters, based on the SNM-18 neutron counter, have been shown and drawbacks of passive control technique have been pointed out. An active neutron control has been found to be the best way to address the problem considered. The system's computational model shows that the signal exceeds the triple background error (both for ambient and intrinsic background from Cm isotopes) more than twelvefold. To improve the signal recording efficiency, the system has been modified to allow for irregularities in the geometrical position of structural materials (SM) in the measuring chamber. The proposed procedure makes it possible to determine in a short time the content of 239 Pu, 242 Cm, and 244 Cm in an SFA. After the quantity of 239 Pu is determined, it is possible to estimate the content of other isotopes (Am, U, Np) due to the constancy of the 239 Pu mass ratio to the mass of the actinide identified.