Production and detection of fission-induced neutrons following fast neutron direct interrogation to various dry materials containing ²³⁵U

For better nuclear material accountancy, we had developed a non-destructive assay system dedicated to uranium waste drums (JAWAS-N: JAEA Waste Assay System at Ningyo-toge). The system is based on a fast neutron direct interrogation (FNDI) method. To clarify the characteristics of the FNDI method and the performance of JAWAS-N, experimental and computational mock-up tests were carried out using various dry materials that contained known amounts of natural uranium. As a result, linearity between the die-away time (τ₂) and the counts of fast neutrons attributed to ²³⁵U fission was confirmed. Moreover, the MCNP simulation was performed to discuss the radial and axial dependences of ²³⁵U fission probability, neutron detection efficiency, and sensitivity on uranium distributions in the drum. The simulation results agreed with the empirical results reported in a previous paper, providing valuable information on the practice of FNDI-based uranium determination. Furthermore, the nominal detection limits of natural uranium in JAWAS-N were estimated to be 15, 4, and 2 g for τ₂ = 0.2, 0.3, and 0.4 msec, respectively. The findings obtained here will contribute to the implementation of the FNDI method to assess the quantities of ²³⁵U in actual uranium waste drums. (author)