MCNP-PoliMi analysis of neutron-source penetrability in uranium-oxide samples measured with an active well coincidence counter

Active well coincidence counters (AWCCs) are widely used for nondestructive assay and nuclear-safeguards applications, mainly for special nuclear material. The measurement method is based on the detection of time-correlated neutrons from fission by 3 He tubes embedded in a polyethylene moderator. The purported advantage of this method over traditional gamma-ray spectral analysis is the greater penetrability of neutrons in the measured sample. However, the degree of this advantage has not been yet fully quantified. In order to accurately characterize the AWCC response, the Monte Carlo simulations must take into account all detailes of an AWCC measurement. In this study, all these issues are addressed and a detailed simulation analysis of the measurement system is presented. The three-dimensional behavior of the neutron flux within the AWCC has been obtained through the analysis with the MCNPX-PoliMi code. Particular interest has been paid to the flux behavior within the analysis sample: a 4-kg can of U 3 O 8 powder. The distribution of fission reactions occurring in the sample is peaked near the bottom of the sample while it is fairly constant in the radial direction. The fission-position data were further analyzed in order to determine the origin of the captured neutrons. Approximately twice as many counted neutrons originate from the sample peripheries (both top and radial) than from the center. This means that the AWCC can ‘see’ the interior of a bulk powder sample only half-as-well as the peripheries.