Replacing a 252C f Source with a Neutron Generator in a Shu??er - A Conceptual Design Performed with MCNPX*

The 252 C f shu??er has been widely used in nuclear safeguards and radioactive waste management to assay ??ssile isotopes, such as 235 U or 239 P u, present in a variety of samples, ranging from small cans of uranium waste to metal samples weighing several kilograms. Like other non-destructive assay instruments, the shu??er uses an interrogating neutron source to induce ??ssions in the sample. Although shu??ers with 252 C f sources have been reliably used for several decades, replacing this isotopic source with a neutron generator presents some distinct advantages. Neutron generators can be run in a continuous or pulsed mode, and may be turned o??, eliminating the need for shielding and a shu??ing mechanism in the shu??er. There is also essentially no dose to personnel during instal- lation, and no reliance on the availability of 252 C f . Despite these advantages, the more energetic neutrons emitted from the neutron generator (14.1 MeV for D-T generators) present some challenges for certain material types. For example when the enrichment of a uranium sample is unknown, the ??ssion of 238 U is generally undesirable. Since measuring uranium is one of the main uses of a shu??er, reducing the delayed neutron contribution from 238 U is desirable. Hence, the shu??er hardware must be modi??ed to accommodate a moderator con??guration near the source to tailor the interrogating spectrum in a manner which promotes sub-threshold ??ssions (below 1 MeV) but avoids the over-moderation of the interrogating neutrons so as to avoid self-shielding. In this study, where there are many material and geometry combinations, we use the Monte Carlo N-Particle eXtended (MCNPX) 1 transport code to model, design, and optimize the moderator con??guration within the shu??er geometry. We then use the code to evaluate and compare the assay performances of both the modi??ed shu??er and the current 252 C f shu??er designs for dif- ferent test samples. We investigate and quantify the matrix e??ect and the non-uniformity of the interrogating ??ux in each case. The modi??ed geometry we propose in this study can serve as a guide in retro??tting shu??ers that are already in use.