SOURCE TERM CHARACTERIZATION FOR SNM PIT STORAGE FACILITIES

In order to properly design a mobile system to validate and verify the presence of special nuclear materials for non-proliferation and safeguards monitoring in SNM storage facilities, accurate modeling of source materials along with optimal detector collimation is imperative. In support of this effort, models were developed for use in design assessments based on an AL-R8 special nuclear material (SNM) standardized container specification to determine the radioactive signatures for both highly enriched uranium (HEU) and weapons plutonium (WGPu) SNM housed in the containers. Intrinsic gamma boundary leakage currents were determined using 3D fixed-source deterministic SN photon transport (PENTRAN) as well as via stochastic Monte Carlo methods (MCNP5). Group-dependent leakage radiation terms were calculated at two interfaces within the model, one directly surrounding the SNM source, and one immediately surrounding the canister. Analysis showed good agreement between the two leakage models for energy groups of interest based on a tuned 24 group gamma library established for HEU and WGPu gamma signatures of interest. Intrinsic and neutron induced gamma leakage was determined from Monte Carlo calculations, and the combined gamma signatures were then treated as a net gamma leakage to be used in subsequent photon transport calculations. Neutron leakage based on the BUGLE-96 47 group structure was determined using Monte Carlo calculations for the WGPu canisters. These results are being used to evaluate the source term from stored nuclear materials to develop the proper detector collimation and time gating by analyzing the angular distribution of the gamma and neutron leakage from the outer container of the SNM source. Moreover, we performed deterministic adjoint transport calculations to determine the importance of photons, efficiency, and relative to the Field of View (FOV) of the detector. Background contributions assumed to appear from nearby sources, cosmic background, and ground scatter were established to determine a minimum Currie-limit necessary to achieve a 95% Probability of Detection and a 5% Probability of False Alarm. This information will be used to determine a signal/noise ratio with the current design, and augment our efforts to design a mobile detection system to validate the presence of these materials for safeguards purposes.