Development & Demonstration of a Waste Vault Directional Assay System (VDAS) for High Gamma Rate Environments

Nuclear facilities that have processed nuclear materials or performed post-irradiation examination of spent fuel often contain waste vaults, hot cells, or other areas with an unknown amount of Special Nuclear Material (SNM). In order to make a mass declaration to the IAEA and/or during the facility decommissioning process, these facilities will attempt to quantify the U and Pu content in the waste vaults. Experimental measurements of the U and Pu mass are complicated by the unknown geometry and distribution of SNM as well as the presence of high gamma activity fission products. The difficulty of performing such a measurement in harsh radiation environments can result in a zero mass declaration and/or the requirement to completely remove all material to make a declaration and terminate safeguards. To address these issues, a new neutron and gamma detector, called VDAS (Vault Directional Assay System), is being developed and tested at Los Alamos National Laboratory to improve safeguards for waste measurements in high gamma radiation environments. VDAS consists of three 3He tubes (5” active length, 4atm) and two ion chambers and can operate in high gamma fields with both neutron and gamma directional sensitivity. The sensitivity of VDAS to the direction of the neutron/gamma source was optimized by using differential moderation of high density polyethylene, cadmium, and tungsten shielding. VDAS was designed such that it can be remotely deployed and adapted to a broad range of possible facility infrastructure to record a 3D array of gross neutron and gamma measurements, including axial and radial rotation, for identifying potential SNM material locations. Monte Carlo N-Particle transport code simulations were performed to optimize the detector design and were benchmarked against the calibration measurements to validate the model. This paper describes the development, calibration and functionality tests performed with VDAS to characterize its capabilities and detection limits for locating SNM. The capabilities of VDAS will help address and further advance the safeguards verification needs for waste and scrap nuclear material measurements.