The WCAS Waste Crate Assay System is a passive neutron coincidence counter designed for low level waste assay to determine the transuranic (TRU) activity of contaminated solid waste in containers up to 1.43 x 1.43 x 1.20 m crates. The WCAS system provides simultaneous passive neutron coincidence and high resolution gamma isotopic analysis allowing rapid characterization of plutonium wastes.
The WCAS system is based on the Mirion Waste Drum Assay System (WDAS) design with integrated High Resolution Gamma-Ray System. Results from individual neutron and gamma-ray assays are combined automatically by the NDA-2000 software. Because the assays are performed in the same system in a single assay sequence there is no confusion over item ID or modification of sample contents between assays.
The system is designed to be operated as an automated counting system which can process batches of drums, or can be incorporated in a facility process line. The following sections describe the various assay sub-systems, hardware, software and indicate typical performance characteristics.
Components of the system include the neutron counter with 3-He detector, Amptek based fast preamplifier/discriminator circuit boards (Model JAB-01), coincidence electronics, computer hardware, high resolution gamma detectors, digital signal processing for the gamma-ray analysis and application software.
The WCAS shield assembly provides a 4p, 20 cm thick High Density Polyethylene (HDPE) moderator/shield as a measurement chamber for the drums and crates. The neutron shield provides an assay chamber that minimizes the effects of outside neutron radiation levels. As configured the neutron shielding is also effective for reducing the gamma-ray background levels (equivalent to a 1 in. thick steel shield) enhancing the performance of the MGA isotopic analysis at low Pu mass loadings.
The software converts the correct neutron coincidence rate to 240-Pu effective. The plutonium isotopic abundances are then calculated and used to convert the measured 240-Pu effective mass to a total plutonium mass.