An Automated Non-Destructive Assay System for the Measurement and Characterization of Radioactive Waste

The paper describes an automated non-destructive assay (NDA) system for the measurement and characterization of radioactive waste. The Waste Characterisation System (WCS) can be adapted to measure a variety of drum sizes: 60, 220 (55 gallon) and 440 liter, the latter with a maximum weight of 1500 kg (1.5 tonnes). The NDA system includes a Tomographic Segmented Gamma Scanner (TSGS) and an active/passive neutron Differential Die-away (DDA or DDT). The system can assay a wide variety of waste types in a range of waste matrices. The assay stations are linked by a heavy duty roller conveyor which incorporates a 20 drum buffer store, a load cell (built into the conveyor), bar code readers and a dose rate measurement station. The Tomographic Segmented Gamma Scanner (TSGS) combines conventional high resolution gamma spectrometry and a tranission source to interrogate a waste drum in vertical slices (segments) as for Segmented Gamma Scanner (SGS) measurements. However, in the case of the TSGS, while the drum is rotated, it is also moved in the horizontal direction leading to an enhanced ability to correct the gamma ray energies, from the nuclides of interest, for the attenuation of the matrix. The TSGS can also be operated as a conventional SGS for the measurement of homogeneous waste drums. The DDA is a very sensitive active neutron interrogation method that uses thermalised neutrons from a pulsed source within the chamber to irradiate a waste drum. Prompt neutrons from fissile material present in the waste (e. g. ²³⁹Pu, ²³⁵U) are detected and provide a measure of the fissile content in the drum. In passive mode, the DDA determines the even Pu nuclides exhibiting significant spontaneous fission (e.g. ²⁴⁰Pu). Measurement accuracy depends on correction algorithms to compensate for self-shielding and matrix effects in waste drums containing hydrogenous materials. In addition, the DDA will be provided with the Fission-Fission Neutron Correlation Analysis System (FFnC) which is an absolute technique eliminating the need for matrix dependent mass calibrations, and allowing separate U and Pu determination using delayed neutron counting. The FFnC technique will be tested for the first time on the WCS. The NDA system incorporates integrated stations to determine the weight and dose rate of each drum, the former built into the conveyor the latter as part of the TSGS. Six Geiger Muller tubes measure the surface dose at three positions on the drum side, one at 1 metre from the drum and one each measuring the surface dose of the top and bottom of the drum. The assay instruments are linked to a heavy duty conveyor system onto which up to 20 waste drums can be loaded for delivery to the various measurement stations, thus permitting unattended, automated operation. Once measured, the drums remain on the conveyer in a holding system waiting to be unloaded. Automation is provided using a programmable logic controller (PLC) and associated computers. A central computer and associated software is used for data acquisition and management.