Calibration and Validation of a Wide Range Segmented Gamma Ray Scanning Instrument for the Measurement of Low and Intermediate Level Waste

This paper describes the design and calibration of a Wide Range Segmented Gamma ray Scanning (WR-SGS) assay instrument for the measurement of both Low and Intermediate Level Waste (LLW and ILW) in 200 litre drums. The instrument employs a single shielded and collimated high purity germanium (HPGe) detector to quantify the radionuclide content of the waste. One of the novel features of the instrument is the use of an automated variable aperture collimator, which allows the vertical segment height to be adjusted, and allows the scanning of intermediate level waste drums, with significant surface dose rates. Conventional SGS measurements may be performed where the drum is rotated and measured in vertical segments. Alternatively, faster measurement can be made using continuous helical scanning of the drum as it rotates. A gamma ray emitting transmission source is used to correct for waste density. In place of a conventional shutter, the shielded transmission source is moved to a shielded storage position to eliminate background radiation arising from the transmission source. Using this approach, higher activity transmission sources may be used in order to achieve adequate density corrections for higher density drums. These new features makes the WR-SGS suitable for the measurement of drums containing exempt level waste to intermediate level waste, as well as drums with a wide range of density. Results will be presented from the calibration of the instrument using horizontally displaced line sources to simulate distributed sources and the results will be compared with benchmarked MCNP Monte Carlo calculations.