Year
2000
Abstract
The Trawsfynydd Nuclear Power Station in Wales was closed in 1993 and work is currently underway to decommission the reactors. As part of this work, the Intermediate Level Waste generated during the operation of the site is to be recovered, conditioned and repackaged ready for storage until final disposal. Magnox Electric are responsible for this decommissioning project[1]. Harwell Instruments Ltd. have supplied a Low Resolution Gamma Spectrometry system to Magnox Electric’s main contractor, AEA Technology, to monitor Intermediate Level Waste in the form of fuel element debris. This paper describes the design of the instrument and how it has been calibrated and tested to ensure proper operation once it has been installed in the waste processing facility. The monitor is designed to detect the presence of pieces of fuel and nimonic springs in trays of fuel element debris as they are sorted prior to tipping into final disposal containers. It determines the fuel content of the waste in each sorting tray (and the amounts of other radionuclides of interest to the operators of the final waste repository) by measuring the 137Cs and 60Co gamma-ray signals and using known relationships (the waste stream fingerprint) between the radionuclides present in the waste. One of the main challenges in designing the instrument was to enable the detection system to cope with the wide range of activities that may be present on the sorting tray in the presence of a variable gamma-ray background from material that is waiting to be processed. Two detectors were used, each optimised for a different range of gamma-ray activities and a special detector collimator/shield was designed to confine the view of both detectors to the sorting tray and so minimise the background from other sources.