Optimisation of ⁴¹Ar environmental monitoring to provide a validation of radiological assessment models

¹ Westlakes Research Institute, Environmental Science, Westlakes Science and Technology Park, Moor Row, Cumbria CA24 3LN, U.K.
² British Nuclear Fuels plc, Sellafield, Seascale, Cumbria CA20 1PG, U.K.
³ Scottish Environment Protection Agency, Erskine Court, Castle Business Park, Stirling FK9 4RT, U.K.

Abstract

The aerial discharges of particulate matter, from Sellafield, have declined by more than three orders of magnitude, since their peak in the mid 1970s, as a result of back-fitting abatement plant to the older installations. The decline in the level of particulates has led to the volatile and inert nuclides being the main contributors to the critical group dose. Of these ⁴¹Ar, from the Calder Hall reactors, dominates with an assessed contribution of 42 µSv a^-1 out of a total critical group dose of 65 µSv a^-1 from both power generation and reprocessing discharges during 1999. This predominance has led to a requirement for field validation of the assessed dose. A programme of field measurements, using a sodium iodide spectrometer, calibrated to measure the external dose rate, has been carried out to determine the ⁴¹Ar gamma flux at the critical group location. Comprehensive meteorological measurements were correlated with ⁴¹Ar emission rates and gamma flux measurements. The resulting database of measurements has provided an extensive validation "toolkit" with which to determine the accuracy of the dose predictions from the radiological assessment models used by both industry and the UK regulators. This paper presents the results of the dose rate measurements of ⁴¹Ar, together with a comparison of both long- and short-term dose rate predictions from a number of radiological assessment models. The relative performance of the models has been tested in a variety of meteorological stability conditions and discharge scenarios.