An Aerial Gamma Ray Survey of the Surrounding Area of Sizewell Nuclear Power Station

An airborne gamma ray survey of the surroundings of the Sizewell nuclear power station was conducted to define the present levels of radiation background for reference purposes. A twin engine helicopter fitted with a high volume NaI detector and two semiconductor detectors was used. A 20x30km area around the site was surveyed with 500 m line spacing, with an inner zone of 6x6 km being investigated with 250 m line spacing. More than 10,000 gamma ray spectra were recorded between 1st and 3rd October 1996 at a survey height of 200-300 feet above ground level and used to prepare maps showing the distribution of 137Cs, 40K,214Bi,208Tl and gamma ray dose rate. The data set has been retained digitally in an archive which can be used in the future should the need arise to measure change resulting from long term site operations, or for emergency response. 137Cs levels are typically around 2 kBq m-2, derived mainly from weapons' testing fallout, with slight Chernobyl input. Peak levels of some 6 kBq m-2 around the tidal inundation limits of estuarine and marsh areas may be associated with marine discharges from nuclear fuel reprocessing at Sellafield. Natural radionuclides show a distribution which reflects the geological and geomorphological variations within the landscape, and are the major contributors to dose rates within the survey zone. The gamma ray dose rate maps also show the position of the 41Ar plume emitted from the Magnox station at Sizewell during the survey. At the time in question the plume was projected over the sea in the SE direction. The gamma ray survey data crossing the plume show clear evidence of dispersion downwind with plume broadening as a result of gaseous diffusion with increasing distance. Ground level gamma ray dose rates were recorded at 8 routine district monitoring sites using a mini-series 680 survey meter operated by Sizewell staff and a portable scintillation spectrometer. The mean dose rates recorded with the 680 meter (24.7 nGy hr-1) are in good agreement with the results of ground based (27.4 nGy hr-1) and the nearest airborne gamma dose rate results (24.6 nGy hr-1). Airborne survey methods are uniquely well suited to rapid environmental data capture from large areas. This has important emergency response potential which is increasingly recognised in the context of nuclear sites. The data recorded in this study provide a frame of reference against which future changes can be measured.