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An Aerial Gamma Ray Survey of Chapelcross and its Surroundings in February 1992

Sanderson, D.C.W., Allyson, J.D., and Tyler, A.N. (1992) An Aerial Gamma Ray Survey of Chapelcross and its Surroundings in February 1992. Project Report. Scottish Universities Research and Reactor Centre, Glasgow, UK.

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Abstract

A short aerial gamma ray survey was conducted in the vicinity of the Chapelcross site from 4th-7th February 1992 to define existing background radiation levels, against which any future changes can be assessed. A twin engine AS 355 "Squirrel" helicopter chartered from Dollar Helicopters was used for this work. It was loaded with a 16 litre NaI gamma spectrometry system at SURRC in East Kilbride on the afternoon of 3rd February and flown to an off-site operational base at Lockerbie that evening. Over the following four days over 3500 gamma ray spectra were recorded from a main survey area of 21 x 25 km surrounding the site, and from a extension examining coastal zones up to 25 km further west. Additional NS and EW "tie lines" out to 40 km from the site were added for the purpose of future extendability. The main survey grid, bounded by OS coordinates NY100590, NY 100800, NY 350800 and NY 350590, was surveyed in a series EW flight lines spaced apart by 500m. Survey speed and height were 120 kilometres per hour and 75 m. respectively. Gamma ray spectra were recorded every 10 seconds, interleaved with positional information collected on-line from a GPS satellite navigation system and time averaged radioaltimetry signals. 3500 spectra were recorded. The combination of line spacing and flight conditions results in a practically complete area survey with 500m spatial resolution. The same flight parameters were adopted for the western coastal extension and the tie lines. Data were recorded in continuous flight tracks through these to minimise flight times. In addition a rapid response flight route was prepared which could be used to define arcs at 10km, 5km and 2km radii from Chapelcross in the event of a future incident. The path has been chosen to be navigable under most weather conditions, and falls within the area which has been mapped for baseline purposes. A survey aircraft arriving from East Kilbride could perform such a survey without pausing to refuel. Survey results have been stored archivally and used to map the naturally occurring nuclides 40K, 214Bi, 208Tl together with 137Cs and total gamma ray flux. In interpreting the maps correctly the spatial averaging of the aerial measurements and the contouring process should be taken into account. This leads to a slight tendency to broaden spatial features and to reduce maximum values particularly for boundaries less than the spatial resolution (500m) of the survey. Small scale features will be underestimated. Radiation from the Chapelcross plant was readily detected at the perimeter and can be clearly seen in the gamma dose rate and 137Cs maps. In the former case direct radiation from 16N in the reactor heat exchangers and the release of 41Ar gas from the reactors is largely responsible. The 137Cs associated with the small-bore pipeline on-site was detected to the SW of the reactors. Signals from 234mPa were detected to the NE of the site, and are due to the on-site depleted uranium store in this area. These local observations are broadly consistent with expectations based on-site dosimetry measurements. Further afield the contamination of the Solway Firth, its intertidal sediments and adjacent tide washed pastures by 137Cs is readily detectable, and the distribution visible on the radiometric maps. This is attributed for the most part to past marine discharges from Sellafield. The most extensively affected areas were near Kirconnel in the Nith, at Caerlaverock, Rockcliffe and Burgh marshes. Smaller areas of local enhancement occur close to the tidal limits of most rivers, notably the Southwick burn, Kirkbean Glen, Burnfoot, the river Annan, the Kirtle Water and river Sark. Terrestrial levels of 137Cs vary from those associated with weapons testing fallout (2-4 kBq m-2) which occur in the main survey area, and the edge of the area contaminated from the Chernobyl accident where levels above 10 kBq m-2 are observed in the western extremities of this survey. Previous SURRC surveys have shown that this component is up to 3-4 times higher further to the West. The natural radionuclides (40K, 214Bi, 208Tl) show variations which reflect the local subsoil geology and surface geomorphology. Prominent features include the eastern edge of the Criffel pluton, Triassic shales are responsible for enhanced levels to the east of the Nith Valley, adjacent negative anomalies are associated with Silurian outcrops to the NE of the Lochar water. Carboniferous limestones and permian sandstones are responsible for the majority of the main survey grid. Natural sources in this survey zone define a relatively low natural background level, against which local signals close to the reactor site and deposited activity on tide washed pastures produce distinct enhancements. Radiation levels at Chapelcross site fall off rapidly with distance from the perimeter, approaching natural levels within approximately 0.5-1 km at the time of the survey. Those from the marine, estuarine and tide washed environments are mostly attributed to past Sellafield discharges. Further ground based investigation of these features would be desirable.

Item Type:Research Report or Paper (Project Report)
Status:Published
Glasgow Author(s) Enlighten ID:Sanderson, Prof David
Authors: Sanderson, D.C.W., Allyson, J.D., and Tyler, A.N.
College/School:College of Science and Engineering > Scottish Universities Environmental Research Centre
Publisher:Scottish Universities Research and Reactor Centre
Copyright Holders:Copyright © 1992 The Authors
Publisher Policy:Reproduced with the permission of the authors
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