An Aerial Gamma Ray Search for a Missing 137Cs Source in the Niger Delta, May 1991

The technique of aerial gamma-ray spectrometry has been applied to the search for a missing 74 GBq 137Cs source in the Niger Delta, Nigeria. Comparisons with a duplicate source confirm that an unshielded source of this type is readily detected at the normal survey height of 100m. It is also possible to detect the shielded source at closer range providing the survey altitude does not exceed about 50m. The detection of an unshielded source is very likely at the normal survey altitude of 100m. A survey of the populated zones of Warri and river routes to Otumara was conducted between 9th and 19th May 1991. The results demonstrate that the missing source was not present in this area in an unshielded form at this time. Signals likely to have been due to a shielded radiocaesium source were recorded. To date no source has been recovered, but investigations are continuing. High radiation readings were recorded over part of Forcados Village, on the banks of the Warri River. They are associated with a high Thorium/Uranium concentrated source. Ground level investigations are recommended to identify the origin of these signals and evaluate their potential radiological implications. If the missing source is not located by further ground based operations, it is suggested that consideration be given to remobilising a wider and more comprehensive aerial search

Review of Past Nuclear Accidents: Source Terms and Recorded Gamma-Ray Spectra

Airborne gamma ray spectrometry using high volume scintillation detectors, optionally in conjunction with Ge detectors, has potential for making rapid environmental measurements in response to nuclear accidents. A literature search on past nuclear accidents has been conducted to define the source terms which have been experienced so far. Selected gamma ray spectra recorded after past accidents have also been collated to examine the complexity of observed behaviour

A Brief Aerial Survey in the Vicinity of Sellafield in September 1990

A two day survey exercise was conducted by SURRC from 25th-26th September 1990 with the joint aims of demonstrating the potential of helicopters for emergency response and beginning the definition of baseline levels in the immediate vicinity of Sellafield. The aircraft used for this work was a twin engined AS 355 "Squirrel" helicopter chartered from Dollar Helicopters. It was loaded with an 8 litre NaI gamma spectrometry system at SURRC in East Kilbride on the afternoon of 24th September and flown down to Sellafield the same day. Over the following two days roughly 1300 gamma ray spectra were recorded from an area ranging south from an EW line linking Ennerdale Fell and St. Bees Head to beyond Ravenglass. Operations were conducted from the Sellafield helipad, the aircraft being refuelled at Barrow in Furness. The flights were arranged to provide nominally 1km spaced parallel NS flight lines throughout the survey area, for the purpose of baseline mapping. Supplementary flights to improve spatial resolution are possible at a later stage. In addition a rapid response flight route was rehearsed involving definition of landward arcs at 10km, 5km and 2km radii from Sellafield plus the beachline from St. Bees Head to Ravenglass. The precise path was chosen to be navigable under most weather conditions and took roughly 40 minutes to fly. A survey aircraft arriving from East Kilbride could perform such a survey without pausing to refuel. The results have been stored archivally and used to map the naturally occurring nuclides 40K, 214Bi, 208Tl together with 137 Cs and total gamma ray flux. The maps presented are spatially smoothed, both by the inherent character of the aerial survey technique and by the colour contouring processes. This leads to a tendency to broaden spatial features, while slightly reducing maximum values, and should be taken into account when interpreting the maps. Greater spatial detail could be achieved with closer flight line spacing. Activity due to Sellafield was readily detected in the area and can be seen clearly in the maps. It is noted that the levels observed are generally comparable with those in BNFL annual reports and other research publications. Furthermore, with a few exceptions, naturally occurring radionuclides are the dominant radiation source in much of the survey area, and show considerable variations from place to place. 137Cs due to marine discharges was most pronounced in the Irt, Mite and Esk estuaries and on the beachline close to the plant. The same nuclide was detected at lower levels in terrestrial areas due to a combination of global fallout, aerial discharges from the plant and the Chernobyl accident. 41Ar and 16N activities were detected in the immediate vicinity of Calder Hall. A small feature was detected at Drigg, probably associated with current operations on the site, and equivalent to an enhancement of less than twice the local natural background. Radiation levels due to current activities on the Sellafield site fall off rapidly with distance from the perimeter, approaching natural levels within 0.5-1km at the time of the survey. Those from the marine, estuarine and tide washed environments are mostly attributed to past marine discharges, and can be expected to continue to receive attention in the future

An Airborne and Vehicular Gamma Survey of Greenham Common, Newbury District and Surrounding Areas

The airborne gamma ray survey recorded more than 40,000 scintillation spectra and 20,000 spectra from semiconductor detectors. The vehicular survey produced a further 1346 and 763 spectral sets respectively. The installation, calibration, recording and analysis followed SURRC procedures which have been developed and validated over many years and are fully documented. Pre flight checks on detector performance for energy calibration, energy resolution and sensitivity were performed on a daily basis. Background readings over water were taken on a daily basis. All data were registered and backed up in duplicate to form a digital archive of the survey. Subsequent analysis and mapping has used a combination of standard procedures established over many years, and new techniques developed to analyse the low energy spectra. All results have been retained to facilitate traceability and further analysis in the future. The sensitivity of the aircraft and vehicle were also checked at Greenham Common by collecting a set of 31 core samples for independent laboratory analysis. The key points arising from the airborne survey of the entire area show that there has been sufficient sensitivity to record variations in the natural background. The levels of 137Cs are consistent with weapons' testing fallout, and are substantially lower than in other parts of the UK and Europe. The average levels of K (0.5%), U (1 ppm) and Th (3 ppm) are lower than national averages and show variations within the area which reflect local geology and landcover. The area as a whole therefore is one of low environmental radiation background compared with national averages. There is no evidence of signals at Greenham Common or in its vicinity which would present a local radiation hazard. However, signals were detected in the vicinity of Harwell and the Rutherford laboratory which would, at the time of the survey, represent radiation projected off-site as a result of materials stored on-site or on-site activities. Examination of the low energy gamma ray spectra recorded from the semiconductor detectors reveals no evidence, within the sensitivity limits of the method, for excess gamma ray signals at the energies associated with 235U around Greenham Common, Newbury and Thatcham. The low energy data are sufficiently sensitive to record variations in the distribution of natural activity in the area. There is tentative evidence for 241Am in the vicinity of AWE Aldermaston. The vehicular survey demonstrated that the grass areas in between the runway and taxi lanes, and around the hardstand associated with the 1958 fire have retained weapons' testing 137Cs. This supports the view that these represent authentic undisturbed areas for sampling. The built surfaces remaining at the time of the survey were of lower natural activity and 137Cs content than their surroundings. High resolution gamma ray spectra at selected sites were also consistent with the known sources of background radioactivity. On the basis of the results, Newbury District and surrounding areas represent an area with low environmental radioactivity compared with national and European averages. There is no evidence to substantiate fears about the quality of the radiation environment in the vicinity of Greenham Common

An Aerial Gamma Ray Survey of Springfields and the Ribble Estuary in September 1992

<p>A short aerial gamma ray survey was conducted in the vicinity of the Springfields site and Ribble Estuary from 1st-5th September 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(Tl) gamma ray detector and spectroscopy system on the 31st August and during the following days over 2700 separate spectra were recorded within a survey area of 20 x 12 km. Gamma ray spectra were recorded every 5 seconds at survey speed and altitude of 120 kph and 75 m respectively. A flight line spacing of 0.3km was chosen for the main survey area. On the 3rd September a low altitude, high spatial resolution (flight line spacing 100m and altitude 30m) was made over Banks Marsh (an area frequented by local wild fowlers).</p> <p>Survey results have been stored archivally and used to map the naturally occurring radionuclides 40K, 214Bi and 208Tl together with 137Cs and total gamma ray flux. In addition, for the first time, estimates of 234mPa in terms of deconvoluted count rate (normalised to 100m altitude) were made in the presence of 228Ac interference probably in disequilibrium with its parent thorium series.</p> <p>The maps provide a clear indication of the distribution and sources of environmental radioactivity in the Ribble at the time of the survey. The Ribble estuary is subject to regular and ongoing ground based studies by BNF, MAFF, HMIP, and University based groups, as a result of the authorised discharges of low level radioactivity from the Springfields site. The results of this survey complement this ground based work, and add to confidence that the estuarine system, it's associated sediments, tide washed pastures, salt marshes and river banks, have been thoroughly examined. There is support for earlier conclusions that the Cs on the salt marshes is the dominant source of external gamma exposure, and that the Springfields contribution to these locations is minor in comparison with this, Sellafield derived, signal. Upstream the situation is more complex, particularly where the dynamic sources of beta radiation are considered. As far as critical group assessments are concerned the survey provides clear evidence that the areas affected by 137Cs, where external gamma dose and possible food chain effects are of greatest interest, are in the lower reaches of the Ribble, whereas, at the time of the survey the 234mPa distribution was in the upper reaches of the river. This not only confirms the findings of ground based work, but provides some assurance that the different exposure paths (external gamma dose, skin dose) are not entirely synergistic. The discovery of possible transient sources of natural 228Ac in the salt marsh environment as a consequence of Th series disequilibrium immediately following spring tides is extremely interesting. If substantiated by further studies using semiconductor detectors this provides a new insight into the dynamic radiation environment of tide washed contexts.</p> <p>Aerial survey can potentially provide a rapid and cost effective means of studying environmentally dynamic sources such as 234mPa. In the case of the Ribble it would be necessary to reduce survey height to below 50m ground clearance to improve spatial resolution. Possible inconvenience to residents and property owners of such low altitude flights would have to be considered in addition to the potential value of environmental knowledge of the behaviour of short lived nuclides in a dynamic system such the Ribble estuary. There is nonetheless considerable potential for time series studies of this location. Recent flight trials by SURRC incorporating high efficiency germanium semiconductor detectors have verified the feasibility and potential a hybrid scintillation⁄ semiconductor spectrometer. Such a device can resolve any ambiguities arising from overlapping gamma ray peaks. This is particularly relevant to the confirmation of 228Ac in salt marshes. Ground based sampling at the time of measurement would enable concentration calibrations to be made for these dynamic sources. Further ground based measurements would be desirable to establish the extent to which low energy photons contribute to external gamma ray dose rates from sources with pronounced subsurface activity maxima.</p&gt

A Brief Aerial Survey in the Vicinity of Sellafield in September 1990

A two day survey exercise was conducted by SURRC from 25th-26th September 1990 with the joint aims of demonstrating the potential of helicopters for emergency response and beginning the definition of baseline levels in the immediate vicinity of Sellafield. The aircraft used for this work was a twin engined AS 355 "Squirrel" helicopter chartered from Dollar Helicopters. It was loaded with an 8 litre NaI gamma spectrometry system at SURRC in East Kilbride on the afternoon of 24th September and flown down to Sellafield the same day. Over the following two days roughly 1300 gamma ray spectra were recorded from an area ranging south from an EW line linking Ennerdale Fell and St. Bees Head to beyond Ravenglass. Operations were conducted from the Sellafield helipad, the aircraft being refuelled at Barrow in Furness. The flights were arranged to provide nominally 1km spaced parallel NS flight lines throughout the survey area, for the purpose of baseline mapping. Supplementary flights to improve spatial resolution are possible at a later stage. In addition a rapid response flight route was rehearsed involving definition of landward arcs at 10km, 5km and 2km radii from Sellafield plus the beachline from St. Bees Head to Ravenglass. The precise path was chosen to be navigable under most weather conditions and took roughly 40 minutes to fly. A survey aircraft arriving from East Kilbride could perform such a survey without pausing to refuel. The results have been stored archivally and used to map the naturally occurring nuclides 40K, 214Bi, 208Tl together with 137 Cs and total gamma ray flux. The maps presented are spatially smoothed, both by the inherent character of the aerial survey technique and by the colour contouring processes. This leads to a tendency to broaden spatial features, while slightly reducing maximum values, and should be taken into account when interpreting the maps. Greater spatial detail could be achieved with closer flight line spacing. Activity due to Sellafield was readily detected in the area and can be seen clearly in the maps. It is noted that the levels observed are generally comparable with those in BNFL annual reports and other research publications. Furthermore, with a few exceptions, naturally occurring radionuclides are the dominant radiation source in much of the survey area, and show considerable variations from place to place. 137Cs due to marine discharges was most pronounced in the Irt, Mite and Esk estuaries and on the beachline close to the plant. The same nuclide was detected at lower levels in terrestrial areas due to a combination of global fallout, aerial discharges from the plant and the Chernobyl accident. 41Ar and 16N activities were detected in the immediate vicinity of Calder Hall. A small feature was detected at Drigg, probably associated with current operations on the site, and equivalent to an enhancement of less than twice the local natural background. Radiation levels due to current activities on the Sellafield site fall off rapidly with distance from the perimeter, approaching natural levels within 0.5-1km at the time of the survey. Those from the marine, estuarine and tide washed environments are mostly attributed to past marine discharges, and can be expected to continue to receive attention in the future

An Airborne and Vehicular Gamma Survey of Greenham Common, Newbury District and Surrounding Areas

The airborne gamma ray survey recorded more than 40,000 scintillation spectra and 20,000 spectra from semiconductor detectors. The vehicular survey produced a further 1346 and 763 spectral sets respectively. The installation, calibration, recording and analysis followed SURRC procedures which have been developed and validated over many years and are fully documented. Pre flight checks on detector performance for energy calibration, energy resolution and sensitivity were performed on a daily basis. Background readings over water were taken on a daily basis. All data were registered and backed up in duplicate to form a digital archive of the survey. Subsequent analysis and mapping has used a combination of standard procedures established over many years, and new techniques developed to analyse the low energy spectra. All results have been retained to facilitate traceability and further analysis in the future. The sensitivity of the aircraft and vehicle were also checked at Greenham Common by collecting a set of 31 core samples for independent laboratory analysis. The key points arising from the airborne survey of the entire area show that there has been sufficient sensitivity to record variations in the natural background. The levels of 137Cs are consistent with weapons' testing fallout, and are substantially lower than in other parts of the UK and Europe. The average levels of K (0.5%), U (1 ppm) and Th (3 ppm) are lower than national averages and show variations within the area which reflect local geology and landcover. The area as a whole therefore is one of low environmental radiation background compared with national averages. There is no evidence of signals at Greenham Common or in its vicinity which would present a local radiation hazard. However, signals were detected in the vicinity of Harwell and the Rutherford laboratory which would, at the time of the survey, represent radiation projected off-site as a result of materials stored on-site or on-site activities. Examination of the low energy gamma ray spectra recorded from the semiconductor detectors reveals no evidence, within the sensitivity limits of the method, for excess gamma ray signals at the energies associated with 235U around Greenham Common, Newbury and Thatcham. The low energy data are sufficiently sensitive to record variations in the distribution of natural activity in the area. There is tentative evidence for 241Am in the vicinity of AWE Aldermaston. The vehicular survey demonstrated that the grass areas in between the runway and taxi lanes, and around the hardstand associated with the 1958 fire have retained weapons' testing 137Cs. This supports the view that these represent authentic undisturbed areas for sampling. The built surfaces remaining at the time of the survey were of lower natural activity and 137Cs content than their surroundings. High resolution gamma ray spectra at selected sites were also consistent with the known sources of background radioactivity. On the basis of the results, Newbury District and surrounding areas represent an area with low environmental radioactivity compared with national and European averages. There is no evidence to substantiate fears about the quality of the radiation environment in the vicinity of Greenham Common