Environ. Sci. Technol. 2000, 34, 4496-4503
Investigation of an Alleged Nuclear Incident at Greenham Common Airbase Using TI-mass Spectrometric Measurements of Uranium Isotopes IAN W. CROUDACE,* PHILLIP E. WARWICK, REX N. TAYLOR, AND ANDREW B. CUNDY School of Ocean and Earth Science, Southampton Oceanography Centre, Southampton, SO14 3ZH, U.K.
Uranium isotopic compositions were determined in over 500 soil samples from the Greenham Common Air Base and in surrounding west Berkshire and north Hampshire, England to detect potential contamination from an alleged 1958 nuclear incident. Owing to the large number of samples collected for the study, the short reporting time expected, and the potentially subtle nature of any contamination, a rapid and highly precise method of uranium analysis was developed. High precision 238U/235U mass spectrometric measurements are required to see slight excesses in any anthropogenic component. The reproducibility for measurements of uranium isotopic ratios in standards and soil samples was better than 0.26% at the 3σ uncertainty level. Despite an intensive study around the airbase no evidence was found for any contamination by uranium of anomalous isotopic composition. Plutonium measurements were also made to support the U analysis and were found to be consistent with atmospheric bomb fallout for the United Kingdom. The detection of subtle (though radiologically insignificant) amounts of U contamination around a nearby nuclear weapons site illustrates the usefulness and effectiveness of this approach in environmental surveys.
Introduction Contamination of the environment by uranium and plutonium around nuclear facilities is a sensitive political problem in many countries with nuclear capability. In terrestrial environments, where U and Pu may be released via stack emissions or on- or off-site spillage, contamination may be highly localized and/or heterogeneously distributed around nuclear sites. Hence, where a suspected nuclear incident or contaminant release has occurred there is a need, for public protection, to carry out high-density sampling both in the immediate vicinity of the site and in more distant areas. Such surveys generate large numbers of samples and so require rapid, precise, and efficient U and Pu measurement. Consideration also has to be given to U and Pu that has been deposited, normally at nanogram/kg levels in soils, from atmospheric fallout from above-ground nuclear weapons testing. Since U is naturally present at ppm values in soils and rocks, there is a strong masking effect of any anthro* Corresponding author phone: +44 23 8059 2780; fax: +44 23 8059 6450; e-mail:
[email protected]. 4496
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pogenic components. Consequently, sensitive methods of source discrimination are needed to distinguish locally released U and Pu from global fallout derived Pu and U and in the case of U from mineral-bound U with a natural isotopic composition. This discrimination may be obtained by determining the isotopic composition of the U and Pu, either by alpha spectrometric or mass spectrometric techniques. This study reports on the development of a rapid and precise method of U isotope composition (and supporting Pu data) and illustrates its effectiveness in assessing low-level contamination around suspect sites. Data are presented for over 500 soil samples from west Berkshire and north Hampshire, southern England, to assess the extent of contamination from an alleged historical nuclear incident in 1958 at the American airbase at Greenham Common. The Alleged Incident. The headline “Revealed: nuclear fallout at U.K. air base” appeared in the U.K. national newspaper the Sunday Telegraph on the 14th of July 1996 and was promptly followed by similar stories in the national media over the following 8 months. The original article gave details of a leaked 1961 report written by two scientists (1) from the Atomic Weapons Research Establishment (AWRE, now called AWE) at Aldermaston, Berkshire, southern England. Their study was part of a secret project code-named Exercise Overture. This project was conceived in the 1950s, at the height of the Cold War, to determine whether it was possible to gather information about foreign nuclear weapon development activities by sampling the environment for traces of nuclear materials in locations near nuclear facilities. To test the hypothesis, measurements were carried out using environmental samples from around the Atomic Weapons Research Establishment at Aldermaston. The work reported that elevated levels of uranium 235U had been detected around the American airbase at Greenham Common and that this activity was possibly linked to events involving a B-47 aircraft fire on the airbase in February 1958 (Figure 1a). It was suggested that a parked B-47 bomber that had caught fire was still loaded with nuclear weapons and that one or more of these had been damaged releasing some of their fissile material. A figure in the 1961 report showed a dumbbellshaped pattern of dispersion to the southwest and northeast of the runways (Figure 1b) which, it was claimed, was consistent with the disturbance of dust particles (contaminated with very low levels of activity) during aircraft takeoff and landing. However, the very limited data-set of the 1961 report (approximately 35 samples) meant that firm conclusions were difficult to draw. Small amounts of excess 235U were reportedly discovered on evergreen leaf samples from around the base at concentrations up to 20 ng/m2. The supposed incident on February 28, 1958 was linked to a catastrophe originating from human error, which destroyed a B-47 bomber (USAF Incident B-47; Nr 53-6216, 28 Feb 1958) and caused other significant damage on the airbase along with the deaths of two ground crew. The commander of a B-47, which had recently taken off with a full fuel load and bound for the U.S., believed there was an engine fire. Permission was requested to jettison full auxiliary fuel tanks on the airbase drop-zone before attempting an emergency landing. This was granted by an inexperienced tower operator who gave misguided advice on the timing of the drop. Following their release, the fuel tanks ignited after hitting a hard-standing, holding parked B-47 aircraft, and a nearby hangar. The airborne B-47 could not land at the Greenham Common airbase because of the conflagration, smoke, and fire fighting operations, but it later landed safely 10.1021/es000032m CCC: $19.00
2000 American Chemical Society Published on Web 09/30/2000
FIGURE 1. (a) Location of study site and pictogram of the 1958 incident at Greenham Common airbase, southern England. (b) Distribution of excess 235U as modeled by ref 1. Numbers around the edge of the figure are U.K. National Grid references (e.g. SU 40000 60000). at RAF Brize Norton (Oxfordshire). The U.K. and U.S. governments persistently denied claims that nuclear material was involved, and a USAF Enquiry Board Report (2) on the incident that was released into the public domain in 1997 made no mention of any nuclear material. Intense public concern in the area, arising from pressure groups and media interest, and associated worries about a leukemia cluster in Newbury led two neighboring public authorities (Newbury District Council and Basingstoke and Deane Borough Council) to commission an independent survey. There was also considerable worry about the potential blight on house prices in the district. The Southampton Oceanography Centre in partnership with the Scottish Universities Research and Reactor Centre (SURRC) was invited to carry out an assessment, and three reports were produced in early 1997 (3-5). It is notable that the U.K. Ministry of Defence commissioned a smaller survey at approximately the same time using the U.K. National Radiological Protection Board (6). In that study traditional alpha spectrometry was used to make an assessment of anomalous uranium and plutonium. This paper presents U isotopic and Pu data from the Southampton Oceanography Centre/SURRC study where higher sensitivity mass spec-
trometric measurements were used to study uranium isotopic compositions. Environmental Plutonium and Uranium. Plutonium and uranium present in the study area may have been derived from a number of sources. For example, U and Pu are present in nuclear weapons, in nuclear weapons’ fallout ,and in lowlevel stack discharges from nuclear sites. There are two nuclear weapons establishments in the area at Aldermaston, some 10 km due east of the Greenham Common airbase, and at Burghfield. These two AWE sites (formerly AWRE) began operations in the early 1950s as part of the U.K. defense nuclear weapons program, and discharge records from these have been presented in environmental reports since the 1980s. The isotopic signatures of these different sources of uranium (and plutonium) will vary (Table 1), and consequently isotope ratios can be used in assessing the contribution of any sources (e.g. ref 7). Isotopic measurements can be performed using radiometric or mass spectrometric methods. Mass spectrometers are more highly sensitive and precise than radiometric methods for long-lived isotopes such as 235U and 238U since they count atoms, rather than radioactive decay events. Hence, mass spectrometry has been used here for U isotope determinations since it can identify much smaller anomalies of depleted and/or enriched U against the natural background U found in soils (∼0.2-2.5 ppm) and other environmental samples. An advantage of the original Cripps and Stimson study (1) was that U isotope measurements, using mass spectrometry, were made on low-U environmental materials such as evergreen leaves (laurel and rhododendron). This sampling strategy was no longer possible nearly 40 years later where any of the original fallout would have long been transferred and mixed into the ground. Soils and sediment were therefore the most suitable materials, which would be likely to preserve any remains of nonnatural uranium in the environment. Since plutonium does not exist naturally a background masking problem does not occur as it does for uranium. Alpha spectrometry was used for the Pu measurements in the survey. However, plutonium can also be measured using mass spectrometry, but it is technically more difficult and a method could not be developed sufficiently quickly at the time of the survey. It is notable that we now have an effective working method suitable for measuring 239Pu/240Pu in environmental samples that uses a multicollector ICP-MS (8).
Methodology Suitable sample sites were chosen on the basis that the locations represented undisturbed (e.g. unploughed) ground. This approach ensured that any significant historical contamination should be recognizable. Each site was sampled by taking a core approximately 15 cm long that was then split into subsamples of 5 cm length. Samples were dried, lightly ground, and sieved to remove flint pebbles greater than 1 mm in size. All analytical results for soils are presented on a dry weight basis (dried to 105 °C). Thirty control samples were also collected from outlying areas of the Newbury and Basingstoke and Deane region as being distant from any nuclear sites. The Savernake Forest west of Hungerford was specifically sampled as an area far removed from any alleged uranium contamination sources, which provided soil material of similar bulk composition to the test locations around Greenham Common (i.e. clay-rich soils derived from underlying Tertiary sands and clays). Over 500 uranium and 350 plutonium measurements were made to identify any possible anthropogenic releases. The survey area was conveniently subdivided into six zones for the purposes of presenting the results (Figure 2). Quality control was monitored throughout the study by measuring subsamples taken from a solution of standard reference VOL. 34, NO. 21, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. U and Pu Isotopes of Interest half-life
natural atomic abundance
typical atomic abundance in nuclear weapon materials ∼0.9% 6% (low enriched U) 93% (high enriched U) 0.35% (depleted U) 94% (low enriched U) 6% (high enriched U) 99.65% (depleted U) 0.04% 93.34% 6%
235U
2.45 × 105 years 7.04 × 108 years
