A new ground-level fallout record of uranium and plutonium isotopes for northern temperate latitudes

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Abstract

Plutonium and uranium isotope ratios can be used to differentiate the sources of nuclear contamination from nuclear weapon establishments (Environ. Sci. Technol. 34 (2000) 4496; Internal Report for AWRE Aldermaston, UK (1961)), weapon fallout (Geochim. Cosmochim. Acta 51 (1987) 2623; Earth Planet. Sci. Lett. 63 (1983) 202; Earth Planet. Sci. Lett. 22 (1974) 111; Geochim. Cosmochim. Acta 64 (2000) 989), reprocessing plants, reactor or satellite accidents (Science 105 (1979) 583; Science 238 (1987) 512) and in addition they provide markers for post-1952 geochronology of environmental systems. A good record of plutonium and uranium isotope ratios of the background resulting from atmospheric nuclear testing is essential for source characterisation studies. Using recently developed mass spectrometric techniques (J. Anal. At. Spectrom. 16 (2001) 279) we present here the first complete records between 1952 and the present day of northern temperate latitude 240Pu/239Pu and 238U/235U atom ratios for atmospheric deposition. Such information was not derived directly during the period of atmospheric testing because suitable mass spectrometric capability was not available. The currently derived records are based on an annual herbage archive and a core from an Alpine glacier. These studies reveal hitherto unseen fluctuations in the 238U/235U atmospheric fallout record, some of which are directly related to nuclear testing. In addition, they also provide the first evidence that plutonium contamination originating from Nevada Desert atmospheric weapon tests in 1952 and 1953 extended eastwards as far as northwestern Europe. The results presented here demonstrate that we now have the capability to detect and precisely identify sources of plutonium in the environment with implications for the development of atmospheric transport models, recent geochronology and environmental studies.

Introduction

The radioactive debris from nuclear explosions (Fig. 1, Fig. 2) was partitioned into the troposphere and stratosphere according to its particle size and the power of the explosion. The subsequent fallout occurred on a time scale of minutes through to years. The amount of debris produced and its particle size distribution depended on the explosive yield of the device and its height above ground when detonated [9]. Fine debris from small yield tests (<100 kt TNT equivalent) produced fallout that was expected to have a maximum residence time of 70 days in the troposphere [10]. Fallout from large yield tests (>500 kt TNT equivalent), however, was almost wholly derived from material injected into the stratosphere and, though falling mostly in the hemisphere of injection, was distributed globally and deposited over several years. A typical residence time for aerosols in the stratosphere is 15–18 months [11]. The transfer of fallout from the stratosphere to the troposphere is seasonally modulated and for the Northern Hemisphere occurs mostly in the late winter and spring with little fallout occurring during summer and autumn. At ground level the maximum fallout, termed the Spring Peak or Spring Maximum [11] occurs between March–June with the main stratosphere–troposphere exchange occurring roughly 1 month before. This input of stratospheric material into the troposphere occurs by seasonal adjustment of the tropopause at temperate to high latitudes, by the Hadley circulation at around 30° and by the Jetstreams at around 30° and 60°. The first process accounts for the exchange of about 10% of the total stratospheric mass per year and the Jetstreams and Hadley circulation account for about 50–60% [11]. Existing records of nuclear fallout, particularly for plutonium, were either derived from sampling of stratospheric air by aircraft or from Polar ice cores and therefore may not represent the ground level fallout seen in temperate latitudes. Detailed measurements of other radionuclides such as 90Sr were made during the testing period and in New York, for example, determinations were made in rainwater going back to 1983 [12], [13], [14], [15]

The two primordial isotopes of uranium are 235U and 238U with 238U/235UPRESENT=137.88 [16] and this atomic abundance ratio has no significant variation in nature except in fossil natural reactors [17]. Deviations from this ratio in environmental materials can therefore only be explained by the addition of technologically modified uranium (Table 1) [1]. Although uranium was a common component of many nuclear weapons there are no published accounts of 238U/235U in weapon fallout.

Plutonium, unlike uranium, is virtually entirely anthropogenic [18] in origin and its main isotopes found in the environment, 238Pu, 239Pu, 240Pu, 241Pu and 242Pu, are derived from civil and military sources (Table 2). Approximately 6 tonnes of 239Pu were introduced into the environment from 541 atmospheric weapon tests [19] with a total explosive yield of 440 megatons (TNT equivalent), with 25 tests accounting for two thirds of the yield. Fallout was distributed globally at a ∼3:1 ratio between the Northern Hemisphere and the Southern Hemisphere.

The current work, though now of wider interest and importance, was initiated by a recent investigation of the alleged nuclear accident at the former USAF Greenham Common airbase in Berkshire, UK in 1958 [1], [7]. A confidential Cold War study [2] was leaked to the UK media in 1996 that reported a small excess of 235U in environmental samples collected around the Greenham Common airbase. One aspect of the study by Croudace et al. [1] was to determine if the small amounts of enriched U reported by Cripps and Stimson could have originated from weapon fallout rather than from a weapon accident. To do this 30 years after the incident would have required knowledge of 238U/235U in 1957–58 weapon fallout in southern England. The results found in this study now make this assessment possible.

Section snippets

Methods

The IACR Rothamsted (Harpenden, UK) archive is a unique herbage collection with continuity from 1843 until now of some 20 000 samples of crops and soil taken from fully documented field experiments. The Rothamsted archive includes summer and autumn harvested herbage samples and the existence of samples from both harvests is important because they provided an ideal means of identifying the source and timing of some nuclear tests due to the ‘Spring Peak’. The first harvest samples are expected to

Results and discussion

Results of the current study (Table 6, Table 7) were compared with records of device testing and contemporary radiochemical monitoring of fallout [21], [22], [23], [24], [25]. The profiles for 137Cs and 239+240Pu activities in the Rothamsted grass (Fig. 3A,B) are very similar to these records of fallout and reflect the yields of atmospheric testing (Fig. 2), if a 1-year transit time for stratospheric deposition is assumed. The data variations reveal clear evidence for the earliest US

Conclusions

Ground level records for 240Pu/239Pu and 238U/235U atom ratios for northern temperate latitudes have been obtained from an annual UK herbage archive and a core from a French Alpine glacier using a recently developed mass spectrometric technique. Evidence is provided, for the first time, that plutonium contamination originating from Nevada Desert atmospheric weapon tests in 1952 and 1953 extended eastwards as far as northwestern Europe. Hitherto unseen fluctuations in 238U/235U atom ratios in

Acknowledgements

We are grateful to IACR Rothamsted, particularly Dr Paul Poulton, for providing access to their valuable grass sample and we also appreciate the help of Dr Pourchet from CNRS ‘Laboratoire de Glaciologie et Géophysique de l’Evironnement’ Grenoble for the ice core samples. We are also grateful to the helpful comments of the journal reviewers, Peter Santschi, Chris Hawkesworth and Gus MacKenzie.[BW]

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