Abstract
The isotopes 239 Pu and 240 Pu are present in surface soils as a result of global fallout from nuclear weapons tests carried out in the 1950's and 1960's. These isotopes constitute artificial tracers of recent soil erosion and sediment movement. In practice the high throughput capabilities and high sensitivity of the AMS technique makes the study of Australia's geographically large areas viable using Pu isotopes. As part of its weapons development program the United Kingdom carried out a series of atmospheric and surface nuclear weapons tests at Maralinga, South Australia in 1956 and 1957. The contribution from the Maralinga tests to the Pu isotopic abundances present in the region around Maralinga is largely unknown. In global fallout, for example, the 240 Pu/ 239 Pu ratio is typically in the range 0.17 - 0.19, but the influence of the regional tests could lead to values outside this range. This would impact on the assessment techniques used in the soil and sediment tracer studies. We report recent measurements on soil samples collected from across the Maralinga Test site.
Highlights
From the point of view of using plutonium as a tracer of soil movement nuclear weapons tests can be divided into two categories: (i) high yield explosions that inject Pu into the stratosphere, where it remains with a residence half-life of 8 - 24 months [1] and becomes relatively well mixed before returning to the surface as “global fallout”, and (ii) low yield detonations (< ~300 kT) for which the bulk of the nuclear debris are confined to the troposphere, for a residence half-life estimated at ~70 days [2] and which leads to “regional fallout” over an area determined largely by the fission yield of the device and by the meteorological conditions at the time of the test and in the period thereafter
Our data are consistent with that of global fallout in Australia, and are consistent with fallout distribution maps from the Australian tests [22, 23] which indicate the bulk of the regional fallout from the Maralinga weapons detonations was to the north and east of the test site
The data from sites T006-T008 show clear signatures of local fallout Pu derived from the safety trials, with indications of the presence of “hot particles” of Pu bearing material
Summary
From the point of view of using plutonium as a tracer of soil movement nuclear weapons tests can be divided into two categories: (i) high yield explosions that inject Pu into the stratosphere, where it remains with a residence half-life of 8 - 24 months [1] and becomes relatively well mixed before returning to the surface as “global fallout”, and (ii) low yield detonations (< ~300 kT) for which the bulk of the nuclear debris are confined to the troposphere, for a residence half-life estimated at ~70 days [2] and which leads to “regional fallout” over an area determined largely by the fission yield of the device and by the meteorological conditions at the time of the test and in the period thereafter. Fallout in the immediate vicinity of tests carried out close to ground level can have relatively large particles derived from bomb fragments or contaminated ground material that are deposited from the atmosphere as “local fallout” within hours or days of the test [3]. In particular the 12 Vixen B trials, each of which involved approximately 4 TBq of 239Pu, left Taranaki the most severely contaminated site at Maralinga, with approximately 22 kg of 239Pu being distributed across the local environment. This is comparable with the ~30 kg of 239Pu deposited across the whole of mainland Australia as a result of global fallout derived from all atmospheric tests world-wide
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