Perturbation in the 240Pu239Pu global fallout ratio in local sediments following the nuclear accidents at Thule (Greenland) and Palomares (Spain)

It is well established that the main source of the plutonium found in marinesediments throughout the Northern Hemisphere is global stratospheric fallout, characterized by a typical 240Pu239Pu atom ratio of ∼ 0.18. Measurement of perturbations in this ratio at various sites which had been subjected to close-in fallout, mainly from surface-based testing (e.g. Bikini Atoll, Nevada test site, Mururoa Atoll), has confirmed the feasibility of using this ratio to distinguish plutonium from different fallout sources. In the present study, the 240Pu239Pu ratio has been examined in samples of sediment (and soil) collected at Thule (Greenland) and Palomares (Spain), where accidents involving the release and dispersion of plutonium from fractured nuclear weapons occurred in 1968 and 1966, respectively. The 240Pu239Pu ratio was measured by high-resolution alpha spectrometry and spectral deconvolution, and confirmed in the case of the most active samples by high-resolution X-ray spectrometry. Only samples which displayed plutonium heterogeneities, i.e. hot particles or concentrations well in excess (at least two orders of magnitude) of those expected from global fallout, were selected for analysis. The analytical results showed that at Thule the mean 240Pu239Pu atom ratio was 0.033 ± 0.004 (n = 4), while at Palomares the equivalent ratio appeared to be significantly higher at 0.056 ± 0.003 (n = 4). Both ratios are indicative of low burn-up plutonium and are consistent with those reported for weapons-grade plutonium. It is noteworthy that the mean 238Pu239Pu activity ratio in the Thule samples, at 0.0150 ± 0.0017 (n = 4), was also lower than that measured in the Palomares samples, namely, 0.0275 ± 0.0012 (n = 4). The 241Pu239Pu ratios were similarly different. Finally, the data show, in contrast to Palomares, that not all of the samples from the Thule accident site were contaminated with plutonium of identical isotopic composition.