PERTURBATION IN THE PU-240 PU-239 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 stratosph eric fallout, characterized by a typical Pu-240/Pu-239 atom ratio of s imilar to 0.18. Measurement of perturbations in this ratio at various sites which had been subjected to close-in fallout, mainly from surfac e-based testing (e.g. Bikini Atoll, Nevada test site, Mururoa Atoll), has confirmed the feasibility of using this ratio to distinguish pluto nium from different fallout sources. In the present study, the Pu-240/ Pu-239 ratio has been examined in samples of sediment (and soil) colle cted at Thule (Greenland) and Palomares (Spain), where accidents invol ving the release and dispersion of plutonium from fractured nuclear we apons occurred in 1968 and 1966, respectively. The Pu-240/Pu-239 ratio was measured by high-resolution alpha spectrometry and spectral decon volution, 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 fallo ut, were selected for analysis. The analytical results showed that at Thule the mean Pu-240/Pu-239 atom ratio was 0.033 +/- 0.004 (n = 4), w hile at Palomares the equivalent ratio appeared to be significantly hi gher at 0.056 +/- 0.003 (n = 4). Both ratios are indicative of low bur n-up plutonium and are consistent with those reported for weapons-grad e plutonium. It is noteworthy that the mean Pu-238/Pu-239 activity rat io 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 Pu-241/Pu-239 ratios were similarly different. Finally, the data show, in contrast to Palomares, that not all of the samples f rom the Thule accident site were contaminated with plutonium of identi cal isotopic composition. (C) 1997 Elsevier Science B.V.