Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere

P. Bouisset (1) , M Nohl (1) , Catherine Cossonnet (2) , Beatrice Boulet (2) , Sandrine Thomas (2) , Nicolas Cariou (2) , Gilles Salaun (3)

1 IRSN/PSE-ENV/SEREN/LESE - Laboratoire d'étude et de suivi de l'environnement
2 IRSN/PSE-ENV/SAME/LMRE - Laboratoire de métrologie de la radioactivité dans l'environnement
3 IRSN/PSE-ENV/SEREN/LEREN - Laboratoire d'étude et d'expertise sur la radioactivité de l'environnement

The inventories of 137Cs (503 ± 34 Bq.m−2), 241Am (11.3 ± 1.2 Bq.m−2), 241Pu (33.7 ± 3.4 Bq.m−2), 238Pu (6.82 ± 0.87 Bq.m−2) and 239+240Pu (113.0 ± 5.9 Bq.m−2), sum of 239Pu (100 ± 11 Bq.m−2) and 240Pu (14.5 ± 1.7 Bq.m−2), in the Gambier archipelago (23°S) of the French Polynesia, are well higher the global fallout at this latitude, in unequal proportions for the different radionuclides. (240Pu/239Pu)AR (AR: atomic ratio) of 0.0394 ± 0.0062, and (241Pu/239Pu)AR of (2.03 ± 0.39)10−4, confirm that the overwhelmingly dominant source of these radionuclides comes from local fallout during the 1970s of the French atmospheric tests of Moruroa and Fangataufa located nearly 400 km from Gambier. The signatures of the local fallout were deduced from the excess of its inventory in 137Cs and from the mixing lines established from the signatures of the global fallout, some of the test sites and the isotopic ratios measured in Gambier. Signatures obtained are 2.0 ± 0.4 for 137Cs/239+240Pu, 0.045 ± 0.008 for 238Pu/239+240Pu, 0.031 ± 0.009 for 241Am/239+240Pu, 0.092 ± 0.027 for 241Pu/239+240Pu, 0.0163 ± 0.0049 for (240Pu/239Pu)AR, (0.78 ± 0.23)10−4 for (241Pu/239Pu)AR. The concordance of the mixing lines of the [(240Pu/239Pu)AR, (241Pu/239Pu)AR] and the linear regression of these ratios measured in the stratosphere (40°S) during the 1970s, indicates that the signatures of the close-in deposition are also those of the stratospheric injections of the French tests. The signatures of stratospheric fallout in the Southern Hemisphere were evaluated by considering that the fission energy of these injections represents 11% and that of the Northern Hemisphere represents 89% and that the isotopic ratios of stratospheric injections remained the same over the period 1970–1974. The activity ratios deducted are 21.9 ± 0.1 in 137Cs/239+240Pu, 0.11 ± 0.05 in 238Pu/239+240Pu, 1.03 ± 0.12 in 241Pu/239+240Pu and 0.35 ± 0.04 in 241Am/239+240Pu. The associated atom ratios are 0.157 ± 0.011 for (240Pu/239Pu)AR and (8.33 ± 0.48)10−4 for (241Pu/239Pu)AR. These signatures appear to be consistent with the results of the inventories at Hiva Oa, located more than 1,000 km north of both French test sites, and with those found in the Australian continent, in regions not impacted by UK-test debris. The proportions of close-in tropospheric fallout from the French tests are about 90% in Gambier. They represent a proportion in the inventories of 40% for the 137Cs, 60% for 241Am and in the range between 80 and 90% for Pu isotopes.

Mots clés: Inventory; Close-in deposition; Stratospheric signatures; Pu atom ratios; 137 Cs; 241 Am; Gambier; French atmospheric tests

Domaines: Physique Atmosphérique et Océanique [physics.ao-ph]; Sciences de l'environnement

P. Bouisset, M Nohl, Catherine Cossonnet, Beatrice Boulet, Sandrine Thomas, et al.. Contribution of close-in fallout from the French atmospheric tests in inventories of 137Cs, 241Am and plutonium (238, 239, 240) in Gambier Islands (French Polynesia) – Signatures of stratospheric fallout in the Southern Hemisphere. Journal of Environmental Radioactivity, 2021, 235-236, pp.106624. ⟨10.1016/j.jenvrad.2021.106624⟩. ⟨hal-03360613⟩