Situation of radioactivity monitoring in French Polynesia in 2016. Synthesis of results from the IRSN's monitoring network. 2018 mission report

The radiological monitoring of the French environment carried out in Polynesia, outside the nuclear test sites of Moruroa and Fangataufa, by IRSN's Laboratory for the Study and Monitoring of the Environment (LESE) since 1962 consists of sampling regularly samples of various types in the atmospheric, terrestrial and marine environments with which the population may be in contact as well as foodstuffs. Since 1998, foodstuffs are collected in seven islands (Tahiti, Maupiti, Hao, Rangiroa, Hiva Oa, Mangareva and Tubuai) representing the five archipelagos. Since 2015, an additional island is included in the annual monitoring. After Raivavae (Australes archipelagos) last year, Nuku Hiva (Marquises archipelagos) was retained in 2016. In addition to radiological measurements of food samples locally produces in the two Marquises Islands, Hiva Oa and Nuku Hiva, external exposure measurements on the whole island are performed and soil samples collected to study the density deposition of plutonium and ¹³⁷Cs from atmospheric fallout of nuclear tests. After a steady decrease in radioactivity levels since the cessation in 1974 of the French atmospheric nuclear tests and the last atmospheric test carried out by China in 1980, the levels of radioactivity measured in 2016 are in continuity with previous years; they are at a very low level. This residual radioactivity is mainly attributable to ¹³⁷Cs. In 2016, the annual effective dose for the adults of Tahiti, including the doses associated with the external irradiation, the ingestion and the inhalation, added by the radioactivity of artificial origin is in the order of 0.1% of the dose due to the natural exposure, less 1.4 mSv.y^-1. This natural exposure due about equally from the external exposure (cosmic radiation and telluric radionuclides at equal parts) and from the ingestion of solid foodstuffs which the majority contributors are ²¹⁰Po and ²²⁶Ra. Since 2010, measures of the external exposure have been made on some islands of the five archipelagos. Exposure levels are low, averaging less than 1.0 mSv.y^-1 for elevated highland and of the order of 0.3 mSv.y^-1 in atolls. Because the very low levels of exposure by ingestion of locally produced foods, the number of foodstuffs collected is now limited to the products that concentrate the most ¹³⁷Cs so as to have the best bioindicators to continue to observe the evolution of the concentration of this radionuclide in the environment. As a consequence, the radiological exposure by ingestion is no longer evaluated with a food ration based on an annual consumption of an amount established for the different foods; it is replaced by an assessment of the global exposure of the Polynesian population from the measurement of a series of meal trays collected in Tahiti during 5 days. There is no longer any specific assessment by archipelago and age ranges (children aged 0-5 years and adults). The advantage is a better reality of the current consumption of different foods, locally produced or imported, since it has changed considerably since the establishment in 1982 of food rations used until now for these calculations. Another advantage is that, by limiting sampling, additional analyzes can be used to obtain a more complete dose evaluation integrating the predominantly natural radioactivity in food. After the Fukushima disaster, enhanced radiological monitoring of the environment was organized and continued in 2016 for the marine environment. The measurement results, for sea water and various pelagic fish species, do not show any impact of the contamination of the Japanese marine domain in Polynesian waters: no increase of ¹³⁷Cs over previous years was observed and ¹³⁴Cs was never detected. A new orientation of the monitoring program was implemented since 2014. It concerns the study of radioactivity of artificial origin in soils, the main route of transfer to food at present. The analyses are mainly aimed at assessing ¹³⁷Cs, a radionuclide which can be still measured in more than one thirty per cent of analyzed food samples, and 238, 239 and 240 plutonium isotopes, scarcely detectable in food samples but remaining in soils at concentration levels quite steady over time (long time radioactivity periods and low leaching by rainwater). The measured concentrations in soils of the high islands sometimes exceed 1 Bq.kg^-1 dry both for ¹³⁷Cs and for plutonium. (authors)