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Artificial radioactivity within the Rhône River waters. Consequences of floods on activity levels and fluxes toward the Sea.

Frédérique Eyrolle, Rolland Benoît, Christelle Antonelli et J.M. Métivier
Environnement, risques et santé, Vol.5, N°2, 83-92


Although analytical strategies allowing today artificial radionuclide measurements at ultra trace levels (i.e., <10-5 Bq l-1), only 137Cs, 60Co and 241Am are today regularly detected by gamma spectrometry in the Rhône waters downstream all the nuclear installations located along the Rhône valley. Since the beginning of the Marcoule-COGEMA reprocessing plant dismantlement in 1997, new radionuclides such as 51Cr, 54Mn, 63Ni, 110mAg and 65Zn are listed in the reports dedicated to liquid release controls according to the regulations. These last elements were released at level 10 to 1000 fold lower than those of 137Cs or 60Co. At the Rhône watershed scale, the Marcoule liquid releases do not constitute any more the main 137Cs source as before the nineties. Significant 137Cs sources now originate first from the Rhône draining basin itself whose soils were submitted to the atmospheric fallout from the past nuclear tests and from the nuclear accident, and second, from the remobilisation of sedimentary stocks previously formed. Indeed, our last researches demonstrate that flood events may generate today an increase of 2 orders of magnitude in 137Cs activity in waters due to inputs of contaminated particles either from watershed erosion or sedimentary stock remobilisation. Flood events also generate high solid fluxes and associated contaminants fluxes towards the Mediterranean Sea. Indeed, among the 173±22 GBq of 137Cs transferred by the Rhône to the Sea from august 2002 to august 2003, 111±9 GBq were exported during the two exceptional floods that occurred in September and November 2002, i.e, 60 % of the annual export in only 15 days and almost ten fold the 137Cs activity released each year today by the reprocessing plant. The contribution of rhodanian sedimentary stocks
contaminated over the last forty years to such fluxes through remobilisation is today the subject of specific researches.


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