Mass balances and transit times of artificial radionuclides transported from the Rhone River to the Mediterranean Sea

The Rhone River (France) is one of the most nuclearized systems over the world with five nuclear power plants and two plants (Uranium conversion and spent fuel treatment) that have released radionuclides along 400 km of the river since the 50's. In this river, the transfer of particulate matter mainly depends on floods which transport up to 90% of the annual suspended matter flux in less than 10% of time. This could induce a disconnection between inputs and output fluxes because releases are not allowed during floods. Since 2005, IRSN collects and measures dissolved and particulate radionuclide with an automated station located 30 km upstream of the river mouth. This station also allows collecting specific samples during flood events, in order to better estimate the particulate radionuclide fluxes. Important results are: - Fluxes of radionuclide linked to releases and atmospheric fallout (²³⁸Pu, ²³⁹+²⁴⁰Pu, ²⁴¹Am, ¹³⁷Cs, ⁹⁰Sr) are slightly higher than those declared by the industries. This is partly due to leaching from the watershed soils but also due to resuspension of past sediment within the river, as evidenced by Pu isotopic ratios. - Fluxes of radionuclide only issued from the nuclear power plant releases (⁵⁸Co, ⁶⁰Co, ^110mAg, ¹²⁴Sb, ¹²⁵Sb, ⁵⁴Mn, ²⁴⁴Cm) are in equilibrium with those declared, traducing probably an efficient and rapid export from the system on a yearly basis, even for particulate radionuclide. Sometimes however a slight deficit is observed, which could be attributed to difficulties to measure radionuclides at trace level and/or to temporary accumulation within the river