Identification of the origin of radiocesium released into the environment in areas remote from nuclear accident and military test sites using the 135 Cs/ 137 Cs isotopic signature.

The isotopic signature of radionuclides provides a powerful tool for discriminating radioactive contamination sources and estimating their respective contributions in the environment. In this context, the 135 Cs/137 Cs ratio has been tested as a very promising isotopic ratio that had not been explored yet in many countries around the world including France. To quantify the levels of radioactivity found in the environment, a new method combining a thorough radiochemical treatment of the sample and an efficient measurement by ICP-MS/MS has been recently developed. This method was successfully applied, for the first time, to soil and sediment samples collected in France in two mountainous regions preferentially impacted either by global fallout from nuclear weapons testing (i.e., the Pyrenees) or by the Chernobyl accident (i.e., the Southern Alps). The 135 Cs/137 Cs ratios measured on twenty-one samples ranged from 0.66 ± 0.04 and 4.29 ± 0.21 (decay-corrected to January 1st, 2022) corresponding to the characteristic signatures of the fallout from Chernobyl and global fallout associated with the nuclear weapons testing, respectively. Moreover, large variations of both the 137 Cs mass activity and the studied isotopic ratio recorded by most samples from the southern Alps suggest varying proportions of these two 137 Cs sources. For these samples, the contribution of each source was estimated using this new tracer (135 Cs/137 Cs) and compared with the mixing contribution given by activity ratio: 239+240 Pu/137 Cs. This work has successfully demonstrated the applicability of the 135 Cs/137 Cs isotopic signature to nuclear forensic studies and could be extended to better evaluate the environmental impact of nuclear facilities (i.e., NPP, waste reprocessing).