Post-accidental riverine dispersion of sediments contaminated by radionuclides: confrontation of lessons learnt from Chernobyl andFukushima case studies in catchments from Russia (1986-2009) and Japan (2011-2012)

International audience; Chernobyl (1986) and Fukushima (2011) nuclear power plant accidents led to the release of important quantitiesof radionuclides (e.g., Cs-134; Cs-137) into the environment, and to the formation of severe contaminationplumes (with initial Cs-137 activities exceeding typically 400 kBq m-2) on soils of the regions exposed to theradioactive fallout. This leads to important consequences for agriculture in strongly contaminated areas wherethe most affected fields should not be cultivated anymore during long periods of time, depending on the half lifeof the emitted radionuclides. Furthermore, sediment transfer in rivers can lead to the dispersion of radioactivecontamination into larger areas over time.In this paper, we propose a methodology to trace and model radioactive contamination in river catchmentsover the short (2 yrs) and the longer term (25 yr) after major nuclear power plant accidents. This methodology isestablished and confronted to two case studies.The most recent study was conducted in the coastal catchments of the Rivers Nitta, Mano and Ota (ca. 600km2) draining the main part of the radioactive pollution plume that deposited across Fukushima Prefecture. Threefield campaigns were conducted to sample riverbed sediment along those rivers after the summer typhoons and thespring snowmelt (i.e. in Nov 2011, April 2012 and Nov 2012). Based on their analysis in gamma spectrometry,we show the rapid dispersion of the inland contamination and its progressive export by coastal rivers to the PacificOcean. This is confirmed by measurements of the Ag-110m: Cs-137 ratio. Analysis of sediment sequences thataccumulated in reservoirs of the region provides additional information on the magnitude on sediment transfers inthose areas.This rapid dispersion of radioactive contamination in Japan is confronted to lessons learnt from a casestudy conducted in the Plava River catchment (ca. 2000 km2) located in the so-called “Plavsk contaminationhotspot”, in western Russia. We used the Landsoil expert-based erosion model, 137Cs inventory profiles andalluvial sediment core analyses to understand and quantify contaminated sediment transfer across the cultivatedcatchment since 1986. Our results show that soil redistribution in the fields was dominant, and that sedimenteroded from cropland mostly re-deposited in dry valleys during the heaviest storms. Overall, only 15 to 25% ofmaterial eroded from the hillslopes was delivered to the river valleys. Accumulation of contaminated sediment indry valley systems therefore constitutes a major problem 25 years after Chernobyl accident.In conclusion, we show how the experience acquired after the Chernobyl accident contributed to facilitatethe urgent analysis of sediment transfers across Fukushima Prefecture, where the possible measurement ofrelatively short-lived radionuclides (Ag-110m, Cs-134) provided a way to conduct a rapid quantitative assessmentof contaminated sediment sources and exports.