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Numerical modelling of the marine dispersion of radionuclides released during the accident at the Fukushima nuclear power plant and their transfer to marine biota

Mokrane Belharet has defended his thesis on 6th October 2015 at CESBIO in Toulouse​.

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/DEI/SESURE/LERCM

Authors > BELHARET Mokrane

Publication Date > 06/10/2015

Summary

Huge amounts of radionuclides, especially 137Cs, were released to the coastal northwestern Pacific ocean after the Fukushima nuclear power plant accident, that occurred on 11 March 2011. The resultant radiocesium contamination was quickly transferred to marine biota resulting in elevated cesium levels in various organisms. Using a modelling approach, this work aims to study the 137Cs transfer to the marine pelagic food chains, from plankton populations to the large piscivorous fishes.

Coupling the radioecological model, developed in this study, with an ecosystem model comprising an NPZD biogeochemical model and a regional ocean circulation model, is appeared to be the most adaptedtool for modelling of plankton populations contamination in this accidental situation. The results of this study showed high contamination levels in the plankton populations, especially in the vincity of the power plant, where the maximal concentrations are estimated to be about 4 orders of magnitude higher than those observed before the accident. In spite of these high contamination levels, the maximum 137Cs absorbed dose rates for phyto- and zooplankton populations were estimated to be well below the 10 µGy.h-1 benchmark value, from which a measurable effect on the marine biota can be observed. This study has also highlighted the predominance of the cesium uptake from food and the presence of biomagnification potential at this trophic level.

The radioecological model developed to study the nektonic species contamination is based on the individual size. In this approach, each species is represented by a set of cohorts. The number of these cohorts is a function of the species life span and reproduction frequency. Unlike traditional approaches,
the organism ingestion rate and diet composition considered in this modelling approach are not constant, but vary over the time according to the size of the organism. The model results are in general satisfactory, and the validation is carried out in both equilibrium and accidental situations. This study highlighted
the importance of the organism migratory movements in the radioecological modelling espicially in the accident situations caracterized by a very high spatial variability of radionuclides concentrations in the seawater. The detailed caracteristics of 137Cs concentration dynamics in the different species are
discussed. The contamination levels estimated for the different species are significantly higher than those observed before the accident, with a clear tendency to increase with individual size.

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