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Enhancing Nuclear Safety


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Radioecological Sensitivity of Permanent Grassland Areas

Benoît BESSON, doctorate of University of Franche-Comté, 325p., defended on the 9th december 2009

Document type > *Mémoire/HDR/Thesis

Keywords > paleoseism, emergency management, pollutant, sphingosine-1-phosphate, synthetic seismograms

Research Unit > IRSN/DEI/SESURE/LERCM

Authors > BESSON Benoît

Publication Date > 09/12/2009

Summary

The project “SENSIB” of the Institute for Radiological Protection and Nuclear Safety (IRSN) aims at characterizing and classifying parameters with significant impact on the transfer of radioactive contaminants in the environment. This thesis is focused on permanent grassland areas. Its objectives are the analysis of the activity variations of two artificial radionuclides (137Cs and 90Sr) in the chain from soil to dairy products as well as the categorization of ecological and anthropogenic parameters, which determine the sensitivity of the studied area.
For this study, in situ sampling is carried out in 15 farms in 3 different French regions (Charente, Puy-de-Dôme and Jura). The sampling sites are chosen according to their natural variations (geology, altitude and climate) and the soil types. Additionally to the radiologic measurements, geographic, soil and vegetation data as well as data concerning cattle-rearing and cheese manufacturing processes are gathered.
From the soil to the grass vegetation, 137Cs transfer factors vary between 3 ×10-3 and 148 ×10-3 Bq kg-1 (dry weight) per Bq kg-1 (dry weight) (N = 73). Theses transfer factors are significantly higher in the region Puy de Dôme than in the Jura region. The 137Cs transfer factor from cattle feed to milk varies from 5.9 ×10-3 to 258 ×10-3 Bq kg-1 (fresh weight) per Bq kg-1 (dry weight) (N = 28). Statistically, it is higher in the region Charente. Finally, the 90Sr transfer factor from milk to cheese ranges from 3.9 to 12.1. The studied site with the highest factor is the Jura (N = 25). The link between milk and dairy products is the stage with the most 137Cs and 90Sr transfers.
A nonlinear approach based on a discretization method of the transfer factor with multiple comparison tests admits a classification of the sensitivity factors from soil to grass vegetation. We can determine 20 factors interfering in the 137Cs transfer into the vegetation, for instance, the clay rate of the soils or a marker for soil particles adhered on vegetation. 137Cs transfers into milk depend on the clay rate of the vegetation sample and on the cattle feed. Finally, during the cheese manufacturing process, 90Sr acts like the calcium, its chemical analogue, what explains the increase of 90Sr transfer in calcium enriched cheeses, like Comté cheese. Based on the classification of these factors, a method is proposed to evaluate the 137Cs sensitivity transfer from soil to dairy product.

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Laurent Pourcelot, thesis supervisor

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