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Origin and becoming of tritium in French non-nuclerized Mediterranean coastal continental hydro-systems

Loïc Ducros has defended his thesis on 14th June 2018, in Aix-en-Provence (France)​

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PSE-ENV/SRTE/LRTA

Authors > DUCROS Loïc

Publication Date > 14/06/2018

Summary

​Initially emitted in the environment by atmospheric nuclear weapons tests between 1945 and 1980, tritium (3H or T) is one of the main radionuclide released by nuclear power plants and spent nuclear fuel reprocessing plants. It is present in all environmental media in several different forms. It can be bound to organic matter (OBT) or found in conjunction with other hydrogen atoms in such free forms, such as (HT, HTO, CH3T), as it has the same physicochemical properties as those of the stable isotope of hydrogen. Furthermore, 3H follows the same dynamics of incorporation and degradation in living organisms as the stable isotope of hydrogen owing to its mobility and is quickly balanced with the surrounding environment. For several years, numerous studies have shown disequilibrium of OBT/HTO in different environmental compartments (terrestrial and aquatic) and their matrices (e.g., soils, sediments and fishes). This research work aims to explain, on one hand, the variability of tritium concentrations, and in the other the origin of disequilibrium of OBT/HTO based on the persistence hypothesis. Indeed, if an organism was exposed in the past to higher ambient concentrations than currently, this contamination can be found in the organic matter (OBT). This persistence induces an apparent disequilibrium compared to the current levels of free forms in the environment, with which the biotic tissues water (HTO) is in equilibrium. This phenomenon can be observed for living organisms or abiotic matrices (organic matter in soils and sediments). In this study, a multi-scale approach was used, focusing on the role of delayed transfer of organic matter between environmental compartments. The main results, at the scale of the Mediterranean region, have established the ranges of tritiated water concentrations (HTO) in several rivers that are not influenced by atmospheric releases from nuclear facilities, and also identified key environmental parameters linked to the observed variability. The results also made it possible to understand the origin of OBT, which was measured in different terrestrial and aquatic samples, using statistical analyses. The observations and statistical analyses tend to confirm that atmospheric nuclear tests led to areas of persistence of tritium (OBT), and can lead to significant disequilibrium between the organically bound and free forms. Moreover, the study of a sediment core has shown a peak of tritium concentrations (OBT) at depth, in all likelihood from the global fallout associated with atmospheric nuclear weapons tests. Also, a study site impacted by industrial releases of tritium for the last fifty years has confirmed the persistence of OBT in organic matter, especially in abiotic matrices.

 


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