Experimental measurements of the bacterial oxidation of HT in soils: impact over a zone influenced by an industrial release of tritium in HT form.
Résumé
Tritium is a radionuclide released to the atmosphere by nuclear industries in various forms, mainly HTO and to a lesser extent HT. However, some nuclear sites may emit predominantly HT in the atmosphere and it is important to understand the behaviour of this HT form when oxided in the top cm of soils, and that this tritium can then be released into the atmosphere in assimilable HTO form, which has a greater impact in terms of dose. In this work, we adapt the bacterial oxidation model of HT in soils of Ota el al (2007) by laboratory experiments on soils typical of western France, and we have in particular adapted the frequency factor A and the Michaelis-Menten enzymatic reaction parameter (Km) on the basis of an Arrhenius equation in function of the porosity of the soil. We then applied this model to the environment near the reprocessing plant of Orano la Hague (France), which emits a significant amount of HT. Based on the adapted model, and knowing the atmospheric variations of HTO and HT over the period 2013-2016, we estimated that the mean HTO activity in soil due to atmospheric HT reached 0.6 Bq.L-1 (with a peak value of 5 Bq.L-1) while the mean value with all sources taken into account is 6.2 Bq.L-1. Then, in an environment such as that surrounding the Orano La Hague plant, where near-field atmospheric HT activity is very high, the bacterial oxydation contribution to produce HTO in the soil can be considered as approximately 10%. The flux to the atmosphere from these source representing approximately. 1.5 Bq.m-2.d-1. If we compared this value with the direct atmospheric release of HTO around the site (2km around i.e. 13 km2), we estimate 218 Bq.s-1 of HTO was released by the soil, representing only 0.1% of available ambient stock. From this work, it appears clear that this secondary source term from the soil is insignificant at this specific site.
Domaines
Sciences de l'environnement
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