Sélénium, radioactivité et risques d'exposition chronique

Selenium, radioactivity and chronic exposure related risks

Institut de Radioprotection et de Sûreté Nucléaire BP 17, 92262 FONTENAY AUX ROSES CEDEX

Résumé

Du fait de l'importante affinité du Se envers la matière organique, les scénarios pessimistes d'impact radioactif des déchets nucléaires doivent supposer que, si du Se fuit, il pourra se bioaccumuler dans le sol superficiel et y rester longtemps. Les quantités notables de Se dans ce type de déchets et la très faible concentration de Se stable dans sol ne permettent pas de garantir, dans l'état actuel des connaissances, que leur mélange conduira à un Se « moyen » de radioactivité insignifiante. Les recherches de l'Institut Radioprotection et de Sûreté Nucléaire (IRSN) se focalisent sur quelques questions importantes pour évaluer l'impact du Se : quel est son temps de résidence dans le sol et comment est-il influencé par l'activité microbienne? Comment le Se traverse-t-il membranes cellulaires ? Comment se propage-t-il dans les chaînes trophiques ? Quels sont les effets d'une exposition chronique sur les populations ? Ces recherches font partie d'un programme plus général dédié aux expositions chroniques aux éléments radioactifs. Ce programme nommé ENVIRHOM est en cours depuis 2003 et il utilise largement les méthodes de Técotoxicologie et de la radioécologie. Les résultats espérés sont une meilleure évaluation des contributions respectives des radionucléides et des autres types de stress auxquels sont confrontés l'homme et les écosystèmes.

Abstract

Taking into account the high affinity of Se to organic matter, pessimistic scenarii of radioactive impact of nuclear waste have to assume that leaking Se, if it happens, can bioaccumulate in the top soil and stay there a very long time. The noticeable amount of Se in this kind of waste and the usually very small concentration of stable Se in soil are not favorable to a negligible radioactivity of the resulting mixture. Ongoing researches of the "Institut de Radioprotection et de Sûrete Nucléaire" (IRSN) are addressing some important questions to evaluate Se impact: what is its residence time in soil and how is it influenced by the microbial activity ? How does the Se go through a cell boundary ? How does the Se move in trophic chains ? What is the effect of a chronic exposure on a population ?

As regards persistence of a contamination, vertical transfer in soil is a key point. Simple experiments show that this migration is not only a function of organic matter content but also of the state (living or dead) of this organic matter. Column experiments show that a small amount of Se can migrate rapidly but that the largest part evolves toward a chemical form that is strongly linked to the soil. To evaluate more precisely this problem of ageing, long experiments (2 years) with a precise control of chemical conditions and a precise control of inputs and outputs are currently in progress.

In a situation of contamination, the second important problem is the health effect of this contamination. Man is not particularly at risk since its Se content is not very high and since even if it is replaced by a contaminated Se according to a pessimistic scenario, the resulting "dose" is in the range of extremely low risk. Of course there is still the possibility that the usual models are not "valid" but a more likely scenario is that more than man, the damage will affect some specific ecosystems. It is well known that in "wet zones" Se trophic concentration processes are able to generate a risk for top predators. The same concentration process can also happen for radioactive Se. This is why experiments are ongoing with typical components of this kind of ecosystems: algae such as Chlamydomonas reinhardtii or mollusks such as Corbicula fluminea. The experiments, still with stable Se, study internal repartition of Se in cells, they study also transfer through cell boundaries in function of chemical conditions and competitive ions. Various chemical forms of Se and various conditions of feeding are studied. An important result is that the bioaccumulation is not a fixed process. It does not depend only of the toxicant concentration. This may seem "obvious " but there are still many data to get before being able to incorporate such phenomena in "operational" models used to foresee the risks caused by toxicant whether they be radioactive or stable.

These researches are part of a more general program devoted to chronic exposures to radioactive elements. This program named ENVIRHOM is running in IRSN since the year 2003 and is widely using methods of ecotoxicology and of radioecology. It is hoped that it will allow a better evaluation of the respective contributions of radionuclides and other kind of stressors to which man and ecosystems are submitted.