Livre blanc du Tritium

“Tritium Impact” working group Dr Patrick Smeesters, Chairman. Background to the group’s work Tritium can be naturally occurring or anthropogenic. It is produced in the form of tritiated hydrogen, tritiated water or tritiated organic molecules. Oxidation converts tritium gas to tritiated water which then joins the water cycle. Since hydrogen is a major constituent element in living matter (along with carbon, oxygen and nitrogen), the tritium can be taken up into organic molecules within cells through processes such as photosynthesis or, in animals, the biosynthesis of molecules in cells or by hydrogen exchanges with the surrounding environment. The differences in bonding forces with the organic matter lead to the definition of two sub- fractions of organically bound tritium (OBT): exchangeable OBT (exchangeable with hydrogen in cell water) and non-exchangeable OBT (more specifically, tritium bound to carbon). The exchange processes can be slowed or even stopped after the death of an organism, causing the organic molecules to remain for a certain time in the soil or sediments. Tritium is a low- energy beta emitter and is generally considered to be an element with low radiotoxicity. Tritium ingested in organic form in food is approximately three times more radiotoxic than tritiated water (dose coefficient per unit of activity ingested is approximately three times higher). This is related to the biological (elimination) half-life. Tritium has been making the news again over the last few years. Firstly, high concentrations of tritium in organic forms have been unexpectedly observed in some marine species (flat fish, crustaceans and molluscs) in Cardiff Bay, an area which has seen industrial discharge of tritium-marked biological molecules. Analogous observations, but somewhat less marked, were also found off Sellafield, an area in which industrial discharges should theoretically be limited to tritiated water. These observations raise the issue of potential tritium accumulation along the marine food chain. Secondly, from a health perspective, recent summary reports (AGIR 2 , Article 31 3 ) have highlighted various difficulties and/or uncertainties in assessing the effects of tritium exposure. Issues include the 2 Health Protection Agency, Review of Risks from Tritium, Report of the Independent Advisory Group on Ionizing Radiation, November 2007 consequences of the highly heterogeneous dose distribution delivered by tritium, particularly when incorporated into DNA or histones, the uncertainties related to quality factors and the RBE (relative biological effectiveness), the value of tritium weighting factor W R (a proposal has been made to increase the factor to 2), the lack of data on the effects of chronic exposure or the spread of results when dealing with tritiated organic molecules, with significant variation depending on molecule type and the biological effect analysed. The ASN’s aim in setting up this working group was to examine current knowledge on all these issues. The issue of bioaccumulation Semantic clarification As is often the case in meetings, regardless of the make- up and expertise of the groups, many discussions stem from the fact that different people understand words differently. It was important, therefore, to agree on the meaning of the terms used. Bioconcentration means the presence of substances in an organism (e.g. aquatic organism) at a higher or lower concentration than the concentration measured in its environment (e.g. water) at the same time. The bioconcentration factor is simply the ratio between contaminant concentration in the living organism (or one of its organs or tissues) and the concentration of the same substance in the organism’s environment. Bioconcentration factors may thus be greater than, equal to or less than 1. Since bioconcentration factors are often defined in the laboratory, they do not take into consideration transmission up the trophic levels of the food chain. Some writers, including the authors of this report, use the terms bioconcentration and bioconcentration factor with a general and purely descriptive meaning to refer to the increased tritium concentrations observed in living organisms than in their environment, without in any way indicating the exact nature of the contamination source, the time at which such contamination occurred or the underlying biological mechanisms. Due prudence should be taken in interpreting the concentration factors. The term bioaccumulation is often used with the same general meaning as bioconcentration. The French 3 European Commission, EU Scientific Seminar 2007 “Emerging Issues on Tritium and Low Energy Beta Emitters, Radiation Protection No 152, Luxembourg, 2008