Nivesse1),2)*, N. Baglan2), G. Montavon1), O. Péron1)
1)Laboratoire Subatech, Groupe Radiochimie, UMR 6457, IN2P3/CNRS/IMT Atlantique/Université́ de Nantes, 4 rue Alfred Kastler, BP20722, 44307 Nantes Cedex 3, France
2)CEA, DAM, DIF, F-91297 Arpajon, France
Tritium is the radioactive isotope of hydrogen and can therefore integrate organics molecules of living organisms to form the organically bound tritium fraction (OBT) [1]. OBT is usually distinguished into two forms: a non-exchangeable fraction (NE-OBT) and an exchangeable fraction (E-OBT) with the near environment [2]. However, there is no consensus on their definition and several variations can be found in the literature, differentiating them from an analytical, structural or kinetic point of view. The main interest of the NE-OBT study related to its non-exchangeability capacity is to enable retrospective studies of tritium releases in the environment.
NE-OBT analysis involves an isotopic exchange step to remove E-OBT from a sample. The current method, where the dehydrated sample is covered with aqueous water, has the disadvantage of causing the dissolution of a part of the organic compounds of the sample, which can lead to an analytical bias [3]. A new method has been developed to overcome this potential solubilisation. It consists of a tritium labelling line, with a controlled and stable ratio (T/H), which allows the determination of the exchangeable hydrogen fraction (α) and the TOL-NE activity’s calculation using equation (1) [4], [5].
(T/H)OBT = a x (T/H)E-OBT + (1-a) x (T/H)NE-OBT (1)
Following previous work on carbohydrate molecules [4], it is accepted that a theoretical (α) calculated from the molecular formula may be different from the experimental obtained one. According to this, investigations are conducted on a similar matrix, starch extracted from tritium-labelled wheat grains, to study the exchangeability parameter (α) and the NE-OBT and E-OBT recovery in this main component of wheat grains. Thereby, isotopic exchange studies are conducted on bio-indicators such as humic substances and obtained (α) parameters are related to the mobility of hydrogen in functional groups of interest for the transfer of metals in soils. Afterwards, the (α) parameter of Myriophyllum Spicatum, an aquatic macrophyte from La Loire (France), samples downstream of Dampierre nuclear power plant with initially 3.4 ±0.1 Bq.kg-1 of fresh material of OBT activity concentration is also highlighted.
The main aim of this work is to improve the global understanding of tritium exchange mechanisms in environmental matrices, to validate the E-OBT and NE-OBT information determined on the tritium labelling line and to develop the knowledge on migration processes of tritium in the environment.
References:
[1] S. Diabaté, S. Strack, Organically bound tritium, Health Phys. 698-712 (1993).
[2] S.B. Kim, N. Baglan, P.A. Davis, Current understanding of organically bound tritium (OBT) in the environment. J. Environ. Radioact. 126, 83-91 (2013).
[3] N. Baglan, E. Ansoborlo, C. Cossonnet, L. Fouhal, I. Deniau, M. Mokili, A. Henry, E. Fourré, A. Olivier,Métrologie du tritium dans différentes matrices: cas du tritium organiquement lié (TOL). Radioprotection, 45(3), 369-390 (2010).
[4] O. Péron, E. Fourre, L. Pastor, C. Gegout, B. Reeves, H.H. Lethi, G. Rousseau, N. Baglan, C. Landesman, F. Siclet, G. Montavon. Towards speciation of organically bound tritium and deuterium: Quantification of non-exchangeable forms in carbohydrate molecules. Chemosphere 196 120-128 (2018).
[5] Feng, X., Krishnamurthy, R. V., & Epstein, S. Determination of DH ratios of nonexchangeable hydrogen in cellulose: A method based on the cellulose-water exchange reaction. Geochimica et cosmochimica acta, 57(17), 4249-4256 (1993).