An Investigation of the Neptunium Complexes Formed Upon Interaction of High Level Waste Glass and Boom Clay Media
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C.Marquardt ** * *
Waste and Disposal Department, SCK-CEN, Boeretang 200, 2400 Mol, Belgium * Institut fuir Nukleare Entsorgungstechnik, FZK, P.O. 3640, D-76021 Karlsruhe, Germany
ABSTRACT Among the actinides which can be released from vitrified HLW, Neptunium-237 is considered as the most critical one in the Belgian performance assessment studies. Therefore, leaching experiments with Np-doped glass in humic acid containing Boom Clay water at different conditions were carried out. For the speciation of Np in the leachate, LPAS (Laser Photoacoustic spectroscopy) and UV-VIS absorption spectroscopy were used. The Np concentration in the leachates, after a slightly decrease, seems to approach a steady-state value at about 5x10 7 M after 6 months of glass corrosion. The highest Np concentrations were observed for the highest S/V value and for the glass doped with the highest Np concentration, the combination of the two separate parameters leading to even higher Np concentrations in solution. Np is mainly bound to colloids. The first spectra of the leachates obtained by LPAS confirm the potentiality of this technique to characterise the Np species at low Np concentration and show the presence of Np(IV) complexes which have still to be identified. We can, therefore, anticipate that the Np steady-state concentration may be controlled by a Np(V) reduction/precipitation process. INTRODUCTION Inorganic and organic complexes are known to form upon interaction of vitrified HLW
and Boom Clay medium, which is the candidate geological formation in Belgium for disposal of HLW. Among the several geochemical reactions, complexes formed between actinides released from the waste matrix and the organic matter may control the actinide solubility and mobility. The interstitial water in Boom Clay contains a high concentration of humic substances (100 mg/i HA), which are naturally occuring polyelectrolytes of not well defined compositions [1]. Among their large variety of functional groups, carboxyl and phenolic groups constitute the larger fraction and appear to be responsible for the complexation of radionuclides increasing the solubility above the predicted levels. Because of its half life of 2.14x10 6 years and its relatively large abundance in nuclear waste, Np is considered as the most critical actinide in the Belgian performance assessment studies. Under the reducing conditions expected in the repository, Np will be in the tetravalent oxidation state. Humic materials, present as colloidal species in natural waters can strongly interact with tetravalent ions. If the complexation of Np(V) by humic acids or carbonates, two ligands susceptible to increase the Np solubility, has been often reported in the literature [2-6] and was studied as well by us [7], few data are available on the Np(IV)-carbonate complexation and no data are found on the Np(IV)-humate complexation. The purpose of this study is to determine the species of the soluble Np formed during the interaction of HLW glass and the clay water. This can be done by using spe
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