The Solubility and Sorption of Uranium (VI) in a Cementitious Repository
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THE SOLUBILITY AND SORPTION OF URANIUM (VI) IN A CEMENTITIOUS REPOSITORY M BROWNSWORD, A B BUCHAN, F T EWART, R McCROHON*, G J ORMEROD, J L SMITHBRIGGS AND H P THOMASON, AEA Technology, Harwell Laboratory, Oxfordshire, OX11 ORA, UK
ABSTRACT The solubility behaviour of uranium(VI) has been studied in a number of waters representative of the pore waters to be expected in candidate repository backfill concretes and also in solutions of calcium and sodium hydroxides. The sorption of uranium has been studied on one of these candidates, a Portland cement with a slag filler. The same cement has been hydrothermally treated to simulate the temperature conditions in the repository and the product used for comparative studies. The paper describes the experimental measurements and the results. The aqueous concentration of uranium over the uranates formed in the presence of sodium and calcium hydroxides was constant above pH 7 at 3 x 10-6 M. In the concrete pore waters the solubility of uranium was equal to or lower than this value. The distribution coefficient, Rd, for the sorption of uranium onto the slag cement was 2.5 x 104 ml g-1 , and was increased by a factor two after hydrothermal treatment.
INTRODUCTION The main thrust of the radioactive waste disposal research in the UK, conducted by UK Nirex Ltd, is directed towards the deep disposal of low and intermediate-level wastes (LLW and ILW) [11].For ILW the waste material will be immobilised in a cementitious matrix and packed in a steel drum. These drums are then stacked in the repository and backfilled with a cementitious grout. A proposed repository structure consists of a large concrete vault which resides in a cavity excavated from the host geology. A combination of the activity of the ILW and the ambient temperature at depth will maintain the vault at an elevated temperature for many years. The research studies can be divided into three broad categories: those which address the environment of living organisms - the biosphere, those which address the geology - the far field, and those which address the vault and the waste form - the near field. It is the last of these studies which is addressed in this paper. One of the objectives of the near field research programme is to obtain an understanding of the processes of solubility and sorption that will control the aqueous concentration of the important radionuclides. The near-field source term for a given radioelement will be determined either by its solubility limit under the prevailing conditions or by its sorption behaviour, depending on the inventory. As the inventory of a solubilitylimited element is exhausted, sorption from solution will reduce the concentration and hence the flux leaving the repository. An important element in the context of radioactive waste disposal is uranium [2]. Together with its daughters, uranium has been shown to contribute significantly to the overall radiological impact of the repository. The aqueous chemistry of uranium is complicated by the multiple valence states [3] that may exist u
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