Thermodynamic Modelling of the Effect of Hydroxycarboxylic Acids on the Solubility of Plutonium at High pH
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THERMODYNAMIC MODELLING OF THE EFFECT OF HYDROXYCARBOXYLIC ACIDS ON THE SOLUBILITY OF PLUTONIUM AT HIGH pH ANTHONY D MORETON, AEA Decommissioning & Radwaste, B.424.4, Harwell Laboratory, Oxfordshire, UK. ABSTRACT A number of the hydroxycarboxylic acids generated by the alkaline degradation of cellulosic wastes under reducing conditions in a cementitious repository can significantly increase the solubility of the actinides at high pH, especially plutonium. The solubility of plutonium at pH 12, in the presence of a range of hydroxycarboxylic acids containing a number of hydroxyl groups and between one and three carboxylate groups, has been modelled using the HARPHRQ code. All the plutonium-organic complexes assumed in the model are based on a stable unit in which a central plutonium ion is bound by four oxygen atoms. The oxygen atoms can be provided either by a deprotonated hydroxyl group on one of the ligands, or by hydroxide ions. INTRODUCTION As part of the UK Nirex Ltd. Safety Assessment Research Programme (NSARP) in support of the proposed UK repository for low-level waste (LLW) and intermediate-level waste (ILW), AEA Technology are investigating the solubility of a number of radioelements shown to be significant in post-closure safety assessments. The programme includes investigations of the effect on solubility of organic degradation products formed from materials present in LLW and ILW. Previous studies [1-4] have indicated that degradation products of cellulose produced by alkaline degradation processes (mainly polyhydroxycarboxylic acids) have the most significant effect on the solubility of actinide elements under expected repository chemical conditions (i.e. high pH, low Eh). In order better to quantify the processes responsible for the often large radionuclide solubility enhancements, thermodynamic modelling of experimental data is being undertaken using the HARPHRQ [5] code in conjunction with the HATCHES database [6] to identify the complexes responsible and determine equilibrium constants for their formation.
MODEL DEVELOPMENT Earlier modelling studies [1,2] assessed the complexation of plutonium by D-saccharate (SAH-), D-gluconate (GLU-) and glycerate (GLY-). The major species responsible for the 2 plutonium solubility enhancement were considered to be PuIV(GLU H4 )-, PuIV(SAH H_4 ) 3 and PuV(GLY H-1 ) +*. Whilst the model developed was largely internally self-consistent and consistent with the limited amount of data identified in the literature, there were two 3 areas where the description was open to question: (a) the incorporation of PuV(GLY H. 1) + as the predominant species in the glyceric acid system appeared anomalous compared to the other two systems where Pu1V species predominate; (b) in the case of gluconate, molecularstick structural models indicated that if four deprotonated hydroxyl groups bind to the plutonium ion, the carboxylate group cannot co-ordinate for steric reasons. Because of the latter point, a literature search was undertaken as part of this study to identify any metal comp
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