Modelling the Sorption of Actinides onto Cement: An Approach with the Surface Co-Precipitation Model
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Modelling the Sorption of Actinides onto Cement: An Approach with the Surface Co-Precipitation Model Daisuke Sugiyama and Tomonari Fujita Central Research Institute of Electric Power Industry (CRIEPI), 2-11-1, Iwado-Kita, Komae, Tokyo 201-8511, Japan. ABSTRACT An approach, assuming that co-precipitation on the surface of the cement phase dominates ‘sorption’, is proposed and discussed to interpret the sorption behaviour of the cement-complex system. This model takes account of the mineralogical composition of Ordinary Portland Cement (OPC) and assumes that Calcium Silicate Hydrates (C-S-H) dominates the sorption (fixation) of actinide onto OPC and that a small amount of surface co-precipitation of actinides with calcium hydroxide in the structure of C-S-H is formed during the late period of the fixation process. Therefore, this will be referred to as the ‘Surface Co-Precipitation Model (SCPM)’. The modelling results show generally good agreement between the predicted and measured distribution ratios of plutonium, thorium and neptunium, of which the dominant species in high pH solution are neutral species, on OPC. INTRODUCTION In the concept for TRU waste disposal in Japan, the sorption of radionuclides onto cement materials is a very important parameter when considering the release of radionuclides from the near field of a repository in a long-term performance assessment [1]. Batch sorption experiments for actinides onto cement material and constituent minerals have been carried out [2] and an approach has been developed to describe the sorption of radionuclides onto cement materials based on the surface-complexation model [3]. However, it has also been suggested that radionuclides were fixed on cement materials by co-precipitation [4,5]. In this paper, an approach assuming that co-precipitation on the surface of the cement phase dominates ‘sorption’ is proposed and discussed to interpret the sorption behaviour of actinides onto hydrated Ordinary Portland Cement (OPC). This model takes account of the mineralogical composition of OPC and assumes that Calcium Silicate Hydrates (C-S-H) dominates the sorption (fixation) of actinides onto OPC and that a small amount of surface co-precipitation of actinides with calcium hydroxide in the structure of C-S-H is formed during the late period of the fixation process. Therefore, this is referred to as the ‘Surface Co-Precipitation Model (SCPM)’. All modelling calculations were carried out using the geochemical code HARPHRQ [6] and the thermodynamic database HATCHES [7] ver. NEA14. MODEL DEVELOPMENT In order to describe the sorption onto OPC it was necessary to consider the underlying mineralogical phases that comprise hydrated OPC. Although the actual composition of OPC is more complicated, C-S-H phases (including Portlandite (Ca(OH)2)) would be the dominant
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phases in OPC and the grain phase would be covered with C-S-H and Ca(OH)2 during the hydration of OPC [8]. Experimental measurements for cement-component minerals also showed that the C-S-H phases were
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