Solubility of Lanthanide-phosphates (LnPO4) and aqueous Lanthanide speciation under repository-relevant conditions

In the last decades, various ceramic materials have been proposed as potential waste forms for the immobilization of special nuclear waste streams, such as separated plutonium from civilian or military sources, or separated minor actinides /1/. Among them, monazites (monoclinic LnPO4) appear as promising candidates due to their specific physico-chemical properties including high structural flexibility, high chemical durability, and high radiation resistance /2, 3/. Within a co-operative research project on the immobilization of long-lived radionuclides by incorporation into repository-relevant ceramics funded by the German Federal Ministry of Education and Research (Support code 02NUK021), we employed geochemical modelling tools to calculate the solubility of lanthanide-phosphates in repository-relevant environments and to support the interpretation of experiments on waste form dissolution. Calculations were performed for ground waters occurring in crystalline and clay environments at potential repository sites and pore waters typically encountered in bentonite buffers or in a cementitious near-field. The results show that the solubility and long-term performance of phosphate waste forms can vary significantly depending on site hydro¬geochemistry and the engineered barrier concept pursued, with the highest LnPO4-solubilities obtained for cementitious near-field conditions.References1. G.R. Lumpkin, Elements 2 (2006) 365–3722. R.C. Ewing, L.M. Wang, Rev. Min. Geochem. 48 (2002) 673–6993. N. Dacheux, N. Clavier, R. Podor, Am. Min. 98 (2013) 833–84