Influence of organic degradation products on the solubilisation of radionuclides in intermediate and low level radioactive wastes

In highly alkaline cement pore waters, the degradation of cellulose and similar organic substances generates a large range of compounds. Some of them have strong complexing properties and might thus affect the confinement of radionuclides. We have identified and quantified the production of these degradation products with temperature and time. We have used conventional potentiometric titrations, HPLC, GC-MS and Capillary Electrophoresis. Among these products, we confirm the importance of isosaccharinic acid (ISA), but many other hydroxyacids, and possibly diketones could also play a role in the complexation of metals. We have experimentally studied the complexation properties of a chemical analogue of ISA, the tetrahydroxypentanoic acid (THPA) towards transition elements (Cu, Co) and rare Earth elements (Sm and Eu), in order to have a basis to derive the behaviour of some long-lived fission products and actinides. We determined the stoichiometries of the complexes and the stability constants at zero-ionic strength. In parallel, we have conducted solubility experiments on real cellulose degradation solutions. The solubility of transition metal cations increases by a factor of 104 to 105, and 106 to 107 for rare Earth elements. These results impose a thorough evaluation of possible failure of the confinement in cement-based waste forms. (orig.)