Alteration of fresh MIMAS MOX and (U, Ce)O2 analogous materials in simplified cementitious water

• The alteration of MIMAS MOX and (U, Ce)O 2 model materials is investigated • Experiments are performed in simplified cementitious water and in anoxic conditions • Leaching experiments are combined with post-alteration characterizations • Significant reduction of uranium release rates is observed compared to alteration in carbonated water • The inhibitory role of silicate ions on the oxidative dissolution of uranium is demonstrated An experimental study was conducted to investigate the effects of altering fresh (U, Pu)O 2 MOX of high α-specific activity under alkaline conditions (pH = 12), in the presence of 2 mmol.L -1 silicate ions and in anoxic environment. This test was conducted in conjunction with analogous experiments using (U, Ce)O 2 simulant materials of homogeneous and heterogeneous microstructure. The results obtained indicated that the oxidative dissolution induced by α-radiolysis of water in contact with (U, Pu)O 2 MOX was suppressed under these alteration conditions. The uranium concentration determined at equilibrium was found to be fully comparable to those obtained with the (U, Ce)O 2 model materials. This observation suggested that the alteration mechanism occurring at the solid/solution interface was independent of the α-activity of the material and highlighted the analogy of the two types of materials in the tested conditions. Post-alteration characterizations of the material surface using Raman spectroscopy, TEM/EDS and SEM were conducted. The results obtained supported the assumption that adsorption of silicate ions onto reactive sites in the material was responsible for the inhibition of uranium oxidation. This study revealed the important role played by silicate ions in reducing uranium release in alkaline conditions and under alpha radiolysis of water. Considering that uranium is a tracer of the MOX matrix alteration, these results had positive implications regarding the long-term management of spent fuel in deep disposal especially in case of contact with cementitious water.