Effect of the Oxygen Potential on the Mo Migration and Speciation in UO2 and UO2+x

Molybdenum is an abundant element produced by fission in the nuclear fuel UO2 in a pressurized water reactor. Although its radiotoxicity is low, this element has a key role on the fuel oxidation and other fission products migration, in particular in the case of an accidental scenario. This study aims to characterize the behavior of molybdenum in uranium dioxide as a function of environmental conditions (oxygen partial pressure, high temperature, UO2 oxidation) typical of an accidental scenario. To do so, molybdenum was introduced in UO2 or UO2+x pellets by ion implantation, a technique that allows us to mimic the production of Mo in the nuclear fuel by fission. Then, thermal treatments at high temperature and different oxygen partial pressures were carried out. The mobility of Mo in UOx samples was followed by secondary ion mass spectrometry (SIMS), while the Mo chemical speciation was investigated by spectroscopic techniques (XANES, Raman). In parallel, ab initio calculations were performed showing the effect of interstitial oxygen atoms on the Mo incorporation sites in UO2. We show that the Mo mobility is directly connected to its chemical state, which in turn, is linked to the redox conditions. Indeed, under reducing atmosphere, Mo is present in UO2 or UO2+x samples under a metallic state Mo(0). Its mobility, being quite low, is driven by a diffusion mechanism. An increase of pO(2) entails the UO2 and Mo oxidation and, as a consequence, a strong release of this element. We show an increase of the Mo release rate with the increase of the UO2+x hyper-stoichiometry x. After thermal treatment, Mo remaining in the samples is located in the grains under the MoO2 form. Our experimental results are assessed by ab initio calculations showing that in the presence of oxygen Mo atoms adopt in UO2 a local structure close to the octahedral local geometry of Mo oxides.