Effect of the Oxygen Potential on the Mo Migration and Speciation in UO and UO.

Molybdenum is an abundant element produced by fission in the nuclear fuel UO 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, UO oxidation) typical of an accidental scenario. To do so, molybdenum was introduced in UO or UO 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 UO 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 UO. 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 UO or UO samples under a metallic state Mo(0). Its mobility, being quite low, is driven by a diffusion mechanism. An increase of pO entails the UO 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 UO hyper-stoichiometry x. After thermal treatment, Mo remaining in the samples is located in the grains under the MoO form. Our experimental results are assessed by ab initio calculations showing that in the presence of oxygen Mo atoms adopt in UO a local structure close to the octahedral local geometry of Mo oxides.