Effect of the Oxygen Potential on the Mo Migration and Speciation in UO₂ and UO_2+x

Abstract

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 pO2 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.

Effect of molybdenum on the behaviour of caesium in uranium dioxide at high temperature

“…Therefore, increasing the quantity of dpa (and thus uranium vacancies) created in UO2 by ion implantation may enhance the Mo migration. This connexion between Mo migration and defects density in UO2 is supported by the results obtained in one of our previous studies, in which we observed a Mo diffusion in UO2 at 1600°C when this element was implanted at a high fluence of 5 10 16 at/cm² (~4 at.% max) [23]. In the present study, defects due to Cs implantation are mainly created in the area where Mo atoms are present.…”

“…Indeed, we have shown in a previous study that, after annealing, the major part of remaining Cs is found accumulated at a 50 nm depth from the surface in the form of bubbles [18]. In the case of Mo, we have observed it to be in a metallic form [23]. Further TEM analysis on an annealed UO2 sample implanted with molybdenum at high concentration confirms these results, by showing the formation of metallic precipitates.…”

“…We can therefore explain the mobility of Mo in presence of Cs by the increase of U vacancies created by the Cs implantation. However, the main Mo migration mechanism observed in presence of Cs is not diffusion, as in the case of [23], but a transport mechanism towards the surface. One may suppose that this movement is driven by the large amount of vacancies present towards the surface.…”

“…This instrument is equipped with an eucentric rotating sample stage (see [22] for more details), to improve the depth resolution by inhibiting the surface roughening during SIMS analysis of poly-crystalline materials. We have previously shown that this device allows one to minimize the preferential grain abrasion of UO2 polycrystalline pellets, therefore yielding reproducible concentration profiles [18,23,24]. A primary beam of 10 keV O2 + ions with a current of 140 nA was used.…”

Thermal behaviour of caesium implanted in UO2: A comparative study with the xenon behaviour

“…The aim is to characterise migration mechanisms depending on the profile evolution induced by the different annealing conditions. The SIMS technique has already been successfully used to study the behaviour of elements easily ionised in UO2, such as iodine [17][18], chlorine [19] but also molybdenum in UO2 and UO2+x [20].…”