The complex formation of nitrate ions with nickel(II) in dry [C4mim][Tf2N] ionic liquid (IL) was investigated by means of UV-visible spectrophotometry, isothermal titration calorimetry (ITC), extended X-ray absorption fine structure spectroscopy (EXAFS), and molecular dynamics (MD) simulations. EXAFS spectroscopy and MD simulations show that the solvated Ni(II) cation is initially coordinated by the oxygens of the [Tf2N](-) anion of IL, which can behave either as mono- or bidentate. Spectroscopic and thermodynamic data show that Ni(II) is able to form up to three stable mononuclear complexes with nitrate in this solvent. The stability constants for Ni(NO3)j complexes (j = 1-3) calculated from spectrophotometry and ITC experiments decrease in the order log K1 > log K2 > log K3. The formation of the first two species is enthalpy-driven, while the third species is entropy-stabilized. The UV-vis spectra of solutions containing different nitrate/Ni(II) ratios show that the metal ion retains the six-coordinate geometry. Furthermore, the EXAFS evidences that nitrate is always bidentate. Molecular dynamics simulations show that the [Tf2N](-) anions bind Ni(II) through the sulfonyl oxygen atoms and can coordinate either as monodentate or chelate. The analysis of the MD data shows that introduction of nitrates in the first coordination sphere of the metal ion results in remarkable structural rearrangement of the ionic liquid.

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.