[en] The speciation of uranium in SON68 glass specimens doped with 0.75-3.5 wt% uranium and in the gels formed by alteration of the specimens was investigated by X-ray absorption spectroscopy. In the glasses, uranium is present at oxidation state VI and coordination number 6 with the same average distances than those found in a UO₃ type environment. The U-O distances and uranium coordination numbers are identical throughout the uranium concentration range. During glass alteration the uranium remains at oxidation state VI in the gels, but was found in the uranyl form. An increase in the equatorial distances (from 2.20 and 2.32 A in the glass to respectively 2.22 and 2.39 A in the gel) and coordination numbers (to about 7 and 8, respectively) was observed

[en] The interaction of salicylic acid with zirconium diphosphate surface and its reactivity toward uranium (VI) was investigated. The interaction of salicylic acid with zirconium diphosphate was firstly studied using several analytical techniques including atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The sorption of uranium (VI) onto surface-modified zirconium diphosphate was evaluated by the classical batch method at room temperature. This study showed that the uranium (VI) sorption onto zirconium diphosphate is influenced by the presence of salicylic acid. A fluorescence spectroscopy study revealed the presence of a uranyl specie onto the modified solid surface. The spectroscopy results were then used to restrain the modeling of experimental sorption data, which are interpreted in terms of a constant capacitance model using the FITEQL code. The results indicated that interaction between the uranium (VI) and the surface of zirconium diphosphate modified with salicylic acid leads to the formation of a ternary surface complex.

[en] This paper presents the main results of a study conducted to quantify and to model the degradation state of zirconium-based fuel claddings submitted to severe-accident conditions in a nuclear reactor core: high temperatures and either pure steam or an air-steam mixture. Due to the progressive thickening of a dense and protective ZrO₂ layer, the oxidation kinetics of zirconium-based claddings in steam at high temperatures typical of severe nuclear accidents are generally cubic or parabolic. However, for some temperature domains, this oxide layer may crack, becoming porous and no longer protective. In these 'breakaway' conditions, the oxidation kinetics change from (sub)parabolic to linear or even accelerated. In addition, the temperature increase can lead core materials to melt and to relocate down to the vessel lower head, threatening its integrity. If it fails, and for specific conditions, air ingress may take place into the reactor. Hence, oxygen and nitrogen both react with zirconium-based claddings successively through the oxidation of zirconium (forming a ZrO₂ layer), nitriding of zirconium (forming zirconium nitride particles), and the oxidation of zirconium nitride (forming ZrO₂ and releasing nitrogen). These self-sustained chemical reactions enhance the deterioration of zirconium-based claddings and their ZrO₂ layers, inducing an increase in their open porosity. To quantify this porosity, a series of two-step experiments was conducted. First, ZIRLO^TM cladding samples were isothermally oxidized in pure steam or in a 50:50 mol% air-steam mixture at several different temperatures and durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation were all confirmed by experimental results. Second, pioneering porosimetry measurements by mercury intrusion were realized for the first time on such corroded cladding samples. In both atmospheres, it was pointed out that 1,200 and 1,250 K led to particularly porous oxide layers, especially due to strong breakaway effects. Moreover, we confirmed that the presence of air strongly enhances the oxide cracking: cladding samples were more porous when oxidized in the air-steam mixture than under pure steam. Finally, we observed that in all conditions, the open porous volume fraction of ZIRLO claddings continuously rose during their corrosion process. Hence, for each experimental condition, porosity correlations were determined through linear regressions, and porosity increase rates were deduced by derivation versus time and validated against porosimetry results of cladding samples corroded in transient nonisothermal) conditions. (authors)

[en] This paper presents the main results of a study conducted to quantify and to model the degradation state of Zr-based fuel claddings submitted to severe accident conditions in a nuclear reactor core: high temperatures and either pure steam or air-steam mixture. Due to the progressive thickening of a dense and protective ZrO₂ layer, the oxidation kinetics of Zr-based claddings in steam at high temperatures typical of nuclear severe accidents, is generally cubic or parabolic. However, for some temperature domains, this oxide layer may crack, becoming porous and non-protective anymore. In these 'breakaway' conditions, the oxidation kinetics change from (sub-)parabolic to linear or even accelerated. Additionally, the temperature increase can lead core materials to melt and to relocate down to the vessel lower head, threatening its integrity. If it fails, and for specific conditions, air ingress may take place into the reactor. Hence, oxygen and nitrogen both react with Zr-based claddings, successively through oxidation of Zr (forming ZrO₂ layer), nitriding of Zr (forming ZrN particles) and oxidation of ZrN (forming ZrO₂ and releasing nitrogen). These self-sustained chemical reactions enhance the deterioration of Zr-based claddings and of their ZrO₂ layers, inducing a rise of their open porosity. To quantify this porosity, a series of two-step experiments was conducted. First, Zirlo^TM cladding samples were isothermally oxidized in various conditions: in pure steam or in 50- 50 mol% air-steam mix, at several temperatures, and for different durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation are all confirmed by experimental results. Second, pioneering porosimetry measurements by Hg intrusion were realized for the first time on such corroded cladding samples. In both atmospheres, it is pointed out that 1200 and 1250 K lead to particularly porous oxide layers, especially due to strong 'breakaway' effects. Moreover, it is confirmed that the presence of air strongly enhances the oxide cracking: cladding samples are more porous when oxidized in the air-steam mixture than under pure steam. Finally, it is observed that in all conditions, the open porous volume fraction of Zirlo^TM claddings continuously rises during their corrosion process. Hence, for each experimental condition, porosity correlations are determined through linear regressions, and porosity increase rates are deduced by derivation versus time and validated against porosimetry results of cladding samples corroded in transient (nonisothermal) conditions. (authors)

[en] Early actinides (U, Np, Pu, Am) show a particular linear bond actinyl-type structure in their highest oxidation state. The multiple-bond nature of this chemical pattern contributes to a drastic diminution of the charge on the metallic core inducing a strong stabilization of these high oxidation states. The potential participation of the early actinide 5f orbitals in the valence molecular shell is supposed to be one of the most important engines of this chemical specificity. In order to progress in the comprehension of this behavior, a study of the electronic and the geometric structures of some actinyl complexes with different electronic configurations is undertaken using theoretical and experimental approaches. (authors)

[en] The threat of a dirty bomb which could cause internal contamination has been of major concern for the past decades. Because of their high chemical toxicity and their presence in the nuclear fuel cycle, uranium and neptunium are two actinides of high interest. Calmodulin (CaM) which is a ubiquitous protein present in all eukaryotic cells and is involved in calcium-dependent signaling pathways has a known affinity for uranyl and neptunyl ions. The impact of the complexation of these actinides on the physiological response of the protein remains, however, largely unknown. An isothermal titration calorimetry (ITC) was developed to monitor in vitro the enzymatic activity of the phosphodiesterase enzyme which is known to be activated by CaM and calcium. This approach showed that addition of actinyl ions (AnO₂ⁿ⁺), uranyl (UO₂²⁺) and neptunyl (NpO₂⁺), resulted in a decrease of the enzymatic activity, due to the formation of CaM-actinide complexes, which inhibit the enzyme and alter its interaction with the substrate by direct interaction. Results from dynamic light scattering rationalized this result by showing that the CaM-actinyl complexes adopted a specific conformation different from that of the CaM-Ca²⁺ complex. The effect of actinides could be reversed using a hydroxypyridonate actinide decorporation agent (5-LIO(Me-3,2-HOPO)) in the experimental medium demonstrating its capacity to efficiently bind the actinides and restore the calcium-dependent enzyme activation. (authors)

[en] The evolution of the sites occupied by cerium and neodymium (coordination numbers and Ce, Nd-O distances) during alteration of simplified SON68 glass specimens was determined by L_III-edge XAS. Cerium and neodymium are situated in a silicate environment in the glass, surrounded by eight oxygen atoms at an average distance of 2.44 and 2.48 A, respectively. These two rare earth elements exhibit different leaching behavior, however. The main environment of cerium becomes a silicate (d _Ce-O = 2.19 A) with a second oxide or more probably oxyhydroxide site (d _Ce-O = 2.32 A). The cerium coordination number increases by 1 to 3 compared with the glass, depending on the leaching conditions. Neodymium is found mainly in a hydroxycarbonate environment (d _Nd-O = 2.46 A); the second site is a silicate (d _Nd-O = 2.54 A). The neodymium coordination number increases by 1 compared with the glass. When glass containing neodymium is doped with phosphorus, Nd is situated in a phosphate environment; this change is also reflected in the coordination number and Nd-O distance (seven oxygen atoms at 2.42 A). During glass leaching, neodymium is present at two different sites, phosphate (d _Nd-O = 2.52 A) and hydroxycarbonate (d _Nd-O = 2.40 A)

[en] In a nuclear power plant, a potential risk in some low probability situations in severe accidents is air ingress into the vessel. Air is a highly oxidizing atmosphere that can lead to an enhanced core oxidation and degradation affecting the release of Fission Products (FP), especially increasing that of ruthenium. This FP is of particular importance because of its high radio-toxicity and its ability to form highly volatile oxides. Oxygen affinity is decreasing between Zircaloy cladding, fuel and ruthenium inclusions in the fuel. It is consequently of great need to understand the phenomena governing cladding oxidation by air as a prerequisite for the source term issues. A review of existing data in the field of Zircaloy-4 oxidation in air-containing atmosphere shows that this phenomenon is quantitatively well understood. The cladding oxidation process can be divided into two kinetic regimes separated by a breakaway transition. Before transition, a protective dense zirconia scale grows following a solid state diffusion-limited regime for which experimental data are well fitted by a parabolic time dependence. For a given thickness, which depends mainly on temperature and the extent of pre-oxidation in steam, the dense scale can potentially breakdown. In case of breakaway combined with oxygen starvation, cladding oxidation can then be much faster because of the combined action of oxygen and nitrogen through a complex self sustaining nitriding-oxidation process. A review of the pre-existing correlations used to simulate zirconia scale growth under air atmospheres shows a high degree of variation from parabolic to accelerated time dependence. Variations also exist in the choice of the breakaway parameter based on zirconia phase change or oxide thickness. Several correlations and breakaway parameters found in the literature were implemented in the MAAP4.07 Severe Accident code. They were assessed by simulation of the QUENCH-10 test, which is a semi-integral test designed to study fuel bundle exposure to steam first and then to air. This paper deals with the main results obtained with MAAP4.07 when simulating QUENCH-10.

[en] The behavior of the UO₂²⁺ uranyl ion at the water/NiO(100) interface was investigated for the first time using Born-Oppenheimer molecular dynamic simulations with the spin polarized DFT +U extension. A water/NiO(100) interface model was first optimized on a defect-free five layers slab thickness, proposed as a reliable surface model, with an explicit treatment of the solvent. Water molecules are adsorbed with a well-defined structure in a thickness of about 4 Å above the surface. The first layer, adsorbed on nickel atoms, remains mainly in molecular form but can partly dissociate at 293 K. Considering low acidic conditions, a bidentate uranyl ion complex was characterized on two surface oxygen species (arising from water molecules adsorption on nickel atoms) with d_{U−Oadsorption}=2.39 Å. This complex is stable at 293 K due to iono-covalent bonds with an estimated charge transfer of 0.58 electron from the surface to the uranyl ion.

[en] Highlights: ► We developed a new modelling of fuel oxidation and ruthenium release in the EDF version of the MAAP4 code. ► We validated this model against some VERCORS experiments. ► Ruthenium release prediction quantitatively and qualitatively well reproduced under air and steam atmospheres. - Abstract: In a nuclear power plant (NPP), a severe accident is a low probability sequence that can lead to core fusion and fission product (FP) release to the environment (source term). For instance during a loss-of-coolant accident, water vaporization and core uncovery can occur due to decay heat. These phenomena enhance core degradation and, subsequently, molten materials can relocate to the lower head of the vessel. Heat exchange between the debris and the vessel may cause its rupture and air ingress. After lower head failure, steam and air entering in the vessel can lead to degradation and oxidation of materials that are still intact in the core. Indeed, Zircaloy-4 cladding oxidation is very exothermic and fuel interaction with the cladding material can decrease its melting temperature by several hundred of Kelvin. FP release can thus be increased, noticeably that of ruthenium under oxidizing conditions. Ruthenium is of particular interest because of its high radio-toxicity due to ¹⁰³Ru and ¹⁰⁶Ru isotopes and its ability to form highly volatile compounds, even at room temperature, such as gaseous ruthenium tetra-oxide (RuO₄). It is consequently of great need to understand phenomena governing steam and air oxidation of the fuel and ruthenium release as prerequisites for the source term issues. A review of existing data on these phenomena shows relatively good understanding. In terms of oxygen affinity, the fuel is oxidized before ruthenium, from UO₂ to UO_2+x. Its oxidation is a rate-controlling surface exchange reaction with the atmosphere, so that the stoichiometric deviation and oxygen partial pressure increase. High temperatures combined with the presence of oxygen in the atmosphere lead to fuel expansion and formation of cracks. In these conditions, intra- and inter-granular diffusions of ruthenium in the fuel matrix are so enhanced that it is possible to consider an instantaneous volatilisation of ruthenium oxides at the fuel surface. Based on these considerations, a completely new model has been implemented in the EDF local version of the MAAP4.07 severe accident code (Modular Accident Analysis Program), owned by EPRI (Electric Power Research Institute). The fuel oxidation modelling takes into account many kinds of atmospheres (steam and/or air and/or hydrogen), the stoichiometric evolution and the oxygen partial pressure of the fuel matrix. The release of ruthenium oxides is calculated considering their particular reaction constants. The model was assessed by the simulation of different CEA-VERCORS experiments in air, steam and mixed atmospheres. These experiments are specifically designed to study FP release from fuel under different atmospheres and temperatures. This paper deals with the main results obtained with MAAP4.07 when simulating these tests.