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Abstract: The radiation enhanced diffusion of chlorine in UO2 during heavy ion irradiation is studied. In order to simulate the behaviour of 36Cl, present as an impurity in UO2, 37Cl has been implanted into the samples (projected range 200nm). The samples were then irradiated with 63.5MeV 127I at two fluxes and two temperatures and the chlorine distribution was analyzed by SIMS. The results show that, during irradiation, the diffusion of the implanted chlorine is enhanced and slightly athermal with respect to pure thermal diffusion. A chlorine gain of 10% accumulating near the surface has been observed at 510K. This corresponds to the displacement of pristine chlorine from a region of maximum defect concentration. This behaviour and the mean value of the apparent diffusion coefficient found for the implanted chlorine, around 2.5×10−14 cm2 s−1, reflect the high mobility of chlorine in UO2 during irradiation with fission products. [Copyright &y& Elsevier]

During reactor processing, fission products, among which iodine, are implanted by recoil inside the Zircaloy cladding tube. At the same time, oxidation of the cladding tube occurs, hence in the waste storage phase, zirconia acts as a migration barrier. Before chemical separation, the cladding tubes are sectioned into pieces called hulls in order to release the UO2 pellets which are rendered soluble in nitric acid. The hulls are collected as a solid waste and were embedded inside a concrete structure until 1995. In the perspective of geological storage, a great interest is given to iodine release processes in order to model and to extrapolate them to large time scales. In order to analyse the mechanisms involved in iodine migration, iodine atoms were introduced in Zircaloy oxidized samples by means of ion implantation. Corrosion tests were performed in order to simulate the impact of infiltrated water in the concrete disposal. Iodine release was measured by Rutherford backscattering spectroscopy. Processes responsible for iodine release are analysed. [Copyright &y& Elsevier]

Boron carbide (B 4 C) behavior under irradiation is widely studied in order to predict the lifetime of this material in future (Generation IV) nuclear fission reactors. This paper is focused on the effects of high electronic stopping powers (S e) on B 4 C structure modifications. Sintered B 4 C samples were irradiated at room temperature with swift heavy ions (between 0.5 and 3 MeV · u −1) corresponding to S e values in the 4.1 to 15.4 keV · nm−1 range at the sample surface. In order to investigate the structural changes as a function of depth, transmission electron microscopy and Raman mapping were performed on the irradiated samples along the path of the incident ions. For the highest S e values, damage results in the creation of large hillocks at the sample surface along with the amorphization of the bulk. These results are explained, in the frame of the inelastic thermal spike model, by local melting in latent tracks that are created only when irradiations are performed above a S e threshold evaluated at around 9 keV nm−1. [ABSTRACT FROM AUTHOR]

Studies the microstructural modifications in alumina induced by implantation of zirconium, copper or titanium ion and by postimplantation thermal annealings. Use of grazing incidence x-ray diffraction in thermal annealings; Dependency of lattice formation on the ion implanted; Effects of implantation before annealing; Result of thermal annealings after implantation.

Presents a study that investigated ion beam mixing at the Fe[sub2]O[sub3]/Al[sub2]O[sub3] interface. Background on ion beam mixing at the interface of metal-insulator systems; Analysis on resulting phases developed through structural and chemical techniques; Description of preliminary ion beam mixing experiments.

Presents a study which examined the thermally induced chemical transformations at an ion implanted α-iron oxide/α-aluminum oxide interface. Analysis of the transformations by conversion electron M&oumi;ssbauer spectroscopy; Experimental procedure; Results and discussion.

Abstract Molybdenum migration was studied in polycrystalline UO 2 pellets under irradiation with swift ions. 95Mo isotope was introduced in the samples by ion implantation at a maximum concentration of 0.8 at.%. Irradiation conditions were chosen in order to understand, from a fundamental point of view, the behaviour of Mo in nuclear fuel during reactor operation. In this paper, the effect of three electronic stopping power values (7, 15 and 30 keV.nm−1) were investigated at different temperatures (room temperature, 600 °C and 1000 °C). The Mo concentration profiles before and after irradiation were measured by SIMS (Secondary Ion Mass Spectrometry). Raman spectroscopy was used to probe the defect creation or annealing in the UO 2 microstructure. It was evidenced that Mo is very stable in the UO 2 matrix as it migrates only for irradiation conditions combining a S e value of 15 or 30 keV.nm−1 and a temperature value of 1000 °C. [ABSTRACT FROM AUTHOR]

Boron carbide (B 4 C) is used as a neutron absorber for nuclear reactor control and installation protection in most types of reactors. When the B 4 C is irradiated in the reactor, large quantities of lithium and helium (up to about 1022 /cm3) are produced due to the neutron absorption reaction 10B (n, α) 7Li. This work aims at studying the Li diffusion in large grain size (20–50 µm) or small grain size (0.2 – 5 µm) B 4 C in the temperature range of 500–800 °C. Lithium was implanted in B 4 C samples at a fluence of 1014 ions/cm2 (maximum concentration of 54 at. ppm). The Li concentration profiles as a function of depth were obtained before and after each heat treatment by Time-Of-Flight Secondary Ionization Mass Spectrometry technique. A heterogeneous diffusion process was observed depending on the implantation depth. Considering diffusion in the non-damaged, bulk material beyond the implanted zone, the thermal diffusion of lithium into the grain boundaries was found to be about four orders of magnitude higher than into the grains. • TOF-SIMS analyses of Li have been performed in implanted and annealed high density, large grain boron carbide. • The diffusion coefficient of Li in boron carbide has been determined in the 500 – 800 °C range, the obtained activation energy, about 2 eV, is close to ab initio estimations. • Li diffusion is highly enhanced in the gain boundaries. [ABSTRACT FROM AUTHOR]

Boron carbide B 4 C is widely used as a neutron absorber in nuclear plants. Most of the post-irradiation examinations have shown that the structure of the material remains crystalline, in spite of very high atomic displacement rates. Here, we have irradiated B 4 C samples with 4 MeV Au ions with different fluences at room temperature. Transmission electron microscopy (TEM) and Raman spectroscopy have been performed. The Raman analyses show a high structural disorder at low fluence, around 10 −2 displacements per atoms (dpa). However, the TEM observations show that the material remains crystalline up to a few dpa. At high fluence, small amorphous areas a few nanometers large appear in the damaged zone but the long range order is preserved. Moreover, the size and density of the amorphous zones do not significantly grow when the damage increases. On the other hand, full amorphisation is observed in the implanted zone at a Au concentration of about 0.0005. It can be inferred from those results that short range and long range damages arise at highly different fluences, that heavy ions implantation has drastic effects on the structure stability and that in this material self-healing mechanisms are active in the damaged zone. [ABSTRACT FROM AUTHOR]

Already used as neutron absorber in the current French nuclear reactors, boron carbide (B 4 C) is also considered in the future Sodium Fast Reactors of the next generation (Gen IV). Due to severe irradiation conditions occurring in these reactors, it is of primary importance that this material presents a high structural resistance under irradiation, both in the ballistic and electronic damage regimes. Previous works have shown an important structural resistance of boron carbide even at high neutron fluences. Nevertheless, the structural modification mechanisms due to irradiation are not well understood. Therefore the aim of this paper is to study structural modifications induced in B 4 C samples in different damage regimes. The boron carbide pellets were shaped and sintered by using spark plasma sintering method. They were then irradiated in several conditions at room temperature or 800 °C, either by favoring the creation of ballistic damage (between 1 and 3 dpa), or by favoring the electronic excitations using 100 MeV swift iodine ions ( S e ≈ 15 keV/nm). Ex situ micro-Raman spectroscopy and Doppler broadening of annihilation radiation technique with variable energy slow positrons were coupled to follow the evolution of the B 4 C structure under irradiation. [ABSTRACT FROM AUTHOR]

Raman spectroscopy has proved to be an appropriate technique to probe defects in carbon-based materials owing to its high sensitivity, most often focused on the commonly used I D /I G parameter. However, this ratio may be activated by various types of defects and in a completely independent manner. Therefore, discriminating between defects is challenging. The central idea of the present work is to provide a better understanding of the Raman response to the various types of defects that may appear in nuclear graphite (carbon–carbon composite) during its manufacturing process, its operation in the nuclear reactor, or even during its preparation process such as polishing which is usually used prior to Raman characterization. This work also demonstrates the discrimination of the defect types using the combination of the I D /I G and FWHM(G), two structural disorder indicators evolving differently according to the type and the concentration of the introduced defects into the carbon network. The ion-beam irradiation was used here as an effective way for creating defects that could be similar to those created by neutrons in the nuclear reactor. [ABSTRACT FROM AUTHOR]

This work aims at studying the impact of D + ion implantation and thermal annealing on the organization of Highly Ordered Pyrolytic Graphite (HOPG). Raman microspectrometry, High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM) were used to characterize the multiscale organization of HOPG. For the first time, HRTEM images were processed, allowing a better estimation of structural parameters such as size of the graphene layers, mean interlayer spacing, distribution of the relative orientations of the graphene layers, defined as the spatial orientation of the stacked layers. Ion implantation induces a strong disordering of the HOPG reflected by a decrease of the length of graphene layers, and an increase of both angular dispersion and interlayer distance and a significant decrease of the Basic Structural Unit (BSU) diameter. Thermal annealing up to 1000 °C leads to partial reordering of the HOPG, with increase of the mean length of the graphene layers, decrease of their angular dispersion and interlayer distance and larger and increasingly parallel oriented BSUs. Even if the conservation of the lamellar nanostructure favors reordering, the latter is only partial since annealing temperature is far from 2000 °C, temperature above which stiff and perfect layers may be obtained. [ABSTRACT FROM AUTHOR]

In this paper, we aim at understanding tritium behavior in the graphite moderator of French CO 2 -cooled nuclear fission reactors (called UNGG for “ Uranium Naturel-Graphite-Gaz ”) to get information on its distribution and inventory in the irradiated graphite waste after their dismantling. These findings should be useful both to improve waste treatment processes and to foresee tritium behavior during reactor decommissioning and waste disposal operations. The purpose of the present work is to elucidate the effects of temperature on the behavior of tritium during reactor operation. Furthermore, it aims at exploring options of thermal decontamination. For both purposes, annealing experiments were carried out in inert atmosphere as well as in thermal conditions as close as possible to those encountered in UNGG reactors and in view of a potential decontamination in humid gas. D + ions were implanted into virgin nuclear graphite in order to simulate tritium displaced from its original structural site through recoil during reactor operation. The effect of thermal treatments on the mobility of the implanted deuterium was then investigated at temperatures ranging from 200 to 1200 °C, in inert atmosphere (vacuum or argon), in a gas simulating the UNGG coolant gas (mainly CO 2 ) or in humid nitrogen. Deuterium was analyzed by Nuclear Reaction Analysis (NRA) both at millimetric and micrometric scales. We have identified three main stages for the deuterium release. The first one corresponds to deuterium permeation through graphite open pores. The second and third ones are controlled by the progressive detrapping of deuterium located at different trapping sites and its successive migration through the crystallites and along crystallites and coke grains edges. Extrapolating the thermal behavior of deuterium to tritium, the results show that the release becomes significant above the maximum UNGG reactor temperature of 500 °C and should be lower than 30% of the total amount produced over a reactor operating time corresponding to about 11 effective full-power years. Moreover, it would mainly concern the tritium located close to the free surfaces. Furthermore, the total extraction of the remaining tritium in graphite waste should be more efficient in dry inert gas than in humid gas, but would require temperatures higher than 1300 °C for the total removal of the most deeply located deuterium. [ABSTRACT FROM AUTHOR]

Chromium silicon CrSi alloys are foreseen as possible materials for spintronic devices. Ion beam mixing could be an efficient technique to produce thin films of such alloys at room temperature while avoiding thermal diffusion. In order to assess this point, we have irradiated 20 nm Cr layer on a (1 0 0) Si wafer with 70 keV Kr ions. The X-ray reflectometry technique combined with Transmission Electron Microscopy and Energy Dispersive X-ray analysis was applied to analyze, at the nanometric scale, the formation of Cr/Si blurred interfaces induced by ion beam mixing. From the analysis of reflectivity curves, it appears that nanometric Cr 5 Si 3 and CrSi 2 phases are produced at the early stage of the process. The existence of these two paramagnetic phases gives some clues to explain the reason why the experimentally observed ferrimagnetism was weaker than predicted. [ABSTRACT FROM AUTHOR]

Abstract: This study aims at elucidating the mechanisms regulating the interaction of Eu and Ni with calcite (CaCO3). Calcite powders or single crystals (some mm sized) were put into contact with Eu or Ni solutions at concentrations ranging from 10−3 to 10−5 molL−1 for Eu and 10−3 molL−1 for Ni. The sorption durations ranged from 1week to 1month. Rutherford Backscattering Spectrometry (RBS) well adapted to discriminate incorporation processes such as: (i) adsorption or co precipitation at the mineral surfaces or, (ii) incorporation into the mineral structure (through diffusion for instance), has been carried out. Moreover, using the fluorescence properties of europium, the results have been compared to those obtained by Time-Resolved Laser Fluorescence Spectroscopy (TRLFS) on calcite powders. For the single crystals, complementary SEM observations of the mineral surfaces at low voltage were also performed. Results showed that Ni accumulates at the calcite surface whereas Eu is also incorporated at a greater depth. Eu seems therefore to be incorporated into two different states in calcite: (i) heterogeneous surface accumulation and (ii) incorporation at depth greater than 160nm after 1month of sorption. Ni was found to accumulate at the surface of calcite without incorporation. [Copyright &y& Elsevier]

This paper focuses on the thermal behavior of deuterium, simulating tritium, implanted into virgin nuclear graphite of French gas-cooled reactors, which are being decommissioned. Deuterium ions D+ were implanted into graphite (around 3at.% at the projected range R p) at two different depths (around 670nm and 2.8μm) and annealed up to about 300h in a temperature range from 200°C to 1200°C under vacuum or argon flow. Before and after heat treatments, D distribution profiles in the samples were followed using the nuclear reaction D(3He,p)4He, with a millimetric beam at the 4 MV Van de Graaff accelerator of IPNL (Institut de Physique Nucléaire de Lyon, France). The results show that the deuterium release becomes significant at temperatures higher than 600°C and is almost totally completed at 1200°C. The comparison of the results, obtained for both implantation depths, points out the role of the porosity with respect to deuterium permeation. The release follows two stages: a rapid step where it occurs within a few hours, followed by a much slower step during which the release of deuterium saturates. The initial stage is characterized by an activation energy of 1.3eV and might correspond to detrapping of D located at crystallite edges and its diffusion at the crystallite surfaces. We assume that the second stage kinetics corresponds to a very slow diffusion of D located inside the crystallites and chemisorbed to carbon atoms through sp2 or sp3 bonds. [ABSTRACT FROM AUTHOR]

This paper quantifies the migration of silver contained within mesostructured hybrid silica films and mesoporous titania films under exposure to modulated light. This quantization allows to demonstrate that all of the initial silver salt can be concentrated and reduced in domains accumulating the higher photonic energy. Entirely reduced in the form of nanoparticles of few nanometers size embedded in the silica matrix, silver is then quite stable even under subsequent homogeneous exposures. It is also shown that thanks to the relatively slow nanoparticle growth, successive multiple exposures can be used to create complex 3D microstructures within silica films using a simple dual beam interferometer. In mesoporous titania films, the UV photo-growth of silver nanoparticles remains limited to the vicinity of the film interface because of the matrix absorption and cannot provide deep 3D patterns of silver nanoparticles. However, 2D refractive index patterns can be obtained under UV exposure, erased with visible light and updated thanks to a reversible photochromic behavior. In such films, opposite migration flows of silver species are proven under UV intensity gradient and homogeneous visible exposure. [ABSTRACT FROM AUTHOR]

Abstract: The effect of annealing on defects and the formation of Xe bubbles were investigated in zirconium oxycarbide implanted with 800-keV136Xe2+ ions at two fluences 1×1015 and 1×1016 Xe/cm2. Doppler broadening technique combined with slow positron beam was used. The analysis of the S depth profiles and S–W maps revealed that in the as-implanted samples at both fluences Xe bubbles are not formed. The post-implantation annealing of the samples implanted at 1×1016 Xe/cm2 caused formation of Xe bubbles. The response of the lower implantation dose samples to this post implantation annealing was found rather complicated and is discussed. [Copyright &y& Elsevier]

Abstract: In this paper, we present an X-ray Absorption Near Edge Structure (XANES) study of chlorine speciation in virgin nuclear graphite of the French UNGG reactors of St. Laurent (SLA2) and Marcoule (G2). Comparable Cl K-edge XANES spectra are obtained for both kinds of virgin nuclear graphite. Spectral analysis shows that chlorine is covalently bound to carbon atom, with a formal valence equal to (−I). Annealing of the samples in the temperature range 200–1000°C induces a release of chlorine but does not change its speciation and valence. [Copyright &y& Elsevier]

Abstract: Graphite was modified by 250keV 37Cl+ ion implantation. Combined Raman microspectrometry/transmission electron microscopy (TEM) studies have been used to characterize the multiscale organization of the graphite structure. The penetration depth of 37Cl+ into the graphite sample was limited to the surface (∼200nm) because of the dissipation of the irradiating ion energy as expected by secondary ion mass spectrometry analysis. Raman microspectrometry appears to be an appropriate tool for studying such scales. Spectra showed a strong increase of defect bands after implantation at a fluence of 5·1013 ions/cm2. In order to examine the structural degradation of the graphite versus the depth at the nanometer scale, the focused ion beam technique seems to be a well-suited method for a relevant coupling of Raman and TEM observations. [Copyright &y& Elsevier]

Abstract: Elaboration of mesostructured silica films with a triblock copolymer polyethylene oxide–polypropylene oxide–polyethylene oxide, (PEO–PPO–PEO) and controlled growth of silver nanoparticles in the mesostructure are described. The films are characterized using UV–visible optical absorption spectroscopy, TEM, AFM, SEM, X-ray diffraction (XRD) and Rutherford backscattering spectrometry (RBS). Organized arrays of spherical silver nanoparticles with diameter between 5 and 8nm have been obtained by NaBH4 reduction. The size and the repartition of silver nanoparticles are controlled by the film mesostructure. The localization of silver nanoparticles exclusively in the upper-side part of the silica-block copolymer film is evidenced by RBS experiment. On the other hand, by using a thermal method, 40nm long silver sticks can be obtained, by diffusion and coalescence of spherical particles in the silica-block copolymer layer. In this case, migration of silver particles toward the glass substrate-film interface is shown by the RBS experiment. [Copyright &y& Elsevier]

Abstract: Titanium nitride has been proposed as a fission product barrier in fuel structures for gas cooled fast reactor (GFR) systems. The thermal migration of Cs was studied by implanting 800keV 133Cs++ ions into sintered samples of TiN at an ion fluence of 5×1015 cm−2. Thermal treatments at temperatures ranging from 1500 to 1650°C were performed under a secondary vacuum. Concentration profiles were determined by 2.5MeV 4He+ elastic backscattering. The results reveal that the global mobility of caesium in the host matrix is low compared to xenon and iodine implanted in the same conditions. Nevertheless, the evolution of caesium depth profile during thermal treatment presents similarities with that of xenon. Both species are homogeneously transported towards the surface and the transport rate increases with the temperature. In comparison, iodine exhibits singular migration behaviour. Several assumptions are proposed to explain the better retention of caesium in comparison with both other species. The potential role played by the oxidation is underlined since even a slight modification of the surface stoichiometry may modify species mobility. More generally, the apparition of square-like shapes on the surface of the samples after implantations and thermal treatments is discussed. [Copyright &y& Elsevier]

Abstract: This study deals with the behaviour of helium in a molybdenum liner dedicated to the retention of fission products. More precisely this work contributes to evaluate the release of implanted helium when the gas has precipitated into nanometric bubbles close to the free surface. A simple model dedicated to calculate the helium release in such a condition is presented. The specificity of this model lays on the assumption that the gas is in equilibrium with a simple distribution of growing bubbles. This effort is encouraging since the calculated helium release fits an experimental dataset with a set of parameters in good agreement with the literature. [Copyright &y& Elsevier]

Abstract: Titanium nitride is one of the inert matrixes proposed to surround the fuel in gas cooled fast reactor (GFR) systems. These reactors will operate at high temperature and refractory materials with a high chemical stability and good mechanical properties are required. Furthermore, a total retention of the most volatile fission products, such as I, Xe or Cs, by the inert matrix is needed during the in-pile process. The isothermal migration of iodine in TiN was studied by implanting 800keV I++ ions in sintered samples at an ion fluence of 5×1015 cm−2. Thermal treatments were performed under secondary vacuum at temperatures ranging from 1200 to 1700°C. Iodine concentration profiles were determined by 2.5MeV α-particle elastic backscattering. The migration of iodine seems to be correlated with point defects created by implanted ions near the surface. The Arrhenius plot corresponding to iodine detrapping is curved with possibly two straight-line regions which could indicate either the presence of two types of traps, or a strong dependence of trap’s concentration on temperature above 1500°C. The activation energies associated with each linear region of the Arrhenius plot were found to be: E a =2.4±0.2eV below 1500°C and above 1500°C. Nitrogen evaporation from TiN surface under secondary vacuum was proposed as a contributing factor to the enhanced mobility of iodine at high temperature. [Copyright &y& Elsevier]

Nuclear fuel devices of pressurised water reactors are composed of uranium oxide pellets which are enclosed in zircaloy-cylinders. During reactor operation in the contact with the fuel the zircaloy oxidises non-uniformly in depths of the order of some micrometers. Furthermore, energy deposition of fission recoils leads to sputtering of uranium onto the inner surface of the cladding material. Thus, sputtered uranium ions start to migrate outwards. Experiments were performed at Institut Laue Langevin (ILL) on the Lohengrin mass spectrometer in order to simulate the alteration of cladding tubes in contact with the nuclear fuel during irradiation. A thin UO2 deposition in direct contact with a zirconium foil was put into the ILL high flux reactor (5×1014 neutrons cm−2 s−1). The variation of energy loss of selected fission products in the foil put in evidence a zirconium oxidation whose kinetics constant is deduced from these experiments. Following oxidation the fission product mean kinetic energy is stabilised, but still a broadening of the energy distribution is observed, which is characteristic of uranium diffusion inside the zirconia target. Using Fick''s model, a uranium diffusion coefficient in zirconia under irradiation is deduced. [Copyright &y& Elsevier]

Abstract: A comparative study of damage accumulation in calcium molybdate (CaMoO4) has been conducted using Rutherford Backscattering/Channeling (RBS/C), cathodoluminescence (CL) and ionoluminescence (IL) techniques. All methods used confirm a two-step character of damage build-up process. Similar threshold fluence values have been extracted from RBS/C, CL and IL measurements. This analysis confirms the huge potential of luminescence techniques for damage analysis in single- and polycrystalline samples and the ability of the IL method to perform fast, in situ analysis of damage accumulation process. [Copyright &y& Elsevier]

• The effect of a metallic fission product on Cs behaviour and its release in UO2 was investigated as a relevant aspect of fuel safety • Mo and Cs were co-implanted in UO2 • Mo metallic precipitates anchor Cs bubbles and hinder their migration • The role of dislocations on the nucleation of bubbles and metallic precipitates is highlighted This study aims to evaluate the influence of a metallic fission product, molybdenum, on the behaviour of caesium in UO 2 at high temperature (1600°C) under reducing atmosphere. Both elements were successively introduced by ion implantation (firstly Mo, then Cs) at room temperature at a fluence of 1016 ions/cm². We show that in these co-implanted samples, the defects created by the Mo implantation render possible the nucleation of nanometric size bubbles during Cs implantation at room temperature. SIMS and TEM techniques were coupled in order to characterize the migration of both elements, as well as the UO 2 microstructure evolution after annealing at 1600°C. We found that Mo is more mobile in presence of Cs, which may be related to an increase of available vacancies in the material produced by Cs implantation. On the contrary, the concentration profiles of Cs remain quite similar than when it is solely implanted, and its release percentage remain the same both in absence and presence of Mo. After annealing, both elements are found to be associated as Mo metallic precipitates and Cs bubbles distributed over the same depth (~150 nm). Comparing with the Cs bubbles distribution in absence of Mo, it is clear that the formation of Mo metallic precipitates hinders the Cs bubble migration. Our study highlights the role of dislocations on the nucleation of bubbles and metallic precipitates. [ABSTRACT FROM AUTHOR]

• SIMS and TEM techniques were combined to compare the thermal behaviour of Cs and Xe in UO 2. • Both elements form bubbles with different growth kinetics. • At 1600 °C, caesium is found to be highly mobile in the UO 2 matrix while Xe distribution does not evolve. Xenon and caesium are among the most impacting fission products when studying the nuclear fuel: xenon for its role on the fuel rod thermomechanical behaviour during reactor operation and caesium in the case of atmospheric radioactive release during an accident in a nuclear power plant. This paper focuses on the comparison of caesium and xenon thermal behaviour in polycrystalline uranium dioxide (UO 2) pellets. Caesium-133 or xenon-136 stable isotopes were introduced in depleted UO 2 samples by ion implantation at a maximum concentration of 0.08 at% at a depth of around 140 nm below the sample surface. Annealing under reducing atmosphere (Ar/H 2 5 %) was performed at 1000 °C or 1600 °C, which corresponds respectively to a representative temperature during nuclear reactor operation (at the centre of the fuel pellets) and during an accident. The caesium migration in UO 2 was investigated by Secondary Ion Mass Spectrometry and compared to the thermal behaviour of xenon in UO 2 at 1600 °C. Transmission Electron Microscopy was performed in order to characterise UO 2 microstructure before and after annealing. The results indicate that caesium has a different behaviour than xenon with which it is often compared for its release from the nuclear fuel. In particular, we highlight a difference between the growth kinetics of caesium and of xenon bubbles at 1600 °C which can be correlated to the availability of thermal vacancies in UO 2 and to the different ability of Xe and Cs atoms for thermal resolution. [ABSTRACT FROM AUTHOR]

This work aims to study the effect of radiolytic species induced by water radiolysis on the passive behavior of 316L stainless steel. For this purpose, the stainless steel/neutral and aerated 0.02 M Na 2 SO 4 , electrolyte solution interface was irradiated with proton beams. A wide range of energies between 2 and 16 MeV was selected, varying the maximum of the energy deposition between 0.5 and 122 μm in water from the interface. The irradiation experiments were performed at the CEMHTI cyclotron in Orléans and the 4 MV Van de Graaff accelerator at IP2I in Lyon (France). A dedicated irradiation device implemented with a 3-electrode cell dedicated to perform electrochemical measurements allows to measure the surface reactivity of the stainless steel as a function of the irradiation conditions. Results show that whatever the beam energy, the corrosion potential remains unchanged. It indicates that the very short-lived, highly reactive radiolytic species drive the corrosion potential and not only the recombination products such H 2 O 2 or H 2 . The stainless steel remains in the passive state whatever the irradiation conditions. However, it is shown that, during irradiation, the passive film is less protective. This evolution is attributed to radiolysis of bound water molecules in the passive film., (© 2024 Wiley-VCH GmbH.)

Molybdenum is an abundant element produced by fission in the nuclear fuel UO 2 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 2 oxidation) typical of an accidental scenario. To do so, molybdenum was introduced in UO 2 or UO 2+ 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 UO X 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 2 . 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 2 or UO 2+ 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 UO 2 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 2+ x hyper-stoichiometry x. After thermal treatment, Mo remaining in the samples is located in the grains under the MoO 2 form. Our experimental results are assessed by ab initio calculations showing that in the presence of oxygen Mo atoms adopt in UO 2 a local structure close to the octahedral local geometry of Mo oxides.

Elaboration of mesoporous titanium oxide film supporting silver nanoparticles is described. Mesoporous titanium oxide films are characterized by TEM analysis. Titania films are infiltrated with a silver salt solution and chemical reduction treatments are performed using either a NaBH(4) or a formaldehyde solution. Infiltrated films are then characterized by TEM, SEM, AFM, UV-visible spectroscopy, X-ray diffraction, and Rutherford Backscattering Spectrometry (RBS). The utilization of a mesoporous titania substrate allows to control the nanoparticle size and the interparticle distance. RBS experiments provide the evidence that NaBH(4) treatment induces a strong accumulation of silver nanoparticles in the subsurface of the layer, while formaldehyde treatment induces the formation of silver nanoparticles embedded into almost the whole depth of the titania film. Large silver nanocrystals are also formed at the film surface whatever the reducer used. A broad visible absorption band related to the surface plasmon resonance (SPR) is obtained in both cases and is strongly red-shifted compared to the SPR obtained for silver nanoparticles inside a silica matrix. Moreover, irradiation with visible light causes the photooxidation of silver nanoparticles by titania and a complete discoloration of the material. The photooxidation is related to a drastic decrease in the silver nanoparticle size and is found to be reversible, particularly in the case of the material obtained by the formaldehyde reduction.