The aluminum alloy AlFeNi used as fuel cladding for the Jules Horowitz Reactor (JHR) may undergo corrosion in the reactor environment. In order to qualify the corrosion behavior of the fuel elements of the JHR in accidental conditions, several specimens of AlFeNi have been corroded at 250°C for different durations (9–34days) in distilled water at various pH (4.9, 5.2 and 5.6) chosen to simulate that currently considered for the JHR. On all specimens, the only crystalline corrosion product formed is boehmite (AlOOH). The corrosion film is composed of three oxide layers which show through thickness chemical composition variations. The iron–nickel precipitates pre-existing in the metal matrix are present in the inner and intermediate oxide layers though oxidized. For long corrosion times, some of the iron and nickel particles are released in the water and some precipitation is observed at the surface of the oxide layer. The effect of surface finish (as received or polished) and thermal treatment (annealed and not annealed) on the oxide growth rate has also been investigated. For durations over 25days, pH=5.6 appears to be more favorable than pH=5.2 and 4.9 in terms of oxide thickness and weight gain limitation. This effect of pH is however reduced on unpolished specimens. The effect of surface finish on the corrosion behavior as measured by optical microscopy appears to be strong, especially for pH=4.9 where polished samples exhibited an accelerated evolution of the oxide thickness and of the mass gain. This could be due to the combined effect of a strong acid solution (pH=4.9) and of the local microstructural changes formed at the interface through polishing. The effect of thermal treatment on the behavior of unpolished AlFeNi specimens during corrosion tests in the conditions investigated was found to be small. In this study, microstructural and chemical analyses were performed on the corroded specimens in order to get a better understanding of the corrosion kinetics. The crystallographic nature of the boehmite layers investigated by X-ray diffraction is unaffected by the pH of the solution. Iron precipitates were identified on the oxide surface beyond 34days of corrosion by Environmental Scanning Electron Microscope (ESEM). Finally, Electron Probe Micro-Analysis (EPMA) was used to determine the chemical composition of the metal matrix and of the different oxide layers and precipitates versus the pH of the solution.

The AlFeNi aluminium alloy (1 wt% Fe, 1 wt% Ni, 1 wt% Mg) is expected to be used as nuclear fuel cladding for the Jules Horowitz experimental reactor. To guarantee a safe behaviour of the fuel, a good understanding of the fuel clad corrosion mechanisms is required. In this field, the experimental characterization of the selected alloy was performed. Then experimental studies of the aluminium alloy corrosion product obtained in autoclaves have shown an oxide film composed of two layers. This duplex structure results from a mixed growth mechanism: an anionic growth to develop the inner oxide and a cationic diffusion parallel to a dissolution–precipitation process to form the outer zone. Dynamic experiments at 70 °C have demonstrated that a solid diffusion step controls the release kinetic. Then post-irradiation exams performed on irradiated fuel plates were used to investigate the effects of the irradiation on the corrosion behaviour in the reactor core.

The 5000 and 6000 series aluminum alloys are extensively used in research reactors. To validate the choice of the suitable Al-alloys for the core components and the experimental devices for the conception of the Jules Horowitz Reactor (RJH), a characterization program of some highly irradiated components was performed. In this paper, we focalise on the 5754NET alloy in the annealed temper (O treatment) which was irradiated under different neutron spectrum, in Osiris and Orphée reactors. Tensile test results on this alloy irradiated for 30 years in OSIRIS are presented. The temperature and fluence effects are discussed. Tensile test results on the 5754-NET alloy irradiated for 15 years in ORPHEE in a cold neutron flux (cold source shell) are provided and compared with results obtained after irradiation with a harder neutron spectrum in Osiris. Neutron calculations in the ORPHEE cold source shell show that the total flux is hardly reduced by the presence of the cold source, but the Si production rate is increased by a mean factor of 1.53. This Si production rate is dependent on the circumferential position relative to the reactor core. The comparison with the data on the alloy irradiated in Osiris shows that the total elongation after irradiation is directly related to the Si content created under neutron flux, whereas the mechanical strength is hardly affected by the high Si production rate due to the cold neutron flux. The neutron spectrum effect is discussed and compared to published results. Microhardness measurements on the TIG-welding joins show that, on the unirradiated sample, the melted zone is slightly harder than the base metal. This difference is smoothed at high neutron fluence. All these results contribute to a better knowledge of aluminium alloys properties under irradiation in research reactors. This topic is very important for better risk and safety analyses of this type of reactors. 1

The conception of the Jules Horowitz Reactor (JHR) requires to qualify at high neutron fluences the alloys which will be used for the tank, the core components and the experimental devices. To validate the choice of an aluminum alloy for the JHR tank, we started an extensive characterization program of Al-alloys irradiated in research reactors during at least 15 years. For this program, we investigated some highly irradiated materials : the AG3-NET in annealed condition (5754-NET-O) and the 6061-T6. The 5754-NET-O is coming from components replaced during the refurbishment of the OSIRIS and ORPHEE reactors operated by CEA at Saclay (France). The components are the lattice structure of OSIRIS irradiated for 30 years (from 1966 to 1996), and the cold source shell of ORPHEE irradiated for 15 years (from 1980 to 1995). The 6061-T6 alloy is extracted from rods used as beryllium plug during 32 years in the BR2 reactor located at Mol (Belgium). This paper describes the mechanical tests (tension, fracture toughness) already performed on these materials. The effect of neutron fluence on mechanical properties of the AG3-NET-O (5754-NET-O) and 6061-T6 alloys is investigated. Finally, an outline of the further program for the qualification of the JHR tank and experimental devices is provided for both alloys. In non-irradiated condition, erosion resistance tests are in progress, and welding ability is tested on different grades of 6061-T6. For the irradiated condition, an irradiation of specimens in the alloy chosen for the tank will be performed in OSIRIS. A great part of these specimens will be transferred into the JHR and used for a surveillance program of the tank.

A molecular dynamics simulation of the perovskite MgSiO3 gives evidence for a phase transition from the orthorhombic to a tetragonal phase at about 2600 K on heating the crystal under a constant pressure of 310 kbar. This phase transition is associated with a phonon acoustic mode, characterized by a precession movement of the SiO6 octahedra. In the tetragonal phase, without introducing defects in the model, we observe a rapid diffusion of oxygen, comparable with that of a solid electrolyte. This high anionic mobility may be associated with the acoustic mode observed at the phase transition. This is supported by the values calculated for oxygen mean square displacements, which become high above the phase transition temperature. More generally, our results suggest that rapid ionic conduction in perovskites is associated with phase transitions by rotations of the anionic octahedra; hence rapid anionic motion should be observed only in the perovskites undergoing ferroelastic transitions.The activation energy computed for oxygen diffusion is 6.3 +/- 0.6 eV at 310 kbar and 6.6 +/- 0.6 eV at 370 kbar. In the tetragonal phase, this cooperative substitution mechanism without stable defects should always be more efficient than the vacancy mechanism. The behaviour of the elastic and thermodynamical properties at the phase transition is investigated at 310 kbar. The shear modulus undergoes a sharp drop from 1.96 +/- 0.11 Mbar at 2400 K to 1.16 +/- 0.12 Mbar at 3075 K, whereas the bulk modulus decreases continuously with temperature; the specific heat C(v) increases slightly from 6.0 +/- 0.2 cal K-1 mol-1 in the orthorhombic structure to 6.3 +/- 0.3 cal K-1 mol-1 in the tetragonal structure; the volumetric thermal expansion coefficient, which is 2.16 (+/- 0.18) x 10(-5) K-1 at room temperature in the orthorhombic phase, becomes higher in the tetragonal phase, with a strong temperature dependence. As in other perovskites undergoing ferroelastic phase transitions, the temperature T(c) of the phase transition increases with pressure; in MgSiO3 we calculate dT(c)/dP = 5.3 +/- 1.4 K kbar-1 in the pressure range 310-370 kbar. At high temperature in the orthorhombic structure, a drop in pressure emphasizes the distortions of the structure; extrapolation of our results to high pressures suggests that at 2400 K, MgSiO3 could be cubic above 740 kbar. At low pressures, the perovskite phase is not stable; this destabilization occurs at less than 50 kbar at 300 K, and at less than 150 kbar at 2400 K. The precision of our calculations does not permit the detection of any pressure dependence of the Slater and Debye Gruneisen parameters at 300 K in the pressure range 50-310 kbar. However, the thermodynamical Gruneisen parameter gamma(th) calculated at 300 K is strongly pressure dependent: it varies from 1.5 at 310 kbar to 2.1 at zero pressure. The product of gamma(th) and crystal density is therefore lower at high pressure. The temperature dependence of gamma is estimated by calculation of the Mie-Gruneisen parameter gamma(M), which is a strongly decreasing function of temperature.

A molecular dynamics study of diffusion in cubic perovskite MgSiO3 at high temperature is presented. We have chosen a pressure of 37 GPa, which is comparable with the values prevailing in the Earth's lower mantle. It is shown that above 3750 K oxygen ions have a high mobility in the absence of any kind of point defects. The calculated diffusion coefficients are comparable to those observed in usual superionic conductors near their melting point. A careful analysis of the diffusion mechanism has revealed that the migration of oxygen ions occurs by a hopping process with very strong correlations between diffusing particles.