Defect and Diffusion Forum Vols. 323-325

Abstract: This paper describes an original method to measure the plutonium self-diffusion coefficient in the mixed oxide U_0.55Pu_0.45O_2±x. This method is based on using ²⁴²Pu as a tracer atom. A thin film of the tracer was deposited on the well-polished surface of the samples and then diffusion annealings were performed from 1500°C to 1700°C, in an Ar-H₂ 5% atmosphere. The oxygen potential was fixed at-395 kJ.mol^-1. After annealing, the ²⁴²Pu self-diffusion profiles were established by means of secondary ion mass spectrometry (SIMS). The ²⁴²Pu concentration profiles were determined by assessing the relative U, Am and Pu ionisation yields according to the experimental parameters. The choice of favourable experimental conditions and the relevance of the resulting concentration profiles are discussed at length.

Abstract: The Cluster Dynamics method is assessed for the investigation of fission gas behaviour in a krypton-implanted and annealed UO₂ sample. The simulation results are then compared to Thermal Desorption Spectroscopy (TDS) data. A release mechanism is proposed: the initial burst is related to krypton migration via an interstitial mechanism, while the second stage of the release process can be accounted for by the diffusion of krypton in a substitutional position. This latter mechanism is compatible with a diffusion coefficient of 4.10^-21 m²/s.

Abstract: Among the various products originating from fission events, the noble gas elements Xe and, in a lesser extent, Kr present important fission yields. The accumulated gas inventory in its various states (e.g. atomically dissolved, precipitated in cavities or released from the fuel) has a strong impact on the performance of LWR fuel and is presently one of the limiting factors for fuel burnup extension. A more fundamental understanding of fission gas behaviour at the atomic scale would enable to improve the modelling of the various mechanisms ultimately leading to fission gas release and to refine conservative safety margins. Lots of efforts have already been undertaken using atomistic computer simulations, ab initio calculations and Empirical Potential Molecular Dynamics (EP-MD) techniques, that relate to the bulk behaviour. This article will discuss EP-MD investigations in nanosized polycrystalline UO₂, constructed from Voronoi cells in a 3-D periodic environment. This study has focused on Xe diffusion at and close to grain boundaries.

Abstract: Polycrystalline Fe 99.95 and 99.5 samples were implanted with helium at 8 keV and 3 MeV. Thermal Desorption Spectroscopy (TDS) and Nuclear Reaction Analysis (NRA) provided a complementary set of techniques to characterize helium-materials interactions within two different implantation depths, respectively close to the surface and in the bulk. Using TDS, it was possible to get information about the nature and the states of the structures where helium was trapped in radiation damaged Fe specimens. Activation energies for every trapping site (mono-vacancies, clusters) have been determined from conventional reaction model. The effect of interstitial carbon was also discussed, and compared with previous ab-initio studies. Moreover, the helium bulk diffusion constants in radiation damaged structures could be derived from non destructive ³He depth profiling. Preliminary observations highlighted that a few part of He remained trapped while helium bubbles migrated in the bulk.

Abstract: In PWR, the Zircaloy based clad is the first safety barrier of the fuel rod, it must prevent the dispersion of the radioactive elements, which are formed by fission inside the UO₂ pellets filling the clad. We focus here on internal corrosion that occurs when the clad is in tight contact with the UO₂ pellet. In this situation, with temperature of 400 °C on the internal surface of the clad, a layer of oxidised Zircaloy is formed with a thickness ranging from 5 to 15 µm. In this paper, we will underline the specific behaviour of this internal corrosion layer compared to wet corrosion of Zircaloy. Simulations will underline the differences of stress field and their influences on corresponding dissolved oxygen profiles. The reasons for these differences will be discussed as function of the mechanical state at inner surface of the clad which is highly compressed. Differences between mechanical conditions generated by an inner or outer corrosion of the clad are studied and their influences on the diffusion phenomena are highlighted.

Abstract: In CeO₂-based solid electrolytes, it has been shown that point defects are directly responsible for oxygen ionic conduction. The ionic conductivity is strongly affected by the anion vacancy concentration which is enhanced by doping with aliovalent cations. When rare earth sesquioxides such as La₂O₃, Gd₂O₃, Sm₂O₃, Y₂O₃ are added to CeO₂, the dopant cation substitutes for the cerium ion, and oxygen vacancies are created for charge compensation. Incorporation of trivalent dopants into CeO₂ at the Ce⁴⁺ sites can be depicted by the following defect reaction (expressed in Kröger-Vink notation):

Abstract: For economical and environmental reasons, hydrogen is considered as a major energetic vector for the future. Hydrogen production via high temperature water vapour electrolysis (HTE) is a promising technology. A major technical difficulty related to high temperature water vapour electrolysis is the development of interconnects working efficiently for a long period. Working temperature of 800°C enables the use of metallic materials as interconnects. Chromia forming alloys are among the best candidates. The interconnect material chosen in the present study is a ferritic stainless steel with 18% chromium content. High temperature corrosion resistance and electrical conductivity of the alloy was tested in both cathode (H₂/H₂O) and anode (O₂/H₂O) atmospheres. Corrosion products were then characterized by SEM-EDX and XRD. Moreover chromium evaporation measurements were carried out under anode atmosphere.

Abstract: Reformer materials are exposed to severe operating conditions at high temperature in aggressive catalyzer and combustion atmospheres. Therefore, materials used for the construction of the reformer reactor have to possess appropriate high temperature resistance. Diffusion coatings improve the high temperature resistance of materials by enrichment of the alloy surface with thermodynamically stable oxide formers. Beside conventional mono-element diffusion coatings (e.g. Al coating), multi-element diffusion coatings can be developed in a single process step. In this work we developed Al diffusion and Al-Si and Al-Si-RE (RE: reactive element Y, Ce) co-diffusion coatings on a low cost austenitic 18Cr10Ni-steel. The high temperature resistance of coated and uncoated 18Cr10Ni-steel, 20Cr31Ni-steel and 23Cr18Fe-Ni base alloy was tested in catalyzer and combustion atmosphere under cyclic operation conditions.

Abstract: Mixed Uranium-Plutonium Carbides are candidate fuels for 4^th generation nuclear reactors. In this frame, a research program is underway at CEA (French alternative and atomic energies commission) to gather basic properties of such material to get a better understanding of in-pile behaviour. In the field of solid state physics, the measurement of transport properties and in particular diffusion coefficient requires the preparation of uranium carbide samples with very specific characteristics: very low oxygen content, mono-phased UC, high density (greater than 95% of the theoretical one), coarse microstructure, accurate samples geometry. In this paper the Process development is presented to manufacture such samples and the specific equipments which have been set up in glove-box. Characterizations of the first samples are also given.

Abstract: The influence of the chemical activity effects on moisture transport in the presence of ions in cementitious materials is studied. Wetting-drying tests are carried out on various hardened concretes (sound and fully carbonated) with water. Moisture profiles are experimentally assessed by gamma-ray attenuation. A multispecies and multiphase numerical model, in which the Lin and Lee approach is applied to compute the chemical activity coefficients of water in presence of ions, is used to simulate the transport of ions and moisture. The drying kinetics is analyzed to assess the intrinsic permeability of the material by inverse analysis. Experimental profiles are compared to the simulations to illustrate the relevance of the modeling.