Caroline Toffolon-Masclet

Studies of second phase particles in different zirconium alloys using extractive carbon replica and an electrolytic anodic dissolution procedure

Zirconium alloys are widely studied for applications as cladding tubes and structural components of PWR fuel assemblies. Due to their influence on some of the alloys properties (corrosion resistance, irradiation growth, …), the crystallographic structure and the chemical stoichiometry of the second phase particles (SPP) precipitated in these alloys have to be well established. The aim of this paper is to present the results obtained using two methods of SPP extractions. The first one, the extractive carbon replica method, allowed us to determine the chemical composition of SPP in different zirconium alloys: Zr–Sn–Fe–Cr (Zircaloy-4®), Zr–Sn–Fe–Cr–(V,Mo), Zr–Nb and Zr–Nb–Fe alloys. The second one, an anodic dissolution procedure of the matrix, is an interesting way of isolating SPP from the surrounding α-Zr matrix, giving access to a precise determination of the crystallographic structure and lattice parameters of the SPP by X-ray diffraction. This procedure was validated for Zy-4 by comparing the SPP size distribution obtained by extraction with that directly measured on a massive Zy-4 alloy (i.e. the SPP size distributions were the same for both measurements).

Simulation of the β→α(O) Phase Transformation due to Oxygen Diffusion during High Temperature Oxidation of Zirconium Alloys

The EKINOX numerical code, formerly developed to simulate high temperature oxidation of Ni alloys, has been recently adapted to solve out the issue of high temperature oxidation of Zirconium alloys. This numerical code is a one dimensional model that simulates the growth of an oxide layer using a specific algorithm for moving boundaries problem. In order to simulate the oxygen diffusion inside Zr alloys, an adaptation of the EKINOX code was necessary. It consisted in adding, first, a non-null oxygen equilibrium concentration in the substrate and second, a new interface in order to simulate the β/α(O) phase transformation due to oxygen diffusion. In this study, EKINOX has also been coupled with the thermodynamic database for zirconium alloys ZIRCOBASE (thermocalc formalism) in order to obtain accurate concentrations values in each phases (considering local equilibrium at each interface). The present paper illustrates the simulation ability of the code by comparing experimental and calculated oxygen diffusion profiles corresponding to different cases, from isothermal oxidations at high temperature (900 < T < 1250°C) to the study of dissolution kinetics of a pre-transient oxide layer under a neutral environment. The influence of pre-hydriding from a few hundreds up to thousands weight-ppm is also derived from the calculations.

Carbide Precipitation in 2.25 Cr-1 Mo Bainitic Steel: Effect of Heating and Isothermal Tempering Conditions

The effect of the tempering heat treatment, including heating prior to the isothermal step, on carbide precipitation has been determined in a 2.25 Cr-1 Mo bainitic steel for thick-walled applications. The carbides were identified using their amount of metallic elements, morphology, nucleation sites, and diffraction patterns. The evolution of carbide phase fraction, morphology, and composition was investigated using transmission electron microscopy, X-ray diffraction, as well as thermodynamic calculations. Upon heating, retained austenite into the as-quenched material decomposes into ferrite and cementite. M7C3 carbides then nucleate at the interface between the cementite and the matrix, triggering the dissolution of cementite. M2C carbides precipitate separately within the bainitic laths during slow heating. M23C6 carbides precipitate at the interfaces (lath boundaries or prior austenite grain boundaries) and grow by attracting nearby chromium atoms, which results in the dissolution of M7C3 and, depending on the temperature, coarsening, or dissolution of M2C carbides, respectively.

Contribution to Modeling of Hydrogen Effect on Oxygen Diffusion in Zy-4 Alloy During High Temperature Steam Oxidation

Previous studies have shown that the numerical model EKINOX-Zr was able to simulate with accuracy oxide growth and oxygen diffusion into the matrix during high-temperature oxidation of Zy-4. In this study, the aim of the development was to evaluate if the observed effect of hydrogen cladding content on the increase of oxygen solubility in the high-temperature βZr was only a thermodynamic effect. Previous experimental studies have shown that hydrogen induces an evolution of equilibrium oxygen concentration at the αZr/βZr interface. The present work showed that EKINOX-Zr linked with the thermodynamic database Zircobase reproduced the evolution induced by hydrogen during the high-temperature steam oxidation. However, the results showed also that additional studies are necessary to better understand hydrogen behavior during high-temperature oxidation of Zr.

Microstructure and Properties of a Three-Layer Nuclear Fuel Cladding Prototype Containing Erbium as a Neutronic Burnable Poison

To increase cycle length and/or fuel burnup, several theoretical and experimental studies have been performed at CEA. Among them, prospective neutronic calculations have shown that the addition of a few weight percents of erbium into the cladding materials could be a promising alternative to the introduction of the neutronic poison directly into the nuclear fuel pellets. Thus, fabrication of homogeneous Zr-Er alloys has been assessed, at least up to 10 wt. % of erbium and, based on the as-received mechanical properties, an optimum erbium concentration ranging from 3 to 6 wt. % has been derived. However, because of the high-oxygen thermodynamic affinity of erbium, thermal treatments have to be controlled during the fabrication route to limit Er2O3 precipitation and coarsening, which may have detrimental effects on the ductility/toughness of Zr-Er alloys. In parallel, to get more fundamental insights into the underlying phase diagrams, thermodynamic studies have been devoted to experimental assessment and modeling of the Zr-Er-(H-O) system. Because of the detrimental influence of erbium on the corrosion resistance, a three-layer sandwich clad prototype has been developed using corrosion-resistant inner/outer Zr-1Nb layers to protect the intermediate Zr-Er layer from direct water exposure. Compared to a reference Zr-1Nb(O) alloy that has been subjected to the same fabrication route, the three-layer clad prototype shows limited decrease in ductility because of pre-hydriding or after high-temperature steam oxidation e.g., in the case of a loss-of-coolant accident). Moreover, the studies performed so far have shown a spectacular hydride trapping capacity of the intermediate Zr-Er layer both for hydrogen coming from nominal outer corrosion or because of massive secondary hydriding in case of the direct access of water to the Zr-Er intermediate layer. Using μ-ERDA (elastic recoil detection analysis) measurements, detailed studies of the hydrogen spatial redistribution upon thermal cycling has been done. A simple model has been successfully used to characterize the cooling rate influence on the through-wall clad thickness partitioning of hydrogen/hydrides between the three layers, after cooling from a temperature corresponding to full dissolution of hydrides

Thermodynamic Study of Al-Li-O-Zr: Experimental Results and Thermodynamic Assessment

To better understand the formation of solid, liquid and gas phases between the Al-Li-O ceramics and zirconium (Zr) at temperatures above 1273 K, we built a thermodynamic database on the Al-Li-O-Zr quaternary system using the CALPHAD method. An accurate knowledge of both phase diagram and thermodynamic data of all the binary and ternary sub-systems is necessary to understand the phase relations in the quaternary. The thermodynamic descriptions of both Al-Li-Zr and Al-O-Zr ternary systems are presented. In addition, the Li-Al-O-Zr system was experimentally investigated using diffusion couples made of [ Zr / AlxLiyOz ] and [ ZrO2 / AlxLiyOz ] between 1373 K and 1573 K. The experimental results are compared with activity diagrams calculated with the thermodynamic database.