Materials under irradiation

Irradiation of materials changes their microstructure. This can be desired or suffered. In the latter case, the irradiation results from the operating conditions of the components made of the materials under consideration. It is then at the origin of an ageing phenomenon under irradiation, most often harmful to the use properties of the materials.

The main projects carried out in ERMEN cover the following fields:

• Ageing of structural materials in pressurized water reactors
• Development of materials for the nuclear reactors of tomorrow

Aging of structural materials in pressurized water reactors

Some of this research is part of the joint EDF-CNRS EM2VM laboratory (Study and Modeling of Mechanisms for the Ageing of Materials). They aim to understand the mechanisms responsible for the ageing under irradiation or at temperature of steels used in pressurized water reactors and to correlate, as far as possible, the evolution of the microstructure and mechanical properties.

• Pressure vessel steels: Since the vessel of a pressurized water reactor is a non-replaceable component, it can be a limiting factor in the life time of a nuclear power plant. Thus we aim at understanding and anticipating the evolution of the microstructure of low alloy steels (bainitic) under irradiation (phase transformations, inter and intragranular segregation, role of metallurgical heterogeneities, effect of flow and incident particles) and its impact on high-flow mechanical properties (hardness, transition temperature...) (Projects in progress: industrial contract with EDF, Project H2020 SOTERIA, Project NEEDS SAFETY).

• Reactor internal structures: The internal structures are made of austenitic stainless steels. In service, these steels age in the primary environment under irradiation and stresses. The aim here is to study the formation of extended defects (dislocation loops, cavities), segregation and transformation of phases under irradiation contributing to stress corrosion cracking and to correlate the evolution of the microstructure with the hardening of the material. (SUPERMEN Project)

• Primary circuit: Austeno-ferritic stainless steels are the cast elbows of the primary circuit of generation II power plants. They are used because of their stainless steel character, their good resistance to impact failure. However, it has been established that these steels age over time and their mechanical properties degraded due to phase transformations: formation of interconnected regions respectively rich and poor in Cr (spinodal decomposition) and precipitation of G phase (a nanoscale intermetallic phase, mainly enriched in Si, Ni and Mo). The aim here is to study the influence of alloy elements on the ageing kinetics and the influence of the phases formed on mechanical properties. (industrial contract with EDF)

• Benchmarking activities: The atom probe tomography is nowadays an essential technique for the study of ageing under irradiation of steels. Atom probe data can identify aging mechanisms; provide quantitative information for modelling microstructure evolution and for curing models. It is therefore essential to check if data collected by different groups around the world are consistent and to identify possible sources of error. With this in mind, we actively participated in 3 benchmarks bringing together international experts in atom probe and irradiation effects (NUGENIA APLUS, MAI "benchmark on vessel steels", EPRI "Round robin on austenitic stainless steels")

Development of materials for tomorrow's nuclear reactors

The construction of the next generations of nuclear reactors (Generation IV, fusion) requires the development of materials that are resistant to more severe temperature and irradiation conditions than today. The work carried out in this theme aims to understand how to optimize the microstructures of these materials, to determine the ageing mechanisms at temperature or under irradiation and to understand to what extent model irradiations (charged particles, high flux) allowing high fluences to be achieved can be extrapolated to the real case of neutron irradiations.

This work contributes to research carried out under the Joint Project on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA). EERA is an alliance of European research centres and universities. The GPM via ERMEN (C. Pareige) coordinates the research carried out in the subprogramme "Physical modelling and experiment oriented for the modelling of structural materials", i.e. 25 research organisations from 14 European countries.

GPM's activities around the reactors of the future deal with different types of steels.

• Ferritic-martensitic steels with high Cr content for 4th generation reactors and nuclear fusion. These steels have better swelling resistance and thermal conductivity than austenitic steels but they suffer from two problems that limit their use in temperature: embrittlement at low temperature (T<350°C) and creep at high temperature (T>550°C). We study the phenomenon of low temperature embrittlement, including investigations of the understanding the role of impurities on the evolution of microstructures under irradiation - Eurofusion Enabling Research NANOHARDENING, H2020/M4F, ICAR pilot project (JPNM), SLIPLOC pilot project (JPNM)

Oxide-dispersion reinforced steels: These are martensitic ferritic or ferritic steels reinforced by a fine and dense dispersion of nanoscaled oxides in order to improve their tensile properties at high temperatures and their resistance to irradiation. They are produced by powder metallurgy. We study in particular the microstructure of these steels in order to better understand the process/microstructure relationships, oxide precipitation, their stability in temperature and under irradiation as well as the evolution of the microstructure under irradiation: a/a' decomposition, formation of solutes clusters... (FP7 Matisse, Eurofusion IREMEV).

• Ultrafine grain materials: Ageing under irradiation is caused by the oversaturation of point defects resulting from collisions between incident particles and atoms of the material. Ultrafine grain materials, because of the very high density of defect wells (grain joints) they contain, are likely to be more resistant to irradiation than conventional materials. The GPM works closely with USUATU (Ufa, Russia) to develop and study such materials nanostructured by intense plastic deformation (EraNet-RUS NANODE)

• Development of F/M steels resistant to irradiation and creep by thermomechanical treatment (TM): A limitation in the use of F/M steels is related to their high temperature tensile properties. One way to improve these tensile properties is to strengthen the material by carbide precipitation. We work on this topic (microstructure/TM treatment relationship, carbide stability) within the CREMAR Pilot Project (JPNM).

• Simulation of neutron irradiation by ion irradiation: the limited accessibility of experimental reactors for conducting neutron irradiations, the radioactivity of neutron irradiated samples, the very high cost and time involved in neutron irradiations, lead the community to conduct irradiations with alternative sources such as ions. However, it is clear that the use of ions as substitute particles does not allow to fully reproducing neutron irradiations, and the transferability of results has to be adressed. It is therefore essential to identify the limits of such irradiations, to identify and understand transferability problems in order to develop strategies to extrapolate the results obtained and thus obtain information equivalent to neutron irradiations. H2020/M4F, SMORE II (IAEA), NEEDS Challenges, IOANIS Pilot Project (JPNM)

• Standard and advanced austenitic stainless steels: Austenitic stainless steels are candidates for many 4th generation reactor structures. Depending on their use, they will be subjected to thermal ageing at medium temperature (~500°C) over a long period of time.