Influence of the Bulk Chemical Composition on the Microstructure Evolution of Irradiated Chemically-tailored Nuclear RPV Steels

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    The growth of the global population and industrialisation request more and more energy production each year. At the same time, humanity is running out of fossil fuels, and there is a demand for a sustainable energy source to stop the detrimental climate changes. In this scenario, sources of electricity production with low carbon emission are essential.
    Nuclear power is a source of energy with low CO2 emission which contributes to around 10 % of the electricity supply worldwide by the fleet of 440 nuclear reactors [1]. Contrary to the general public perception, nuclear energy is the safest way of producing electricity used so far [2]. By 2020s almost 100 nuclear reactors will surpass 40 years from the day they started to produce electricity. Currently, almost 70% of the active reactors are over 30 years old [3]. The design of nuclear power plants (NPP) of the next generation is not finalised yet, and the first fusion reactor is still under development. Thus, the lifetime of contemporary pressurized water reactors (PWRs) should be safely prolonged.
    One of the critical long-term operation issues of NPPs is the neutron irradiation embrittlement of reactor pressure vessel (RPV) materials. The RPV is a steel-made container of nuclear fuels, internal structures and coolant water maintained under high pressure (about 15 MPa) at high temperature (around 300 °C) [4]. The RPV is one of the most important parts of the NPP because it acts as a barrier between radioactive materials and the environment and cannot be replaced.
    During the operation of NPP, neutrons produced by nuclear fission in the reactor core can interact with the RPV steel, resulting in the formation of self-interstitial atoms (SIA) and vacancies (V) which will enhance diffusion and lead to the formation of irradiation enhanced precipitates. Also, a continuous flux of these point defects can drag solute atoms resulting in the formation of irradiation-induced solute clusters or segregations. Precipitates and clusters act as barriers for dislocation motion resulting in the hardening and the consequent embrittlement of material [5]. Alloying elements such as Mn, Ni and Si, and impurities like Cu and P are often associated with the clusters and precipitates formed under irradiation.
    Different parameters influence the magnitude of irradiation hardening. The most noteworthy ones are irradiation temperature and flux, accumulated dose and bulk chemical composition. The irradiation temperature (~290 °C) and flux (~1014 n/m2s) are similar for most commercial nuclear power plants [4]. The accumulated dose increases with the operation time. Finally, the bulk chemical composition significantly varies depending on the country and the time of NPP construction.
    The main objective of this PhD research is to understand the influence of the bulk chemical composition (focusing on Mn, Ni, Cu and P) on the microstructure evolution and consequently its impact on the irradiation hardening of RPV steels.
    To do so, seven chemically-tailored (CT) RPV steels, divided into four groups are selected. The first group aims to evaluate the effect of Mn, the second to account for the effect of Ni and Mn/Ni synergy, the third one to study the effect of P content and its synergy with Cu, and finally the fourth one to measure the effect of high bulk concentrations of Cu, P, Mn, Ni combined. These materials were irradiated at 290 °C inside the BR2 material test reactor (MTR) at high flux conditions (from 1 to 2.5 x 1017 n/m2s, E > 1 MeV) [6]. In order to evaluate the evolution of solute cluster characteristics with irradiation dose, three distinct irradiation doses of about 0.06, 0.08 and 0.13 dpa, representing typical exposures for nearly 25, 30 and 50 years of operation in commercial NPPs are studied. Microstructural features are characterised by Atom Probe Tomography (APT) using a CAMECA LEAP 4000X HR with electrical pulses (GPM Rouen). The magnitude of irradiation hardening was measured using tensile specimens tested at room temperature, in hot cells at SCK CEN. The irradiation hardening due to solute cluster formation is estimated with several hardening models. Predicted values are compared with the experimental data measured by tensile tests.
    The manuscript is composed of five chapters. In the first chapter, the literature review on the microstructure evolution and mechanical properties degradation of RPV steels under the effect of irradiation will be delivered. The irradiation related processes occurring at the atomic scale will be reported. The description of the radiation enhanced and radiation-induced mechanisms driving the microstructure evolution under irradiation will be given. The main metallurgical and irradiation parameters influencing the microstructure evolution will be listed. The correlation between microstructure evolution and detrimental irradiation hardening and embrittlement processes will be reported.
    In Chapter II, definitive information about composition, thermo-mechanical treatment, irradiation conditions and irradiation hardening, of chemically-tailored RPV steels selected to study the effect of bulk chemical composition on microstructure evolution and irradiation hardening will be presented. A comprehensive description of the APT technique, selected to study the irradiation-induced microstructure evolution on the nanoscale, its limitations and sample preparation procedure will also be described.
    In Chapter III, the results of the microstructure investigation on chemically-tailored RPV steels at the micro- and nano-scales will be delivered. The results of the chemical composition measurements and cluster identification procedures in both reference and irradiated states will be reported. For the neutron irradiated materials, the number density, size and chemical composition of solute clusters evaluated by APT will be reported.
    In the following Chapter IV, the discussion of the observed APT results will be delivered. Cross-comparison between chemically-tailored RPV steels to understand the effect of bulk solute concentration of Mn, Ni, Cu and P will be performed. The dose effect on the microstructure evolution will also be discussed. The more probable cluster / precipitates formation mechanisms will be proposed.
    Further, in Chapter V, the obtained microstructural data will be compared with the results of tensile tests, to evaluate the effect of bulk chemical composition on the irradiation hardening in CT RPV steels.
    Finally, the manuscript will be summarised by a general conclusion and the future perspective of this study.
    Original languageEnglish
    Awarding Institution
    • INSA Rouen - Normandie Université
    • Stergar, Erich, SCK CEN Mentor
    • Chaouadi, Rachid, SCK CEN Mentor
    • Pareige, Philippe, Supervisor, External person
    Date of Award2 Nov 2022
    StatePublished - 2 Nov 2022

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