PhD Defense | Aidar Zakirov | Influence of the Bulk Chemical Composition on the Microstructure Evolution of Irradiated Chemically-tailored Nuclear RPV Steels

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

Lifetime of the nuclear power plant (NPP) is limited by the embrittlement of the reactor pressure vessel steel (RPV) steel. For the most of commercial RPV steels this process is directly related to the formation of high number density of nanosized solute clusters and precipitates. To ensure the safe operation conditions and to prolong the lifetime of current pressurised water reactor (PWR) fleet, the nature of these nanosized features should be understand. One of the most important parameters determining the microstructure evolution under irradiation is the bulk concentration of irradiation sensitive solutes such as Mn, Ni, Cu and P.

To fulfil the presented goal, a set of chemically-tailored (CT) steels with the tuned bulk solute contents were forged following the industrial recipes and neutron irradiated at 290 °C in high flux (~10¹⁷ n/m²s, E > 1 MeV) BR2 reactor up to the doses of 0.14 dpa. Further, irradiation hardening was measured by tensile tests at SCK CEN. To study the nanosized solute clusters the state of the art Atop Probe Tomography (APT) technique was utilised.

The results of this study show that:

• Solute clusters enriched in Mn, Ni, Si and to the less extent in Cu and P were observed in 11 out of 12 conditions. These clusters were formed inside the ferritic matrix, alongside dislocation lines and at low-angle grain boundaries (LAGBs);
• Addition of all solutes of interest results in the increase in solute cluster number density. The stronger effect was observed for Ni and Cu that of Mn and P. In addition, a positive synergy between Mn and Ni was evaluated;
• High Fe content inside the clusters was measured by APT. Cluster formation mechanism is rather radiation-induced segregation than radiation enhanced precipitation. Also, important role of P on early stages of solute clustering was evaluated;
• Measured by tensile testes irradiation hardening is well explained by the formation of solute clusters. Solute clusters are weak obstacles with an obstacle strength of around 0.14.

Promotor:

• Philippe Pareige (UROUEN)

• Pascale Jacques (UCLouvain)

SCK CEN mentors:

• Erich Stergar (SCK CEN)

• Rachid Chaouadi (SCK CEN)

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