Ferritic/Martensitic steels, with Chromium contents ranging between 9 and 12%, were introduced into fusion material programmes about 30 years ago, when it became evident from research in fast reactor programmes that they possessed better swelling resistance and excellent thermal properties with respect to austenitic stainless steels.

In the course of the 80s, attention was redirected towards so-called “low activation” structural materials, which during irradiation would either not activate or give rise to induced radioactivity rapidly decaying to allow safe operation or at least hands-on reactor maintenance. However, truly low activation materials were not feasible and “reduced activation” material were proposed instead, whose chemical composition would carefully exclude all elements which could transmute by interaction with high-energy neutrons into long-life radioactive elements; low enough radioactivity levels should be achieved in about 100 years, as compared to several thousand for conventional structural steels.

EUROFER (or EUROFER97 as it is sometimes referred to, on account of the year when its chemical composition was defined) is a 9% Cr, 1% Mo steel. It represents the final developmental stage of low activation steels in Europe, which evolved through the MANET, OPTIMAX, BATMAN and OPTIFER series.

The material is used in the normalised and tempered condition and has a tempered martensitic microstructure which allows operation at relatively high temperatures (500 °C), offers good dimensional stability under high neutron doses and exhibits higher swelling resistance with respect to austenitic steels.

Goal of the work and methods The goal of the work is to understand the fundamentals for the design of a Eurofer grade with high creep resistance. Mechanical properties including creep, are strongly controlled by the presence of precipitates. It is therefore of great importance to understand the behaviour and characteristics of the precipitates in reduced activation steels. The precipitates of importance in Eurofer are Chromium rich iron carbides (M23C6), Vanadium carbonitrides and Tantalum carbides (MC). This study aims at the atomistic simulation to quantify essential properties like interfacial energy, solution enthalpy and diffusivity. These properties play a role in the formation of the precipitates but also in the evolution during prolonged exploitation at elevated temperature (creep). Such simulations can pave the way to identify the most promising alloying elements to design a fusion-compatible low-creep grade of Eurofer. The first important challenge is to construct a simulation protocol to predict interfacial energies, solution enthalpies and diffusion speeds in steel from first principles, and to assess the accuracy of these predictions. Such a well-tested protocol will then become the work horse in follow-up studies that search for the most promising (combinations of) alloying element(s).