# Technological Processes for Steel Applications in Nuclear Fusion
## 1 Introduction
In this area, at least three main damage modes have to be taken into account: (1) thermal fatigue due to the pulsed plasma operation with some ten-thousand cycles, (2) neutron irradiation hardening and embrittlement as a consequence of displacement damage, and (3) further material damage due to transmutation products, i.e., in steels, this mainly concerns the formation of helium bubbles. Hydrogen is also a transmutation product and may have an additional negative impact on the material properties. A relatively comprising set of irradiation data give a clear view on the displacement damage, which is mainly responsible for the observed irradiation hardening. The effect of helium transmutation in ferritic/martenstic 9 Cr-steels, such as EUROFER97, has not been explored yet, sufficiently. Nevertheless, available results indicate that such steels will reach the end of their lifetime when the formation and growth of helium bubbles come to a critical amount or size. At this point, the component will fail by brittle fracturing [9; 10; 11]. For future fusion power plants, first wall operating temperatures in the range from 450 \({}^{\circ}\)C to 550 \({}^{\circ}\)C or even higher, would be an effective measure to reduce irradiation hardening significantly (see, for example, [12; 13; 14; 15; 16; 17]). In this case, the lifetime of helium-cooled breeding blankets would then mainly be determined by the thermo-mechanical load response (thermal fatigue) and by the accumulation and growth of helium bubbles, but not by irradiation hardening and embrittlement. However, the possible effect of thermal aging, which might be enhanced by the neutron irradiation, has to be considered and investigated. Based on the following facts and assumptions, we have developed a possible strategy for the mitigation of the before-mentioned design limiting material damage scenarios and first wall operating regimes: (1) Neutron dose and related transmutation rates decrease significantly from the first wall towards the back plates, that is, only a plasma-near volume of the blanket receives a critical neutron dose [18,19]. (2) Nanostructured oxide dispersion strengthened (ODS) steels can have an enormous trapping effect on helium, which slows down the formation of helium bubbles beyond a critical size [20]. (3) Compared to conventional steels, ODS steels have a better fatigue and creep resistance. As a consequence, they can be operated at higher temperatures, which would substantially reduce irradiation hardening [21, 22, 23]. In addition, 9 Cr-ODS steels show higher uniform elongations under irradiation, which extends certain design ranges when applying codes such as RCC-MRx [17,24,25]. In summary, producing the plasma facing, highly neutron and heat loaded parts in blankets by an ODS steel in combination with EUROFER97 steel for the remaining structure, would allow a higher heat flux, a longer and safer operating period, and possibly more flexibility in the design. However, in the present study, we follow only a first step in this direction, which is not quite so ambitious, but which provides valuable information nevertheless. If we focus just on the superior thermo-mechanical properties, an ODS steel layer on top of the blanket first wall could, in principle, allow higher coolant temperatures and longer component operating-times. For this, an ODS steel plating of 2-3 mm on top of a EUROFER97 based cooling structure--the breeding blanket with cooling channels according to the current designs--would be required. A first verification of the suggested strategy took about 7 years, starting with the idea and ending after the final destructive post-high-heat-flux-test analyses, during which we followed three main lines of research, development, and investigation: (1) production of thin ODS steel plates on the lab-scale as well as industrial size in the order of 100 kg; (2) design and manufacture of a testable DEMO breeding blanket first wall mock-up; and (3) cyclic high heat flux loading in a helium cooling loop and subsequent analyses.