Increasing Irradiation and Thermo-Hydraulic Performance of Breeding Blankets by ODS Steel Plating

Structural components of future nuclear fusion reactors have to sustain specific neutron damage. While the majority of irradiation data gives a relatively clear view on the displacement damage, the effect of helium transmutation is not yet explored very well. Nevertheless, available results from simulation experiments indicate that 9%-chromium steels will reach their operating limit as soon as the formation of helium bubbles extents a critical size. At that point, the material would fail due to grain boundary embrittlement. Here we present a strategy for the mitigation of the before-mentioned problem using the following facts. (1) The neutron dose and related transmutation rate decreases quickly inside the first wall of the breeding blankets, that is, only a plasma-near area is extremely loaded. (2) Nanostructured oxide dispersion strengthened (ODS) steels may have an enormous trapping effect on helium, which would suppress the formation of large helium bubbles. (3) Compared to conventional steels, ODS steels show improved irradiation tensile ductility and creep strength. In summary, producing the plasma facing, highly neutron and heat loaded part of blankets by an ODS steel, while using EUROFER97 for everything else, would allow a higher heat flux as well as a longer operating period. Consequently, we (1) developed and produced 14% Cr ferritic ODS steel plates. (2) We fabricated a mock-up with 5 cooling channels and a plated first wall of ODS steel, using the same production processes as for a real component. (3) Finally, we performed high heat flux tests in the Helium Loop Karlsruhe at KIT, applying short and up to 2 h long pulses, in which the operating temperature limit for EUROFER97 (i.e., 550 °C) was finally exceeded by 100 K. Thereafter, microstructure and defect analyses did not reveal critical defects or recognizable damage. Only a heat affected zone in the EUROFER/ODS steel interface could be detected. This demonstrates that the use of ODS steel could make a decisive difference in the future design and performance of breeding blankets.