Fabrication routes for advanced first wall design alternatives

In future nuclear fusion reactors, plasma facing components have to sustain specific neutron
damage. While the majority of irradiation data provides a relatively clear picture of the
displacement damage, the effect of helium transmutation is not yet explored in detail.
Nevertheless, available results from simulation experiments indicate that 9%-chromium steels
will reach their operating limit as soon as the growing helium bubbles extent a critical size. At
that point, the material would most probably fail due to grain boundary embrittlement. In this
contribution, 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 for a
much longer period. (3) Compared to conventional steels, ODS steels also provide improved
irradiation tensile ductility and creep strength. ... mehrTherefore, a design, based on the fabrication of
the plasma facing and highly neutron and heat loaded parts of blankets by an ODS steel, while
using EUROFER97 for everything else, would extend the operating time and enable a higher
heat flux. Consequently, we (i) developed and produced 14%Cr ferritic ODS steel plates and
(ii) optimized and demonstrated a scalable industrial production route. (iii) We fabricated a
mock-up with five cooling channels and a plated first wall of ODS steel, using the same
production processes as for a real component. (iv) Finally, we performed high heat flux tests in
the Helium Loop Karlsruhe, applying a few hundred short and a few 2 h long pulses, in which
the operating temperature limit for EUROFER97 (i.e. 550 ◦C) was finally exceeded by 100 K.
(v) 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. However, a solution to prohibit the formation of such heat affected zones is given. These research contributions demonstrate that the use of ODS steel is not only feasible and affordable but could make a decisive difference in the future design and performance of breeding blankets.