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Radiation effects

The plasma facing components and structural materials in future fusion reactors will be exposed to intense fluxes of 14 MeV neutrons due to D-T fusion reactions, which results in swelling, microstructural modifications, and transmutation effects. Radiation resistance and low activation materials have to be developed and evaluated under reactor relevant neutron fluxes to minimise these effects.

Neutron irradiation studies are nowadays performed in material test reactors (e.g. in the HFR-Petten or the BR2 reactor in SCK•CEN) due to the lack of powerful 14 MeV neutron sources. Here, neutron fluxes up to approximately 1 dpa which simulate the conditions in the next step fusion experiment ITER are applied to test samples and wall components. An extensive post irradiation examination program is set up to quantify the neutron induced material degradation and changes in high heat flux performance of irradiated plasma facing materials and components.

Figure 1 shows the result of thermal shock tests performed on neutron irradiated beryllium samples in the JUDITH 1 facility. A large network of random cracks was found for all samples with a power density of 0.55 GW/m2 (fig. 1(a) and (c)). The same cracking behaviour could be found for the 0.66 GW/m2 loaded samples but with an increase in crack width and density (fig. 1(b) and (d)).

leeres Bild

Surface morphology of neutron irradiated berylliumSurface morphology of neutron irradiated beryllium after thermal shock testing. A maximum neutron fluence of 5.20×1020 n/cm² or a 0.78 dpa (steel) value was obtained. The thermal shock loading was performed with an absorbed power density of (a) 0.55 GW/m² and (b) 0.66 GW/m² after 1 pulse with a 5 ms pulse length on a loaded area of 4 mm × 4 mm. The SEM images (c) and (d) show a detail from the crack width from (a) and (b), respectively.