Fig. 9-6 Divertor in ITER Plasma facing materials cover the plasma facing surface of the divertor.

Fig. 9-7 Illustration of thermal shock test by using an electron beam An incident electron beam deposits heat in the plasma-facing material. Specimens are irradiated by neutron in the reactor before the thermal shock test.

(a) Relationships between dpa and weight loss (b) Observation of eroded surface

Fig. 9-8 Experimental results A weight loss of CFC increases with increasing dpa. An eroded area on CFC also increases with increasing dpa.

Plasma facing components in a fusion device are irradiated with neutrons and the other particles under a high heat loading. Carbon-fiber reinforced composite (CFC) is one of the candidate materials in plasma facing components; however, the thermal conductivity of material may decrease under neutron exposure. If CFC with a degraded thermal conductivity is heated extensively, it may lose its weight by erosion. It has been unclear how the neutron exposure under high heat loading will damage CFC. Thermal shock tests were carried out for the neutron-irradiated material in order to investigate the effects of thermal conductivity degradation. The materials were irradiated to 0.4 dpa (displacement per atom) in temperature range of 556 - 569 K at JMTR. Then the materials were thermally shocked with a high heat flux of 500 MW/m2 during 40 ms. Two types of CFC were used: one is called 1D (one-dimensional) where all fibers are arranged in the same direction and the other one is called 2D (two-dimensional) where fibers are piled up in layers of randomly arranged-fiber planes. Figure 9-8 shows the results of weight loss as a function of dpa and surface observation. Weight loss and eroded area of specimens increase with increasing dpa. This result made clear the damage of CFC materials after neutron irradiation for the first time.

Reference M. Uda et al., Disruption and Erosion on Plasma Facing Materials with Oarai Hot-Cell Electron Beam Irradiating System (OHBIS), Proc. 20th Symp. On Fusion Technology, Sep. 7-11, 1998, Marseille, France 161 (1998).

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