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10 Selection of Plasma-Facing Materials
A concern on W from the PMI aspect is its accumulation in plasma center resulting
in huge radiation loss which could cause plasma claps. In recent experience with
using W as PFM in ASDEX, EEST, and JET, the central accumulation could be
avoided by appropriate plasma control. However, the power load was kept rather low
to avoid material damage. Accordingly, the material’s response of W to the highpower load in burning plasma remains quite uncertain. Unclear direct responses to the
high-power load are sublimation, melting, movements of melt layer and droplets, resolidification, and recrystallization. Reponses to repetitive power loads are various.
They are loss of ductility due to grain growth or recrystallization, surface modification by He injection like blistering, swelling and fatigue, erosion by melting,
and particle release by cracking of recrystallized grains. Neutron irradiation gives
radiation damages, activation and nuclear transformation to produce Re and Os of
which alloying in W enhances brittleness. Together with cyclic heat load swelling and
fatigue limit the lifetime of PFM W. High chemical activity with hot water produces
volatile molecules of WO 3 in LOCA which is very dangerous owing to high activation of W by neutron irradiation. These characters do not allow W to use as heat sink,
or direct contact to water. Radioactivity of released droplets produced by melting
and cracking peace from W surface requires particular care on their safe handling in
addition to their T retention. High D and T retention could cause hydrogen embrittlement, in particular, their trapping at grain boundaries is concerned but not studied
yet.
Generally, W used as armor tiles should be welded or brazed to heat sink or cooling
channel materials, which is not easy because of large difference in thermal expansion
coefficients of W and the heat sink materials. It should also be mentioned that W is
heavy, nearly ten times compared to C materials and is very hard to machine. This
clearly appears as difficulty of manufacturing of the ITER divertor. Thus, significant
efforts are still required to design and manufacture W divertor mainly in materials
aspect.
10.3 Use of Carbon Materials as PFM
10.3.1 Character of C as PFM
Carbon materials (C) can be very good armor tiles of PFW, if they are used at higher
temperatures. Considering the temperature of cooling water for power generation in
DEMO [2], around 700 K or higher, the temperature of heat sink is likely above 800 K.
Because of their porous nature, the C armor tiles must be adhered to heat sink metals
having cooling channels or connected to cooling systems. For adhesion, both brazing
and mechanical fixing would be possible, while welding is not available because of
carbide formation or large difference in thermal expansion between C and the heat
sink metals resulting easy rupture of the bonding. Although the brazing would be
better for heat removal, mechanical fixing is recommended for easy exchange of the
10 Selection of Plasma-Facing Materials
A concern on W from the PMI aspect is its accumulation in plasma center resulting
in huge radiation loss which could cause plasma claps. In recent experience with
using W as PFM in ASDEX, EEST, and JET, the central accumulation could be
avoided by appropriate plasma control. However, the power load was kept rather low
to avoid material damage. Accordingly, the material’s response of W to the highpower load in burning plasma remains quite uncertain. Unclear direct responses to the
high-power load are sublimation, melting, movements of melt layer and droplets, resolidification, and recrystallization. Reponses to repetitive power loads are various.
They are loss of ductility due to grain growth or recrystallization, surface modification by He injection like blistering, swelling and fatigue, erosion by melting,
and particle release by cracking of recrystallized grains. Neutron irradiation gives
radiation damages, activation and nuclear transformation to produce Re and Os of
which alloying in W enhances brittleness. Together with cyclic heat load swelling and
fatigue limit the lifetime of PFM W. High chemical activity with hot water produces
volatile molecules of WO 3 in LOCA which is very dangerous owing to high activation of W by neutron irradiation. These characters do not allow W to use as heat sink,
or direct contact to water. Radioactivity of released droplets produced by melting
and cracking peace from W surface requires particular care on their safe handling in
addition to their T retention. High D and T retention could cause hydrogen embrittlement, in particular, their trapping at grain boundaries is concerned but not studied
yet.
Generally, W used as armor tiles should be welded or brazed to heat sink or cooling
channel materials, which is not easy because of large difference in thermal expansion
coefficients of W and the heat sink materials. It should also be mentioned that W is
heavy, nearly ten times compared to C materials and is very hard to machine. This
clearly appears as difficulty of manufacturing of the ITER divertor. Thus, significant
efforts are still required to design and manufacture W divertor mainly in materials
aspect.
10.3 Use of Carbon Materials as PFM
10.3.1 Character of C as PFM
Carbon materials (C) can be very good armor tiles of PFW, if they are used at higher
temperatures. Considering the temperature of cooling water for power generation in
DEMO [2], around 700 K or higher, the temperature of heat sink is likely above 800 K.
Because of their porous nature, the C armor tiles must be adhered to heat sink metals
having cooling channels or connected to cooling systems. For adhesion, both brazing
and mechanical fixing would be possible, while welding is not available because of
carbide formation or large difference in thermal expansion between C and the heat
sink metals resulting easy rupture of the bonding. Although the brazing would be
better for heat removal, mechanical fixing is recommended for easy exchange of the