196
10 Selection of Plasma-Facing Materials
through the cracking from W to cooling tubes. Hence, we should keep C materials
as an alternative for PFM in a reactor or even in ITER [11, 12].
According to the present estimation of the in-vessel T inventory, it seems impossible to keep it below 1 kg, ITER safety limit, after certain period of operation. Hence,
periodic removal or reduction of the in-vessel T inventory will be indispensable. This
means that the total T retention might not be a good figure of merit for the selection
of PFM in a reactor. Instead, readiness to remove T once retained should be taken
into account. Carbon, at ITER operation temperature, would retain larger amount of
T compared with Be and W, while T retention is limited in surface and subsurface
regions including deposited layers and inner surfaces facing open pores, and in-grain
or bulk retention is quite low. Accordingly, T retained in C is rather easily replaced
by hydrogen isotopes subsequently impinging by isotopic exchange. Even T in the
deposited layers could be replaced isotopically, if their temperature was above 800 K.
In the aspect of T management, C if used above 800 K (in a reactor) seems better
than W. Heavy neutron activation of W would result in much dangerous dust than
carbon. Dimensional change of carbon by neutron irradiation was claimed, but intentional and periodic replacement in the occasion of reactor opening for maintenance
would relieve the problem. As such, figure of merit for the selection of PFM of
DEMO reactor might be reconsidered [11].
10.6 Summary
In a fusion reactor, power load (given by radiation and particle fluxes) to PFS from
burning plasma must be extraordinarily high such like the surface of a rocket running
in the sun is exposed to. For any materials, there is the maximum tolerable power
load and the reactor should be designed not to give higher power load than the
maximum. Until now, however, such high-power load to large area like divertor has
been hardly realized to test the PFM candidate materials. Therefore, extrapolation
of current experience and observations in PSI to reactor condition is indispensable.
Nevertheless, expected particle flux to the wall of 10
24 m
−2 is too large to make
reliable extrapolation. Recently, linear plasma machines giving extremely high power
are available and various heat load tests are under way for the development of the
ITER divertor. However, details of power load given to the divertor area are still
uncertain, i.e. energy spectra of incident ions, electrons, and photons are still quite
uncertain. One example is vapor shielding. At present, only ITER can be used as
a testbed for PMI in a rector. Hence, it is desired to have a chance to change PFM
materials from W to other candidate materials, in particular, carbon-based materials
even in ITER.
Still operational conditions of ITER are somewhat different from that of a reactor.
Simultaneous establishments of production of electricity, T breeding, and keeping
nuclear safety under economically acceptable efficiency should be attained. Owing
to shortage in T resource, satisfying T fuel self-sufficiency and keeping T safety
following T regulation law are must and require more details in PMI. In addition,
10 Selection of Plasma-Facing Materials
through the cracking from W to cooling tubes. Hence, we should keep C materials
as an alternative for PFM in a reactor or even in ITER [11, 12].
According to the present estimation of the in-vessel T inventory, it seems impossible to keep it below 1 kg, ITER safety limit, after certain period of operation. Hence,
periodic removal or reduction of the in-vessel T inventory will be indispensable. This
means that the total T retention might not be a good figure of merit for the selection
of PFM in a reactor. Instead, readiness to remove T once retained should be taken
into account. Carbon, at ITER operation temperature, would retain larger amount of
T compared with Be and W, while T retention is limited in surface and subsurface
regions including deposited layers and inner surfaces facing open pores, and in-grain
or bulk retention is quite low. Accordingly, T retained in C is rather easily replaced
by hydrogen isotopes subsequently impinging by isotopic exchange. Even T in the
deposited layers could be replaced isotopically, if their temperature was above 800 K.
In the aspect of T management, C if used above 800 K (in a reactor) seems better
than W. Heavy neutron activation of W would result in much dangerous dust than
carbon. Dimensional change of carbon by neutron irradiation was claimed, but intentional and periodic replacement in the occasion of reactor opening for maintenance
would relieve the problem. As such, figure of merit for the selection of PFM of
DEMO reactor might be reconsidered [11].
10.6 Summary
In a fusion reactor, power load (given by radiation and particle fluxes) to PFS from
burning plasma must be extraordinarily high such like the surface of a rocket running
in the sun is exposed to. For any materials, there is the maximum tolerable power
load and the reactor should be designed not to give higher power load than the
maximum. Until now, however, such high-power load to large area like divertor has
been hardly realized to test the PFM candidate materials. Therefore, extrapolation
of current experience and observations in PSI to reactor condition is indispensable.
Nevertheless, expected particle flux to the wall of 10
24 m
−2 is too large to make
reliable extrapolation. Recently, linear plasma machines giving extremely high power
are available and various heat load tests are under way for the development of the
ITER divertor. However, details of power load given to the divertor area are still
uncertain, i.e. energy spectra of incident ions, electrons, and photons are still quite
uncertain. One example is vapor shielding. At present, only ITER can be used as
a testbed for PMI in a rector. Hence, it is desired to have a chance to change PFM
materials from W to other candidate materials, in particular, carbon-based materials
even in ITER.
Still operational conditions of ITER are somewhat different from that of a reactor.
Simultaneous establishments of production of electricity, T breeding, and keeping
nuclear safety under economically acceptable efficiency should be attained. Owing
to shortage in T resource, satisfying T fuel self-sufficiency and keeping T safety
following T regulation law are must and require more details in PMI. In addition,