10.3 Use of Carbon Materials as PFM
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armor tiles by remote handling. The poorer heat transfer by the mechanical fixing
than that by the brazing must result in higher surface temperature. Moreover, poor
thermal conductivity of C at higher temperatures would enhance surface temperature
rise. One of the benefits of C is non-melting, if significant temperature rise of plasmafacing surface is allowed. In a reactor, periodical breaks for reactor maintenance, for
example, at every one or two years, are required and defective tiles can be replaced
during the maintenance. This also keeps the neutron irradiation level rather low.
One of the benefits of C is their small hydrogen diffusion in crystallites, which
inhibits hydrogen penetration into the crystallites, even though C is porous, resulting
quite small bulk retention. In addition, chemical erosion could be significantly
reduced, if the temperature of the C PFC was above ~ 800 K, near reactor operating temperature. Still radiation enhanced sublimation at higher temperatures is
concerned Less erosion would result in less redeposition. Moreover, the deposited
carbon layers at such high temperatures are rather graphitized and their T up-take
becomes significantly less. Consequently, hydrogen retention in C seems more easily
saturated compared to those for any metals in which no saturation seems to occur
owing to hydrogen diffusion in deep inside and even permeation.
It is well known that neutron irradiation degrades thermal conductivity of C and
gives dimensional change. However, their high-temperature usage would mitigate
the degradation. In particular, their usage as armor tiles allows temperature rise and
even the dimensional change. Furthermore, the temperature increase due to the loss
of thermal conductivity by neutron irradiation would reduce tritium retention and
anneal the damage.
10.3.2 Possible Use of C as PFM in a Reactor
As described above, C can be armor tiles of PFC in a fusion reactor with temperature
of cooling water or gas being above 700 K. Mechanical fixing of the C armor tiles
to heat sink or cooling channels is recommended allowing surface temperature rise
to reduce chemical erosion, T retention, and neutron irradiation effects. Nevertheless, the degradation of thermal conductivity and some dimensional change of C
by neutron irradiation are unavoidable, which requires replacement of the tiles. The
mechanical fixing ensures easy replacement of failed tiles by remote handling, and
periodic break for system maintenance gives opportunity for routine replacements of
the tiles. Still concerns remain; how to fix the carbon armor tiles to PFC mechanically,
tolerance of the mechanical fixing to surface temperature escalation due to its poor
thermal contact and any effects caused by irradiation. It should be mentioned that
less activation of C by neutron irradiation than any other candidate plasma-facing
metals makes system maintenance safer in aspect of radiation exposure of workers.
In order to reduce T inventory in a reactor, additionally proposed are routine
T removal by isotopic exchange with D discharges and installation of replaceable
cooled plates at divertor region to acquire carbon deposition and T included in.
191
armor tiles by remote handling. The poorer heat transfer by the mechanical fixing
than that by the brazing must result in higher surface temperature. Moreover, poor
thermal conductivity of C at higher temperatures would enhance surface temperature
rise. One of the benefits of C is non-melting, if significant temperature rise of plasmafacing surface is allowed. In a reactor, periodical breaks for reactor maintenance, for
example, at every one or two years, are required and defective tiles can be replaced
during the maintenance. This also keeps the neutron irradiation level rather low.
One of the benefits of C is their small hydrogen diffusion in crystallites, which
inhibits hydrogen penetration into the crystallites, even though C is porous, resulting
quite small bulk retention. In addition, chemical erosion could be significantly
reduced, if the temperature of the C PFC was above ~ 800 K, near reactor operating temperature. Still radiation enhanced sublimation at higher temperatures is
concerned Less erosion would result in less redeposition. Moreover, the deposited
carbon layers at such high temperatures are rather graphitized and their T up-take
becomes significantly less. Consequently, hydrogen retention in C seems more easily
saturated compared to those for any metals in which no saturation seems to occur
owing to hydrogen diffusion in deep inside and even permeation.
It is well known that neutron irradiation degrades thermal conductivity of C and
gives dimensional change. However, their high-temperature usage would mitigate
the degradation. In particular, their usage as armor tiles allows temperature rise and
even the dimensional change. Furthermore, the temperature increase due to the loss
of thermal conductivity by neutron irradiation would reduce tritium retention and
anneal the damage.
10.3.2 Possible Use of C as PFM in a Reactor
As described above, C can be armor tiles of PFC in a fusion reactor with temperature
of cooling water or gas being above 700 K. Mechanical fixing of the C armor tiles
to heat sink or cooling channels is recommended allowing surface temperature rise
to reduce chemical erosion, T retention, and neutron irradiation effects. Nevertheless, the degradation of thermal conductivity and some dimensional change of C
by neutron irradiation are unavoidable, which requires replacement of the tiles. The
mechanical fixing ensures easy replacement of failed tiles by remote handling, and
periodic break for system maintenance gives opportunity for routine replacements of
the tiles. Still concerns remain; how to fix the carbon armor tiles to PFC mechanically,
tolerance of the mechanical fixing to surface temperature escalation due to its poor
thermal contact and any effects caused by irradiation. It should be mentioned that
less activation of C by neutron irradiation than any other candidate plasma-facing
metals makes system maintenance safer in aspect of radiation exposure of workers.
In order to reduce T inventory in a reactor, additionally proposed are routine
T removal by isotopic exchange with D discharges and installation of replaceable
cooled plates at divertor region to acquire carbon deposition and T included in.