182
9 Fuel Retention in a Rector with Full …
ablation; (3) chemical removal by reactions with reactive gases like H 2 O and O 2 or
their ions or plasma; and (4) isotopic exchange of T with D.
Although thermal desorption is one of the most reliable ways to remove T from
whole volume, required temperature for full removal is quite high. Because of thermal
stress caused by heating, simple heating of plasma-facing tiles in tokamak to very high
temperature is not realistic. Although laser irradiation works as surface heating for
thermal desorption of surface recession by ablation, its effects are limited near surface
layers [44, 45]. He grow discharges have been successfully applied in tokamaks for
cleaning of their PFS [46, 47]. Unfortunately, it would not work for removal of T in
deposited C layers on plasma shadowed area. Introduction of O 2 gas or O 2 discharge is
a good chemical way to remove surface layers including T making volatile molecules
of CO, CO 2 , and DTO [48]. Nevertheless, remaining O in tokamak and surface
oxidation of in-vessel components are concerned for the following discharges, i.e.
oxygen impurity in plasma could cause instability of plasma or even disruption.
Although isotope exchange of T with H 2 O or D 2 O is encouraging as observed in
JT-60U [49], resultant HTO and DTO are hazardous and recovery of T form them
requires large effort. Residual D 2 O is also concern for following discharges.
9.7.2 Removal/Recovery of T Retained in W
T removal from graphite is not easy, so as T removal from plasma-facing W. Most
of T in W are trapped with larger trapping energy. Therefore, high temperature is
required to thermally desorb T in W [20, 50]. Furthermore, depth distribution of T in
W would be significantly different from that in C. Because of faster T diffusion in W,
T can easily penetrate in deep and be trapped. This means only thermal desorption
would work as reliable method to remove T in plasma-facing W.
Efficiency of thermal desorption for tritium retention measurement and removal
in ITER was studied [32]. Although isotopic exchange with H or D would work, T in
deep inside is quite hard to remove by the isotope exchange [51]. Ogorodnikova, et al.
[52] have tried the removal of D trapped at radiation damages in tungsten by isotopic
exchange with H atomic beam. Although the efficiency of isotopic exchange increases
with increasing the sample temperature, incident ion/atomic flux, and incident H
energy, full removal was difficult. Roth et al. [51] have concluded that the isotopic
exchange is not a viable method to reduce the tritium inventory in ITER.
Laser ablation is tried to remove H implanted in W. Owing to heating the
surrounding area, some hydrogen is thermally released in addition to the ablation.
Hence, laser power near ablation threshold is recommended [53]. Deuterium desorption by laser heating was studied by Yu et al. [54], and comparison of laser desorption
with the TDS was made by Zlobinski et al. [55].
Finally, it should be mentioned that C would be easier to remove T than W.
Although small T retention has been one of the most important criteria for selection
of PFM, easy removal of retained T would be more important in the aspect of T
safety and fuel self-sufficiency. In both respects, C would be a better PFM candidate
than W [11, 20].
9 Fuel Retention in a Rector with Full …
ablation; (3) chemical removal by reactions with reactive gases like H 2 O and O 2 or
their ions or plasma; and (4) isotopic exchange of T with D.
Although thermal desorption is one of the most reliable ways to remove T from
whole volume, required temperature for full removal is quite high. Because of thermal
stress caused by heating, simple heating of plasma-facing tiles in tokamak to very high
temperature is not realistic. Although laser irradiation works as surface heating for
thermal desorption of surface recession by ablation, its effects are limited near surface
layers [44, 45]. He grow discharges have been successfully applied in tokamaks for
cleaning of their PFS [46, 47]. Unfortunately, it would not work for removal of T in
deposited C layers on plasma shadowed area. Introduction of O 2 gas or O 2 discharge is
a good chemical way to remove surface layers including T making volatile molecules
of CO, CO 2 , and DTO [48]. Nevertheless, remaining O in tokamak and surface
oxidation of in-vessel components are concerned for the following discharges, i.e.
oxygen impurity in plasma could cause instability of plasma or even disruption.
Although isotope exchange of T with H 2 O or D 2 O is encouraging as observed in
JT-60U [49], resultant HTO and DTO are hazardous and recovery of T form them
requires large effort. Residual D 2 O is also concern for following discharges.
9.7.2 Removal/Recovery of T Retained in W
T removal from graphite is not easy, so as T removal from plasma-facing W. Most
of T in W are trapped with larger trapping energy. Therefore, high temperature is
required to thermally desorb T in W [20, 50]. Furthermore, depth distribution of T in
W would be significantly different from that in C. Because of faster T diffusion in W,
T can easily penetrate in deep and be trapped. This means only thermal desorption
would work as reliable method to remove T in plasma-facing W.
Efficiency of thermal desorption for tritium retention measurement and removal
in ITER was studied [32]. Although isotopic exchange with H or D would work, T in
deep inside is quite hard to remove by the isotope exchange [51]. Ogorodnikova, et al.
[52] have tried the removal of D trapped at radiation damages in tungsten by isotopic
exchange with H atomic beam. Although the efficiency of isotopic exchange increases
with increasing the sample temperature, incident ion/atomic flux, and incident H
energy, full removal was difficult. Roth et al. [51] have concluded that the isotopic
exchange is not a viable method to reduce the tritium inventory in ITER.
Laser ablation is tried to remove H implanted in W. Owing to heating the
surrounding area, some hydrogen is thermally released in addition to the ablation.
Hence, laser power near ablation threshold is recommended [53]. Deuterium desorption by laser heating was studied by Yu et al. [54], and comparison of laser desorption
with the TDS was made by Zlobinski et al. [55].
Finally, it should be mentioned that C would be easier to remove T than W.
Although small T retention has been one of the most important criteria for selection
of PFM, easy removal of retained T would be more important in the aspect of T
safety and fuel self-sufficiency. In both respects, C would be a better PFM candidate
than W [11, 20].