8.4 Recycling and Retention of Fuels
155
already discussed. In addition, there appears line of site deposition at remote areas
from plasma; tile gaps or tile sides and divertor opening for pumping. The deposition
in tile gaps is much larger for surface eroded tiles compared to that for surface
deposited tiles [21], which is the evidence of prompt deposition of eroded materials.
D is incorporated in the deposited layers. But this is not due to simultaneous
deposition (co-deposition) of eroded carbon with D. The concentration of D in the
deposited layers is varied afterwards. Generally, injecting D flux to PFS from plasma
is significantly higher than that of impurity carbon flux. Accordingly, most of injected
D is spontaneously reflected and/or reemitted and only part of the incoming D remains
in the deposited C layers with saturation concentration of 0.4 in D/C at maximum (see
Fig. 5.7) [21]. Except far-remote area from plasma, the deposited carbon layers are
subsequently exposed to boundary plasmas. And if the temperature of the redeposited
carbon layers is raised, part of the retained D is desorbed. In a reactor, incident fuel
flux to PFS is so large that the surface is immediately saturated with its concentration
determined by the PFS temperature. In addition, the retained fuels in the saturate
layers near surface are subjected to succeedingly injected fuels and part of them
are always replaced. This means the fuel concentration of surface saturated layers
was determined by the last exposure before the termination of discharges with little
influence of previous discharges.
The situation for the eroded region is the same, i.e. the surface is saturated with D
with the saturation concentration determined by the surface temperature but not the
injected flux ratio of D and C. In eroded area, because of the surface recession, the
thickness of the surface saturated layers would be limited only at near-surface regions
and would not linearly increase with time [21]. It also suggests that the isotopic
ratios of retained hydrogen near-surface layers are always equilibrated with those of
incoming hydrogen isotopes fluxes (H/D/T), when different hydrogen isotopes are
impinging. And the depth attaining this equilibrium is quite thick owing to the porous
nature of carbon materials and is increased by temperature rise. Hence, D retention in
plasma-facing surfaces (both eroded and redeposited) would be significantly reduced
by isotopic replacement by HH discharges subsequently made after DD discharges
in JT-60U as depicted in Fig. 8.12 [36].
In Table 8.1, D retention rates are added to the carbon deposition rates in current
tokamaks [19–22]. As seen in the table, the fuel retention rates of 3–50% are found
using gas balance measurements. A lower retention rate (~10%) is often obtained
using post-mortem analysis of plasma-facing tiles. Since carbon erosion and deposition rates are nearly the same for most tokamaks, the fuel retention rates are totally
dependent on temperatures of PFS or operation temperature owing to the temperature
dependence of the saturation concentration of D in the deposited layers. Accordingly,
JT-60U operated at the highest temperature (523–573 K) shows the lowest retention
rate. Sometimes, JT-60U showed wall saturation, i.e. 0% retention rate.
The operating temperature of ITER, around 423 K, is still lower than the reactor
operation temperature (above ~700 K) or even that of JT-60U (573 K). Accordingly, D
retention in its redeposited carbon layers would be significantly large. Other tokamaks
(except JET) operated at near RT show similar retention rates of 10–20%. The largest
155
already discussed. In addition, there appears line of site deposition at remote areas
from plasma; tile gaps or tile sides and divertor opening for pumping. The deposition
in tile gaps is much larger for surface eroded tiles compared to that for surface
deposited tiles [21], which is the evidence of prompt deposition of eroded materials.
D is incorporated in the deposited layers. But this is not due to simultaneous
deposition (co-deposition) of eroded carbon with D. The concentration of D in the
deposited layers is varied afterwards. Generally, injecting D flux to PFS from plasma
is significantly higher than that of impurity carbon flux. Accordingly, most of injected
D is spontaneously reflected and/or reemitted and only part of the incoming D remains
in the deposited C layers with saturation concentration of 0.4 in D/C at maximum (see
Fig. 5.7) [21]. Except far-remote area from plasma, the deposited carbon layers are
subsequently exposed to boundary plasmas. And if the temperature of the redeposited
carbon layers is raised, part of the retained D is desorbed. In a reactor, incident fuel
flux to PFS is so large that the surface is immediately saturated with its concentration
determined by the PFS temperature. In addition, the retained fuels in the saturate
layers near surface are subjected to succeedingly injected fuels and part of them
are always replaced. This means the fuel concentration of surface saturated layers
was determined by the last exposure before the termination of discharges with little
influence of previous discharges.
The situation for the eroded region is the same, i.e. the surface is saturated with D
with the saturation concentration determined by the surface temperature but not the
injected flux ratio of D and C. In eroded area, because of the surface recession, the
thickness of the surface saturated layers would be limited only at near-surface regions
and would not linearly increase with time [21]. It also suggests that the isotopic
ratios of retained hydrogen near-surface layers are always equilibrated with those of
incoming hydrogen isotopes fluxes (H/D/T), when different hydrogen isotopes are
impinging. And the depth attaining this equilibrium is quite thick owing to the porous
nature of carbon materials and is increased by temperature rise. Hence, D retention in
plasma-facing surfaces (both eroded and redeposited) would be significantly reduced
by isotopic replacement by HH discharges subsequently made after DD discharges
in JT-60U as depicted in Fig. 8.12 [36].
In Table 8.1, D retention rates are added to the carbon deposition rates in current
tokamaks [19–22]. As seen in the table, the fuel retention rates of 3–50% are found
using gas balance measurements. A lower retention rate (~10%) is often obtained
using post-mortem analysis of plasma-facing tiles. Since carbon erosion and deposition rates are nearly the same for most tokamaks, the fuel retention rates are totally
dependent on temperatures of PFS or operation temperature owing to the temperature
dependence of the saturation concentration of D in the deposited layers. Accordingly,
JT-60U operated at the highest temperature (523–573 K) shows the lowest retention
rate. Sometimes, JT-60U showed wall saturation, i.e. 0% retention rate.
The operating temperature of ITER, around 423 K, is still lower than the reactor
operation temperature (above ~700 K) or even that of JT-60U (573 K). Accordingly, D
retention in its redeposited carbon layers would be significantly large. Other tokamaks
(except JET) operated at near RT show similar retention rates of 10–20%. The largest