180
9 Fuel Retention in a Rector with Full …
However, the fuel retention rate is not necessarily constant with fluence and could
change from nearly 100% (except direct reflection) to 0% and the fuel retention rate
should be quite small if PFM was saturated with the fuel, i.e. no more retention
in the saturated wall. The initial fuel retention rate is very large because PFM is
empty and most of injected fuels are retained, while for saturated PFM with fuels
most of injected fuels are reemitted. Because of higher temperature operation of
a reactor, the saturation concentration is much less than that observed in most of
present tokamaks operated at ambient temperature without cooling. Consequently,
the surface saturation will be attained at less fluence. Actually, the surface saturation
appeared in JT-60U resulting in loss of wall pumping as shown in Table 8.1.
Thus, the fuel retention rate changes with fluence as appeared in its fluence dependence. Only when it shows linear flux dependence, the fuel retention rate stays
constant, otherwise it changes with fluence. The linear fluence dependence appears
for the fuel retention in deposited layers at plasma shadowed area for the C-wall,
because of no erosion. When the fuel retention is caused by penetration (diffusion) into inner side, the flux dependence deviates from the liner one, depending on
penetration mechanism it would change.
In the fluence dependence of the fuel retention given in Fig. 9.7, C and W clearly
show difference. For W, the retention in deposited layers is not included, while the
retention in carbon wall is dominated in the deposited layers. Accordingly, the fluence
dependence for W is between φ
0.5 and φ
1.0 and that for the carbon wall nearly φ
1.0
for higher fluence. Although the fluence for W stays still lower level compared to
the carbon wall, the effect of its surface saturation appears as downward deviation
from φ
1 at higher fluence. In a reactor with the full W-wall, the fuel retention in
the deposited layers would be less than that in the bulk. Therefore, to keep the
fuel retention rate to be less than 0.1% seems possible in both C-wall and W wall.
However, it should be mentioned that neutron irradiation could give the upward
deviation due to H trapping at defects produced by the irradiation.
Thus, the fuel retention rate could be reduced to be less than 0.1% at the steadystate operation. However, integrated fuel retention continuously increases. Owing to
T regulation low, the maximum allowable T inventory in a rector will be set (in ITER
it is 1 kg), which requires occasional T removal from PFM in addition to necessary
T recovery to attain fuel self-sufficiency.
9.7 Fuel Removal/Recovery
Recovery of T retained in PFM is directly connected to T safety and T regulation.
Furthermore, for establishment of fuel self-sufficiency, the recovery should be as
much as possible. Although variety of techniques have been developed for T removal,
they are mainly for detritiation of contaminated surfaces [31]. However, considering
reuse or recycling of T in a fusion reactor, techniques would be limited, i.e. recovered
T should not be diluted by H or other gases.
9 Fuel Retention in a Rector with Full …
However, the fuel retention rate is not necessarily constant with fluence and could
change from nearly 100% (except direct reflection) to 0% and the fuel retention rate
should be quite small if PFM was saturated with the fuel, i.e. no more retention
in the saturated wall. The initial fuel retention rate is very large because PFM is
empty and most of injected fuels are retained, while for saturated PFM with fuels
most of injected fuels are reemitted. Because of higher temperature operation of
a reactor, the saturation concentration is much less than that observed in most of
present tokamaks operated at ambient temperature without cooling. Consequently,
the surface saturation will be attained at less fluence. Actually, the surface saturation
appeared in JT-60U resulting in loss of wall pumping as shown in Table 8.1.
Thus, the fuel retention rate changes with fluence as appeared in its fluence dependence. Only when it shows linear flux dependence, the fuel retention rate stays
constant, otherwise it changes with fluence. The linear fluence dependence appears
for the fuel retention in deposited layers at plasma shadowed area for the C-wall,
because of no erosion. When the fuel retention is caused by penetration (diffusion) into inner side, the flux dependence deviates from the liner one, depending on
penetration mechanism it would change.
In the fluence dependence of the fuel retention given in Fig. 9.7, C and W clearly
show difference. For W, the retention in deposited layers is not included, while the
retention in carbon wall is dominated in the deposited layers. Accordingly, the fluence
dependence for W is between φ
0.5 and φ
1.0 and that for the carbon wall nearly φ
1.0
for higher fluence. Although the fluence for W stays still lower level compared to
the carbon wall, the effect of its surface saturation appears as downward deviation
from φ
1 at higher fluence. In a reactor with the full W-wall, the fuel retention in
the deposited layers would be less than that in the bulk. Therefore, to keep the
fuel retention rate to be less than 0.1% seems possible in both C-wall and W wall.
However, it should be mentioned that neutron irradiation could give the upward
deviation due to H trapping at defects produced by the irradiation.
Thus, the fuel retention rate could be reduced to be less than 0.1% at the steadystate operation. However, integrated fuel retention continuously increases. Owing to
T regulation low, the maximum allowable T inventory in a rector will be set (in ITER
it is 1 kg), which requires occasional T removal from PFM in addition to necessary
T recovery to attain fuel self-sufficiency.
9.7 Fuel Removal/Recovery
Recovery of T retained in PFM is directly connected to T safety and T regulation.
Furthermore, for establishment of fuel self-sufficiency, the recovery should be as
much as possible. Although variety of techniques have been developed for T removal,
they are mainly for detritiation of contaminated surfaces [31]. However, considering
reuse or recycling of T in a fusion reactor, techniques would be limited, i.e. recovered
T should not be diluted by H or other gases.