172
9 Fuel Retention in a Rector with Full …
9.4.3.3 Bulk Retention
For carbon tiles, bulk retention does not likely change with fluence significantly.
Because the bulk retention is mostly caused by surface and near-surface region in
crystallites or filler particles (grains). Once tiles were exposed gaseous hydrogen,
most of their surface is saturated and penetration inside by diffusion is hardly possible.
Therefore, the contribution of the bulk retention dominates only earlier discharges.
The hydrogen concentration in the bulk of carbon tiles at the eroded divertor regions
and the first wall are around to be 2 × 10
−4 and 1 × 10
−5 , respectively, according to
the data obtained in JT-60U.
9.4.3.4 Direct Injection of Energetic Fuel Particles
Based on the result that 13% of T produced by the DD reactions was retained in
the outer first wall, the annual amount of T retention in the whole outer first wall
was estimated. On the assumption that 17.6 MeV of fusion energy produced by one
DT reaction are fully converted to useful power supply, about 20 kg of T and 13 kg
of D are required to be DT reaction annually in ITER. In ITER, the DD reactions
simultaneously occur with the DT reactions. At the plasma temperature of about 10
9
K, cross section of the DD reaction is around 10
−1 of that of the DT reaction. This
means that about 1.3 kg of D will be used for the DD reactions to produce about
1 kg of energetic T. Applying the value of 13%, about 130 g of T at maximum will
be retained annually in the whole outer first wall. Stronger magnetic field and less
ripple loss of ITER would significantly reduce the value. Still the T retention by the
direct implantation would be a few tens gram. Since the direct implantation piles
up and is hardly removed, this type of T retention contributes after several years of
operation.
9.4.3.5 Total Fuel Retention
Adding all the above-estimated fuel retention, the total fuel retention in ITER scale
full-C reactor operated above 573 K becomes as shown in Fig. 9.7b. In the figure, the
retention is normalized with 100 m
2 for easy comparison. For a 3MWth reactor, the
total retention would be 3 times larger. After long time operation, the fuel retention
in the deposits dominate, and the total fuel retention build-ups were estimated to be
~5 × 10
20 s
−1 which is mostly owing to the retention in deposited layers on plasma
shadowed area. As seen in the figure, the total inventory for full carbon stays below
the safety limit of 700 g T for more than 1000 discharges and are smaller by a factor
of ~10 than those for the full C-wall ITER’s estimated by Roth et al. [2] in Fig. 9.1a.
This is simply because the temperature of PFM in the reactor is around 150 K higher
than ITER. In Roth’s estimation, the saturation concentration of (D + T)/C = 0.4 in
C deposited layers was employed, because of lower temperature operation of ITER.
But in the estimation based on JT-60U, the saturation concentration in deposited C
9 Fuel Retention in a Rector with Full …
9.4.3.3 Bulk Retention
For carbon tiles, bulk retention does not likely change with fluence significantly.
Because the bulk retention is mostly caused by surface and near-surface region in
crystallites or filler particles (grains). Once tiles were exposed gaseous hydrogen,
most of their surface is saturated and penetration inside by diffusion is hardly possible.
Therefore, the contribution of the bulk retention dominates only earlier discharges.
The hydrogen concentration in the bulk of carbon tiles at the eroded divertor regions
and the first wall are around to be 2 × 10
−4 and 1 × 10
−5 , respectively, according to
the data obtained in JT-60U.
9.4.3.4 Direct Injection of Energetic Fuel Particles
Based on the result that 13% of T produced by the DD reactions was retained in
the outer first wall, the annual amount of T retention in the whole outer first wall
was estimated. On the assumption that 17.6 MeV of fusion energy produced by one
DT reaction are fully converted to useful power supply, about 20 kg of T and 13 kg
of D are required to be DT reaction annually in ITER. In ITER, the DD reactions
simultaneously occur with the DT reactions. At the plasma temperature of about 10
9
K, cross section of the DD reaction is around 10
−1 of that of the DT reaction. This
means that about 1.3 kg of D will be used for the DD reactions to produce about
1 kg of energetic T. Applying the value of 13%, about 130 g of T at maximum will
be retained annually in the whole outer first wall. Stronger magnetic field and less
ripple loss of ITER would significantly reduce the value. Still the T retention by the
direct implantation would be a few tens gram. Since the direct implantation piles
up and is hardly removed, this type of T retention contributes after several years of
operation.
9.4.3.5 Total Fuel Retention
Adding all the above-estimated fuel retention, the total fuel retention in ITER scale
full-C reactor operated above 573 K becomes as shown in Fig. 9.7b. In the figure, the
retention is normalized with 100 m
2 for easy comparison. For a 3MWth reactor, the
total retention would be 3 times larger. After long time operation, the fuel retention
in the deposits dominate, and the total fuel retention build-ups were estimated to be
~5 × 10
20 s
−1 which is mostly owing to the retention in deposited layers on plasma
shadowed area. As seen in the figure, the total inventory for full carbon stays below
the safety limit of 700 g T for more than 1000 discharges and are smaller by a factor
of ~10 than those for the full C-wall ITER’s estimated by Roth et al. [2] in Fig. 9.1a.
This is simply because the temperature of PFM in the reactor is around 150 K higher
than ITER. In Roth’s estimation, the saturation concentration of (D + T)/C = 0.4 in
C deposited layers was employed, because of lower temperature operation of ITER.
But in the estimation based on JT-60U, the saturation concentration in deposited C