126
7 Fundamentals of Hydrogen Recycling
Furthermore, retention rates of D and T in PFM are not necessarily the same, so as
their release rates from PFM are. This means that if some local power load like ELM
hits the wall, thermally released D and T are different in their amounts and disturb
the D/T ratio of the plasma. Owing to the huge wall inventory, a small change in
its D/T ratio in PFM would cause significant change in the D/T ratio of the plasma.
Thus, we are going to face difficulty in controlling the D/T ratio in burning plasma.
Until now few systematic studies using intentionally mixed hydrogen (H and
D) plasma have been done. Only in JT-60U and JET, retentions of all hydrogen
isotopes of H, D, and T have been systematically studied and proved that all isotopes
do not necessarily behave similar and their behaviors reflect the history of plasma
operations. In D operating tokamaks, a few % of H always remained in their D
discharges after intense D discharges [19, 20]. For study of D plasmas, effects of the
few% H are negligible, but from T aspects it is unacceptable. In the initial phase of
ITER, discharges with intentional control of H/D ratios should be made.
7.7 Long-Term Retention and Trapping
In Sect. 7.4, H retention in PFM is discussed from the fuel recycling point of view.
Since injected H flux is so large, the retention rate relative to the injected flux would be
only a few % or less. In addition, most of retained H would be thermally released after
the termination of the injection. Therefore, the increasing rate of the static inventory
with discharge times would not large. Nevertheless, after long time exposure to
plasma, the static inventory would pile up to be far larger than H in whole plasma.
Long-term H retention concerns on density control of burning plasma and T safety.
A little temperature up of PFM would result in significant amount of T release and
make density control of burning plasma difficult, because the trapping decreases with
temperature, which is discussed more detail in Chap. 8.
Here considered is the T safety. Because of T regulation, any small amount of T
remaining in a system must be accounted. When T inventory in any system becomes
over the regulated amount, for example, in ITER it is 1 kg according to the French
regulation [21], the system is not allowed to continue the operation and forced to
remove T. The T inventory in a reactor, in general, corresponds to the static inventory
discussed in Sect. 7.4. In addition, the initial inventory which would be significantly
large should be taken into account. That is because that any materials contain H as an
impurity and surface absorbed H 2 O, which can be replaced by D and T injected by
exposure to burning plasma. For a simple estimation, assuming that 10 ppm of H is
included in stainless steel (SS) as a structure material, which is not the overestimation,
and they were isotopically exchanged by D and T, the D+T retention in a whole reactor
vessel consisting of 1000 tons of stainless steels could be easily over few kg [18].
This is significantly larger than the allowable T inventory in ITER. Of course, the
isotopic replacement must not be spontaneous, but would proceed very slowly. This
kind of inventory referred to as the initial inventory might not be distinguished from
7 Fundamentals of Hydrogen Recycling
Furthermore, retention rates of D and T in PFM are not necessarily the same, so as
their release rates from PFM are. This means that if some local power load like ELM
hits the wall, thermally released D and T are different in their amounts and disturb
the D/T ratio of the plasma. Owing to the huge wall inventory, a small change in
its D/T ratio in PFM would cause significant change in the D/T ratio of the plasma.
Thus, we are going to face difficulty in controlling the D/T ratio in burning plasma.
Until now few systematic studies using intentionally mixed hydrogen (H and
D) plasma have been done. Only in JT-60U and JET, retentions of all hydrogen
isotopes of H, D, and T have been systematically studied and proved that all isotopes
do not necessarily behave similar and their behaviors reflect the history of plasma
operations. In D operating tokamaks, a few % of H always remained in their D
discharges after intense D discharges [19, 20]. For study of D plasmas, effects of the
few% H are negligible, but from T aspects it is unacceptable. In the initial phase of
ITER, discharges with intentional control of H/D ratios should be made.
7.7 Long-Term Retention and Trapping
In Sect. 7.4, H retention in PFM is discussed from the fuel recycling point of view.
Since injected H flux is so large, the retention rate relative to the injected flux would be
only a few % or less. In addition, most of retained H would be thermally released after
the termination of the injection. Therefore, the increasing rate of the static inventory
with discharge times would not large. Nevertheless, after long time exposure to
plasma, the static inventory would pile up to be far larger than H in whole plasma.
Long-term H retention concerns on density control of burning plasma and T safety.
A little temperature up of PFM would result in significant amount of T release and
make density control of burning plasma difficult, because the trapping decreases with
temperature, which is discussed more detail in Chap. 8.
Here considered is the T safety. Because of T regulation, any small amount of T
remaining in a system must be accounted. When T inventory in any system becomes
over the regulated amount, for example, in ITER it is 1 kg according to the French
regulation [21], the system is not allowed to continue the operation and forced to
remove T. The T inventory in a reactor, in general, corresponds to the static inventory
discussed in Sect. 7.4. In addition, the initial inventory which would be significantly
large should be taken into account. That is because that any materials contain H as an
impurity and surface absorbed H 2 O, which can be replaced by D and T injected by
exposure to burning plasma. For a simple estimation, assuming that 10 ppm of H is
included in stainless steel (SS) as a structure material, which is not the overestimation,
and they were isotopically exchanged by D and T, the D+T retention in a whole reactor
vessel consisting of 1000 tons of stainless steels could be easily over few kg [18].
This is significantly larger than the allowable T inventory in ITER. Of course, the
isotopic replacement must not be spontaneous, but would proceed very slowly. This
kind of inventory referred to as the initial inventory might not be distinguished from