8.4 Recycling and Retention of Fuels
147
recovered T became 100 (=100−1−0.2 + 1.2) and the T fuel self-sufficiency was
attained. Otherwise, recovery of T retained in PFM must be done. In the present
tokamaks, the fuel retention rate is as larger as a few % or more. Owing to higher
temperature operation of a reactor, the fuel retention rate could be reduced. Still
some T loss in other T handling systems cannot be avoided. The fuel retention rate
of 0.2% is too large, while attaining 0.2% seems quite hard. Therefore, the recovery
of retained T in the reactor is necessary.
In Fig. 8.10, the same fueling rates are assumed for T and D. However, behaviors
of D and T would be significantly different from each other, not only in plasma but
also in materials, owing to their very large mass difference. The differences would
appear in confinement times in plasma, fueling efficiencies, escaping fluxes from
plasma, retention rates in materials, and evacuation rates from the reactor vessel. A
simple kinetic theory indicates that the square root of mass ratio of D/T could be
correlated to those properties as described in Chap. 2. Nevertheless, more or less no
data are available for D and T until now.
Furthermore, the different confinement times of D and T in burning plasma would
result in their inhomogeneous radial distribution. Therefore, it must not be easy to
keep appropriate D/T ratio in the burning plasma to get the highest burning efficiency.
The concentrations of D and T in the burning plasma must be separately measured
to make feedback fueling of D and T independently. It is, however, quite difficult to
measure the concentrations of D and T in plasma separately and a new task for ITER
[34].
The retention rates of D and T in PFM are not necessarily the same, so as their
release rates from the wall are. Behavior of fuels in PFM is separately described in
Chap. 5. Thus, we are going to face difficulty in controlling D/T ratio in the burning
plasma to attain and keep efficient burning. However, until now no systematic efforts
have been done to control two isotopes, even for H/D in plasma. All these problems
remain to be solved in ITER.
In the following, detailed fuel flows in a fusion reactor are described focusing
on T with special care on its radioactivity. Particularly, requirements of precise T
accountancy in all T handling systems and reducing T retention in PFM as small as
possible for the T fuel self-sufficiency in a fusion reactor are considered. Although
handling of D/T mixture might give additional difficulty in the fuel flow control, it
is not considered here.
8.4.2 Recycling
Fuel particles incident to PFS are either released back or retained in PFM. The
released ones consist of two components; one reflected keeping some of their incident energy and the other reemitted mostly as molecules nearly thermalized at the
temperature of PFS. Most of the reflected ones are directly return to plasma, while
among the reemitted ones only ionized ones in the boundary plasma return to plasma.
From plasma side, fuel particles returning to the plasma are referred to as recycled
147
recovered T became 100 (=100−1−0.2 + 1.2) and the T fuel self-sufficiency was
attained. Otherwise, recovery of T retained in PFM must be done. In the present
tokamaks, the fuel retention rate is as larger as a few % or more. Owing to higher
temperature operation of a reactor, the fuel retention rate could be reduced. Still
some T loss in other T handling systems cannot be avoided. The fuel retention rate
of 0.2% is too large, while attaining 0.2% seems quite hard. Therefore, the recovery
of retained T in the reactor is necessary.
In Fig. 8.10, the same fueling rates are assumed for T and D. However, behaviors
of D and T would be significantly different from each other, not only in plasma but
also in materials, owing to their very large mass difference. The differences would
appear in confinement times in plasma, fueling efficiencies, escaping fluxes from
plasma, retention rates in materials, and evacuation rates from the reactor vessel. A
simple kinetic theory indicates that the square root of mass ratio of D/T could be
correlated to those properties as described in Chap. 2. Nevertheless, more or less no
data are available for D and T until now.
Furthermore, the different confinement times of D and T in burning plasma would
result in their inhomogeneous radial distribution. Therefore, it must not be easy to
keep appropriate D/T ratio in the burning plasma to get the highest burning efficiency.
The concentrations of D and T in the burning plasma must be separately measured
to make feedback fueling of D and T independently. It is, however, quite difficult to
measure the concentrations of D and T in plasma separately and a new task for ITER
[34].
The retention rates of D and T in PFM are not necessarily the same, so as their
release rates from the wall are. Behavior of fuels in PFM is separately described in
Chap. 5. Thus, we are going to face difficulty in controlling D/T ratio in the burning
plasma to attain and keep efficient burning. However, until now no systematic efforts
have been done to control two isotopes, even for H/D in plasma. All these problems
remain to be solved in ITER.
In the following, detailed fuel flows in a fusion reactor are described focusing
on T with special care on its radioactivity. Particularly, requirements of precise T
accountancy in all T handling systems and reducing T retention in PFM as small as
possible for the T fuel self-sufficiency in a fusion reactor are considered. Although
handling of D/T mixture might give additional difficulty in the fuel flow control, it
is not considered here.
8.4.2 Recycling
Fuel particles incident to PFS are either released back or retained in PFM. The
released ones consist of two components; one reflected keeping some of their incident energy and the other reemitted mostly as molecules nearly thermalized at the
temperature of PFS. Most of the reflected ones are directly return to plasma, while
among the reemitted ones only ionized ones in the boundary plasma return to plasma.
From plasma side, fuel particles returning to the plasma are referred to as recycled