146
8 PMI in Large Tokamaks
8.4.1 Consideration of Fuel Retention Rate
Figure 8.10 shows simplified fuel flow based on Fig. 7.1 with normalized fuel
throughput of 200 fuel particles (100 (D) + 100 (T)) per unit time under steadystate burning [33]. In the figure, the fueling efficiency, the burning efficiency, the
fuel retention rate in a vacuum vessel (VV), and the T breeding ratio are set to be
0.2, 0.05, 0.02, and 1.2, respectively, as desirable values. For the total throughput
of 100/100 (D/T), only 20/20 are going into the plasma and 1/1 fuels are burned to
produce 1 neutron with energy of 14 MeV, and 1/1 (D/T) are retained in VV. Consequently, 98/98 (D/T) are exhausted. In present plasma apparatuses, light hydrogen
(H) is always remaining as an impurity as described in Sect. 8.4.2.2. Its origin is partly
residual H 2 O in VV and adsorbed one on materials surfaces. Another source is backfeed from vacuum pumps. Here is assumed around 2% always remains in fuels.
Whatever the sources are, additional effort to remove H in an isotope separation
process is required.
Neutrons produced by the DT reactions enter blanket systems to deposit their
energy to coolant for power generation and to breed T simultaneously. In Fig. 8.10,
under the assumption of the breeding rate of 1.2, 1.2 T atoms are bred from 1 neutron.
As a whole, 99.2 (=98 +1.2) T could be recovered for the refueling. Therefore, in
this simplified case, the T fuel self-sufficiency is not attained. D can be employed
from outside. Among all efficiencies in the figure, only the fuel retention rate can
be changed by engineering. If the fuel retention rate could be reduced one order
of magnitude to be 0.002 which is far less than the presently observed ones, the
Vacuum Vessel
D
100
T
100
1
Neutron
Blanket
RetenƟon rate
1
1
EvacuaƟon
98=79+19
D
98=79+19
T
20
20
H
4
4
H
Burning
Residual
gas
1
He
Ne /Ar
Cooling
Ne
Ar
T
1.2
Permeation & leakage << 10 -10
Refining
CH, Ne, Ar
Isotope
separation
T
98
D
98
Delivery
Storage
Loss in T
processing
systems
Delivery
Storage
D+T=He+n
1 1 1
1
Fig. 8.10 Fuel flow and balance in a reactor at steady state burning under assumptions for overall
burning efficiency: 1%, breeding ratio:1.2, fueling efficiency: 20%, and fuel retention rate: 1%. Fuel
throughput of D/T is normalized to be 100/100. H always remains as a residual gas. Ne and Ar are
seeded for edge cooling
8 PMI in Large Tokamaks
8.4.1 Consideration of Fuel Retention Rate
Figure 8.10 shows simplified fuel flow based on Fig. 7.1 with normalized fuel
throughput of 200 fuel particles (100 (D) + 100 (T)) per unit time under steadystate burning [33]. In the figure, the fueling efficiency, the burning efficiency, the
fuel retention rate in a vacuum vessel (VV), and the T breeding ratio are set to be
0.2, 0.05, 0.02, and 1.2, respectively, as desirable values. For the total throughput
of 100/100 (D/T), only 20/20 are going into the plasma and 1/1 fuels are burned to
produce 1 neutron with energy of 14 MeV, and 1/1 (D/T) are retained in VV. Consequently, 98/98 (D/T) are exhausted. In present plasma apparatuses, light hydrogen
(H) is always remaining as an impurity as described in Sect. 8.4.2.2. Its origin is partly
residual H 2 O in VV and adsorbed one on materials surfaces. Another source is backfeed from vacuum pumps. Here is assumed around 2% always remains in fuels.
Whatever the sources are, additional effort to remove H in an isotope separation
process is required.
Neutrons produced by the DT reactions enter blanket systems to deposit their
energy to coolant for power generation and to breed T simultaneously. In Fig. 8.10,
under the assumption of the breeding rate of 1.2, 1.2 T atoms are bred from 1 neutron.
As a whole, 99.2 (=98 +1.2) T could be recovered for the refueling. Therefore, in
this simplified case, the T fuel self-sufficiency is not attained. D can be employed
from outside. Among all efficiencies in the figure, only the fuel retention rate can
be changed by engineering. If the fuel retention rate could be reduced one order
of magnitude to be 0.002 which is far less than the presently observed ones, the
Vacuum Vessel
D
100
T
100
1
Neutron
Blanket
RetenƟon rate
1
1
EvacuaƟon
98=79+19
D
98=79+19
T
20
20
H
4
4
H
Burning
Residual
gas
1
He
Ne /Ar
Cooling
Ne
Ar
T
1.2
Permeation & leakage << 10 -10
Refining
CH, Ne, Ar
Isotope
separation
T
98
D
98
Delivery
Storage
Loss in T
processing
systems
Delivery
Storage
D+T=He+n
1 1 1
1
Fig. 8.10 Fuel flow and balance in a reactor at steady state burning under assumptions for overall
burning efficiency: 1%, breeding ratio:1.2, fueling efficiency: 20%, and fuel retention rate: 1%. Fuel
throughput of D/T is normalized to be 100/100. H always remains as a residual gas. Ne and Ar are
seeded for edge cooling