38
3 Power Load on Plasma-Facing Materials
3.2 Estimation of Power Load and Its Distribution
in a Fusion Reactor
A very simple calculation gives power load to plasma-facing surface (PFS) in a
fusion reactor with a thermal output power of 3 GWth or electrical output of 1 GWe
in Table 3.1. In the table, the reactor is assumed to be spherical for simplicity with its
radius of 3 m or 5 m of which volume and surface area of the reactor are 113 m
2 and
113 m
3 , and 314 m
2 and 523 m
3 , respectively. Accordingly, averaged power flux and
produced energy density are 26.5 MW m
−2 and 26.5 MW m
−2 , and 5.7 MW m
−2
and 9.5 MW m
−2 , respectively. Nearly 2/3 of the output power is carried by neutron
and is deposited in a blanket, while 1/3 (8.8 MW m
−2 or 3.2 MW m
−2 ) is carried
by energetic ions and neutral, electrons, and photons and deposited to PFS. Since
any materials have their maximum tolerable power load to avoid melting damage,
around 10–20 MW m
−2 under intensive cooling, the power load is one of the limiting
factors on designing the reactor size. The larger is better to reduce the power load,
while the smaller is better for energy density increasing energy efficiency. This limits
the minimum size of a reactor. For example, it is unrealistic to build a power reactor
with 3 GWth having a diameter of less than 2.5 m.
In Fig. 3.1, power loads to PFS and blanket are specified for a tokamak reactor
having a divertor structure with the output power of 3 GWth under steady-state
operation. Nearly 80% of the fusion power, 2.4 GW, is carried by 14 MeV neutrons
and deposited in the blanket to be converted to heat or electric power and to breed
tritium. The remaining 20%, 600 MW, is carried by energetic ions and neutral,
electrons, and photons. In addition, to sustain the burning plasma, external heating
of around 50 MW is put in. Hence, the power of 650 MW should be exhausted
as radiations and particle energies to PFS. Around 500 MW is carried by He ions
produced by D-T reaction and used to sustain high temperature burning plasma,
which turns to the core radiation mostly by Bremsstrahlung and some by impurity
radiation as seen in Fig. 1.4. The core radiation is rather homogeneously loaded to
PFS resulting in the power load of 0.5 MW m
−2 on average. The remaining 150 MW
are exhausted as radiation and particle energies, a large part of which are loaded to
the divertor region. Consequently, the power loads to the first wall and the divertor
Table 3.1 Simple estimation of power load and energy density for spherical reactors with thermal
output of 3 GWth with radius of 3 or 5 m
Radius (r) m
Surface area
(4πr 2) m 2
Power flux
MW m −2
Volume (4πr 3 /3) m 3 Energy density
MW m −3
3
113
26.5
113
26.5
5
314
9.5
523
5.7
2/3 of output power (6-16 MW m −2 ) is carried by 14 MeV neutrons and deposited in a blanket having
large volume, and 1/3 by particles and radiation (3–10 MW m −2 ) and deposited to plasma-facing
surface
3 Power Load on Plasma-Facing Materials
3.2 Estimation of Power Load and Its Distribution
in a Fusion Reactor
A very simple calculation gives power load to plasma-facing surface (PFS) in a
fusion reactor with a thermal output power of 3 GWth or electrical output of 1 GWe
in Table 3.1. In the table, the reactor is assumed to be spherical for simplicity with its
radius of 3 m or 5 m of which volume and surface area of the reactor are 113 m
2 and
113 m
3 , and 314 m
2 and 523 m
3 , respectively. Accordingly, averaged power flux and
produced energy density are 26.5 MW m
−2 and 26.5 MW m
−2 , and 5.7 MW m
−2
and 9.5 MW m
−2 , respectively. Nearly 2/3 of the output power is carried by neutron
and is deposited in a blanket, while 1/3 (8.8 MW m
−2 or 3.2 MW m
−2 ) is carried
by energetic ions and neutral, electrons, and photons and deposited to PFS. Since
any materials have their maximum tolerable power load to avoid melting damage,
around 10–20 MW m
−2 under intensive cooling, the power load is one of the limiting
factors on designing the reactor size. The larger is better to reduce the power load,
while the smaller is better for energy density increasing energy efficiency. This limits
the minimum size of a reactor. For example, it is unrealistic to build a power reactor
with 3 GWth having a diameter of less than 2.5 m.
In Fig. 3.1, power loads to PFS and blanket are specified for a tokamak reactor
having a divertor structure with the output power of 3 GWth under steady-state
operation. Nearly 80% of the fusion power, 2.4 GW, is carried by 14 MeV neutrons
and deposited in the blanket to be converted to heat or electric power and to breed
tritium. The remaining 20%, 600 MW, is carried by energetic ions and neutral,
electrons, and photons. In addition, to sustain the burning plasma, external heating
of around 50 MW is put in. Hence, the power of 650 MW should be exhausted
as radiations and particle energies to PFS. Around 500 MW is carried by He ions
produced by D-T reaction and used to sustain high temperature burning plasma,
which turns to the core radiation mostly by Bremsstrahlung and some by impurity
radiation as seen in Fig. 1.4. The core radiation is rather homogeneously loaded to
PFS resulting in the power load of 0.5 MW m
−2 on average. The remaining 150 MW
are exhausted as radiation and particle energies, a large part of which are loaded to
the divertor region. Consequently, the power loads to the first wall and the divertor
Table 3.1 Simple estimation of power load and energy density for spherical reactors with thermal
output of 3 GWth with radius of 3 or 5 m
Radius (r) m
Surface area
(4πr 2) m 2
Power flux
MW m −2
Volume (4πr 3 /3) m 3 Energy density
MW m −3
3
113
26.5
113
26.5
5
314
9.5
523
5.7
2/3 of output power (6-16 MW m −2 ) is carried by 14 MeV neutrons and deposited in a blanket having
large volume, and 1/3 by particles and radiation (3–10 MW m −2 ) and deposited to plasma-facing
surface