44
3 Power Load on Plasma-Facing Materials
wall saturation, i.e. no retention of the incident particles makes plasma density control
difficult. However, the wall saturation is unavoidable after a long discharge duration.
Temperature increase during the discharge also makes the wall saturation earlier
owing to the reduction of the saturation concentration with increasing temperature,
which makes plasma density control difficult.
In current tokamak experiments, their discharge duration has been limited within
a few minutes, i.e. steady power and particle balance have not been established well.
In most tokamaks, cooling of vacuum vessel is ambient. Therefore, some tokamaks
require breaks between discharges of more than a half hour to cool down the vessel
or magnetic coils. In a fusion reactor, continuous discharge is planned, even in ITER
discharge duration is 400 s. Gradual temperature increase could cause sudden fuel
release owing to sudden detrapping of hydrogen such like decomposition of chemical
compound. The effects of wall temperature change on boundary plasma are one of
the main remaining issues to be clarified.
3.5 Power Load by Neutrons
As indicated in Fig. 3.1, the power load by D-T fusion neutrons, 2.4 MW m
−2 ,
is not small. However, the neutrons penetrate through plasma-facing wall into the
blanket region and deposit their energy volumetrically. Then, the deposited energy
is transformed to heat in the blanket to generate electricity. Therefore, the neutron
power load to PFS is much less than that given by energetic particles and radiation
from plasma, and the direct influence of neutron loading on PFS and PMI would not
be significant.
14 MeV neutrons directly incident to materials without deceleration in water like
a fission reactor and give both displacement and transmutation of constituent atoms
in materials used in a fusion reactor. Such damaging effects are much higher than
those in the fission reactor. Therefore, degradation of material properties caused by
the irradiation of the 14 MeV neutrons is seriously concerned [11]. They will limit
the lifetime of the structure materials through materials hardening or loss of ductility
and resultant embrittlement.
Comparing with the volumetric damages in the structure materials, degradation of
material properties of PFM by 14 MeV neutron irradiation is less concerned because
they are usually employed as thin plates and do not function as a structure material.
However, accumulated damages in PFM work as trapping sites of hydrogen fuels and
consequently, alter fuel recycling at PFS and increase T retention. Since this book
focuses on PMI, materials damages are not discussed except those appearing at or
near PFS to influence PSI.
It should be noted that T breeding and recovery in the blanket system shall be
significantly influenced by additional damages accompanied by nuclear reactions for
T breeding, i.e. the energy of 14 MeV neutron is converted to heat and used to breed
tritium through the reactions with
6 Li and
7 Li,
3 Power Load on Plasma-Facing Materials
wall saturation, i.e. no retention of the incident particles makes plasma density control
difficult. However, the wall saturation is unavoidable after a long discharge duration.
Temperature increase during the discharge also makes the wall saturation earlier
owing to the reduction of the saturation concentration with increasing temperature,
which makes plasma density control difficult.
In current tokamak experiments, their discharge duration has been limited within
a few minutes, i.e. steady power and particle balance have not been established well.
In most tokamaks, cooling of vacuum vessel is ambient. Therefore, some tokamaks
require breaks between discharges of more than a half hour to cool down the vessel
or magnetic coils. In a fusion reactor, continuous discharge is planned, even in ITER
discharge duration is 400 s. Gradual temperature increase could cause sudden fuel
release owing to sudden detrapping of hydrogen such like decomposition of chemical
compound. The effects of wall temperature change on boundary plasma are one of
the main remaining issues to be clarified.
3.5 Power Load by Neutrons
As indicated in Fig. 3.1, the power load by D-T fusion neutrons, 2.4 MW m
−2 ,
is not small. However, the neutrons penetrate through plasma-facing wall into the
blanket region and deposit their energy volumetrically. Then, the deposited energy
is transformed to heat in the blanket to generate electricity. Therefore, the neutron
power load to PFS is much less than that given by energetic particles and radiation
from plasma, and the direct influence of neutron loading on PFS and PMI would not
be significant.
14 MeV neutrons directly incident to materials without deceleration in water like
a fission reactor and give both displacement and transmutation of constituent atoms
in materials used in a fusion reactor. Such damaging effects are much higher than
those in the fission reactor. Therefore, degradation of material properties caused by
the irradiation of the 14 MeV neutrons is seriously concerned [11]. They will limit
the lifetime of the structure materials through materials hardening or loss of ductility
and resultant embrittlement.
Comparing with the volumetric damages in the structure materials, degradation of
material properties of PFM by 14 MeV neutron irradiation is less concerned because
they are usually employed as thin plates and do not function as a structure material.
However, accumulated damages in PFM work as trapping sites of hydrogen fuels and
consequently, alter fuel recycling at PFS and increase T retention. Since this book
focuses on PMI, materials damages are not discussed except those appearing at or
near PFS to influence PSI.
It should be noted that T breeding and recovery in the blanket system shall be
significantly influenced by additional damages accompanied by nuclear reactions for
T breeding, i.e. the energy of 14 MeV neutron is converted to heat and used to breed
tritium through the reactions with
6 Li and
7 Li,