6.3 Damaging and Degradation of PFM
109
results in dome-shaped He blister. In case of He injection at higher temperature, not
only He but also vacancies produced by He injection and they move to the surface.
Accordingly, characteristic structure referred to as the fuzz structure is formed as
noted in Sect. 4.6.2. However, such structure would not be stable under high-power
load on PFS, because heat transfer through its needle-like structure could not be high
enough to avoid the melting. Once the fuzz structure is formed whatever the causes
are, its melting and evaporation influence the edge plasma. Researches on this point
are on-going.
6.3.3 Other PFM Candidates (Be and Li)
It used to be quite important to reduce oxygen impurity in plasma and materials
working as oxygen getters, like Li, Be, Al, and Ti have been PFM candidates. Be
is used in ITER like wall in JET considering its use in ITER. However, its low
melting point would not allow to use in a reactor. Be as a metal would behave like
W, brittle and having quite low hydrogen solubility and diffusivity. Different from
W, Be reacts with hydrogen to make volatile hydride, BeH 2 , and chemical sputtering
of Be by H like C could result in large erosion [49, 50]. However, BeH 2 is not
stable at higher temperature, and erosion of Be in JET ITER-like wall is not likely
significant. Because of their low melting points, other PFM candidates except Li
will not be used in a reactor. A concept to use liquid Li wall attracts much interest,
because Li works as both coolant and breeder [51–54]. Except significant evaporation
by high-power load, material modifications occurred in solid materials like radiation
damages are not concerned in liquid. Evaporated Li would return to PFM by gyration
after ionization. Furthermore, massive evaporation would work as vapor shielding to
reduce the power load. Hydrogen gettering effect with forming LiH which stays as
tiny solid particles in liquid Li reduces hydrogen recycling and realizes wall pumping
to make plasma density control easier. However, large wall pumping makes in both
T fuel throughput and T retention in Li very large. Hence quick recovery of T from
Li is critically important to use liquid Li as PFS, which is unfortunately not easy
because LiH particles dispersed in Li is quite hard to recover.
6.3.4 Structure Materials
As a structure material, iron-based steels are most reliable. Particularly, low activation
ferrite martensitic steels are the candidate first wall or structure material. Although
martensitic steels are another candidate, their toughness at higher temperature is
concerned to use them in a reactor. Unfortunately, melting points of these ironbased steels are not high enough to use them as PFM and some protective layers
are necessary. In most of present reactor designs, W is employed as armor tiles and
the tiles should be joined to a heat sink or structure material like the steels or Cu
109
results in dome-shaped He blister. In case of He injection at higher temperature, not
only He but also vacancies produced by He injection and they move to the surface.
Accordingly, characteristic structure referred to as the fuzz structure is formed as
noted in Sect. 4.6.2. However, such structure would not be stable under high-power
load on PFS, because heat transfer through its needle-like structure could not be high
enough to avoid the melting. Once the fuzz structure is formed whatever the causes
are, its melting and evaporation influence the edge plasma. Researches on this point
are on-going.
6.3.3 Other PFM Candidates (Be and Li)
It used to be quite important to reduce oxygen impurity in plasma and materials
working as oxygen getters, like Li, Be, Al, and Ti have been PFM candidates. Be
is used in ITER like wall in JET considering its use in ITER. However, its low
melting point would not allow to use in a reactor. Be as a metal would behave like
W, brittle and having quite low hydrogen solubility and diffusivity. Different from
W, Be reacts with hydrogen to make volatile hydride, BeH 2 , and chemical sputtering
of Be by H like C could result in large erosion [49, 50]. However, BeH 2 is not
stable at higher temperature, and erosion of Be in JET ITER-like wall is not likely
significant. Because of their low melting points, other PFM candidates except Li
will not be used in a reactor. A concept to use liquid Li wall attracts much interest,
because Li works as both coolant and breeder [51–54]. Except significant evaporation
by high-power load, material modifications occurred in solid materials like radiation
damages are not concerned in liquid. Evaporated Li would return to PFM by gyration
after ionization. Furthermore, massive evaporation would work as vapor shielding to
reduce the power load. Hydrogen gettering effect with forming LiH which stays as
tiny solid particles in liquid Li reduces hydrogen recycling and realizes wall pumping
to make plasma density control easier. However, large wall pumping makes in both
T fuel throughput and T retention in Li very large. Hence quick recovery of T from
Li is critically important to use liquid Li as PFS, which is unfortunately not easy
because LiH particles dispersed in Li is quite hard to recover.
6.3.4 Structure Materials
As a structure material, iron-based steels are most reliable. Particularly, low activation
ferrite martensitic steels are the candidate first wall or structure material. Although
martensitic steels are another candidate, their toughness at higher temperature is
concerned to use them in a reactor. Unfortunately, melting points of these ironbased steels are not high enough to use them as PFM and some protective layers
are necessary. In most of present reactor designs, W is employed as armor tiles and
the tiles should be joined to a heat sink or structure material like the steels or Cu