164
9 Fuel Retention in a Rector with Full …
However, direct measurements of retained H and D in PFM and plasma-facing
components (PFC) in present tokamaks have been scarcely done as already mentioned
several times, except systematic studies in JT-60U with PFS of full carbon [5–10],
in which direct measurements of the H and D retention in selected plasma-facing
tiles located above mentioned four areas. Assuming the toroidal symmetry in the
retention, the piling-up of the retention with exposed fluence was determined, which
is described in the next section together with the explanation on how H and D were
retained in carbon materials.
Unfortunately, JT-60U was a full carbon device in which all PFS were covered by
carbon materials, and no such data are available for a full W machine. Therefore, for
the full W machine, the estimation of the fuel retention was done from the balance
of fueling and exhaust in some tokamak devices together with the extrapolation of
laboratory experiments, which is described in Sect. 9.5.
In these estimations for the C and W and walls, the fluence of impinging fuel
particles has remained still low compared to that of a reactor. Nevertheless, the
comparison of estimated T retention in a C-wall machine and a W-wall machine gives
important information and ideas for what shall be done to get reliable T retention in
the reactor.
9.4 Fuel Retention in Carbon Materials
9.4.1 Characteristics of Hydrogen Retention in Carbon
Materials [11]
Figure 9.2a shows how hydrogen (referred to as H hereafter) is penetrating and
retained in carbon materials which are constructed of mainly filler (graphite like
crystallite) and binder in nuclear graphite and carbon fibers (graphite-like fibers) and
binder in CFC (carbon fiber enforced carbon materials) [12] and contain open and
closed pores. Hydrogen molecules (H 2 ) can easily migrate through the open pores
(Path 1), and hence C has significantly large gas permeability compared to metallic
materials. H 2 also penetrates through boundaries between the crystallites (Path 2).
Therefore, all surfaces of the crystallites are simultaneously exposed to gaseous
H 2 . H 2 can also migrate intercalantly in between the basal planes of the crystallites
(Path 3), when the pressure of H 2 atmosphere is raised extremely high, which is not
likely to occur in fusion environment. C atoms in the crystallites are inactive and
only C atoms at their edges are very likely terminated by H making C-H bonds in
hydrogen atmosphere. In carbon materials, the crystallites are not fully graphitized
and include various defects which can trap hydrogen atoms. Neutron irradiation
introduces additional defects in the crystallites, which also trap hydrogen to increase
the retention. In normal condition (or exposure to H 2 gas), it seems very hard for H 2
to dissociate to atoms and penetrate or diffuses into the crystallites interstitially as
depicted from quite small solubility and diffusivity of H in graphite.
9 Fuel Retention in a Rector with Full …
However, direct measurements of retained H and D in PFM and plasma-facing
components (PFC) in present tokamaks have been scarcely done as already mentioned
several times, except systematic studies in JT-60U with PFS of full carbon [5–10],
in which direct measurements of the H and D retention in selected plasma-facing
tiles located above mentioned four areas. Assuming the toroidal symmetry in the
retention, the piling-up of the retention with exposed fluence was determined, which
is described in the next section together with the explanation on how H and D were
retained in carbon materials.
Unfortunately, JT-60U was a full carbon device in which all PFS were covered by
carbon materials, and no such data are available for a full W machine. Therefore, for
the full W machine, the estimation of the fuel retention was done from the balance
of fueling and exhaust in some tokamak devices together with the extrapolation of
laboratory experiments, which is described in Sect. 9.5.
In these estimations for the C and W and walls, the fluence of impinging fuel
particles has remained still low compared to that of a reactor. Nevertheless, the
comparison of estimated T retention in a C-wall machine and a W-wall machine gives
important information and ideas for what shall be done to get reliable T retention in
the reactor.
9.4 Fuel Retention in Carbon Materials
9.4.1 Characteristics of Hydrogen Retention in Carbon
Materials [11]
Figure 9.2a shows how hydrogen (referred to as H hereafter) is penetrating and
retained in carbon materials which are constructed of mainly filler (graphite like
crystallite) and binder in nuclear graphite and carbon fibers (graphite-like fibers) and
binder in CFC (carbon fiber enforced carbon materials) [12] and contain open and
closed pores. Hydrogen molecules (H 2 ) can easily migrate through the open pores
(Path 1), and hence C has significantly large gas permeability compared to metallic
materials. H 2 also penetrates through boundaries between the crystallites (Path 2).
Therefore, all surfaces of the crystallites are simultaneously exposed to gaseous
H 2 . H 2 can also migrate intercalantly in between the basal planes of the crystallites
(Path 3), when the pressure of H 2 atmosphere is raised extremely high, which is not
likely to occur in fusion environment. C atoms in the crystallites are inactive and
only C atoms at their edges are very likely terminated by H making C-H bonds in
hydrogen atmosphere. In carbon materials, the crystallites are not fully graphitized
and include various defects which can trap hydrogen atoms. Neutron irradiation
introduces additional defects in the crystallites, which also trap hydrogen to increase
the retention. In normal condition (or exposure to H 2 gas), it seems very hard for H 2
to dissociate to atoms and penetrate or diffuses into the crystallites interstitially as
depicted from quite small solubility and diffusivity of H in graphite.