174
9 Fuel Retention in a Rector with Full …
W layers in tokamak and their H retention characteristics are quite uncertain. Therefore, the estimation of H recycling and retention in a full-W wall reactor are quite
uncertain. In the following sections, after summarizing the present understanding on
characteristics of H in W, the fuel retention in a full W-wall reactor is estimated and
compared with that for the ITER scale full-C reactor given in the previous section.
Present understanding of H retention in W is summarized by Tanabe [20].
9.5.1 Characteristics of Hydrogen in W
Since hydrogen diffusion coefficient in W is not small, H dissolved in bulk can be
released even at RT after long storage. However, H solubility in W is quite small,
while large amount of hydrogen is trapped at defects and impurities. The trapping
energy is rather large. Hence, H migration in W is strongly influenced by trapping
and trapped H is hardly released near RT. Because W is usually made by powder
metallurgy, the density of commercially available W is a little less than that of fully
melted W and included some pores along grain boundaries and impurities like C
used at sintering of W powders. In addition to these intrinsic defects and impurities, defects produced by injection of energetic hydrogen and neutron irradiation
work as trapping sites. The fuel trapped at defects produced by neutron irradiation
and distributed throughout W bulk contribute as T inventory. In addition, defects
produced by energetic hydrogen injection accumulate near surface and significantly
increase H retention of PFM making H saturated surface layers with H/W of ~0.1.
The thickness of the H saturated layers grows with H fluence. Accordingly, H retention in W used as PFM is not likely much smaller than that in C owing to the bulk
trapping and the formation of H saturated layers in near suface region.
Figure 9.8 schematically shows how H injected into W is retained. Different from
C, W is strongly influenced by power load. In particular, PFS subjected to highpower load is recrystallized or their grain sizes grow, as indicated in the figure. In
addition, H retention with very high concentration completely changes the crystalline
structure. Near room temperature hydrogen blisters are formed and their exfoliation
could result in heavy surface erosion. Depth profile of H retained in W under exposure
of H plasma are schematically described in Fig. 9.9 [20]. More than 1 mono-layer
of adsorbed H (around 10
19 /m
2 ) always exists on the top surface. Beneath the top
surface, H saturated layers are formed. Although the amount of retained H in the
H saturated layers with the concentration of around 0.1 in H/W ratio changes with
the incident energy of H, most of laboratory experiments show saturation in nearsurface region with 5 × 10
23 m
−2 (see Fig. 9.11). Since the thickness of the saturated
layers can grow with the plasma exposure time, H retention near-surface region would
further increase. In addition, H can be easily diffuse into deep showing exponentially
decaying concentration profile from the surface. Since the H solubility is very small,
the contribution of the dissolved H in the deeper region would not exceed trapped H
in near-surface region. However, defects produced by neutron irradiation could have
large contribution [21]. Fluence dependence of H retention in W is given again in
the next section.
9 Fuel Retention in a Rector with Full …
W layers in tokamak and their H retention characteristics are quite uncertain. Therefore, the estimation of H recycling and retention in a full-W wall reactor are quite
uncertain. In the following sections, after summarizing the present understanding on
characteristics of H in W, the fuel retention in a full W-wall reactor is estimated and
compared with that for the ITER scale full-C reactor given in the previous section.
Present understanding of H retention in W is summarized by Tanabe [20].
9.5.1 Characteristics of Hydrogen in W
Since hydrogen diffusion coefficient in W is not small, H dissolved in bulk can be
released even at RT after long storage. However, H solubility in W is quite small,
while large amount of hydrogen is trapped at defects and impurities. The trapping
energy is rather large. Hence, H migration in W is strongly influenced by trapping
and trapped H is hardly released near RT. Because W is usually made by powder
metallurgy, the density of commercially available W is a little less than that of fully
melted W and included some pores along grain boundaries and impurities like C
used at sintering of W powders. In addition to these intrinsic defects and impurities, defects produced by injection of energetic hydrogen and neutron irradiation
work as trapping sites. The fuel trapped at defects produced by neutron irradiation
and distributed throughout W bulk contribute as T inventory. In addition, defects
produced by energetic hydrogen injection accumulate near surface and significantly
increase H retention of PFM making H saturated surface layers with H/W of ~0.1.
The thickness of the H saturated layers grows with H fluence. Accordingly, H retention in W used as PFM is not likely much smaller than that in C owing to the bulk
trapping and the formation of H saturated layers in near suface region.
Figure 9.8 schematically shows how H injected into W is retained. Different from
C, W is strongly influenced by power load. In particular, PFS subjected to highpower load is recrystallized or their grain sizes grow, as indicated in the figure. In
addition, H retention with very high concentration completely changes the crystalline
structure. Near room temperature hydrogen blisters are formed and their exfoliation
could result in heavy surface erosion. Depth profile of H retained in W under exposure
of H plasma are schematically described in Fig. 9.9 [20]. More than 1 mono-layer
of adsorbed H (around 10
19 /m
2 ) always exists on the top surface. Beneath the top
surface, H saturated layers are formed. Although the amount of retained H in the
H saturated layers with the concentration of around 0.1 in H/W ratio changes with
the incident energy of H, most of laboratory experiments show saturation in nearsurface region with 5 × 10
23 m
−2 (see Fig. 9.11). Since the thickness of the saturated
layers can grow with the plasma exposure time, H retention near-surface region would
further increase. In addition, H can be easily diffuse into deep showing exponentially
decaying concentration profile from the surface. Since the H solubility is very small,
the contribution of the dissolved H in the deeper region would not exceed trapped H
in near-surface region. However, defects produced by neutron irradiation could have
large contribution [21]. Fluence dependence of H retention in W is given again in
the next section.