Structural materials for a fusion reactor are simply those that comprise a majority of the plant. They are not directly exposed to the plasma, but most are exposed to high doses of neutrons and electromag­netic radiation. Many of these materials are used in the reactor blanket where the tritium is bred by the nuclear reaction 6Li(n, a)3H. It is in the blanket and in the fuel reprocessing area that the structural materials are most likely to be exposed to tritium. The following sections review the structural materi­als that have been considered for fusion reactors.

4.16.3.2.1 Austenitic stainless steels

Austenitic stainless steels, particularly type 316, have been used extensively as a construction mate­rial for nuclear reactors (see Chapter 2.09, Proper­ties of Austenitic Steels for Nuclear Reactor Applications). The type 300-series austenitic stain­less steels (Fe-Cr—Ni) have relatively high nickel content (8-12 wt% for the 304 family of austenitic stainless steels and 10-14 wt% for 316 alloys), which is a detriment for fusion applications for several reasons including the susceptibility of nickel to acti­vation (induced radioactivity).88-90 The solubility and the diffusivity of gaseous hydrogen and its iso­topes through type 300-series austenitic stainless steels have been extensively studied and reviewed in San Marchi et a/.1 Higher strength austenitic stainless steels (such as the Fe-Cr-Ni-Mn alloys, which have not been widely considered for fusion applications) feature solubility and diffusivity that differ by a factor of about 2 compared to the type 300-series alloys.1 The so-called prime candidate alloy (PCA) is a variant of type 316 austenitic stain­less steel modified for fusion applications (although interestingly enough with higher nickel content); from a permeation perspective PCA is anticipated to behave in a manner essentially similar to conven­tional type 316 alloys.91

The Fe-Cr-Mn austenitic stainless steels have been considered as a substitute for the more com­mon grades of austenitic stainless steels since

they have only a nominal nickel content,88,89,90

although low-activation ferritic/martensitic steels have received more attention (see subsequent sec­tion). Alloys that have been considered typically contain both chromium and Mn in the range 10-20 wt%, often with small amounts of other alloying elements (Sahin and Uebeyli90 provides a list of a number of alloys that have been explored for fusion applications). Unlike the Fe-Cr-Ni austenitic stainless steels, there are few reports of transport properties for the Fe-Cr-Mn austenitic alloys; data for oxidized Fe-16Cr-16Mn are reported in Gro­mov and Kovneristyi.92

Austenitic stainless steels can contain ferritic phases in the form of residual ferrite from alloy production, ferrite in welds formed during solidifica­tion, and in some cases, strain-induced martensite from deformation processing. The ferritic phases can result in a fast pathway for the transport of hydrogen and its isotopes at a relatively low temper­ature because the ferritic phases have a much higher diffusivity for hydrogen and its isotopes than austen — ite.93,94 In the absence of ferritic second phases, how­ever, hydrogen transport in austenitic stainless steels is independent of whether the material is annealed or heavily cold-worked95-97 and relatively insensitive to composition for the type 300-series alloys.1

Reported values of hydrogen diffusivity in aus­tenitic stainless steels are less consistent than per­meability as a consequence of surface effects and trapping, as mentioned earlier and elsewhere.1 Figure 10 shows the reported diffusivity of hydro­gen from a number of studies in which special pre­cautions were taken to control surface conditions. The activation energy for diffusion is relatively large for austenitic stainless steels (ED = 49.3 kJ mol-1), and thus the diffusivity is sensitive to temperature, approaching the values of the ferritic steels at very high temperatures (>1000K), while being many orders of magnitude lower at room temperature.

The exceptionally low diffusivity of hydrogen near room temperature results in austenitic stainless steels having significantly lower permeability of hydrogen than other structural steels.

The solubility of hydrogen and its isotopes in the type 300-series austenitic stainless steels is high rela­tive to most structural materials. Compilation of data from gas permeation studies shows that most studies are consistent with one another,93,95,96 while studies that considered a variety of alloys within this class show that the solubility of hydrogen is essentially the same for a wide range of type 300-series austenitic stainless.93,95,96 The heat of solution of hydrogen in austenitic stainless steels is relatively low (AHs = 6.9 kJ mol-1), and thus the equilibrium content of hydro­gen in the metal remains high even at room tempera­ture. The solubility of hydrogen and its isotopes is plotted in Figure 11, while Table 1 lists the recom­mended transport properties for austenitic stainless steels (and a number of other metals and alloys).

The primary traps in type 300-series austenitic stainless steels are dislocations with relatively low binding energy 10kJ mol-. Therefore, the

amount of trapped hydrogen (in the absence of irra­diation and implantation damage) is relatively low at elevated temperatures. Moreover, due to the high sol­ubility of hydrogen and its isotopes in austenitic
stainless steels, the density of trapping sites would need to be impractically high to measurably increase the inventory of hydrogen and its isotopes in the metal.20 For these reasons, trapping from a microstruc­tural origin is anticipated to have little, if any, impact on the transport and inventory of hydrogen and its isotopes in austenitic stainless steels at temperatures greater than ambient.

The recombination-rate constant (kr) for austenitic stainless steels near ambient temperature is typically less than about 10-9 m4 s-1 per mol of H2.113 At higher temperatures (^700 K), the value varies between ~10-5 and 10-7 m4 s 1 per mol of H2, depending on the surface condition.80,113-116