Reduced Activation Alloy Development for Fusion
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and maintenance — for also wanting to reduce the short-lived induced radioactivity of reactor components. 5 These aspects, although important, are outside the scope of this article.
Criteria for Material Selection To qualify any waste as Class C waste (the least restrictive category under 10CFR61), limits are placed on the sum of the concentrations (Ci/m3) of certain long-lived radionuclides. Some of these limits are in dispute, either because they are not included explicitly in 10CFR61 and therefore have been calculated by others using a variety of assumptions, or because some aspects of the NRC methodology used in deriving them have been questioned. 2 We will only summarize here by stating that there is general agreement that niobium and molybdenum must be essentially eliminated from alloys and that the nickel and nitrogen concentrations should be limited. Programs to develop r e d u c e d activation steels are in progress in Europe, 69 Japan, 1011 and the United States.12"15 The objective is to modify the conventional Cr-Mo ferritic steels6,10"15 and austenitic stainless steels7"9'11'1314 by replacing Ni, Mo, Nb, and N with more benign elements. Higher operating temperatures than attainable with steels is a desirable goal, so refractory alloys have always been considered potential structural materials. Even before the emphasis on reduced activation materials, research had centered on vanadium alloys, which are intrinsically low activation for waste management purposes. An essential aspect of fusion reactor materials development is irradiation testing. Keep in mind that no testing with 14 MeV neutrons to reactor relevant fluences is now possible. The
assumption is that poor irradiation behavior of an alloy in a fission reactor test justifies rejection of the alloy. Demonstration of acceptable behavior, on the other hand, awaits testing in a fusion materials irradiation facility.
Ferritic/Martensitic Steels Conventional Cr-Mo ferritic steels now being considered for fusion reactor applications include 2.25Cr-lMo, 9CrlMoVNb, and 12Cr-lMoVW steels in the U.S., 12Cr-lMoV in England, 12CrMoVNb in Europe, and duplex 9Cr-2Mo steel in Europe and Japan. Molybdenum and Nb are the primary alloying elements that keep these steels from meeting the criteria for nearsurface burial, although Ni must also be minimized. Ni does not play a significant role in determining the properties of Cr-Mo steels, although 0.5% Ni (all compositions are in weight percent) is added to 12Cr-lMo steels to control delta-ferrite. In most alloy development programs, Mo is replaced by W (often on an atomfor-atom basis),61114 although V has been emphasized in other programs.10'12-13 The strengthening function of Nb has been replaced by V, Ti, and Ta, 61114 although the high short-term activity of Ta makes such steels difficult to study immediately after irradiation. Several development programs have limited their investigations to only a few alloys with Cr compositions between 8 and 12%,6'10'1114 while other programs have considered a range of Cr from
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