The effects of high He levels on microstructure and mechanical properties have been extensively studied in mixed fast-thermal spectrum fission reactor irradia­tions of alloys naturally containing, or doped with, Ni and B. In these cases, high He levels are produced by thermal neutron nth, a reactions, either by (a) the two-step reaction with 58Ni(nth, g) («68% of elemental Ni with a nth, g cross-section of «0.7 barns) and 59Ni
(nth, a) (bred from 58Ni with a n, a cross-section of «10 barns) cited in Section 1.06.1; (b) or by the 10B + nth! 7Li + a reaction («20% of elemental B with a cross-section of «4010 barns) (1 barn = 10-24 cm2). Significant quantities of He can also be generated by epithermal-fast spectrum neutron reac­tions with B as well as prebred 59Ni.29

Figure 4(a) shows calculated and measured He production in natural Ni in the HFIR target capsule position.30 Figure 4(b) shows the corresponding He/dpa ratio for a Fe alloy doped with 2% natural Ni. Two Ni doping characteristics are evident: (a) there is a transient phase in He production regime prior to a He/dpa peak at about «20dpa in HFIR; (b) if the alloy contains more than a few percent Ni, like in AuSS, the He/dpa is much higher than that for fast fission and higher than that for fusion spectra but is comparable to, or slightly less than, the He/dpa for SPNI.

Modifying the amounts of 58Ni and 60Ni (isotope tailoring) can control and target He/dpa ratios (e. g., to fusion). , , An approximately constant He generation rate can be obtained by using irra­diated Ni pre-enriched in 59Ni.29,31 Various amounts of 58Ni, 59Ni, and 60Ni can also be used to control the He/dpa ratio in fast spectrum reactors, like the Fast Flux Test Facility (FFTF), as well as in mixed spec­trum reactors, like HFIR.29,31,32

Boron is not normally added to steels used for nuclear applications, but it has been used in a number of doping studies.33,34 A major advantage of B doping is that significant amounts of He are produced by the 10B, but not the 11B, isotope. Thus, the effect of doping with 10B versus 11B can be used to isolate this effect of

He, in a B-containing alloy. However, the issues asso­ciated with B doping are even more problematic than those for Ni. In mixed spectrum reactors, all the 10B is quickly converted to He and Li by the thermal neutrons. In this case, the He is initially introduced at much too high a rate per dpa but then saturates at the 10B content. The other major limitations are that B is virtually insoluble in steels and primarily resides in Fe and alloy boride phases.35 Boron also segregates to GBs. Thus, He from B reactions is not homo­geneously distributed. Recently, nitrogen additions to FMS steels to form fine-scale BN phases have been used to increase the homogeneity of B and He distributions.36

Varying the He/dpa ratio in Ni — and B-containing alloys can also be achieved by attenuating thermal neutron fluxes (spectral tailoring) in mixed spectrum reactors as well as selecting appropriate fast reactor irradiation positions.31,37,38 Spectral tailoring, either by attenuating thermal neutrons or irradiating in epithermal-fast reactor spectra, is especially helpful in B doping. , ,

However, doping alloys that do not normally con­tain Ni or B can affect both their properties and microstructures, including their response to He and displacement damage. For example, transformation kinetics during heat treatments (hardenability) and the baseline properties of FMS are strongly affected by both Ni and B. Ni also has a strong effect on refining irradiation-induced microstructures and enhancing irradiation hardening.20, 1 As noted previously, to some extent these confounding factors can be evalu­ated by comparing the effects of various amounts of 10B/11B45 and 58Ni/60Ni. However, doped alloys are inherently ‘different’ from those of direct interest. Note that excess dpa due to n, a reaction recoils must be accounted for,46 and in the case of B doping the Li reaction product may play some role as well.