The swelling behavior in a material depends on composition. An example is shown in Figure 6.17 for nickel-based materials at 425 °C. Nickel with just 0.4% impurities shows greater resistance to swelling than the high-purity nickel, and Inconel-600 (73Ni-17Cr-8Fe, in wt%) does not show any swelling (actually shows densifica — tion!) during irradiation. The swelling resistance observed in Inconel is due to the fine coherent precipitates present in this type of material. In coherent precipitates, the precipitate-matrix interface is continuously bonded. Coherent precipitates (var­iable bias sink) act as recombination sites for vacancies and interstitials, thus reduc­ing swelling. However, precipitates need to be used as direct control of swelling due to instability of the structure in radiation environment.

FERRITIC ALLOYS

FLUENCE (1022 n/cm2, E > 0.1 MeV)

Figure 6.18 Comparison between the swelling behavior of a ferritic/martensitic steel with austenitic stainless steels (316 type) as a function of fast neutron fluence at 420 °C Ref. [16].

It has generally been a common knowledge found out from the EBR-II irradiation testing that ferritic/martensitic (F-M) alloys show greater resistance to void swelling compared to austenitic steels. Swelling behavior of six com­mercial ferritic/martensitic steels is compared with a 316-type austenitic stain­less steel as a function of neutron fluence at ~420 °C in Figure 6.18. At the peak swelling temperature (400-420 °C), only <2% swelling was noted in F-M alloys like HT-9 (12Cr-1MoWV) and T-91 (9Cr-1Mo-V-Nb) steels even after irradiation dose of 200 dpa. The complex microstructure of the ferri — tic/martensitic steels involving lath boundaries, various types of precipitates, high dislocation density, and so on provides numerous defect sink sites that help in limiting void swelling. On the other hand, in austenitic stainless steels, presence of Cr, Ni, and other elements in higher amounts leads to the forma­tion of helium through transmutation reactions and promote void swelling. However, recent studies have shown that F-M steels also swell at a greater rate at very high radiation doses; the phenomenon is generally not observed in conventional irradiation experiments as the incubation dose required is rela­tively large compared to that of the austenitic stainless steels.

Void swelling characteristics in a 16Cr-4Al-Y2O3 oxide dispersion- strength­ened (ODS) steel were studied by Kimura et al. [17]. They also compared the results with those of a non-ODS, reduced activation martensitic steel (JLF-1, 9Cr-WVTa alloy). The ODS alloys contain a high number density of nanometric (<5 nm) Y-Ti based oxide precipitates. They used dual ion irradiation at 500 °C (773 K) at a higher dose rate to accumulate extensive displacement damage. The

and (b) afterthedual ion irradiation up to 60dpa, respectively. (c and d) JLF-1 steel before and after the same irradiation condition, respectively [17].

volumetric swelling has been plotted against the displacement damage levels in Figure 6.19a. Interestingly, the ODS steel showed not much void swelling and was almost independent of the dose level. On the other hand, the JLF-1 steel exhibited comparatively higher volumetric swelling levels. They also presented evidence from TEM studies of both alloys irradiated up to 60 dpa, as shown in Figure 6.19b. The helium bubbles formed were much finer and numerous in the ODS alloy compared to the JLF-1 that showed larger bubbles. This micro­structural evidence supports the hypothesis that helium bubbles are created at the Y-Ti-O nanoprecipitates, and thus help keep a reduced level of volumetric swelling in the ODS alloys.