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Isotopic alloying is examined as a means of manipulating the ratio of helium transmutations to atom displacements in mixed spectrum reactors. The application explored is based on artificially altering the relative abundances of the stable isotopes of nickel to systematically vary the fraction of 58Ni in nickel-bearing alloys. The method of calculating helium production rates is described. Results of example calculations for several experiments are discussed. Both increased understanding of mechanisms and the development of materials for fusion reactor applications, by testing in parameter regimes of direct interest, appear to be possible.

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A theoretical model is described to evaluate the effects of spatially and temporally discrete production of point defects in collision cascades. Large fluctuations occur with time in the vacancy concentration. The interstitial concentration is nearly always zero except for extremely large spikes of short duration, corresponding to the occurrence of a cascade anywhere within the sphere beyond which all generated defects are absorbed by sinks before reaching the reference point. The growth rate of a void in this environment is compared to that given by the more approximate rate theory. The difference is small, but increases at high temperature. This work has implications for void nucleation, irradiation creep, and analysis of pulsed irradiations.

Experimental observations of the void volume at several depths along the range of 4 MeV Ni ions in 316 stainless steel are reported. The specimens were first preconditioned by neutron irradiation at temperatures of 450 and 584°C to fluences of approximately 8 × 1026n/m−2. The void volume after ion bombardment to 60 dpa at the peak damage depth is significantly lower at the peak damage depth than in the region between that and the free surface. The ratio of the step height to void volume at the depth of peak energy deposition between regions masked from and exposed to the beam is strongly dependent on bombardment temperature. The reduction of void volume near the peak damage depth is larger for the 584°C than for the 450°C neutron irradiated material. These observations are consistent with recent theoretical results which account for the injection of the bombarding ions as self-interstitials.

The effect of mobile helium on swelling has been studied by using a rate theory model of void growth. The partitioning of mobile helium to various internal sinks and the trapping of interstitial helium atoms by vacancies were included in the model. Helium gas generated during neutron irradiation increases swelling at temperatures above the peak void swelling temperature. Two temperature regimes of enhanced swelling are related to gas-assisted void growth and gas-driven bubble growth. Swelling due to void and bubble growth in a fusion reactor first wall is predicted using nickel as an example, and the effects of dislocation density and grain size on swelling are discussed. It is found that, as compared to simultaneous helium injection during heavy-ion bombardment, the method of helium preinjection is inadequate in simulating a fusion reactor condition.

Recent advances in the theory of swelling are reviewed. These include (1) the development of a cascade diffusion theory to treat quantitatively the fluctuations in point defect concentrations caused by locally inhomogeneous point defect production, (2) the incorporation of mobile helium and its effects on void growth into the theory, and (3) the spatial variation in swelling during charged particle bombardment. Summaries of (1) and (2) are given. A more complete development of (3) is presented.

AbstractThis paper will present a brief overview of the present understanding of radiation effects in materials. Progress in fundamental understanding of void swelling, phase formation and stability, irradiation creep, and post-irradiation mechanical properties has been gained through comparison of experimental data with the predictions of theoretical models. Applications of the understanding of radiation effects in materials have led to the development of candidate structural materials for use in aggressive radiation environments. Compositional and microstructural controls have been used to achieve the necessary properties and radiation damage resistance. Some of the key issues facing pressure vessel steels, ceramics, and fusion structural materials are described.

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