Innovations in Testing Methodology for Fusion Reactor Materials Development
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MRS BULLETIN/JULY 1989
certain solute elements, often resulting in atypical phase changes. These microscopic changes lead directly to macroscopic phenomena such as irradiationinduced creep and swelling, radiation hardening, and embrittlement. Neutron exposures are usually characterized in terms of the total displacements per atom (dpa). The dpa exposure unit is computed by integrating the product of the neutron flux spectrum and the energy-dependent displacement cross section over all relevant neutron energies. Physically, the dpa is equivalent to the total kinetic energy deposited in atomic recoils, assuming a constant energyper-displacement. Hence, dpa provides a partial basis for comparing the effects of displacement damage resulting from irradiations in different neutron spectra. However, the PKA energy distribution and related details of the displacement cascade a n d defect p r o d u c t i o n p r o cesses— such as the fraction of residual defects in clusters, cluster size distribution and morphology, etc.—are functions of the neutron spectrum. Thus, dpa is not able to fully characterize effects of spectral differences on displacement defect production.' Neutrons also generate both solid and gaseous transmutation products. The range and rate of transmutation is also a strong function of the neutron spectrum.
In combination with displacement damage, the t r a n s m u t a n t s — particularly helium from (n,a) reactions—are known to strongly affect microstructural evolution and mechanical properties, in some cases for concentrations at the parts-per million level. 2 The (n,a) cross section has a threshold energy of about 6 MeV for elements commonly used in structural alloys; for some like nickel the effective (n,a) cross section is relatively large for thermal neutrons as well. Thus, transmutant helium content and the consequent effects of helium can strongly depend on neutron spectrum. Extended exposures in fusion environments will produce hundreds of dpa a n d t h o u s a n d s of parts-per-million (ppm) of helium. The potential significance of such exposures can be illustrated by considering only one of many possible examples. Beyond a threshold exposure, typically in the range of 40 to 60 dpa, growth of voids can lead to macroscopic swelling rates approaching 1%/dpa in austenitic stainless steels. Thus, beyond this threshold, macroscopic dimensional changes can rapidly become unacceptably large for design purposes; since the nature of the displacement damage and the transmutant population can significantly affect this behavior, it is essential to develop data in a prototypic irradiation environment. Unfortunately, there are no fusion reactors for prototypical testing of candidate fusion materials. Over the intermediate term, accelerator-based neutron sources 3 that can reproduce many of the key features of fusion irradiations may become available. However, for the near term, irradiations must be carried out in existing high flux fission test reactors. Several available test reactors can produce about 30 d
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