Workshop Summary: Radiation Damage Correlations - 13. ISRD

As a basis for discussion, the organizers provided a list of potential topics and a table summarizing irradiation environments that need to be considered. In opening remarks, the organizers commented on the continuing use of a broad range of threshold-based fluence exposure parameters for damage correlation, including fluence greater than 1.0 MeV, 0.5 MeV, 0.18 MeV, and 0.1 MeV. The dpa unit is the only exposure or correlation parameter that attempts to account for the complete neutron energy spectrum. When the irradiation environments are 'similar enough' (an ill-defined condition) all of these exposure parameters have exhibited some success as correlation parameters for radiation-induced changes in mechanical properties. A lingering question in the damage correlation community is the relative efficiency of thermal neutrons. Exposure environments such as the HFIR pressure vessel in the USA and the MAGNOX reactors in the UK have clearly demonstrated that dpa from low energy events can not always be neglected. In such cases parameters such as fluence greater than 1.0 MeV will fail to correlate with measured property changes. In the case of the UK reactors, it was further shown that the thermal-induced dpa needed to be more heavily weighted (∼x2) than fast dpa. This is consistent with displacement cascade simulations carried out by molecular dynamics, but the experimental data is not generally sufficient to statistically establish a weighting factor. In contrast, dpa was not found to be superior to fluence greater than 1.0 MeV in an experiment intended to investigate neutron energy spectrum effects carried out by the CEA using irradiations in SILOE and OSIRIS, and fluence greater than 0.1 MeV was significantly worse. The limitations of dpa, or more generally any single correlation parameter, were discussed in the context of the impact of nuclear transmutation and neutron flux effects. In situations where a transmutation influences material response (e.g. helium in many materials and silicon in aluminum alloys), dpa or neutron fluence alone will not be sufficient to correlate material response if differences in the neutron energy spectra are sufficiently large to produce substantial differences in critical transmutation products. This was demonstrated in one of the keynote lectures in which swelling of austenitic stainless steel at a fixed dose in dpa was shown to be a strong function of the He/dpa ratio. Similarly, comparisons of data obtained in low and high flux environments may not be correlated on the basis of total dpa exposure alone. For example, low copper RPV steels have been shown to be less sensitive to flux effects than high copper steels. In addition, compared to high flux irradiations, time-at-temperature effects may influence long-term low flux irradiations. Attenuation of neutron fluence through a RPV was also discussed. This issue comprises both a change in flux and a change in spectrum. In a recently completed experiment called KORPUS it was found that the fluence greater than 0.5 MeV was attenuated at nearly the same rate as the calculated dpa. Both of these values are attenuated more slowly than fluence greater than 1.0 MeV, but mechanical response of the material is not simply related to either exposure parameter. Further experiments and analysis are needed to determine how mechanical properties are attenuated in this environment. Additional issues were discussed as relevant to advanced fission, fusion, and spallation neutron sources. These included the larger uncertainties in neutron cross sections for energies above 20 MeV, and the cross sections required for advanced materials such as SiC that may be employed in such systems. The possible use of correlation monitor materials was discussed. Such materials should be irradiated in test reactor or accelerator experiments and surveillance programs for testing at the same time as other specimens. Finally, the participants expressed an interest in continuing this discussion in a workshop at the 14. ISRD