Developing fusion as a large-scale energy source and high-energy proton accelerator-based technologies are the primary motivations for studying He effects in structural alloys. These environments produce copious quantities of He (and H) by transmutation reactions. High levels of He, coupled with displacement (dpa) radiation damage, lead to a wide variety of property degradation phenomena over a wide range of irradia­tion temperatures, including both severe embrittle­ment and dimensional instabilities of various types.

Indeed, there is growing evidence that He-dpa syner — gisms will severely limit the operating window for leading candidate FMS in fusion first wall structures.

Ultimate resolution of He effect issues in fusion applications will require a dedicated high-energy neutron source to develop an information base to construct and verify rigorous physically based pre­dictive models of the effects of the fusion environ­ment on performance-sustaining properties. These models must account for the interactions of a large number of variables, characterizing both the irra­diation service environment and alloy of interest. However, in the interim, there are a number of irradiation techniques that can be used to simulta­neously introduce high levels of He and dpa into materials. The most notable irradiation techniques include multibeam charged particle irradiations (CPI), ISHI in mixed spectrum fission reactors, and spallation neutron sources. Each of these methods has limitations, but significant progress in understanding He effects has been, and will continue to be, achieved by closely integrating all of these irradiation tools with advanced physical models.

A primary objective of such modeling, and asso­ciated experiments, is to understand and predict the transport, fate, and consequences of He, and its inter­actions with displacement damage. There is a very large historical literature on irradiation effects in AuSS, including the key role played by He. Helium is critical to void swelling and high-temperature embrittlement (HTHE), where the latter is mani­fested as severe reductions in creep rupture times and strains. Standard AuSS provide an excellent basis
for developing an understanding of these phenomena, since they are (a) sensitive to many manifestations of irradiation damage and especially He effects; and (b) generate high levels of He from two-step Ni thermal neutron reactions in mixed spectrum fission reactors (typically of the order 50appm He/dpa) and much lower, but significant, amounts of He (<1 appm He/dpa) in fast reactors. Both void formation and HTHE are characterized by a significant incubation period prior to forming growing cavities followed by rapid swelling or creep rupture.

Helium bubbles are typically the formation sites for both voids and grain boundary creep cavities. RT-based thermodynamic-kinetic models rationalize many important trends in these phenomena. In par­ticular, the critical bubble concept relates the combi­nation irradiation variables, of temperature and dpa rates, along with a number of material-defect vari­ables and parameters, to the critical size and helium content of bubbles that convert to voids, due to dislo­cation bias for SIA or grain boundary creep cavities due to stress. The critical bubble concept rationalizes and provides a basis to quantitatively model the incu­bation periods for both of these phenomena. RT can also be used to model post-incubation swelling rates and creep rupture times and strains. These models predict a number of important observations like bimodal bubble-void cavity size distributions and precipitate-associated bubbles and voids.

Critical bubble-cavity growth models highlight the critical roles played by the overall He bubble microstructures on irradiation effects. Bubbles are necessary for the formation of voids and large
numbers of creep cavities. However, high concentra­tions of bubbles, as generally associated with high He generation rates, can prolong incubation times by increasing the number of He partitioning-trapping sites in small highly subcritical bubbles; and, in the case of swelling and other manifestations of matrix radiation displacement damage. High bubble densi­ties reduce the excess vacancy supersaturations and excess defect fluxes by acting as dominant defect sinks. These insights provide important guidance to developing irradiation-tolerant alloys. Enhanced damage tolerance can be achieved by creating fine — scale and stable microstructural features that can form small, harmless bubbles that sequester high levels of He suppressing swelling and protecting GBs from HTHE and grain boundary decohesion, leading to enormous DBTT shifts. Swelling-resistant advanced stainless steels, with extended incubation times, have used alloy carbide and phosphide phases to manage He in this manner. FMS, with high disloca­tion densities and fine lath structures, are intrinsically more damage resistant than standard austenitic alloys for a variety of reasons. These reasons include low He generation rates in fission reactors. However, the rates of He generation are much higher in D-T fusion spectra, and the irradiation damage tolerance of FMS may be significantly degraded in this case.

SPNI have been the primary source of recent insight into the effects of He in structural alloys, especially mechanical property effects. Perhaps the most significant result of the SPNI studies is that high He-hardening synergisms can lead to enormous shifts in the DBTT shifts and IG fracture in FMS. The mechanisms responsible for such synergistic embrittlement, which can lead to transition tempera­ture elevations of 600 °C or more, are He-induced weakening of GBs as well as enhanced hardening at higher temperatures and higher dpa levels. Thus, there is concern that severe embrittlement and void swelling may close the window for useful application of these alloys in fusion environments.

A parallel set of activities is needed to develop predictive models of He effects. Integrated master models, based on a multiscale-multiphysics paradigm, are being developed to predict He transport, fate, and consequences in realistic alloy microstructures. These master models contain a number ofparameters and must be both mechanistically and microstructu­rally informed. First-principles electronic structure theory and atomistic simulations can provide required model parameters and mechanistic insights. These tools include DFT, embedded atom-based

MD and MS, various Monte Carlo methods, and RT. For example, recent first principles and atomistic research has provided important information on He-vacancy cluster energetics, He interactions with a range of microstructural features, He diffusion mechanisms, and rates in the matrix, along dislocations and in GBs. These models show high He-vacancy binding energies even at the smallest cluster sizes, strong interactions between He and dislocations and dislocation jogs, and both homo — and heterophase interfaces. MD methods have also been used to char­acterize phenomena such as resolutioning of He from bubbles, showing this to be a minor mechanism; and determining the dislocation interaction strength of cavities, ranging from under to overpressurized con­ditions, showing that near-equilibrium bubbles are the strongest obstacles. Recent MD studies have also sug­gested that capillary models may overpredict He gas pressures in equilibrium bubbles.

As noted above, the concepts and models described above can be used to guide the development of irradiation-tolerant alloys. The most promising class of such materials have been dubbed NFA, which contain an ultrahigh density of remarkably stable, nanometer-scale Ti-Y-O NF. The NF are now believed to be primarily a complex oxide Y2Ti2O7 pyrochlore phase. The NF provide remarkable high — temperature creep strength, so that NFA can operate above the displacement damage regime, where recov­ery processes are much faster than defect accumula­tion rates. More significantly the NF trap helium in a very high density of very fine-scale bubbles. In prin­ciple, the bubbles suppress or mitigate essentially all manifestation of radiation damage and property deg­radation. Limited proof in principle validation of the ability of NFA to manage He has been provided by ISHI irradiations that were carried out in the HFIR. For example, an irradiation of NFA MA957 to 9 dpa and 380 appm He produced a very high density of nanometer-scale bubbles on the NFs and dislocations. The same irradiation of the FMS F82H produced roughly an order of magnitude lower density of cav­ities, composed of a bimodal distribution of bubbles and larger voids. Bubbles primarily form on disloca­tions in this case. Further, limited data showed rela­tively bubble-free interfaces in MA957, in contrast to interfaces in F82H that are highly decorated with small bubbles.

The results ofthe ISHI experiments compare very favorably with a He transport and fate master model that is under development. The master model is both microstructurally informed and incorporates parameters derived from the atomistic/electronic models cited above. Preliminary RT models have also been used to extrapolate the ISHI data to predict void swelling at higher dpa and He levels. These models suggest that FMS may experience significant swelling at damage levels greater than 50 dpa, while the NFA will remain void free. Indeed the combination of the experimental and modeling results suggest that He can be transformed from a liability to an asset in NFA.

Although these conclusions represent an optimis­tic view of the status of research on measuring, mod­eling, and managing He-dpa effects in structural alloys, especially for fusion applications, there are a number of outstanding issues that require additional science-based research to resolve. Space does not permit a complete listing, but these issues and research needs can generally be classified into broad categories related to He effects per se and those related to developing, optimizing, and qualifying alloys that can manage He in a way that may provide near immunity to radiation damage. He-related issues and research needs include the following:

• Extension of experimental observations of He transport, fate, and consequences to more alloys for a wide range of microstructures and to higher damage (He and dpa) levels and higher temperatures over a range of He/dpa, using all the irradiation techniques: CPI, ISHI, and SPNI. Characterization of the management of He in NFA at temperatures up to 750 °C, or more, is particularly critical.

• Intercomparisons of accelerated high-rate CPI and lower reactor relevant rate ISHI and SPNI data and development of experimental-modeling approaches that will permit reliable predictions of He-dpa effects at very high damage levels, up to several hundred dpa, that cannot be accessed prac­tically in neutron ISHI and SPNI.

• Very detailed characterization of He and He bub­ble distributions, along with the balance of irra­diation microstructures, with particular emphasis on quantitative evaluations, like the distribution of He and bubbles on interfaces, using a suite of advanced characterization methods tailored to this application.

• Focused mechanism studies that can provide both parameters and insights into key mechanisms that govern the transport, fate, and consequences of He in various complex alloy and model systems.

• Continued development and refinement of multi­scale master models of He transport fate and con­sequences, along with the narrower first principles and atomistic studies (models and experiments) needed to better parameterize and mechanistically inform the master models. One of many examples of modeling needs is the resolution of differ­ences between continuum capillary versus atomis­tic MD-based models of helium-vacancy cluster and bubble properties.

• Developing models and basic experiments that relate the microstructural consequences of the fate of He to the degradation of performance — sustaining mechanical properties. One of many examples is the detailed model of the behavior of He on GBs and the corresponding reduction in the grain boundary fracture strength.

However, it is clear that He must be managed as well as understood. Thus, issues and research needs also include development of practical NFA or alloys with similar attributes. These include the following items:

• Identify alloy composition-synthesis designs and thermal mechanical processing paths that optimize the NF, and the balance of NFA microstructures, so as to provide a balanced suite ofoutstanding and isotropic properties.

• Resolve issues of low fracture toughness and aniso­tropic properties in extruded NFA product forms.

• Demonstrate and understand the thermal and irra­diation stability of far-from-equilibrium NF and NFA microstructures.

• Develop practical fabrication and joining methods, which preserve optimal NFA microstructures and yield defect-free components.

• Reduce costs, improve alloy homogeneity and reproducibility, and establish industrial-scale sup­ply sources.

• Qualify new alloys for nuclear service for extended lifetimes.

We believe that there is a science-based framework to resolve these and other critical issues related to the effects and management of He in practical alloy sys­tems. (see Chapter 1.03, Radiation-Induced Effects on Microstructure and Chapter 1.09, Molecular Dynamics)