The effect of irradiation on materials is a classic example of an inherently multiscale phenomenon, as schematically illustrated in Figure 1. Pertinent processes span over more than 10 orders of magni­tude in length scale from the subatomic nuclear to

Nano/microstructure and local chemistry changes; nucleation and growth of extended defects and precipitates I

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Defect

recombination, clustering, and migration Primary defect production and short-term annealing

Underlying
microstructure
(preexisting and evolving)
impacts defect and solute
fate

structural component level, and span 22 orders of magnitude in time from the subpicosecond of nuclear collisions to the decade-long component service life­times.1,2 Many different variables control the mix of nano/microstructural features formed and the corresponding degradation of physical and mechanical properties of nuclear fuels, cladding, and structural materials. The most important variables include the initial material composition and microstructure, the thermomechanical loads, and the irradiation history. While the initial material state and thermomechanical loading are of concern in all materials performance — limited engineering applications, the added complexity introduced by the effects of radiation is clearly the distinguishing and overarching concern for materials in advanced nuclear energy systems.

At the smallest scales, radiation damage is continu­ally initiated by the formation of energetic primary knock-on atoms (PKAs) primarily through elastic col­lisions with high-energy neutrons. Concurrently, high concentrations of fission products (in fuels) and trans­mutants (in cladding and structural materials) are gen­erated and can cause pronounced effects in the overall chemistry of the material, especially at high burnup. The PKAs, as well as recoiling fission products and transmutant nuclei quickly lose kinetic energy through electronic excitations (that are not generally believed to produce atomic defects) and a chain of atomic col­lision displacements, generating a cascade of vacancy and self-interstitial defects. High-energy displacement cascades evolve over very short times, 100 ps or less, and small volumes, with characteristic length scales of 50 nm or less, and are directly amenable to MD simula­tions if accurate potentials are available.

The physics of primary damage production in high-energy displacement cascades has been exten­sively studied with MD simulations.3-8 (see Chapter

1.11, Primary Radiation Damage Formation) The key conclusions from the MD studies of cascade evo­lution have been that (1) intracascade recombination of vacancies and self-interstitial atoms (SIAs) results in ^30% of the defect production expected from displacement theory, (2) many-body collision effects produce a spatial correlation (separation) of the va­cancy and SIA defects, (3) substantial clustering of the SIAs and to a lesser extent the vacancies occur within the cascade volume, and (4) high-energy displace­ment cascades tend to break up into lobes or subcas­cades, which may also enhance recombination.4-7

Nevertheless, it is the subsequent diffusional trans­port and evolution of the defects produced during displacement cascades, in addition to solutes and transmutant impurities, that ultimately dictate radia­tion effects in materials and changes in material micro — structure.1, Spatial correlations associated with the displacement cascades continue to play an important role in much larger scales as do processes including defect recombination, clustering, migration, and gas and solute diffusion and trapping. Evolution of the underlying materials structure is thus governed by the time and temperature kinetics of diffusive and reactive processes, albeit strongly influenced by spatial correlations associated with the microstructure and the continuous production of new radiation damage.

The inherently wide range of time scales and the ‘rare-event’ nature of many of the controlling mech­anisms make modeling radiation effects in materials extremely challenging and experimental characteri­zation often unattainable. Indeed, accurate models of microstructure (point defects, dislocations, and grain boundaries) evolution during service are still lacking. To understand the irradiation effects and microstructure evolution to the extent required for a high fidelity nuclear materials performance model will require a combination of experimental, theoreti­cal, and computational tools.

Furthermore, the kinetic processes controlling defect cluster and microstructure evolution, as well as the materials degradation and failure modes may not entirely be known. Thus, a substantial challenge is to discover the controlling processes so that they can be included within the models to avoid the detri­mental consequences of in-service surprises. High performance computing can enable such discovery of class simulations, but care must also be taken to assess the accuracy of the models in capturing critical physical phenomena. The remainder of this chapter will thus focus on a description of KMC modeling, along with a few select examples of the application of KMC models to predict irradiation effects on materials and to identify opportunities for additional research to achieve the goal of accelerating the devel­opment of advanced computational approaches to simulate nucleation, growth, and coarsening of mi­crostructure in complex engineering materials.