(84d) Development of a Hierarchical Multiscale Modeling Framework to Address Materials Grand Challenges for Fusion

Harnessing fusion energy is a demanding process and requires materials that can tolerate extreme environments. The key challenge for the development of fusion materials is a lack of representative laboratory-scale environments which can truly mimic the harsh fusion reactor conditions to reliably test the performance of candidate materials. As a result, fusion materials research relies heavily on computational materials modeling and simulation. Research in nuclear materials is inherently spatiotemporally multiscale in nature, and requires connecting atomic-scale phenomena, such as ion- or neutron-induced damage, to defect clustering and material damage evolution over mesoscopic scales, to leading all the way up to the engineering/reactor scale. Here, we focus on developing a hierarchical multiscale modeling framework capable of simulating the dynamical response of plasma-facing components (PFCs) under conditions representative of the environment in a fusion reactor, as an effort to tackle one of the major engineering grand challenges for the 21^st century. On this front, we will discuss two representative multiscale modeling efforts related to the divertor PFC of a nuclear fusion reactor, namely, ‘fuzz’ formation and subsurface helium bubble evolution in the tungsten divertor.

In the tungsten divertor, experiments aiming to investigate PFC performance under continuous operation and high first-wall temperature have shown that a nanostructure with a fuzz-like morphology develops on the helium-plasma-exposed tungsten surface under conditions relevant to fusion reactor operating conditions. The formation of this extremely brittle fuzz-like structure will seriously compromise the operation and performance of a fusion reactor. To first understand and then mitigate this deleterious effect, we have developed an atomistically informed, predictive continuum-scale model that is capable of capturing the formation and the early stage of evolution of the fuzz-like complex surface morphology mediated by various underlying dynamical processes characterized by disparate spatiotemporal scales. In our modeling framework, large-scale molecular-dynamics (MD) simulation results are used to develop the datasets required to parameterize the constitutive equations for the closure of the continuum-scale model for the surface morphological response of the plasma-facing material and targeted MD simulations are used to determine the thermophysical properties of low-energy-helium damaged tungsten.

Helium is insoluble in metals and forms self-clusters and bubbles in PFC tungsten, which causes significant deterioration in the thermophysical properties of tungsten. We will present a systematic study of the subsurface bubble dynamics based on a continuum-scale, spatially distributed drift-diffusion-reaction cluster dynamics model of helium self-cluster and bubble evolution, which can predict the helium retention in low-energy helium plasma-exposed tungsten. Our cluster dynamics model, as implemented in our code, Xolotl, has been benchmarked based on large-scale MD simulation results and experimental measurements available in the literature. We will also discuss an improvement in the predictive capabilities of Xolotl incorporationg into the cluster dynamics model the interaction energy landscape between a migrating cluster with another migrating cluster or a helium bubble, leading to an enhanced helium cluster and bubble growth, respectively. To incorporate this new physics into our cluster dynamics model following the hierarchical multiscale modeling framework, a systematic parameterization of defect interaction energetics was conducted based on molecular-statics (MS) computations of helium cluster-cluster and cluster-bubble energetics.

Furthermore, we will discuss a strategy for coupling the physics of subsurface helium bubble dynamics and gas retention in PFC tungsten, as modeled by Xolotl, with the surface evolution model and computer simulator of surface morphological response to study the feedback effect of this complex interconnected physics of helium-irradiation-induced damage in divertor tungsten. Such model and code couplings will pave the way for ultimately developing a digital twin of the fusion reactor.