Title:	Mechanical, fracture and termodynamic modelling of reactor materials
DNr:	NAISS 2023/5-443
Project Type:	NAISS Medium Compute
Principal Investigator:	Pär Olsson <par.olsson@mau.se>
Affiliation:	Malmö universitet
Duration:	2023-11-30 – 2024-12-01
Classification:	10304
Keywords:

Abstract

The purpose of this project is to investigate (i) the impact of impurities and transmutation products on the mechanical and fracture mechanical properties fusion reactor materials and (ii) the vibrational and thermodynamic properties of uranium, titanium, yttrium and zirconium hydrides. The first project is a collaboration with the ICAMS-group (at Ruhr-University, Bochum) in which we aim to use classical and quantum mechanical modelling to probe the effects of transmutation elements on the grain boundary strength of tungsten alloys. Owing to the fact that empirical potentials for tungsten (W) are notoriously difficult to generate, with severely limiting transferability and predictability, our purpose is to generate a machine learning potential for W within the atomic cluster expansion (ACE) formalism, which can be expanded to the ternary W-Re-Os system to enable classical modelling with close to ab initio accuracy. This requires the systematic establishment of a large database comprising DFT data that covers a large range of low to high coordination configurations and accounts for different local bond orders. This potential will be used to explore the role of precipitation hardening on the brittle to ductile transition of W, while in operation. The generated potential will be also used to investigate the impact of impurities grain boundary decohesion. Such data will be generated to train a scale-bridging machine-learning surrogate that describes the mechanical properties of interfaces and can be used in continuum modelling to investigate the fracture of grain boundaries. For the final part of the project, we will compute the phonon spectra of yttrium and uranium hydrides. This serves two purposes: (i) to support experimental activities associated to devising a new method to use ultra-cold neutrons for characterizing the hydrogen content in hydride phases and (ii)) to generate thermal scattering laws (TSLs) for neutron interaction. The experimental activities will be performed under the supervision of Dr. Takeyasu Ito at the Neutron Science Center at Los Alamos Science Lab, while the researchers in Lund and Malmö will support the experimental activities with first-principles DFT data regarding phonon and thermodynamic properties. Owing to their relevance in nuclear reactor and light weight material applications, we are particularly interested in probing the vibrational properties of uranium hydrides (UHx) and yttrium (YHx) hydrides. Because of the difficulty to reproduce experimental hydrogen phonon spectra using conventional force-constant methods, we need to use ab initio molecular dynamics (AIMD) modelling to probe the dynamics of the system. For postprocessing purposes we will rely mainly on the TDEP package, which enables the extraction of finite temperature phonon data, including anharmonic effects, and has proven reliable to extract hydrogen phonon density of states. For the TSL generation, we have established a collaboration with another research group at Los Alamos, which we will support by providing additional thermodynamic and vibrational data associated with hydrides.