Thermomigration of Hydrogen in Reactor Fuel Cladding Materials

About the Project

This project is part of cohort 3 of the EPSRC CDT in Developing National Capability for Materials 4.0, with the Henry Royce Institute.

This project will study how temperature gradients drive hydrogen diffusion. This little-studied effect, called thermomigration, can strongly impact hydrogen embrittlement. It is especially important in systems with steep thermal gradients, such as hydrogen fuel systems, nuclear fuel cladding and fusion reactor armour. This project will develop a machine-learning-enhanced digital material twin to correctly capture this effect, leveraging new thermo-migration measurements. The transport of solutes through metals due to a temperature gradient is known as thermomigration. It is important for structural metals exposed to hydrogen (H), as H embrittlement and hydride precipitation are sensitive to local concentration. To correctly predict H transport, especially in the presence of steep temperature gradients, thermomigration must be accounted for. In reactor fuel cladding, steep temperature gradients arise radially from internal heating and water-side cooling. Thermomigration is likely to dominate the H concentration profile. For high fidelity modelling of H and hydride embrittlement in cladding materials this effect must be properly understood. Unfortunately, there is a lack of physical clarity regarding of the driving force(s) for thermomigration, including the complex associated electronic effects. The development of new modelling capabilities is urgently needed. These in turn require reliable experimental data across broad temperature and temperature gradient ranges.

The heat of transport, 𝑄∗, is used to quantify the direction and magnitude of thermomigration. Surprisingly, there is little experimental 𝑄∗data, with the most prominent examples found in Zr cladding alloy [1-3]. However, this data was attained post hoc by measuring H content at ambient temperature [4], introducing considerable uncertainties. The most robust heat of transport measurements were reported by Gonzalez and Oriani in the 1960’s for pure Fe and pure Ni, using a thermo-osmosis technique to measure 𝑄∗ in the 400 − 600 °C range [5]. The central goal of this project is to develop a digital material twin that accurately captures hydrogen transport and trapping in Zr under complex conditions (stress, temperature, hydrogen, irradiation etc.). To support this, a new rig will be constructed to allow robust 𝑄∗ measurements across broad temperature ranges. The rig will consist of a permeation cell with precise temperature gradient control across thin membrane samples, coupled with a mass spectrometer for highly sensitive H flux measurements. A detailed digital twin of the experiment will be constructed to allow rapid inversion of experimental measurements into Q* data.

The new heat-of-transport data will allow the student to test a physically-based model we recently proposed to capture the temperature dependence of heat-of-transport [6], and to calibrate this model for Zr. A key challenge will be to develop approaches to capture and explain differences between the model predictions and the experimental measurement, for example due to the transient interplay between H thermomigration and hydride precipitation at low temperatures, or evolution of the microstructure. A key complication is that our previous results suggest that molecular dynamics simulations are unlikely to accurately capture thermomigration due to the lack of electronic effects. Here, we propose to leverage the use of machine learning descriptors of atomistic configurations as a proxy to predict evolution in heat-of-transport (similar to an approach we developed to predict thermal conductivity from MD simulations of defective metals).

To maximise impact, the “thermomigration” digital material twin developed in this project will be integrated into the comprehensive material model being continuously developed by Rolls- Royce for fuel cladding design and optimisation. The combination of sophisticated new experiments with cutting-edge material simulation and surrogate modelling to tackle a complex materials challenge with direct industrial impact strongly reflects the core motivation of the Materials 4.0 CDT.

Further details about the project can be found at https://www.materials.ox.ac.uk/article/thermomigration-of-hydrogen-in-reactor-fuel-cladding-materials.

Funding Notes

This PhD project is fully funded through the EPSRC Centre for Doctoral Training (CDT) in Materials 4.0.

The studentship covers tuition fees, a tax-free stipend at the UKRI rate, and a generous research and training support grant.

Enquiries

For general enquiries, please contact doctoral-training@royce.ac.uk .

For application-related queries, please contact graduate.admissions@materials.ox.ac.uk. Please note that each partner of the CDT in Materials 4.0 will have its own application process.

If you have specific technical or scientific queries about this PhD, we encourage you to contact the lead supervisor, Professor Felix Hofmann (felix.hofmann@eng.ox.ac.uk).

Application webpage

https://www.ox.ac.uk/admissions/graduate/courses/materials-4-0. Application instructions can be found on the 'How to Apply' tab.