Bridging the gap between small scale mechanical testing and bulk material property measurements of advanced, structural nuclear materials

The development of next-generation fission plants require advanced structural materials for higher temperature reactors, fast-neutron Generation IV reactors and compact Small Modular Reactors (SMR's). Understanding and measuring the mechanical properties of these structural materials, and how they evolve under extreme conditions is crucial for their successful operation for large-scale carbon-free electricity generation. Traditional methods of mechanical testing (e.g. tensile testing, impact testing) are not straightforward when handling active materials. The use of hot cell facilities can be extremely costly, and safely handling the volumes of material required for such tests requires complex operational protocols. The use of energetic ions to emulate neutron irradiation damage can yield high damage rates with residual activity that is negligible (proton irradiation) or absent (heavy ion irradiation) and at a low cost. Although a useful tool, ion irradiation has challenges; The most significant challenge for mechanical property measurement arises because heavy ion irradiation penetrates only a very shallow depth into the material (approximately a few microns) and proton irradiation can achieve only slightly more (approximately tens of microns) - this is not adequate for traditional mechanical testing techniques. In addition to the volume being small, the damage level varies substantially through the thickness of the ion-irradiated layer. Hence there is a critical need for a reliable method of measuring the full mechanical response of these materials, from a small-scale technique and the principle subject of this research is to develop a micromechanical testing technique that can reliably measure the bulk mechanical properties, up to and beyond the yield point of nuclear materials.

The proposed research is based on spherical indentation, an early micromechanical testing technique. Taking a step back from the more advanced techniques that have been developed in recent years, this method will eliminate the fabrication complexities associated with the current state-of-the art micromechanical techniques and provide a statistically rich, non-destructive and simple method of mechanical property measurement. Development of this method, will successfully allow the irradiation induced mechanical property changes to be measured from both reactor irradiated, and ion irradiated materials through a combination of experimental validation, computational modelling and advanced characterisation. In addition to this, the mechanical properties of nuclear materials at operating temperatures will be obtained, to understand how these materials will respond in service conditions.

This work will primarily be carried out in the Department of Materials at the University of Oxford working closely with the Materials for Fusion and Fission Power (MFFP) group consisting of talented scientists working on characterising and understanding irradiation damage in a range of nuclear materials. Experimental work on active material will be carried out at the Materials Research Facility (MRF) that is part of the Culham Centre for Fusion Energy (CCFE); this research will be carried out in close collaboration with the materials scientists working at the MRF who have substantial knowledge and expertise in irradiation damage of nuclear materials. Access to large specimens, for bulk mechanical property measurement that have been reactor irradiated, has been made possible through collaboration with the US. A research visit to Oak Ridge National Laboratory will feature in this Fellowship, to enable use of these materials in a world-class research facility. The Fellowship will form a crucial contribution to the Integrated Research Project, titled 'High Fidelity Ion Beam Simulation of High Dose Neutron Irradiation' and will measure the mechanical properties of the key materials of interest in the study.

Who will benefit from this research?

The proposed work will have a profound impact on the way in which structural materials are tested in the nuclear industry and within materials research. This technique will be preferred over traditional mechanical testing techniques by both researchers and design engineers working towards the development of next generation fission reactors. The work is also relevant to researchers working on materials for fusion technology, as the candidate materials of interest to this study are relevant to both technologies.

Directly, this work will impact the wider nuclear materials community, within the UK and the US and they will benefit from the development and implementation of this work. The Oxford Materials 'super group' as well as other academic groups and industrial partners will benefit from this Fellowship. Many of these research groups already have access to the basic equipment required to implement this technique, a Nanoindenter, and development of the method will enable them to generate further information from their experiments. Furthermore, the Materials Research Facility (MRF) at the Culham Centre for Fusion Energy (CCFE) will be provided with a technique that they and their users can implement. The data generated from this work will directly complement the NEUP IRP project and the developed technique can be implemented and shared amongst the partners in this project.

How might they benefit from this research?

The developed method will be preferred over traditional mechanical testing techniques due to its ability to ability to deliver equivalent mechanical property information, from a non-destructive method. The technique will be standardised and a method of 'best practise' will be established in order to aid researchers and design engineers when implementing this technique. The work will provide a full set of mechanical properties to benchmark key candidate materials and their ability to perform when exposed to high doses and extreme conditions.

The MRF has the capability to handle and test active materials with the use of hot cells, in an accessible manner that does not require complex protocols and requirements. This is highly desirable for researchers in the nuclear community who wish to study active materials. The development of this technique will enable users of the MRF to measure the full mechanical response of their materials, particularly when specimens are limited to small volumes.

The IRP project currently lacks the ability to measure the full mechanical response of their key specimens consistently; development of this method, and the measurements obtained from the materials within the project will provide the project with the essential data required to benchmark the materials and develop predictive models.

The workshop held at the end of the Fellowship will give postdocs and students working in this field the opportunity to become up-to-date with the latest research from key academics. They will benefit from networking with other members of the nuclear materials community, initiating collaborations and insightful discussions.