Micromechanical Creep - Improving Experimental and modelling capabilities

This project aims to develop a method for measuring creep deformation of in-core nuclear components in the presence of irradiation damage. Creep deformation is time-dependent permanent deformation of materials under load nominally at temperatures higher than half the material melting point. Creep deformation plays a crucial role in the structural integrity of engineering components that work at high temperature such as those in aerospace propulsion and energy generation. While it is known to be one of the main life limiting factors of nuclear fission power plants that work at high temperature little work has been applied to fusion relevant materials. This is despite the fact that any future nuclear fusion power systems rely on the development of materials which can withstand some of the most extreme engineering environments. These include temperatures up to 1500oC, high fluxes of high energy neutrons and effects of gaseous elements produced by transmutation and implantation from the plasmas. Due to efforts to minimise the production of nuclear waste by such reactors the elements which may be used in structural components is limited and in many cases there is a lack of understanding of the basic deformation processes occur in ether pure materials or alloys and importantly how these are effected by temperature, radiation damage and gas content. Due to the long time periods required for neutron irradiation campaigns, plus the associated cost and difficulty of working with active materials there is a need to develop robust methods for the characterisation of irradiated materials on the microscale. There are two advantages to this. Firstly it allows the maximum data return from small volumes of neutron irradiated materials. Secondly it allows the use of heavy ion irradiation to mimic neutron damage. In this case while the damage is similar to that of neutrons and can be built up in much shorter time frames the damage is only over a few 10's of microns. This precludes the use of traditional mechanical testing methods. While the methods required for understanding micro-scale plastic deformation are well developed micro-fracture testing has lagged behind. While there have been extensive efforts to develop micromechanical methods for measuring all major mechanical properties including, fracture stress and toughness, yield behaviour and elastic properties. Much less effort has been focused on creep. This is due to the fact that traditional micromechanical methods have only been able to operate at room temperature. This project will aim to develop an experimentally validated model of creep deformation on the microscale. This will be fed into a bigger project aiming to develop multiscale model for the complete nuclear industry.


We now have the ability to carry out these tests routinely up to 1273K in Oxford on three different systems. One of these systems allows simultaneous imaging using the secondary electron microscope, for imaged based analysis. This project will use these nanoindenters in conjunction with novel and newly designed micro-scale test specimens cut using focused ion beam machining to provide well describe geometries with simple stress states. Creep tests will be performed, both in and ex-situ, on fusion materials (initially reduced activation steels and 304 stainless steel, which are both also of wider interest in the nuclear field) and then the results validated by development of discrete dislocation dynamics modelling and finite element analysis. Once calibrated on unirradiated materials the method will be applied to irradiated steels. This will be the first time microscale creep tests on irradiated materials has been undertaken. This data will be used to inform high scale models being developed as part of a wider EPSC grant.

This project is funded by the EPSRC CDT in Science and Technology of Fusion Energy. This project falls within the EPSRC Energy research area.

Identifying a solution to the energy problem is crucial to the UK economy and quality of life. In the near term a range of renewable options must be developed, eg wind and solar, but it is unlikely that these will provide the base-load supply required. Nuclear is an option for a carbon-free base-load and, in particular, fusion energy is safe and relatively clean. If it can be achieved, fusion would bring the largest economic benefits to those countries that lead the way to build the first fusion power plants, but ultimately most people in the world will benefit from fusion in some way.
ITER, the largest international science project on Earth, will operate from 2020 to answer the final physics questions and most technology questions required to construct the first demonstration magnetic fusion energy (MFE) power plant, DEMO. We will train the ITER generation of UK fusion scientists who will have the expertise to win time on this key facility against international competition. This is crucial to build experience that will feed into the design of DEMO, ensuring the UK remains at the forefront. EU design studies for DEMO are already under way, with manufacture of prototype components likely to follow soon. There are a number of beneficiaries from this training: (1) it will benefit Culham Centre for Fusion Energy (CCFE), providing well-trained new staff to replace those retiring, keeping the UK at the forefront of fusion energy research, competitive for ITER time and leading elements of DEMO design/prototype development; (2) it will provide expertise for the growing UK industry involvement in fusion, helping to win contracts for ITER and DEMO prototype components; (3) it will ensure the UK has a cadre of fusion experts to advise Government on future directions. We expect to train 60 students in MFE, approximately balanced across plasmas, materials (relevant for IFE also, see below) and related fusion technologies.
For inertial fusion energy (IFE), NIF in the US is the most advanced device in the world, and some expected it would achieve fusion conditions, i.e. ignition. In its 2012 ignition experiments, this did not happen, but the reason why is still uncertain. The immediate need is to understand this, which requires experts to win time on international facilities (including NIF), understand why ignition did not occur and so develop a roadmap to IFE based on the new knowledge. This will benefit the UK Government by providing experts to advise on an appropriate strategy, able to compare the relative merits of IFE and MFE because of our training across both areas. If IFE proves viable, then it will need to integrate fusion technologies in a similar way to ITER and DEMO, bringing benefits to industry. We expect to train 15 students in high energy density physics (HEDP), spanning IFE and lab astrophysics; the MFE materials students' expertise is also relevant for IFE reactor design.
Expertise in HEDP is required by AWE for its science-based approach to underpinning the UK's nuclear deterrent, and is a key element of the UK's strategy to comply with the Test Ban Treaty. The new Orion laser facility at AWE can replicate the conditions in a nuclear warhead, enabling advanced computer codes to be tested. Our students will have the expertise to work with Orion, which requires skilled scientists as it establishes its programme. Also the materials and computational scientists amongst the ~60 MFE students will be of value to AWE.
We will train students in the cooler exhaust plasma of a tokamak. Similar plasma conditions are used in manufacturing industries (coatings, computer chips, etc) so we will develop a skill base that will benefit a number of such companies. Materials research for fusion is also relevant for fission. The popularity of fusion amongst students is a good way to bring outstanding students into the field, providing expertise that benefits the growing nuclear industries and supporting the Government's nuclear policy.