Degradation of thermal conductivity in tungsten alloys under irradiation

Tungsten-based alloys are currently the principal candidates for plasma facing components in fusion reactors due to their unique combination of properties such as neutron irradiation resistance, low sputtering yield, good high-temperature strength and thermal conductivity. The relatively high ductile-to-brittle transition temperature of tungsten, which even increases under neutron irradiation, is one of the main problems of using tungsten as plasma-facing material. Alloying tungsten with elements such as Ti, V or Ta improves the material's ductility. Unfortunately, our limited neutron irradiation data proves the degradation in thermal properties of pure W, and the effect of irradiation in thermal performance of those alloys remains unexplored. Changes in thermal conductivity due to irradiation damage is one of the more pressing areas of information required for the deployment of fusion reactor technology. The aim of the project is to detect changes in thermal conductivity in irradiated tungsten-based alloys, and to correlate those changes in conductivity to the lattice defects induced by radiation fields simulating those expected in the proximity of the hot plasma in fusion reactors. This project involves the use of intense ion beams to simulate the neutron damage that W alloys will be experiencing in future reactor conditions. Ion irradiation allows us to achieve the damage doses that will be experienced inside the reactor after several years of operation. The student will characterise the damaged areas of the material at the atomic-to-nanometer scale using advanced analytical electron microscopy and positron annihilation spectroscopy. The nature and concentration of radiation-induced lattice defects, such as dislocation loops or voids, will be correlated with variations in thermal conductivity. The student will have opportunities to learn fundamental radiation damage mechanisms to nuclear materials at an advanced level, to use upfront particle accelerators at large scale international user facilities, and to become an advanced user of electron and positron probe techniques.

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.