Development of Advanced Ceramic Breeder Materials for Fusion Energy

Lead Research Organisation: Lancaster University
Department Name: Engineering

Abstract

Nuclear fusion offers the promise of abundant, clean, low cost energy. The fusion process involves fusing together two nuclei releasing large amounts of energy that can be harnessed for electricity generation. Future power stations will employ the reaction between two isotopes of hydrogen, tritium and deuterium, creating a helium atom and a neutron. Deuterium is available from seawater, however, tritium does not occur naturally due to its short half-life. Therefore, tritium will be created, or bred, in the reactor from lithium in a process called transmutation.

Transmutation will occur immediately outside the main chamber of the reactor, in a region called the breeder blanket. One of the leading breeder blanket designs will use lithium containing pebbles, such as lithium metatitanate and lithium orthosilicate. Solid breeder materials are attractive as they have high lithium densities that will ensure excellent tritium production and their low reactivity with other reactor materials means they are safe. However the use of a solid breeder material means that following transmutation the tritium will be trapped in the pebbles and must be extracted from the crystal.

For recovery the tritium must diffuse to the pebble surface where it can be carried away by the coolant. The rate at which the tritium can escape from the pebbles is a very important parameter to consider when designing a fusion reactor because if the rate drops too low and tritium is retained in the pebble the fusion reaction will be unsustainable. Therefore, the main goal of this research to understand the process of tritium diffusion in lithium ceramics to design materials that have high tritium release rates.

The exact mechanism of tritium release will depend on the microstructure of the host material.
All crystals contain defects, such as missing atoms (called vacancies), and these defects can either promote tritium release or act as traps and inhibit it. The types and concentrations of defects in a material depend on the exact conditions (i.e. temperature) and will evolve over time. Therefore, to understand the tritium release process we must first understand the microstructure of the ceramics and what defects are present

Previous studies of tritium release have adopted a top down approach where experimentally observed tritium release rates under different conditions are used to infer the exact atomic level mechanism responsible. By contrast this proposal adopts a novel bottom up approach that uses advanced electronic structure calculations to build a tritium release model from first principles.
A key advantage of this approach is that the calculations provide detailed understanding of the atomic rearrangement processes that constitute tritium diffusion and allow a rate to be determined for each process.

Initially the intrinsic defect chemistry of the host materials will be examined. This will allow the identification of the defects present in the ceramic under different conditions. Once the intrinsic defect populations are established the interaction of tritium atoms with the defects will be studied. By examining the bonding between tritium and the defects it is possible to determine exactly where the tritium will sit in the crystal and to identify which defects will act as traps.

The information gathered so far considers where tritium will sit in the crystal but it does not provide information about how quickly the tritium can move through the crystal. Therefore, the next step in the process is to understand how tritium hops between the defects available and to determine which types of hop are most likely under certain conditions.

Finally, all of this information will be used to create a tritium release model from lithium ceramics. This model will be used to optimise the microstructure of the ceramics to deliver maximum tritium release to ensure the fusion process is sustainable.

Planned Impact

This proposal will develop advanced breeder materials for fusion energy and will have the positive impacts listed below, some of which are already being realised as a consequence of writing this statement:

Society

The rise in the global population coupled with rapid economic growth in emerging economies indicates a dramatic increase in global energy demand. Fusion could contribute a large percentage of our future energy needs at a competitive price without generating vast quantities of CO2. Realisation of these societal benefits will require significant sums of research funding. Ensuring the continuation of this funding depends on public support, therefore, engaging with the public to maintain this goodwill will be an important aspect of this project. Principally, this will involve giving the work a high visibility in the popular press, specialist science outlets and via social media (see Pathways to Impact).

Economy

Fusion energy may still be 30 years away but there are key decisions and developments occurring now that will establish the economic beneficiaries in the future. To capitalise on the UK's leading position in fusion Culham Centre for Fusion Energy (CCFE) are designing a Demonstration fusion power plant (DEMO) that could meet domestic energy demand and be built under licence around the world, contributing to future economic prosperity. The proposed scheme of work will aid the design of the breeder blanket region offering a significant advantage in the design of the overall reactor. The proposed research will aid the design of the fuel pebbles themselves and may result in patents and other intellectual property that will support large scale manufacture of fusion fuels in the UK for export, creating high value jobs. These could replace existing nuclear fuel fabrication such as Westinghouse's Springfield site in Preston, helping to maintain the local economy in one of the poorer parts of the UK.

There are some small and medium sized enterprises working on developing small fusions devices, such as Tokamak Energy. Tokamak energy plan to produce electricity from their reactor in 2025. The reactor will also use deuterium and tritium reaction and will require tritium breeder blankets. Dr. Murphy will engage with Tokamak Energy to develop breeder blankets for reactor with the intention of generating intellectual property and patents that will support UK prosperity.

People

This research proposal will generate three highly trained research staff. As a PI for the first time, Dr. Murphy will gain experience of the management of a research project enabling him to generate larger grants in the future. The project will also foster collaboration between the PI and CCFE, enabling Dr. Murphy to establish his reputation in the fusion community and raise his research profile internationally, for example joining the EUROfusion programme. Joining the international Fusion research community will open up possibilities for international collaboration and exploitation of international funding opportunities.

A Post Graduate Research Associate (PDRA) will be hired to work on the project and Lancaster University will fund a PhD student if the bid is successful. The PDRA and the student will gain experience of working in a cutting edge research environment, where they will develop their critical analysis and writing skills as well as gaining experience of high performance computing and working with large datasets. Dr. Murphy will involve undergraduate project students in the project, exposing them to a research environment helping them develop analysis and writing skills that will benefit their future careers. The involvement of undergraduate students will provide the PDRA and student experience of co-supervision of small research projects helping to develop their leadership skills. Therefore, all people involved in the project will develop new skills and capabilities to contribute to the future prosperity of the UK.

Publications

10 25 50
 
Description The most significant outcome of this project is the prediction that it will become more difficult to extract tritium from the ceramic breeder Li2TiO3 as the material ages. This is due to the trapping of tritium at lithium vacancy defects. The project revealed that the most energetically favourable mechanism for tritium diffusion in the presence of a high concentration of lithium vacancies is a collaborative mechanism where the two defects move through the lattice together rather than via a trapping and detraining process as previously believed. Despite the collaborative mechanism being lower in energy than the trapping and detraining mechanism it is still significantly less favourable than tritium interstitial diffusion which implies that as the number of vacancies increases the tritium release rate will slow. This may lead to retention of tritium in the ceramic pebbles and ultimately restrict the pebbles operational lifetime.
Exploitation Route The outcomes will have an important impact on the development of tritium breeding materials for fusion as it may limit the operational lifetime of the material. This will require further experimental confirmation.
Sectors Energy

 
Description Atomistic study of radiation induced degradation of thermal conductivity in fusion materials
Amount £35,000 (GBP)
Organisation UK Atomic Energy Authority 
Sector Public
Country United Kingdom
Start 10/2018 
End 09/2021