Uranium Silicide/Uranium Diboride Composite Fuel Development

Light Water Reactors (LWRs) comprise around 90% of the world's current nuclear generating capacity. The use of Accident Tolerant Fuels (ATFs) could offer significantly improved high temperature capability and operational safety. One way this can be achieved is by reducing the potential for reactivity with high temperature steam by replacing zirconium cladding with a ceramic composite material. This necessitates the replacement of existing UO2 fuel with a silcide; U3Si2, which is the focus of a concerted international research effort and aggressive commercial deployment plan \(see for example "Fuel of the Future", Nuclear Engineering International 2018).

This research effort includes a national research programme within the United States, including large efforts at Idaho National Laboratory, Oak Ridge National Laboratory, The University of South Carolina, Westinghouse and General Atomics (amongst others). It includes a range of ATF concepts, some very close to current fuel designs (UO2, engineered to have a larger grain structure and coated claddings) as well as more research intensive efforts to deploy a true step-change (U3Si2 fuel pellets with composite silicon carbide cladding). International fuel vendors (Westinghouse, GE, Areva) are particuarly interested in devloping ATFs, as they may also provide the key to reducing nuclear capital cost by removing the need for currently expensive and multiply-redundant safety systems. The aim of this research is to establish if the introduction of a boride phase to U3Si2 could improve the materials high temperature water performance.


This is a key challenge currently facing the ATF community Recent work performed by Sooby-Wood et al at Oak Ridge National Laboratory has demonstrated that U3Si2 rapidly and energetically pulverises when exposed to high temperature steam (E. Sooby Wood et al (2018). Journal of Nuclear Materials, 501, 404-412). Atomic scale modelling at the Universities of Bangor suggests that this is the result of a hydrogen reaction within the material, which leads to a rapid volume change and increases the surface area available for oxidation.

Similar atomic modelling of the UB2 system performed at the University of Bangor (in support of the present work and currently being prepared for joint publication) suggests it will not be susceptible to this damage mechanism, and so could provide protection to U3Si2 while maintaining the benefits of an ATF fuel material (high thermal conductivity, high uranium density etc). Preliminary work has produced several test pellets by coarse mixing U3Si2 and UB2 powders in various weight percentages. On investigation, the boride and silicide phases do not have a detectable interaction layer around them, and the UB2 phase appears to have been successfully sintered within the U3Si2 phase.

Preliminary steam tests on these pellets shows the steam reaction with composite pellets onsets at significantly higher temperatures than pure U3Si2 and appears to react much more slowly. This suggests that the presence of UB2 within U3Si2 may do more than simply limit the physical pathways by which U3Si2 is attacked by steam, as it appears to change the reaction mechanism in some way as noted by the lack of any reaction below 500C.

The proposed work will build upon these preliminary studies and assess the feasibility of utilising UB2 within U3Si2 fuel material to improves its steam-corrosion resistance to acceptable levels. This will include pellet manufacture and characterisation, steam testing, proton irradiation tests, autoclave studies and characterisation of the effects of temperature cycles.

The proposed research will create a significant body of knowledge on the performance of advanced nuclear fuel materials. Nuclear fuel has been awarded a high priority by the Government's Nuclear Industry Research Advisory Board (NIRAB), and Accident Tolerant Fuels (ATFs) the highest priority within the proposed body of fuels research. This is largely due to their potential to provide a step-change in both the economic viability and safety of nuclear power.

It will be of great importance to ensure that the results of this research are disseminated amongst as wide an audience as possible; not only amongst peers within the nuclear field, but non-technical parties who take an interest in nuclear matters. Therefore our pathway to impact will be through two routes: 1) Knowledge management, and 2) Outreach and public engagement.


Knowledge Management
- Knowledge management for the proposed research will need to be considered within three environments:
- The UK nuclear community of academic and industry partners;
- The international nuclear community;
- The broader non-nuclear community who may benefit from access to the research.
The priority will be to disseminate the results of the research by publication in open-access journals. We believe that knowledge retention and dissemination is best achieved using public domain systems. Whilst this covers documented knowledge, much of the learning from R&D comes from interaction with fellow researchers, and therefore engagement with fellow academic and industry researchers and practitioners will be sought throughout the project.

Outreach and Engagement
The proposed strategy for outreach and engagement is driven by three criteria; who (needs to be informed), why (will they want to listen) and how (will we get them the information they need, in the form that bests suits them).

Who and Why
There are two non-academic constituencies that will benefit from learning about this research:
Industry: Directly there are key U.K. industrial stakeholders (Westinghouse U.K.) who have an interest in the support of a U.K. nuclear fuels capability and engagement with these stakeholders will be sought as a matter of priority. More broadly, a wide range of non-academic organisations, including regulators who will have a stake in the future of nuclear energy in the UK can also benefit. This group will be interested in the technical knowledge and skills created during the research and in a form which they can use to support their own commercial and technical strategies.
Public. Nuclear energy raises a broad spectrum of interest across society. Our public engagement will focus on how the research will address issues such as safety and energy security. In particular, the focus of the work on developing accident tolerance is likely to be of interest to the public, as nuclear safety is of high concern following Fukushima

How
Participation in existing academic seminars - Experience has shown that open seminars with good attendance from industrial participants can bring great benefit to both industry and academia. The key existing research programme, with which the proposed research would closely relate is the ATLANTIC programme on nuclear fuel and fuel cycle research we therefore propose to disseminate the results of our work via the workshops and seminars that are organised in the frame of this programmes.
In addition to this, two dedicated workshops will be held as part of the proposed work, focused more directly on the water performance of Accident Tolerant Fuel candidates. These will be open to both academic and industrial partners with an interest in the field, and take place at the end of years one and two of the work.