Fundamentals of the Behaviour of Fission Products in Oxide Nuclear Fuels

With the UK committed to reducing its reliance on fossil fuels by 2050, there must be a commensurate increase from other energy sources to maintain and provide for increased energy requirements, at the same time increasing the diversity in energy sources. As a mature and clean energy production method, nuclear power will no doubt make a significant contribution to the UKs energy portfolio. The UK currently operates a civil nuclear fleet consisting primarily of advanced gas reactors (AGRs). Any new nuclear build in the forseable future will likely be new Pressurised Water Reactor (PRW), such as the Westinghouse AP1000 or Areva European Pressurised Reactor (EPR). Both reactors are designed to run using UO2 fuel types enriched with fissile uranium to ~3-5%. In addition to these generation III and III+ type of reactors, some fourth generation (GenIV) reactor types are designed to operate using UO2 fuels (3 out of the 6 final designs). Furthermore it is becoming more desirable by plant operators to increase the level of burn-up in the fuel, not only increasing the efficiency of the reactor, but also reducing operating costs and minimising the volume of nuclear waste produced.

One key factor limiting higher burn-up of nuclear fuel is the production and accumulation of fission products within the fuel that can significantly affect the physical properties and limit performance. Broadly speaking, there are four different characteristic fission products produced during burn-up: gaseous, metallic precipitates, oxide precipitates, and those in solid solution. The work to be undertaken here examines the behaviour of fission products under irradiation in both a non-radioactive model nuclear fuel simulant (ceria) and in simulated and real spent nuclear fuel. These materials will be fully characterised, both structurally and chemically, through a combination of transmission electron microscopy and atom probe tomography, with the results being fed back to modellers to validate and/or benchmark predictive models for in-core performance of the fuels, such as ENIGMA.

The behaviour of nanocrystalline materials under irradiation will also be investigated. Nanocrystalline materials are viewed to be a viable route towards radiation tolerance, and may therefore improve safety, due to the high number of grain boundaries and interfaces present acting as efficient sinks for defects. It is therefore critical that information on the radiation response of nanocrystalline materials is obtained if this class of materials is to be used in nuclear reactors as a means to improve reactor safety.

This characterisation of the materials in the as-received and irradiated state will be performed using the electron microscopy and atom probe capabilities at Oxford University. The ceria samples will be provided by the Universities of Nebraksa and Sheffield, and the real/synthetic/simulated fuels samples will be provided by the National Nuclear Laboratory. The NNL - who will benefit significantly from the results to be obtained from this project - will provide advice in the safe handling of radioactive material, and will provide access to the hot/active transmission electron microscope at the Sellafield central laboratory for characterisation of active samples.

WHO MIGHT BENEFIT FROM THIS RESEARCH?
The behaviour of nuclear fuel during burn-up within a reactor core varies with operating conditions. However, the common factor is that fission products are produced, can accumulate and can result in a reduction of fuel performance. This could be through the accumulation of metallic fission fragments into particles, the accumulation of inert gases into bubbles and solid solution fission products segregating to microstructural defects such as grain boundaries, resulting in detrimental effects such as swelling and grain boundary embrittlement.

There is a desire to burn nuclear fuels to higher burn-up levels by nuclear power plant operators such as EDF by leaving the fuel in pile for longer time periods. This helps offset the cost in fuel production, and reduces the fraction of refuelling time per cycle. If less fuel is being used, as each fuel rod is burnt for longer, the costs for disposal will also reduce. All of these factors can act to reduce the cost of energy to the end consumer.

Furthermore, the safety of nuclear reactor cores is paramount. One possible pathway to improve reactor safety is through the use of nanocrystalline materials due to their improved radiation tolerance. However there is little publicly available information on the radiation tolerance of nanocrystalline materials, and none-whatsoever about fission fragment accumulation.

This work will directly address the questions of how fission products in fuels and nanocrystalline materials - as potentially safe fuel matrices - behave in reactors cores under irradiation and at temperature.

HOW MIGHT THEY BENEFIT FROM THIS RESEARCH?
Through the examination of fission products in fuel surrogates, and their behaviour under irradiation and at high temperature, using advanced atomic scale experimental techniques will provide enhanced understanding of the behaviour of these materials. Such information can then be integrated into fuel simulation codes such as ENIGMA-B, used by the National Nuclear Laboratory. This will then improve predictions on the behaviour of real fuel in the core. Characterising real nuclear fuel in collaboration with the Australian Nuclear Science & Technology Organisation (ANSTO) will benchmark the use of non-radioactive surrogates.

A study of the radiation response of nanocrystalline materials will be undertaken during this project. This is an important study in the application of advanced ceramic materials for next-generation fuel systems as behaviour of these materials under irradiation must be understood if they are to be used as a viable means to improve reactor safety.

A key area of impact that this project will accomplish is the training of the next generation of skilled researchers in the nuclear materials field. This will be achieved through the training of a doctoral student who upon completion of their doctorate will be knowledgeable in the field of radiation damage in materials, and the application of advanced microscopy techniques in the analysis of radiation damage. Interactions with undergraduates will also be sought through the undertaking of final year (Part II) projects in the field of radiation damage in materials. The UK is lacking in these core skills and as such, this project will provide a means by which the UK can once again become a leader in the generation of new nuclear technology.