Fundamental Properties of Thoria Based Mixed Oxides

There is considerable concern that uranium reserves are not sufficient to facilitate large scale international nuclear new build. Thorium is around four times more abundant than uranium, and could offer a potential alternative fuel cycle. Just as importantly, beyond ~500 years, thorium based spent fuel and associated reprocessed wastes are much less radioactive than those arising from conventional uranium fuels. Furthermore, because they generate only very small amounts of plutonium, thorium based fuels are not useful for the production of conventional nuclear weapons, which rely on plutonium - in this regard they are much more proliferation resistant.

At present, except in India, thorium based fuels have only used in research or prototype energy generating reactors. This is because there have been and remain sufficient supplies of uranium. Conversely, in India, the lack of an indigenous uranium supply has driven the development of a thorium dioxide based approach to civil nuclear energy. India is on the verge of completing the second stage in that development. It will continue to develop experience in thorium based fuels for civil nuclear energy applications rapidly over the next decade. This requires a predictive capability to establish that fuel being irradiated in a civil reactor will behave in a manner that is compatible with its design criteria, especially the safety systems of the reactor. In a general sense, this mirrors the requirement for uranium dioxide based energy generation.

The manner in which a safety case for civil reactor operation evolves is complex but takes advantage of developments over decades. For uranium dioxide based fuels, this has resulted in safe and secure operation that has seen a steady improvement in the efficiency with which nuclear fuel is utilised. Further increases in efficiency are certain, but will require modifications to existing strategies. In particular, more research must be undertaken that satisfies regulators that fission products are retained safely and securely within the fuel assembly as it spends more time within the reactor core. This translates to fuel that can retain the fission products within its crystal lattice for longer and that the thermal conductivity of the fuel does not deteriorate. However, unlike in the past where we only had access to experimental work on which to base the fuel performance predictions, we now have advanced modelling techniques that together with experiment can provide better understanding of the fundamental processes responsible for fuel behaviour.

In this project we will use advanced materials simulation techniques to investigate the behaviour of thorium dioxide based materials. This includes the movement of fission products through the lattice and thermal conductivity. These will then be compared to predictions that are being made on civil uranium dioxide based materials in related projects. Comparison will also be made to experimental data already available concerning uranium dioxide and thorium dioxide but also data being generated by collaborators in India on thorium dioxide. This has the advantage of testing existing models that have been developed for uranium dioxide on a different system. We have developed models for existing fuels that include assumptions. Comparison to thorium dioxide provides a more stringent test of those models. It also allows us to understand to what extent it might be possible to translate the uranium dioxide based models to predict the evolution of thorium dioxide fuels. Collaboration will also proceed with modelling being carried out in India.

The computer simulation work described in this proposal will lead to developments of models that can better describe the behaviour and microstructural evolution of nuclear under the extreme conditions experienced within a reactor. Whilst the performance of models has been adequate for current generations of uranium dioxide based fuels, future fuel duties are likely to extend beyond the currently available experimental data-base, especially to higher burn-up. This is also true for thorium dioxide based mixed oxide fuels, where high burn-ups are anticipated to facilitate better utilisation of uranium and thorium (this also leads to more proliferation resistant waste). Thus, the civil sector in India will benefit from this work because it will gain insight into the behaviour of thorium based fuels that will be used exclusively within their civil sector (although it is possible that other countries may follow a thorium route in the future). The UK civil nuclear sector will benefit from the development of more robust and transferable models that will be tested well beyond normal UK experience of fuels.

Significantly, further experimental work is both expensive and time consuming. It is therefore highly desirable to use modelling to inform experimental efforts, thereby reducing the burden of future experimental work. In addition, modelling can identify the underlying physical processes responsible for fuel performance and hence extend the validity of physically-based models. It can also support the investigation of severe accident conditions that are currently beyond the capabilities of current experimental facilities, and also highlight the critical features of the underlying physical processes that need to be simulated and validated using future experimental facilities.