Investigation of the safe removal of fuel debris: multi-physics simulation

Modern computational codes can be very useful in assessing the behaviour of nuclear power facilities and ensuring that they present minimal hazard to; the public, facility workers and the environment when they enter unintended operating scenarios. This proposal will develop such methods, and will establish a simulation tool that is; accurate, robust, efficient and validated, and able to determine the levels of confidence that we can place on the models. It will be used for predicting possible consequences of reactor decommissioning and clean up following a severe nuclear accident. It also seeks to help establish advanced computational methods to investigate aspects of reactor behaviour during severe accidents. The technology proposed will help provide accuracy that is beyond what is currently possible, and will allow the physics to be explored that cannot be reproduced through experiment. The work proposed here seeks to achieve this by developing a basis for the verification and validation of computational tools against benchmark cases that will then be used to simulate more complex/realistic scenarios. The project will combine the expertise from the UK and Japan, both within academia and industry.

This project will investigate two aspects of decommissioning of its debris from severe nuclear accidents as well as the prediction of the evolution of core's materials during an accident. The evolution of the core material is important for decommissioning as it helps determine the final state of the internal structures within the reactor. Due to the condition of one of the Fukushima's stricken reactors, dry decommissioning, where the core is not flooded with water, may be necessary, and this novel method of fuel removal will be investigated here. An issue with dry removal is that it introduces the problem associated with radioactive dust being released into the atmosphere. Dust emissions will occur when the core is opened and parts of its debris are cut and removed. These particles will disperse and move within the air and so will present dangers to both site personnel and the immediate environment. To mitigate the severity of dust propagation fine mist sprays will be deployed within the core's surroundings to capture and remove dust particles from the air. The suitability of such an approach (and whether sufficient shielding to the environment is maintained) will be determined here where advanced modelling methods will be developed to simulate dust dispersion within the reactor and the particles' interaction with the water droplets. Using fine sprays may also overcome a second issue regarding dry removal by providing sufficient heat removal from the debris, which would have been otherwise been sufficiently managed had the core been flooded. A modelling framework will also be developed within this proposal to investigate this safety aspect.

The outcomes of this work will help scientists and engineers understand the processes during decommissioning activities as well as accident scenarios. They will help improve future designs and aid operators' responses to such events. In addition, they will help to enhance safety, limit damage and inform policy makers on design integrity. Importantly, the outcomes of this work will demonstrate to the public our commitment to safety in order to strengthen their confidence in nuclear technology.

This work will benefit those scientists, government bodies and industries concerned with nuclear power safety. It will also be of interest to those interested in multi-phase flows, structural models, damage models, geological safety of waste repositories and state-of-the-art computational modelling. Specific organisations that will benefit from the modelling and increased understanding include AWE, AREVA, NNL, NDA, HSE, Rolls-Royce, AMEC-FW, CGN (China) and EDF (now running UK nuclear reactors) and Hitachi-GE (building the UKs ABWRs). Example areas that will benefit include: nuclear safety, reactor core design analysis, training as well as decommissioning. It is the research outputs of the kind planned here that are important in view of the central role that nuclear power is expected to play over the coming decades. New reactors and reactor types, and those undergoing life extensions, must meet ever more stringent economic and safety criteria, and assessing their ability to meet these must increasingly rely on advanced computational modelling. The outcome of this research will help determine the strategy for nuclear accident modelling which aims to enhance safety in nuclear reactors and their decommissioning. Designers, assessors, regulators and operators of nuclear plants will benefit through better analysis tools, properly validated, which are essential to realize the continuing benefits of nuclear new build. In the unlikely event of an accident the framework proposed here will have helped the scientist and engineers in predicting the events that follow, thus improving their preparations for recovery. It would provide a key component in enabling optimized mitigation strategies to be developed during an evolving accident.

Society will also benefit from the economical and carbon-free energy that nuclear plants provide as well as reassurance that the science and tools behind their safety are of the highest quality. This in turn will help improve public confidence and perception of nuclear safety. The economic impacts will be seen from the potential use of the software and associated consultancy, both within the UK and abroad, and in the prevention of overseas codes dominating the market. The impacts also include addressing the urgent need to train scientists and engineers, at all levels including doctoral and postdoctoral levels, to undertake the necessary activities for the next generation of nuclear power. Besides the research outputs, programmes like this are valuable in training such people.

This work will also be of interest to wider EPSRC and NERC communities. For the EPSRC community, areas where our work will be of benefit include: plasma physics, nuclear waste modelling, industrial processing, combustion, and oil and gas. NERC communities will benefit from this next generation multi-phase technology for research in predictive flows in atmospheres and oceans. Many of the techniques and tools we will be working on are also of interest to the wider computational physics community. There is strong potential for their re-application to resolve other physical phenomena using the general adaptive discretisations and solver technologies. There is also a strong interest in using this software for multi-phase simulations of nuclear systems in National Laboratories including those in the USA, France and Japan, where our researchers have acted as consultants often using our models.