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

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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.