Reactor core-structure re-location modelling for severe nuclear accidents

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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 is to develop such methods, as well as to 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 seeks to help establish advanced computational methods to address problems in fault conditions, as well as to investigate aspects of system behaviour in severe accident situations. It 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.The specific situation that this project intends to investigate is in a civil reactor's response to severe accident scenarios following a loss of its coolant. Under such situations internal structures can be compromised and the melting of control rods and/or fuel pins may occur. In extreme situations whole sections of the core may melt resulting in large quantities of molten materials accumulating in the vessel's lower head. In all these situations the relocation of the core's materials will affect the functioning of the reactor. There is a possibility of achieving a sustained critical reaction, resulting in extensive heating, and the coolant flows will be diverted thus preventing the heat removal from parts of its core. In addition, chemical reactions can occur, some of which pose significant hazards. Examples include oxidation processes between the air and/or steam with the fuel cladding (zirconium in particular) or control rod materials resulting in hydrogen production - as what occurred in Fukushima.The aims of this project will be to develop a generic framework for accident modelling and validate it through the study of control rod and fuel pin melt. A computer model will be built capable of resolving, in unprecedented detail, the melting, relocating and re-solidification process of the pins when overheated. The objectives are to build the specialist numerical tools that enable the complex physics and chemistry to be resolved. The objective also includes a validation process by a comparison with experiment. This will be through collaboration with academics and industry in Japan.The outcomes of this work will help scientists and engineers understand the processes during accident and melting 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 regain or strengthen their confidence in nuclear technology.