OECD/NEA THAI-3 Project Final Report on Fission Product Behaviour, Hydrogen Mitigation, and Hydrogen Combustion in Water Cooled Reactors under Severe Accident Conditions

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The objective of the NEA THAI-3 project was to address open questions concerning the behaviour of hydrogen, iodine and aerosols under severe accident conditions in the containment of water cooled reactors. Understanding the processes taking place during such events is essential to evaluating the challenge posed on containment integrity (due to hydrogen combustion) and for evaluating the amount of airborne radioactivity (iodine and aerosols) during such severe accidents. The project also aimed to contribute to the validation and further development of advanced codes used for reactor applications, e.g. by providing experimental data for code benchmark exercises.

The NEA THAI-3 project has extended the knowledge and data base of severe accident-related issues. As regards hydrogen recombiners, further data about their behaviour under counter-current flow conditions have been obtained, which allow for better assessment of hydrogen mitigation measures and implementation of appropriate models in accident analysis codes. The tests regarding hydrogen deflagration behaviour in the two-vessel geometry have provided data for a sound safety-related assessment of flame acceleration and pressure increase in multi-compartment geometry. The tests concerning both molecular and aerosol-bound iodine release from a boiling sump have provided valuable data for the quantification of a potential accident source term.  

Remaining open questions with a specific focus on the late phases of severe accidents are currently being addressed in the NEA THEMIS project.