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Progress in nuclear reactor containment safety research has led to a deeper understanding of the phenomena that may occur under potential accident conditions. However, there is still a need for additional information due to limitations in experimental databases. The uncertainties are related to the distribution of combustible hydrogen and to the behaviour of fission products, in particular iodine and aerosols.
In the case of hydrogen, uncertainties emerge mainly in the determination of conditions for the occurrence of deflagration flames and in the performance of devices designed to reduce the concentration of hydrogen gas developed in a hypothetical accident, such as passive autocatalytic recombiners. Some concern also prevails regarding the applicability of several previous experiments where helium was used to simulate hydrogen. The relevance to reactor safety is connected with the destructive potential of fast deflagrations.
In the case of fission products, a number of transport processes have not yet been investigated to a level of detail sufficient to set up reliable transport models. Such processes include the exchange of iodine between a turbulent atmosphere and the walls, relocation by wash-down, i.e. washing of walls by condensate water, airborne chemical reaction of iodine with radiolytic ozone, and aerosol re-suspension from a boiling sump. The control of volatile radioactive species is relevant to the potential accident source term and radioactivity management.
The experiments of the OECD/NEA ThAI Project were designed to fill these knowledge gaps by delivering data for the evaluation and simulation of the hydrogen and fission product interactions mentioned above, thereby supporting the validation of accident simulation codes and models. The experiments were conducted in the ThAI facility in Frankfurt, the name of which is an acronym for "Thermal-hydraulics, Hydrogen, Aerosols, Iodine". Within the OECD/NEA ThAI project, the following test series have been performed:
Helium/Hydrogen Material scaling (HM)
Hydrogen Deflagration (HD)
Hydrogen Recombiner (HR)
Interaction of Metal Iodides with Passive Autocatalytic Recombiner
Passive Autocatalytic Recombiner Poisoning
Aerosol Wash-Down (Scoping Test)
Altogether 70 tests have been performed between January 2007 and December 2009.
In addition to the experimental work, an analytical workgroup was established in the frame of the OECD-ThAI project, aimed at the evaluation of the test results for further development and validation of the predictive capabilities of advanced LP codes and CFD codes currently in use in the reactor safety field. For this purpose, a number of experiments have been selected for blind and open post-test calculations, some of them for an ISP.
A concluding seminar on the main outcome of the ThAI Project will be organised on 6-7 October 2010. Remaining open questions shall be addressed in future investigations. An OECD/NEA ThAI-II project is envisaged to be started in late 2010.
Project participants: The project will be supported by safety organisations, research laboratories and industry in the following countries: Canada, Finland, France, Germany, Hungary, Republic of Korea, the Netherlands and Switzerland.
Project period: January 2007 to December 2009
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Keywords: aerosols, containment, fission products, hydrogen, iodine, severe accident, thermal hydraulics.