Numerical Analysis of Innovative Hydrogen Mitigation Measure and Passive Autocatalytic Recombiner in Nuclear Power Plant

Abstract

The basic goal of severe accident management in nuclear power plants (NPPs) is the protection of the containment integrity and the containment becomes the ultimate barrier against the release of fission products to the environment. There are various potential challenges to the containment integrity during a severe accident in a light water reactor (LWR). For most NPPs, severe accidents lead to hydrogen

release rates that exceed the capacity of hydrogen control measures at conventional design basis accident. High local hydrogen concentrations can be reached in a short time, leading to combustible gas mixtures in the containment. Moreover, a long term pressure build-up may occur due to steam generation through decay heat and/or through the generation non-condensable gas from the interaction of the molten core with the containment basement concrete.

The implementation of hydrogen mitigation measures is aimed in general to prevent and limit hydrogen explosion consequences for the containment, the reactor and auxiliary buildings. Therefore, depending on the NPP type, hydrogen mitigation measures are designed to meet specific safety criteria and requirements. In addition to mitigation measures, gas composition monitoring system is often used to check if the requirements are satisfied and to provide relevant information to NPP operators during accident and severe accident conditions.

Passive autocatalytic recombiners (PARs) have been developed and have become commercially available hardware in the last decade. PARs are simple devices, consisting of a catalyst surfaces arranged in an open-ended enclosure. In the presence of hydrogen (with available oxygen), a catalytic reaction occurs spontaneously at the catalyst surfaces and the heat of reaction produces natural convection flow through the enclosure, exhausting the warm, humid hydrogen-depleted air and drawing fresh gas from below. Thus, PARs do not need external power or operator action. PAR capabilities are ultimately subject to mass transfer limitations and may not keep up with high hydrogen release rates in small volumes, for example, as could exist in the immediate vicinity of the hydrogen release.

In this study, varies of tests were conducted on different PAR’s designs to investigate and improve the hydrogen recombination rate. The innovative modifications on the current PAR model were carried out to foresee the unpredictable conditions and potential risks in the NPPs, and hence be adaptable in any circumstances to mitigate the hydrogen mitigation consequences. Although there is none hydrogen mitigation measures could be the best resolution in every single NPPs, the lessons learnt from the Fukushima accidents that hydrogen safety inspection was carried out on all Korean NPPs by Korean government and PARs will be

implemented in all operating and under-construction plants.