Considerations on passive autocatalytic recombiners (PAR) system sizing in Spanish NPP and regulatory assessment

The accident which occurred on March 11th 2011 at the Fukushima Daiichi nuclear power plant in Japan highlighted the importance of combustible gases (hydrogen and carbon monoxide) management in the framework of severe accidents, which may constitute a threat to containment integrity. There are different means for combustible gases management: containment atmosphere homogenization, containment atmosphere inertization, elimination of combustible gases through the containment venting or the action of components that recombine or burn them in a gradual and controlled manner. In order to minimize the risks associated with the combustible gases in containment, all Spanish NPPs have installed Passive Autocatalytic Recombiners (PAR). The reason for choosing this technology is that PARs are passive and do not need any supporting systems to perform its safety function. When sizing the PAR system, two main steps are involved: the calculation of the combustible gases generated and its distribution. This process has shown the importance of an adequate scenario selection, the code used and other aspects such as the specific composition of the containment's concrete. The calculations carried out have shown the importance of having adequate procedures for hydrogen management. An example of this would be to consider the delay in initiating the spray of the containment, which would allow the complete recombination of the oxygen present, reaching an inert containment atmosphere. In the presentation, a brief explanation of the method followed by the Spanish NPP for PAR sizing will be presented. Additionally, the general criteria for the evaluation of this process established by the Spanish regulator (CSN) will be presented. (author)