Methodology developed for the definition of the design parameters and associated safety criteria of the Filtered Containment Venting Systems for Belgian NPPs

Following the post-Fukushima stress tests performed in Belgium, Filtered Containment Venting Systems (FCVS) are foreseen to be installed on Belgian Nuclear Power Plants (NPPs) as ultimate means to cope with slow pressurizations of the containments that could occur during potential Severe Accidents (SA).The present paper discusses the methodology supporting the definition of the FCVS main design parameters and associated safety criteria.This methodology, developed by Tractebel Engineering and accepted by the Belgian nuclear Safety Authorities, followed a series of subsequent steps starting with the definition of the safety referential applicable to the FCVS and leading to the identification of a reference sequence, namely the Complete Station BlackOut (CSBO).Then, key parameters for the FCVS design were defined (e.g. pressure range, venting flow, stored decay heat, stored mass, Decontamination Factors (DF)) along with the associated safety criteria. Several of them were assessed through a well-defined set of supporting calculations. This set included both best estimate representative cases as well as conservative bounding cases performed with state-of-the-art SA codes.The SA codes used were first of all the MELCOR 1.8.6 code and, subsequently, the ASTEC V2.0 code (CPA/IODE modules), considering a chaining methodology. The aim of this approach was to take advantage of the iodine chemistry models features in the CPA/IODE modules of ASTEC to calculate the iodine speciation in the containment and in the FCVS, without having to run a full scope ASTEC calculation.In conclusion, appropriate values for the safety criteria of each design parameters were assessed. In particular, the ones related to the minimum DF relied on a marginal gain approach to cope with the large uncertainties existing when performing dedicated radiological evaluations.Finally, the next steps of this continuous safety assessment of the FCVS that are foreseen to be installed on Belgian NPPs are highlighted at the end of this paper.