Filtered Containment Venting System for Existing Nuclear Power Plants: Design Parameters and Associated Safety Criteria in Belgium

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 pressurization of the containment that could occur during potential Severe Accidents (SA). After an introduction about the Belgian context, information about the regulatory framework is provided, as well as the main objectives of the FCVS. Then, the main part of the paper discusses the methodology supporting the definition of the FCVS main design parameters and associated safety criteria developed by Tractebel Engineering, on behalf of the licensee Electrabel – GDF Suez, and validated by the Belgian nuclear Safety Authorities. This methodology 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 these key parameters 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 the state of the art SA codes MELCOR 1.8.6 and, subsequently, ASTEC V2.0 (CPA/IODE modules), in 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. Finally, an overview of the assessment by the Belgian TSO, Bel V, as well as the follow-up actions and next steps are reported, with the aim to provide information about the current status of the actions and implementation of the requirements related to the FCVS. (author)