Czech Republic: Designing the Hydrogen Mitigation System for Temelin NPP: From Numerical Model Verification Towards PARS' Deployment Optimization

Since the Fukushima Daiichi severe accident (SA) in March 2011, great attention has been paid to evaluation of risks linked to hydrogen deflagration/detonation in course of an accident. In European countries, stress tests were carried out to assess the nuclear power plants' (NPP) vulnerabilities and, as consequence, national action plans for back-fitting the tested NPPs with SA management measures were launched. In Czech Republic, 2 NPPs are being operated by the utility CEZ: the Dukovany NPP (4x the VVER-440/213 type of reactor) and the Temelin NPP (2x the VVER-1000/320 type of reactor). As one of the outcomes from the stress tests, it was decided to equip both NPPs with passive autocatalytic recombiners (PAR) to minimize the risk of occurrence of highly energetic events linked to H₂ production, which takes place during the core degradation as the product of Zr-H₂O reaction: flame acceleration (FA) and deflagration to detonation transition (DDT). This paper consists of 2 parts: the first part deals with the verification and validation of the MELCOR 1.8.6 code and the UJV Rezdeveloped H₂ recombination model for containment analyses of PAR units' operation, whereas the second part describes the process of hydrogen mitigation system (HMS) designing and optimization for the Temelin NPP. (author)