Analysis of flammability in the attached buildings to containment under severe accident conditions

Right after the events unfolded in Fukushima Daiichi, the European Union countries agreed in subjecting Nuclear Power Plants to Stress Tests as developed by WENRA and ENSREG organizations. One of the results as implemented in many European countries derived from such tests consisted of mandatory technical instructions issued by nuclear regulatory bodies on the analysis of potential risk of flammable gases in attached buildings to containment. The current study addresses the key aspects of the analysis of flammable gases leaking to auxiliary buildings attached to Westinghouse large-dry PWR containment for the specific situation where mitigating systems to prevent flammable gases to grow up inside containment are available, and containment integrity is preserved - hence avoiding isolation system failure. It also provides a full practical exercise where lessons learned derived from the current study - hence limited to the imposed boundary conditions - are applied. The leakage of gas from the containment to the support buildings is based on separate calculations using the EPRI-owned Modular Accident Analysis Program, MAAP4.07. The FATE (TM) code (facility Flow, Aerosol, Thermal, and Explosion) was used to model the transport and distribution of leaked flammable gas (H-2 and CO) in the penetration buildings. FATE models the significant mixing (dilution) which occurs as the released buoyant gas rises and entrains air. Also, FATE accounts for the condensation of steam on room surfaces, an effect which acts to concentrate flammable gas. The results of the analysis show that during a severe accident, flammable conditions are unlikely to occur in compartmentalized buildings such as the one used in the analyzed exercise provided three conditions are met: H-2 and CO recombiner devices are found inside the containment; corium is submerged and cooled down to quenching by flooding the reactor cavity; and the containment remains isolated along the accident evolution so that gases flowing into attached buildings to containment are limited to the so-called allowable leakage. (C) 2016 European Commission Joint Research Centre. Published by Elsevier B.V.