CFD analyses of hydrogen risk within PWR containments

Hydrogen and steam releases into a PWR containment under severe accident conditions may lead to gas concentrations close to flammability limits or well within them. This fact constitutes a threat for the containment integrity. Computational treatments of substantial aspects in such situations are presented in this paper. A hydrogen risk assessment methodology is currently being developed based on the combined application of both CFD and 'lumped parameter' codes in a hybrid strategy. In this paper some CFD simulations with the CFX-4 code, as a basic part of this methodology, will be shown applied to a large-dry PWR containment during a severe accident. Experiments have been simulated to validate CFD calculations for buoyancy-driven plumes, stratification phenomena and local accumulation of hydrogen, steam and other light gases. Moreover, a condensation model has been implemented in the code and the MICOCO test has been used for benchmarking purposes. Besides, other tests concerning the hydrogen mixing in a multi-compartment test facility with mitigation devices such as Passive Autocatalytic Recombiners (PARs) have been analysed. Finally, an application to an actual Spanish PWR plant is performed in two scales of accuracy: detailed studies in the release room and a full-scale 3D plant application. The studies have showed the capability of CFD codes for analysing the hydrogen behaviour within containments. These codes solve the short-scale inaccuracies of the simplified 'lumped parameter' codes, which have been widely used in the nuclear safety studies. The level of risk arisen is assessed using the criteria from FzK for Flame Acceleration (FA) and Deflagration to Detonation Transition (DDT), which have been implemented in the CFX-4 code. (author)