[en] After a beyond design basis accident, the prime objective of the operable engineered safety systems is to maintain the containment integrity, i.e. to control the escape of radioactive materials to a rate lower than or equal to that fixed for the design basis accident (LOCA). If we except the direct containment heating scenario, i.e. a scenario affected by a very low probability and against which little if anything can be done, hydrogen appears to be the first threat that can endanger the containment integrity. Different hydrogen release patterns have been identified in the frame of the PRA conducted for Doel 3 (900 MW PWR Belgian reference unit). The analysis of these patterns together with consideration for results gained from safety studies in other countries (ex: Germany) enable to size a hydrogen catalytic recombiner system able to cope with the most credible severe accidents scenarios. In a later phase, the accumulation of steam and non condensable gases may require containment venting, should the pressure exceed the LOCA peak pressure. The venting system has to be sized so that the release of radiologically significative isotopes (iodine, caesium, strontium, actinides) integrated over the venting duration does not exceed the corresponding integrated releases due to the normal containment leak. The sizing of the venting system, i.e. namely, the definition of the decontamination factors, requires the definition of a source term taking into account: the containment atmosphere activity by the time of the release, this activity depending on the behaviour of the fission products during the time elapsed between the accident occurrence and the venting initiation (aerosols deposition and coalescence, organic compounds generation etc.); the total mass of active and inactive aerosols; the aerosols activity size spectrum. This paper describes the Belgian approach concerning the here above-mentioned items and based on the so far available scientific data and calculations. (author). 2 refs, 13 figs