5.49. The management of radionuclides present in the containment after a severe accident is similar to the management of radionuclides in the event of a design basis accident. The aim is still to limit leakage from the containment and to avoid, as far as possible, the creation of unfiltered leakage paths to the environment. The main differences in comparison with design basis accidents are the source term (the magnitudes and physicochemical forms of the radioactive releases to the containment) and the possible unavailability of some containment systems.

5.50. An assessment of possible radioactive releases from the containment should be made for selected severe accident sequences in order to identify any potential weaknesses with regard to the leaktightness of the containment and to determine ways to eliminate them. In this assessment, a best estimate approach should be used to evaluate possible leaks from the containment and the systems that may be unavailable for each specific sequence (such as the potential loss of containment isolation in the event of a plant power blackout).

5.51. For existing plants, any release through the containment vents should be filtered. Moreover, a strategy should be adopted to optimize the effectiveness of passive features (such as the retention capacity of compartments and buildings) and of active systems (such as dynamic confinement by means of an internal filtered ventilation system, if available).