WENRA Reactor Safety Reference Levels January 2007

to enable a timely assessment of the plant status and critical safety functions in severeaccident conditions.4. Protection of the containment against selected beyond design basis accidents 314.1 Isolation of the containment shall be possible in a beyond design basis accident. 32However, if an event leads to bypass of the containment, consequences shall bemitigated.4.2 The leaktightness of the containment shall not degrade significantly for a reasonable timeafter a severe accident.4.3 Pressure and temperature in the containment shall be managed in a severe accident.4.4 Combustible gases shall be managed in a severe accident.4.5 The containment shall be protected from overpressure in a severe accident 33 .4.6 High pressure core melt scenarios shall be prevented.4.7 Containment degradation by molten fuel shall be prevented or mitigated as far asreasonably practicable.AppendixInterpretation of the reference level 2.1, for the purpose of benchmarking of implementation,in terms of types events to be analysed for design extension as a minimum, if not alreadyconsidered in the design basis:- anticipated transient without scram (ATWS)- station black out- total loss of feed water- LOCA together with the complete loss of one emergency core cooling system 34- uncontrolled level drop during mid-loop operation (PWR) or during refuelling- total loss of the component cooling water system- loss of core cooling in the residual heat removal mode- loss of fuel pool cooling- loss of ultimate heat sink function- uncontrolled boron dilution (PWR)- multiple steam generator tube ruptures (PWR, PHWR)- loss of required safety systems in the long term after a Postulated Initiating Event31 These reference levels aim at providing protection at the level 4 of the defence-in-depth. Such protection could be provided byexisting equipment that has been assessed, and if needed modified, to perform the relevant function in a severe accidentcondition or additional equipment on a best estimate basis.32 Special attention needs to be given for certain reactor types to the analysis of severe accident conditions with an opencontainment during certain shutdown states. Should such an accident occur, it should be possible to achieve timely containmentisolation or implement equally effective compensatory measures. Therefore consideration has to be given to the time needed forthe restoration of containment isolation and effective leaktightness, taking into account factors such as the progression of theaccident sequences.33 This reference level could be seen as a special case of reference level 4.3. However, it is kept for clarity as a separate referencelevel since it might call for specific measures to protect against fast as well as slow containment overpressurization.34 Either the high pressure or the low pressure emergency core cooling system18