4.235. The containment systems should be designed to continue fulfilling their functions following a loss of off-site power with a single failure taken into account. Electrical isolation valves that would have to be closed using electric power in a design basis accident should be provided with non-interruptible power supplies.

Compressed air systems

4.236. Containment isolation valves with a clear safe position should be designed to move to their safe positions in the event of a loss of pneumatic pressure.

4.237. If the operation of pneumatic valves is necessary during a design basis accident, the autonomy of the compressed air system (such as by means of having reserve air tanks) should be demonstrated. Otherwise, installation of a backup compressed air system should be considered. Where reserve air supply tanks are installed inside the containment, the increased internal pressures caused by the high temperatures in the containment during design basis accidents should be taken into account in their design.

4.238. The compressed air systems should be designed in such a way as to avoid a containment bypass or pressurization of the containment. Safety systems that are needed in the long term after a design basis accident should therefore not depend on compressed air systems for fulfilling their safety functions. To avoid gradual pressurization of the containment due to the leakage of compressed air systems, consideration should be given to the instal­lation of a dedicated post-accident compressed air system to supply instruments inside the containment with air exhausted from the containment.