4.112. Some energy management systems use the external recirculation of sump water or wet well water through heat exchangers to remove the residual heat from the containment over the medium term (after about one hour). These external recirculation loops are part of the containment envelope. They should be subject to specifications for structural integrity and leaktightness comparable with those of the containment structure itself.

4.113. The specification of the volume of water to be stored in the sump and the design of the suction points should be such that an adequate net pump suction head will be available to the recirculation pumps at any time. The possibility of water boiling in the sump should be considered in the design of the recirculation system if relevant.

4.114. The recirculation loops and their support systems should be redundant so as to satisfy the single failure criterion, and they should be spatially separated so as to reduce the potential for common cause failure. The devices at which suction takes place should be designed to minimize cavitation and to prevent the ingress of foreign material (such as thermal insulation), which could block or damage the recirculation system.

4.115. To avoid the clogging of screens and filters, special care should be taken in the design of piping, component insulation and the intake screens and filters themselves, and consideration should be given to the behaviour under accident conditions of organic paints and coating materials.

4.116. The recirculation loops should be equipped with leakage detection and isolation devices outside the containment and close to the containment penetrations so as to be able to isolate any leaks in the external recirculation loops and therefore to maintain a sufficient water inventory for cooling. Any leakage between the containment penetration and the isolation valve should be prevented by design, for example (a) by means of the provision of a guard pipe or by locating the isolation valve close to the penetrations; (b) by means of quality control in the production of devices to prevent leaks. Strict inspections, maintenance and test controls should be instituted.

4.117. An intermediate cooling system should be provided for heat transport to the ultimate heat sink. This cooling system should be equipped with features to detect and isolate leaks within the recirculation loop heat exchangers. This system should be classified as a safety system.

4.118. Some containment designs do not make use of containment atmosphere cooling systems such as spray or air cooler systems. In the event of a LOCA, they rely on passive heat dissipation and on the release of steam from the reactor coolant systems to the containment atmosphere, limited in time by means of safety injection systems (especially hot leg injection) of appropriate designs. In such cases, it should be demonstrated that energy management for the containment in the medium term and long term can be provided by means of sump recirculation cooling performed by the safety injection system.

4.119. The design of the safety injection system should be such that the release of steam from a broken pipe is sufficiently limited in time, with account taken of the available passive heat sinks provided by the containment and its internal structures.