4.96. In LOCA conditions, containment air cooling systems may operate largely in the condensing heat transfer mode. Appropriate analytical correla­tions of heat transfer rates with temperatures, pressures and steam content should therefore be used in the design and for the testing.

4.97. The evolution of the atmospheric density during a design basis accident should be taken into account in the design of the air cooler fans. The heat removal capacity of the cooling water supplied to the air coolers should be such as to preclude boiling on the coolant side. In addition, the cooling water system for the air coolers should be designed to allow the resumption of cooling water flow following a temporary interruption to that flow.

Pressure suppression pool systems

Bubble condenser suppression pool systems

4.98. Containments of a design with a suppression pool system are divided into two separate compartments called the dry well (which contains the reactor) and the wet well (which contains the suppression pool). The two compartments are normally isolated from one another. When the pressure in the dry well is sufficiently higher than the pressure in the wet well, steam and gases flow from the dry well to the wet well and the steam condenses into the pool of water. In some designs, communication between the dry well and the wet well can also occur if the pressure in the wet well is higher than the pressure in the dry well. In the containments of some designs the suppression pools are also used to collect the steam discharged from the safety valves or the relief valves, or to provide water for recirculation in the emergency core cooling system. Complex hydraulic and pressure transients occur when steam and gases are vented into the suppression pool water. The design of the dry and wet wells should be such that the hydraulic responses and the dynamic loads can be reliably determined by analysis and tests.

4.99. The hydraulic response and the loading function associated with various likely combinations of normal operating events and anticipated operational occurrences should be determined.

4.100. The structural design of the pressure suppression pool system should be such as to ensure that the pool, as well as the containment system as a whole and other safety systems, remains functional in all operational states and/or all postulated accident conditions.

4.101. The pressure suppression pool system should be designed in such a way that the pathway for steam and gases from the dry well to enter the wet well following a postulated LOCA is through submerged vents in the wet well water pool.

4.102. The leakage between the dry well and the wet well that bypasses the submerged venting lines should be minimized and should be taken into account in the design.

4.103. The use of the pressure suppression pool system for other functions should not impair the performance of its main function of providing a means of control in LOCAs.

4.104. The dry well should be designed to withstand, or should be protected from (e. g. by automatic vacuum breaker valves), excessive underpressure caused by operation of the spray system either inadvertently or following a LOCA.

Jet condenser suppression pool systems

4.105. Jet condensers are pressure suppression devices installed to cope with LOCAs. Condensation of steam released from the reactor cooling circuits is achieved by direct contact with and/or mixing with cold water in a mixing chamber of the condenser. The condenser is often located in a water pool that is also used for other purposes, for example as an emergency water tank. Construction of the condenser should be such as to ensure that the mixing and condensation processes take place in the upper part of the condenser, and that warm condensate is released to the top of the water pool while the cold water necessary for condensation is drawn from the bottom of the pool.

4.106. Jet condensers should have the following characteristics:

— The design should be such as to enable the containment structure to withstand thermal loads and pressure loads throughout a design basis accident, including those during the very first few seconds.

— Mixing and condensation should be localized in the condenser, without affecting the walls and equipment in the large water pool.

— The entire volume of water available for pressure suppression should be used effectively.

— The condenser should work efficiently over the wide range of mass flow rates of steam to be condensed.

— Condensation of steam should be stable, without large oscillations, as a result of needing only a low pressure differential for maintaining the flow through the condenser.

— The formation and rapid condensation of large steam bubbles, which could cause pressure waves in the water pool, should be avoided.

— The carryover of water from the water pool into the venting line should be minimized.