Pre-pressurized core flooding tanks, or accumulators, are used in existing nuclear power plants and they constitute part of the emergency core cooling systems. They typically consist of large tanks having about 75% of the volume filled with cold borated water and the remaining volume filled with pressurized nitrogen or an inert gas. As shown in Figure 1, the contents of the tank are isolated from the reactor coolant system (RCS) by a series of check valves that are normally held shut by the pressure difference between the RCS and the fill gas in the tank. In the event of a loss of coolant accident (LOCA), the core pressure will drop below the fill gas pressure. This results in opening the check valves and discharging the borated water into the reactor vessel. This is a Category C passive safety system for conditions mentioned above.

Natural circulation loops represent an effective means of providing core cooling. Several advanced reactor designs implement elevated tanks connected to the reactor vessel or primary loop at the top and bottom of the tank as shown in Figure 2. The tanks are filled with borated water to provide coolant injection at system pressure. The tanks are normally isolated from the reactor vessel by an isolation valve located along the discharge line departing from the bottom of the tank itself. The fluid is always sensing full system pressure through the top connection line. In the event of an emergency, the bottom isolation valve is opened to complete the natural circulation loop and to permit cold borated water to flow to the core. In order to reduce the number of pipelines connected with the reactor pressure vessel, the delivery (or bottom) line of the core make-up tank (CMT) is in common with the emergency core coolant delivery line. In case of a number of accident scenarios, the CMT delivery can start before the accumulator delivery and end-up after the accumulator emptying. In those situations the CMT delivered flow-rate can be affected by the accumulator delivered flow-rate to a noticeable extent. Furthermore, specifically when the CMT delivery line is connected with the cold or hot leg (i. e. without the presence of the direct vessel injection), the direction of the fluid motion in the discharge line should be checked: in other terms there is the possibility that CMT liquid is used to cool the steam generator, or in any case, is diverted from its principal mission that is core cooling. This is a Category D passive safety system.