The safety function “fuel cooling during transients and accidents” is ensured by provision of sufficient coolant inventory, by coolant injection, sufficient heat transfer, by circulation of the coolant, and by provision of an ultimate heat sink. Depending on the type of transient/accident, a subset of these function or all of them may be required. Various passive systems and components are proposed for WWER-1000/V-392 and WWER-640/V-407 reactor concepts to fulfill these functions.

For WWER-640/V-407 reactor, steam generator passive heat removal system (SG-PHRS) which does not require the electricity supply is designed to remove the decay heat in case of non-LOCA events and to support the emergency core cooling function in case of LOCAs. Reactor coolant system and passive heat removal equipment layouts provide heat removal from the core following reactor shutdown via steam generator to the tanks of chemically demineralized water outside the containment and further to the atmosphere by natural circulation as it is shown in Figure 1. Reactor power that can be removed from the core by coolant natural circulation is about 10% of the nominal value, which guarantees a reliable residual heat removal. Thus, in case of non-LOCAs the decay heat is removed by coolant natural circulation to steam generator boiler water. The steam generated comes into the passive heat removal system where steam is condensed on the internal surface of the tubes that are cooled on the outside surface by the water stored in the demineralized water tank outside the containment. The water inventory in this tank is sufficient for the long-term heat removal (at least 24 hours) and can be replenished if necessary from an external source.

Containment passive heat removal system (C-PHRS) of V-407 reactor removes heat from the containment in case of a LOCA and is designed to fulfill the following functions: (1) emergency isolation of service lines penetrating the containment and not pertaining to systems intended to cope with the accident; (2) condensation of the steam from the containment atmosphere; (3) retention of radioactive products released into the containment; (4) fixing of the iodine released into the containment atmosphere. The steam from the containment atmosphere condenses on the internal steel wall of the double-containment being cooled from the outside surface by the water stored in the tank. So, the system operates due to natural circulation of the containment atmosphere and water storage tank. The design basis of this system is to condense the amount of steam equivalent to decay heat release during 24 hours after reactor trip without the water storage tank replenishment.

The emergency core cooling system of V-407 reactor comprises three automatically initiated subsystems: (1) hydroaccumulators with nitrogen under pressure, which are the traditional ECCS accumulators being used at operating WWER-1000 reactors, (2) elevated hydrotanks open to the containment, and (3) equipment for deliberate emergency depressurization of the primary circuit. All these subsystems are based on the principle of passive operation providing for long-term residual heat removal in case of a loss-of-coolant accident accompanied by the plant blackout (i. e. AC power supply is not needed for ECCS operation). In the first stage of the accident, primary pressure is decreased due to loss of coolant and operation of the passive heat removal system. The further cooling down and pressure decrease are realized via steam generator PHRS and containment PHRS. When the pressure difference between primary circuit and containment has decreased to 0.6 MPa, the passive valves of the emergency depressurization system open connecting reactor inlet and outlet with the fuel pond space. When the reactor and containment pressure difference has decreased below 0.3 MPa, the ECCS hydrotanks begin to flood the reactor. This sequence results in creating of so called emergency pool where the reactor coolant system is submerged to and in connection of this emergency pool with the spent fuel pond. The natural circulation along the flow path shown in Figure 2 (reactor inlet plenum — core — reactor outlet plenum — “hot” depressurization pipe — fuel pond — “cold” depressurization pipe — reactor inlet plenum) provides the long-term heat removal from the core in case of a LOCA combined with loss of all electric power. The water in the emergency pool and spent fuel pool reaches the saturation point in about 10 hours. The steam generated will condense on the internal surface of the steel inner containment wall, and condensate flows back into the emergency pool. This configuration ensures also the heat removal from reactor vessel bottom to keep the corium inside the reactor in case of postulated core melt event.

The passive residual heat removal system (PHRS) is included in the V-392 design to remove heat from the reactor plant. The design basis of this system is that in case of station blackout, including loss of emergency power supply, the removal of residual heat should be provided without damage of the fuel and of the reactor coolant system boundary for a long time period. The PHRS consist of four independent trains; each of them is connected to the respective loop of the reactor plant via the secondary side of the steam generator. Each train has pipes for steam and condensate, valves and modular air-cooled heat exchanger installed outside of the containment as it is shown in Figure 3. The steam that is generated in the steam generator due to the heat released in the core condenses in the air-cooled heat exchanger, and condensate is returned back to the steam generator. The motion of the cooling media (steam, condensate and air) takes place in natural circulation.

FIG. 3. V-392. Passive heat removal system

The passive system for reactor flooding during LOCA in V-392 design comprises two groups of hydroaccumulators as it is shown in Figure 4. First group (so called first stage accumulators) consists of four traditional ECCS accumulators being used at operating WWER-1000 reactors; these accumulators are pressurized by nitrogen to 6 MPa and connected in pairs to the upper and lower plenums through special nozzles in the reactor pressure vessel. Second stage accumulators are 8 tanks connected to the reactor coolant system through the check valves and special spring-type valves. These valves are kept closed by the primary pressure; when the primary pressure drops below 1,5 MPa, the spring open the valve. Such a connection configuration and valve design ensures continuity of hydrostatic head irrespective of the primary pressure change during an accident. Installation of hydraulic profiling of the outlet route ensures a step-wise limitation of the water flow rate from the tank when the water level in the tank is decreasing. The water inventory in the second stage accumulators (about 1000 t) ensures the core cooling for 24 hours during a LOCA even if all active ECCS mechanisms are inoperable. Joint operation of the second stage accumulators and SPOT gives a possibility to increase the period indicated.