Suppression pool cooling

Steam condensing Containment spray Suppression pool cooling High-pressure core spray (HPCS) system Low-pressure core spray (LPCS) system Automatic depressurization... 

USNRC Bulletin 95-02, Unexpected clogging of a residual heat removal (RHR) strainer while operating in a suppression pool cooling mode (October 17, 1995). 

USNRC Bulletin 95-02, Unexpected Clogging of a Residual Heat Removal (RHR) Strainer while Operating in a Suppression Pool Cooling Mode (October 17,1995). 

The water in the suppression pool can be recycled through the core cooling systems, much the same as sump water is recycled in a PWR. Long term containment heat removal can be provided by sprays or suppression pool cooling systems either of which can be aligned with appropriate heat exchangers. In addition, Mark I containments are equipped with lines... 

If the pressure in the outer containment exceeds the pressure in the drywell, then vacuum breakers open to equalize the pressure. Long-term containment heat removal can be accomplished with suppression pool cooling or by containment sprays (with appropriate circulation of the water through heat exchangers) in the outer containment. 

Several other systems are available to control room operators to keep the reactor cool during an emergency. These include the High Pressure Core Spray System, Low Pressure Core Spray System, Residual Heat Removal System, Low Pressure Coolant Injection, and Suppression Pool Cooling. All of these systems require both AC and DC power, plus a sufficient flow of coolant water connected to the ultimate heat sink. At Fukushima this ultimate heat sink was the Pacific Ocean (US Nuclear Regulatory Commission, n.d.a). 

Reactor 2, Steam separators 3. Inlet header 4. Main circulation pump 5. Outlet header 6. Pressure suppression pool 7, ECCS vessels 8, ECCS pumps for cooling damaged half of reactor 9. Heat exchangers 10. Clean condensate container 11. ECCS pumps for cooling undamaged half of reactor 12. De-aerator 13. Feed pump,... 

The suppression pool is supplied with heat exchangers to provide cooling in the event of prolonged operation. 

In the unlikely event that the RHR shutdown suction line is unavailable during reactor shutdown to cool reactor water and during the period when the LPCI function of the RHR system and/or the LPCS system pumps are injecting water into the reactor vessel, safety/relief valves used for automatic depressurization can be used to pass water from the reactor vessel to the suppression pool via valve discharge lines. For this to occur, the reactor vessel floods to a level above the vessel main steam line nozzles, selected safety/relief valves are opened from the control room to pass reactor water to the suppression pool. 

The low-pressure ECCSs consist of two separate and independent systems, the CS system and the LPCI mode of the residual heat removal system. The CS system consists of two separate and independent pumping loops, each capable of pumping water from the suppression pool into the reactor vessel. Core cooling is accomplished by spraying water on top of the fuel assemblies. The LPCI mode of the residual heat removal system provides makeup water to the reactor vessel for core cooling under LOCA conditions. 

This small BWR concept (Figure 1) uses an isolation condenser to improve transient response. Gravity-driven control rods and gravity-driven borated water injection are used to simplify and provide diversity to the shutdown function. Core cooling and decay heat removal are provided by depressurizing the reactor to an elevated suppression pool. The drywell and pool gas spaces are inert. 

The suppression pool contains borated water to provide a diverse backup to the gravity-driven control rods. Core cooling and decay heat removal is assured, with water returned to the reactor vessel and steam produced by decay heat vented to the suppression pool. The containment overpressure relief periodically opens to vent steam from the suppression pool. There is a three-day supply of water available to accept decay heat. No operator action is required during this time. For longer periods the suppression pool is manually refilled. Emergency diesel generators and core cooling pumps are not required. 

The newer VVER-440 model 213 differs from the 230 in that it has an emergency core cooling system with limited capacity and a bubbler/condenser tower which is connected with the accident localization compartments of each unit to mitigate the effects of severe accidents. This tower has a rectangular cross section and contains 12 levels of suppression pool trays (in total about 1960 trays). The tower also houses four large receiver volumes referred to as gas holders or air traps. 

Refuelling machine 2) Reactor wet well 3) Reactor pressure vessel 4) Control rod drive mechanisms S) Main coolant pumps 6) Containment 7) Air recirculation system 8) Pipe floors 9) Fuel pool cooling heat exchanger 10) Pressure suppression pool II) Residual heat removal cooler 12) Lock (By courtesy of Siemens/KWU)... 

The PC control EOF is designed to provide a barrier to the uncontrolled release of fission products, contain and condense steam discharged through the safety relief valves and primary cooling system breaks, shield personnel from radiation emitted by the reactor, and provide a protected environment for key equipment important to safety. Entry into this procedure is required at a suppression pool temperature above the limiting condition for operation (LCO), a drywell temperature above LCO, a containment temperature above LCO, a drywell pressure above the high pressure scram set point, a suppression pool water level above maximum level LCO, a suppression pool water level below minimum level LCO, and an SC hydrogen concentration above the alarm set point. 

Since the drain piping is located at the bottom of the pressure vessel, this accident is the most severe one with respect to core flooding. The results show that the accumulator injection system can keep the water level above the top of the core for one day after initiation of the LOCA. Although the active component of the FLS is not activated in the present analysis to show the performance of the ACC, the FLS is supposed to be activated to maintain long-term core cooling by injecting water from the suppression pool, even one day after initiation of the LOCA. For isolation of the reactor, the passive isolation condenser (IC) is introduced for core cooling whereas the RCIC system driven by the steam turbine is eliminated. 

The heat released in the containment vessel during the accident is absorbed in the suppression pool for one day further heat removal is possible over a longer period by evaporation of the water in the outside pool of the containment. This system gives sufficient time margins to cool the core for severe accidents as well as design base accidents. 

Containment, designed with pressure suppression pools and with emergency cooling and residual heat removal systems, can he significantly smaller than those of current water-cooled reactors. 

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