American Nuclear Society Committee Report on Fukushima Diiachi fukushima.ans.org/report/accident-analysis II.B. Accident Details for Fukushima Daiichi NPS: Units 1 Through 4 II.B.1. Fukushima Daiichi Unit 1 “After scram4 and loss of AC power due to the earthquake, both trains of the isolation condenser system were started because of closure of the main steam isolation valves (MSIVs) and subsequent pressurization of the RPV. Operators determined that with both trains operating, the reactor cooldown rate exceeded the technical specification rate of 55°C/hour (100°F/hour), so the isolation condensers were shut down by the operators. Subsequently, one train of the isolation condenser system was restarted and stopped several times to control the reactor pressure and cool the reactor. The HPCI system was not started during this time period as the water level in the RPV was adequate. After the tsunami struck, there was major flooding. In addition to the loss of heat removal function, the EDGs and direct-current (DC) batteries for both power and instrumentation, which were located in the basement of the turbine building, were also flooded and lost. All the instrumentation that was needed to monitor and control the emergency became unavailable; in addition, the HPCI system was not able to operate because of the loss of DC power and not yet needed because the isolation condenser system had just been shut down. Several attempts were made to open the steam supply and condensate return valves of the previously operating train of the isolation condenser system. There is some evidence that this isolation condenser was at least partially working, because of observed steam evolution from the shell side of the heat exchanger. However, by 10:00 p.m., March 11, rising radiation levels were observed in the reactor and turbine buildings, which was an indication that core damage was occurring. In addition, at 12:49 a.m. on March 12, local measurements confirmed that the containment pressure had exceeded the design pressure, which was further evidence of core damage and hydrogen production from the zirconium fuel cladding metal-water reaction. Therefore, processes were started to evacuate local residents and to prepare the containment for venting, in accordance with the NPP emergency procedures. Operators began preparations for primary containment vessel (PCV) venting, but the work ran into trouble because the radiation level in the reactor building was already high. At ~2:30 p.m. on March 12, a small decrease in the PCV pressure level was actually confirmed, which could have been due to leakage paths in the PCV that opened because of the PCV being at high containment pressure and temperature or because of the vent rupture disk opening. Subsequently, at 3:36 p.m., a hydrogen explosion5 occurred in the upper part of the Unit 1 reactor building. The source of the hydrogen in the reactor building is thought to be containment leakage due to the high containment pressure and temperature that occurred, which were well in excess of the design. The records do not show any deliberate attempt to depressurize the RPV, which would be necessary to allow emergency pumps, such as fire pumps, to add water. However, by 2:45 a.m. on March 12, the RPV pressure was found to be low, and by 5:46 a.m. on March 12, the operators began adding freshwater using fire engines. It is not clear whether the RPV depressurization occurred because of damage to the RPV by the molten core, a break in an attached low-elevation pipe, or SRVs that had stuck open. By this time, the fuel was already significantly damaged. Longer term, the water level in the RPV did not recover to more than core midplane regardless of the makeup water quantity being added, indicating a low-elevation leak in the RPV pressure boundary.” Please see the original link for the rest of the analysis. Appendix F of the report is entitled “Safety System Descriptions for Station Blackout Mitigation: Isolation Condenser, Reactor Core Isolation Cooling, and High-Pressure Coolant Injection” download link: fukushima.ans.org/inc/Fukushima_Appendix_F.pdf This Appendix states : “NOTE: Unless otherwise indicated, all dates in this appendix are for 2011. Important systems that can remove decay heat and/or add water to the boiling water reactor (BWR) reactor pressure vessel (RPV) without the need for alternating-current (AC) power are • the isolation condenser system, which is used in BWR/2s and some BWR/3s, including Fukushima Daiichi Unit 1 • the reactor core isolation cooling (RCIC) system, which is used in BWR/4s, BWR/5s, BWR/6s, and the Advanced Boiling Water Reactor (ABWR)1 • the high-pressure coolant injection (HPCI) system, which is used in BWR/3s and BWR/4s. Summary descriptions of these systems are given below. Other safety systems, such as the core spray, residual heat removal (RHR), and containment cooling systems, rely on significant AC power for operation and are not discussed here because the focus of this appendix is on systems that could be available in an extended station blackout (SBO). I. THE ISOLATION CONDENSER SYSTEM The primary purpose of the isolation condenser system (Fig. 1) is to remove decay heat and conserve reactor water inventory when the reactor becomes isolated from the turbine condenser. The isolation condenser system consists of two trains of equipment. Each train consists of a large heat exchanger located in the reactor building, outside of containment and above the RPV in elevation, which condenses steam produced by decay heat and returns it to the reactor by natural circulation. The primary side of the heat exchanger is fed by a steam line from the RPV, and a condensate return line that returns the condensed steam back to the RPV through one of the recirculation pump lines, together with appropriate isolation valves. In the ready state, the steam line is open and the condensate return line is closed, which allows the condensate line to be kept filled and eliminates the potential for water hammer during start-up. When the appropriate signal (high reactor pressure) is given, the condensate return valve is opened and a natural-circulation circuit is completed. The secondary (shell) side of the isolation condenser consists of a large tank of water with sufficient capacity for several hours of decay heat removal by boiling and venting to the atmosphere. From this point the system can operate without any electrical power or operator action for several hours. Longer term, the shell side can be replenished by the nuclear power plant (NPP) operatoNOTE: Unless otherwise indicated, all dates in this appendix are for 2011. Important systems that can remove decay heat and/or add water to the boiling water reactor (BWR) reactor pressure vessel (RPV) without the need for alternating-current (AC) power are • the isolation condenser system, which is used in BWR/2s and some BWR/3s, including Fukushima Daiichi Unit 1 • the reactor core isolation cooling (RCIC) system, which is used in BWR/4s, BWR/5s, BWR/6s, and the Advanced Boiling Water Reactor (ABWR)1 • the high-pressure coolant injection (HPCI) system, which is used in BWR/3s and BWR/4s. Summary descriptions of these systems are given below. Other safety systems, such as the core spray, residual heat removal (RHR), and containment cooling systems, rely on significant AC power for operation and are not discussed here because the focus of this appendix is on systems that could be available in an extended station blackout (SBO). I. THE ISOLATION CONDENSER SYSTEM The primary purpose of the isolation condenser system (Fig. 1) is to remove decay heat and conserve reactor water inventory when the reactor becomes isolated from the turbine condenser. The isolation condenser system consists of two trains of equipment. Each train consists of a large heat exchanger located in the reactor building, outside of containment and above the RPV in elevation, which condenses steam produced by decay heat and returns it to the reactor by natural circulation. The primary side of the heat exchanger is fed by a steam line from the RPV, and a condensate return line that returns the condensed steam back to the RPV through one of the recirculation pump lines, together with appropriate isolation valves. In the ready state, the steam line is open and the condensate return line is closed, which allows the condensate line to be kept filled and eliminates the potential for water hammer during start-up. When the appropriate signal (high reactor pressure) is given, the condensate return valve is opened and a natural-circulation circuit is completed. The secondary (shell) side of the isolation condenser consists of a large tank of water with sufficient capacity for several hours of decay heat removal by boiling and venting to the atmosphere. From this point the system can operate without any electrical power or operator action for several hours. Longer term, the shell side can be replenished by the nuclear power plant (NPP) operators using the NPP makeup water system, the fire protection system, or fire trucks.” II. THE RCIC SYSTEM The primary purpose of the RCIC system (Fig. 2) is to provide makeup water to the RPV when the RPV is isolated from the turbine-condenser. The RCIC system uses a steam-driven turbine-pump unit and operates automatically in time and with sufficient coolant flow to maintain adequate water level in the RPV for the following events: • RPV isolated and maintained at hot standby • complete NPP shutdown with loss of normal feedwater before the reactor is depressurized to a level where the shutdown cooling system can be placed in operation • loss of AC power. The RCIC system is sized to keep up with decay heat inventory losses from the RPV [90 to 180 m3/hour (400 to 800 gal/minute), depending on reactor design power level]. Since the reactor decay heat reduces rapidly after an NPP scram, the RCIC system quickly has more than enough capacity to keep up with decay heat steam production losses through the safety and relief valves (SRVs)—the RCIC system does not control reactor pressure, so the generated steam from decay heat lifts the SRVs, and the steam is routed to the suppression pool. The RCIC system is contained within one electrical division and consists of a steam-driven turbine that drives a pump assembly and the turbine and pump accessories. The RCIC system also includes piping, valves, and instrumentation necessary to implement several flow paths. The RCIC system steam supply II. THE RCIC SYSTEM The primary purpose of the RCIC system (Fig. 2) is to provide makeup water to the RPV when the RPV is isolated from the turbine-condenser. The RCIC system uses a steam-driven turbine-pump unit and operates automatically in time and with sufficient coolant flow to maintain adequate water level in the RPV for the following events: • RPV isolated and maintained at hot standby • complete NPP shutdown with loss of normal feedwater before the reactor is depressurized to a level where the shutdown cooling system can be placed in operation • loss of AC power. The RCIC system is sized to keep up with decay heat inventory losses from the RPV [90 to 180 m3/hour (400 to 800 gal/minute), depending on reactor design power level]. Since the reactor decay heat reduces rapidly after an NPP scram, the RCIC system quickly has more than enough capacity to keep up with decay heat steam production losses through the safety and relief valves (SRVs)—the RCIC system does not control reactor pressure, so the generated steam from decay heat lifts the SRVs, and the steam is routed to the suppression pool. The RCIC system is contained within one electrical division and consists of a steam-driven turbine that drives a pump assembly and the turbine and pump accessories. The RCIC system also includes piping, valves, and instrumentation necessary to implement several flow paths. The RCIC system steam supply line branches off one of the main steam lines (leaving the RPV) and goes to the RCIC turbine with drainage provision to the main condenser. The turbine exhausts to the suppression pool with vacuum breaking protection. Makeup water is supplied from the condensate storage tank (CST) or the suppression pool with the preferred source being the CST. RCIC system flow is discharged to the feedwater injection line. Following a reactor scram, steam generation in the reactor core continues, although at a reduced rate because of the core fission product decay heat. The turbine condenser and the feedwater system supply the makeup water required to maintain RPV inventory. In the event the RPV is isolated and the feedwater supply is unavailable, SRVs automatically maintain the RPV pressure within desirable limits. The water level in the RPV drops because of continued steam generation by decay heat. Upon reaching a predetermined low level, the RCIC system is initiated automatically. The turbine-driven pump supplies water from the CST (preferred) or from the suppression pool to the RPV. The turbine is driven with a portion of the decay heat steam from the RPV and exhausts to the suppression pool. The RCIC system is designed to pump water into the RPV from full operating pressure down to ~1 MPa (150 psia). During RCIC operation, the wetwell suppression pool acts as the heat sink for steam generated by reactor decay heat. This results in a rise in the suppression pool water temperature. When AC power is available, heat exchangers in the RHR system are used to maintain the suppression pool water temperature within acceptable limits by cooling the suppression pool water directly. A design flow functional test of the RCIC system may be performed during normal NPP operation by drawing suction from the CST and discharging through a full flow test return line to the CST (not shown in Fig. 1).2 The discharge valve to the reactor feedwater line remains closed during the test, and reactor operation remains undisturbed. lf the system requires initiation while in the test mode, the control system automatically returns to the operating mode. Cooling water for pump and turbine operations and for the lube oil cooler and the gland seal condenser is supplied from the discharge of the pump. Two turbine control systems include a speed governor limiting the speed to its maximum operating level and a control governor with automatic set-point adjustment that is positioned by a demand signal from a flow controller. Manual operation of the control governor is possible when in the test mode but is automatically repositioned (the governor valve goes back to its normal operating position) by the demand signal from the controller if system initiation is required. The operator has the capability to select manual control of the governor and adjust the power and flow to match decay heat steam generation. Several U.S. NPPs have developed procedures to override RCIC system valves and controls and manually run the RCIC system in case of an SBO. The turbine and pump automatically shut down upon • turbine overspeed • high water level in the RPV • low pump suction pressure • high turbine exhaust pressure • automatic isolation signal. The steam supply system to the turbine is automatically isolated upon • high pressure drop across two pipe elbows in the steam supply line • high area temperature • low reactor pressure • high pressure between the turbine exhaust rupture diaphragms. The RCIC system operates independently of auxiliary AC power, NPP service air, or external cooling water systems. System valves and auxiliary pumps are designed to operate by direct-current (DC) power from the nuclear power station batteries, except for the inboard containment isolation valve, which is powered by AC power (fail as-is), and the hydraulically operated valves, which are operated by the turbine control system through mechanical linkages. In the case of an extended SBO, the RCIC system may stop operation for one of a number of reasons: • The DC power for valves is available, but the DC power for instrumentation has failed, causing the DC-controlled valves to close. • The suppression pool temperature is too high, leading to inadequate pump net positive suction head or inadequate lube oil cooling. • The containment pressure is too high, causing the RCIC system turbine to trip. III. THE HPCI SYSTEM Philosophically, the HPCI system is similar to the RCIC system, except that the HPCI system has about seven times the flow (680 to 1270 m3/hour) and is part of the ECCS network (see Fig. 3). In ECCS use, small breaks in which the reactor does not depressurize through the break are helpful. However, the ECCS can also act as a backup to the RCIC system for isolation transients. Because of the larger steam consumption, the HPCI system can be manually controlled to use the full flow test line to limit the water being pumped to the reactor while depressurizing the reactor through the HPCI turbine. The same types of signals initiate and terminate the HPCI system as do the RCIC system, and DC power is needed to operate some of the HPCI system valves. emphasis added.
Posted on: Fri, 26 Dec 2014 10:35:51 +0000
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