Lessons Learned from the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants (2014) / Chapter Skim
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4 Fukushima Daiichi Nuclear Accident
4 Fukushima Daiichi Nuclear Accident
Pages 101-152
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From page 101... ...
Many accounts of the Fukushima Daiichi nuclear accident have already been published. These accounts provided the factual information used in this chapter and informed committee judgments about accident causes and 101
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From page 102... ...
The committee published an interim report in 2011 and a final report in 2012. 2 The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC)
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From page 103... ...
The fourth section provides a discussion of six issues that stand out from the committee's analysis of the accident. The fifth and final section provides a committee finding on the causes of the Fukushima Daiichi nuclear accident to address the first charge of the study task.
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From page 104... ...
Estimated time of +4 h to +7 h +75 h to +85 h +36 h to +40 h core damage First indication +8.2 to +14.1 h of offsite release of radioactive materials Containment +9.7 h/~+24 h +26.7 h/not +29.5 h/+42 h venting successful preparation/success Hydrogen +24.8 h None +68.2 h explosion Initial injection +15.0/+28.8 h None/+77.2 h +42.6/+46.4 h of freshwater/ seawater Restoration of March 20 March 20 March 22 offsite AC power NOTES: EDGs = emergency diesel generators, HPCI = high-pressure coolant injection system, IC = isolation condenser, MSIV = main steam isolation valve, RCIC = reactor core isolation cooling system, RPV = reactor pressure vessel.
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From page 105... ...
RCIC = Reactor Core Isolation Cooling HPCI = High-Pressure Coolant Injection IC = Isolation Condenser Cooling system active Containment venting Core melting Hydrogen backflow into unit Hydrogen explosion Length of bar indicates duration FIGURE 4.1 Graphical depictions of accident time lines for Units 1-4 at the Fukushima Daiichi plant. The key events shown in the time lines are described in the text.
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From page 106... ...
Fuel from the Unit 4 reactor had been relocated to the spent fuel pool in the reactor building. • Units 5 and 6 were in inspection outages.
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From page 107... ...
As a result of these warnings, field personnel at the plant were evacuated to the onsite Emergency Response Center (onsite ERC; see Appendix D) or to higher ground.
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From page 108... ...
These included "primarily [the] recovery team responsible for restoring power and monitoring instruments, fire brigade units that used fire engines to inject cooling water into reactors, a health physics team that controlled radiation levels within the Fukushima Daiichi NPS [Nuclear Power Station]
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From page 109... ...
Moreover, its accident-management emergency operating procedures did not address accident scenarios involving the complete loss of onsite power, instrumentation, and reactor controls; and reactor operators had not been trained to respond to such scenarios. Indeed, the Fukushima Daiichi nuclear accident was "off the map" in terms of preparation, planning, and training for severe nuclear accidents.
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From page 110... ...
• Limited means of communication between the control rooms and the onsite ERC and between the control rooms and the field made it difficult to plan and carry out response efforts across the site. • Hydrogen explosions, radioactive contamination, and high temperatures limited access to some parts of the Unit 1-4 reactor buildings.
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From page 111... ...
, the loss of telecommunication infrastructures led to the increased involvement of the central government in the response to the accident, partly because the government perceived that it was not receiving accurate and timely information. The Japanese government contacted the headquarters and onsite ERCs directly to get information.
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From page 112... ...
They initially shut down both isolation condensers because reactor cooldown rates were too high; they then cycled one of the isolation condensers (the Train A isolation condenser in Figure 4.3) to maintain reactor pressure and cooldown rates within prescribed specifications.13 The Train B isolation condenser was on standby at the time of the tsunami.
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From page 113... ...
Main-steam safety relief valve Generator Regular power source Stack Isolation Batteries Condenser Turbine Off-site power system C/B B1F Motor operated Main steam vent valve isolation valve Condenser Circulating water pump Dry well vent valve Standby Feedwater pipe liquid control vessel (RPV) system Reactor pressure Sea Feedwater pump Condensate pump Suppression Primary chamber vent containment valve vessel Control rod drive R/B Reactor building Core spray system PCV cooling hydraulic Condensate T/B Turbine building pump system pump storage pool C/B Control building High-pressure coolant Make-up water Heat exchanger injection system pump condensate system pump Power C/B B1F panels Filtered water tank Seawater pump T/B B1F Diesel generator Diesel-driven fire Sea protection system pump FIGURE 4.2 Schematic illustration of major safety systems in Unit 1 of the Fukushima Daiichi plant.
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From page 114... ...
The valves outside of containment operate with DC power. A fuller description of isolation condenser operation is provided in Chapter 2.
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From page 115... ...
The site superintendent directed onsite ERC staff to give priority to restoring plant indicators, particularly reactor water level and pressure. At approximately 17:10 on March 11, he instructed onsite ERC staff to begin preparation for two alternative water injection strategies: water injection via the diesel-driven fire protection system (this system is depicted in Figure 4.2)
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From page 116... ...
In fact, the onsite ERC did not act on this request for several hours. Miscommunications, combined with misleading water-level indicators in the reactor pressure vessel (e.g., at 21:19 the water level was shown to be 200 mm above the top of active fuel,17 which was likely not the case18)
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From page 117... ...
. By 02:45 on March 12 the pressure in the reactor pressure vessel was determined to be near containment pressure21; freshwater injection was initiated at 05:46.22 By this time, however, the fuel in the reactor had already been damaged and hydrogen and radioactive materials had likely already leaked into the reactor building.
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From page 118... ...
Additionally, the apparent miscommunication between operators and onsite ERC about the urgency of supplying batteries for opening the safety relief valves quite possibly led to delays in depressurizing the reactor pressure vessel. TEPCO has argued that efforts to vent and set up alternative water sources were initiated in spite of these communication problems.
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From page 119... ...
. At around 11:36 on March 12, after running for approximately 20 hours, the reactor core isolation cooling system stopped and could not be restarted.26 The safety relief valves cycled to control reactor pressures; as a result, water levels in the reactor pressure vessel dropped and the highpressure coolant injection system started automatically at 12:35.
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From page 120... ...
FIGURE 4.4 Schematic illustration of major safety systems in Units 2 and 3 of the Fukushima Daiichi plant. SOURCE: Courtesy of TEPCO.
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From page 121... ...
It was not until 03:55 on March 13 that the site superintendent and the headquarters ERC learned these facts.31 The onsite ERC immediately recognized the need to obtain batteries to operate the safety relief valves and fire engines to inject water into the reac 28 The operators were concerned specifically about the potential for a steam leak resulting from damage to the high-pressure coolant injection system caused by excessively low speed of the turbine. 29 The safety relief valves can be manually opened by remote control only if the pressure in the reactor pressure vessel is over 0.686 MPa (gauge)
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From page 122... ...
The high pressure reduced water injection flow rates into the reactor pressure vessel and likely caused hydrogen and fission products to leak from the containment into the reactor building. A hydrogen explosion occurred in the Unit 3 reactor building at 11:01 on March 14.
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From page 123... ...
. It could be presumed that, however, if depressurization of Unit 3 had been performed much earlier than it actually had and the alternative method of water injection using fire engines had been conducted smoothly, the progress of core damage might have been slower, radiation dose in the RPV [reactor pressure vessel]
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From page 124... ...
More details on the reactor core isolation cooling system and failsafe control logic are provided in Section 2.2.3.2 in Chapter 2. 39 Specifically, operators could not monitor or control the rate at which the system delivered water to the reactor pressure vessel.
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From page 125... ...
At this point the onsite ERC staff realized that the reactor core isolation cooling system must be functioning; the ERC's focus then shifted away from providing emergency injection water for Unit 2. By 23:35, operators obtained further indirect confirmation that the reactor core isolation cooling system was functioning when they were able to connect emergency power to a drywell pressure gauge; the gauge reading was 0.14 MPa (abs)
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From page 126... ...
The reactor core isolation cooling system in Unit 2 continued to operate until about 13:30 on March 14.45 After the system stopped, the safety relief valves operated mechanically to vent steam from the reactor pressure vessel to the suppression pool. Steam loss from the reactor pressure vessel caused its water levels to drop continuously for the next 5 hours.
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From page 127... ...
TEPCO's president ordered the site superintendent to depressurize the reactor pressure vessel without waiting to vent containment. The site superintendent accepted the president's directive and gave instructions to start venting and water injection into the Unit 2 reactor pressure vessel while concurrently continuing preparations for containment venting.48 Operators struggled to depressurize the reactor pressure vessel.
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From page 128... ...
, for substantial periods of time, preventing seawater injection from taking place and placing the containment vessel under significant thermal and pressure stresses. The operators and onsite ERC struggled through the evening of March 14 and early morning of March 15 to vent containment.
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From page 129... ...
• The Unit 2 operators had to depressurize the reactor pressure vessel and vent containment to enable injection of low-pressure cooling water.52 Venting of the Unit 2 containment was difficult to implement on an ad hoc basis: emergency air supplies were inadequate; the torus room environment was too hot, humid, and contaminated for the staff to manually operate the suppression chamber vent valves; and the rupture disks were designed to operate at higher containment overpressures than were achieved and could not be bypassed. The hydrogen explosion in the Unit 3 reactor building further impeded efforts to vent the Unit 2 containment.
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From page 130... ...
The MSIV apparently was also open. A number of safety systems were either unavailable (i.e., reactor core isolation cooling system, high-pressure coolant injection system; residual heat removal system)
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From page 131... ...
It involved brute-force prying open of the nitrogen supply line to the vent valve on top of the reactor head from outside the containment.55 Workers apparently had to enter the reactor building or containment to connect an ad hoc nitrogen supply line that could be used to activate the safety relief valve that was ultimately used to maintain the reactor pressure at desired levels (Investigation Committee, 2012, p.
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From page 132... ...
The Unit 6 containment was closed but its vent line was open. Water was supplied to the reactor pressure vessel and the spent fuel pool on a reliable basis from March 13 onward.
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From page 133... ...
, or more complex events. The explosions caused extensive damage to the reactor buildings (Figure 4.5)
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From page 134... ...
Regardless of combustion speed, hydrogen explosions can be very destructive when large volumes of combustible gas within confining structures are involved, as was the case for Units 1, 3, and 4 at the Fukushima Daiichi nuclear plant.
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From page 135... ...
. For large volumes, such as the refueling areas on the upper floors of the Fukushima Daiichi reactor buildings, there may be potential for transition to detonation for some mixtures that are within the blue-shaded region marked "deflagration" (see Chapter 3 of NEA (2000)
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From page 136... ...
Unit 3 exterior; (C) Unit 3 interior at the operating floor level (i.e., the upper floor of the building where the spent fuel pool is located)
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From page 137... ...
There is extensive experience with modeling severe accidents in Mark I containments59 for the relatively simple accident scenario that occurred in Unit 1: the reactor pressure vessel was isolated except for mechanical venting into the suppression chamber through automatic operation of the safety relief valves, and there were no active cooling measures for at least 14 hours after the earthquake. Consequently, the MELCOR and MAAP simulations for Unit 1 likely have better fidelity to reality than the simulations for the other units.
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From page 138... ...
Continued depressurization lowered water levels in the reactor pressure vessel. At about +2.5 to +3 hours, all simulations predict that the water level in the Unit 1 reactor pressure vessel dropped enough to expose the active portion of the fuel in the reactor core; within +4.5 to +5 hours, the liquid level dropped below the bottom of the active portion of the fuel.
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From page 139... ...
• The corium flowed downward onto the lower head of the reactor pressure vessel causing it to melt and fail. • The molten mass flowed onto the concrete floor of the containment.
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From page 140... ...
between +80 and +90 hours after the reactor core isolation cooling system stopped (at +70 hours) and seawater injection was initiated.
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From page 141... ...
It also forced field workers to retreat to the onsite ERC, further delaying the accident response. The hydrogen explosion in Unit 4 (see Figure 4.5D,E)
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From page 142... ...
Efforts are now under way by TEPCO to move spent fuel from the Unit 4 pool into the common pool. Nevertheless, the events at the Fukushima Daiichi plant highlight concerns about the vulnerability of spent fuel pools to severe accidents.
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From page 143... ...
For example, procedures to cool the reactors using various installed emergency core cooling systems (e.g., isolation condenser system, reactor core isolation cooling system, high-pressure coolant injection system) were specified in accident management procedures.
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From page 144... ...
Onsite ERC staff training assumed that the safety display parameter system and communication lines with control rooms would provide good situational awareness of plant state and operator actions. Operators could not take critical control actions from the control room; instead, they had to take manual actions in the field.
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From page 145... ...
. These include the use of fire engines for water injection and batteries to restore water-level gauges and operate steam safety relief valves (TEPCO, 2012b, p.
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From page 146... ...
Operators used safety relief valves and reactor core isolation cooling systems to lower reactor pressures in Units 1, 2, and 4 following the tsunami; reactor pres sures were less than 1 MPa 8 hours after the tsunami. Cooling was then transi tioned seamlessly to low-pressure water injection with an alternative water supply (the makeup water condensate system)
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From page 147... ...
Operators also did not have to enter dark and contaminated reactor buildings to mount a response, but could monitor and control reactors from their control rooms. The communications and command structure functioned properly: the onsite ERC had a functional safety parameter display system and continuous communication with the control rooms.
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From page 148... ...
The hydrogen explosions in the Unit 3 and Unit 4 buildings also affected the management of the accident at all units because personnel at the site were reduced to a bare minimum for a time and recovery operations at the reactor units were halted. The units also competed for physical resources and attention and/or services of the onsite ERC staff.
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From page 149... ...
(A notable aspect of the accident was the fact that the plant personnel remained onsite and worked diligently without news about their families.) Some notable examples of communication failures were mentioned previously in this chapter: • Miscommunications about operations of valves and status of the isolation condenser in Unit 1; • Miscommunications about the need for batteries to operate the safety relief valves in Unit 1; • Lack of coordination between shift team and firefighters because neither understood the responsibility given to them by the site superintendent for hooking up the fire truck pump to the Unit 1 fire protection system; • Incorrect battery types (2V instead of 12V)
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From page 150... ...
was effective for situations that were explicitly covered by the accident management procedures, but they proved to be inadequate for the performance of tasks that fell outside the procedures. In particular, defining roles and responsibilities for tasks that were not covered by the procedures (e.g., water injection using fire engines)
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From page 151... ...
that it is taking measures to strengthen the organizational structure for handling simultaneous and compound accidents at multiple units. This includes increasing the number of technical support personnel at the onsite ERC and establishing two technical support rooms in the headquarters ERC to handle the simultaneous occurrence of a nuclear accident and a natural disaster.
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From page 152... ...
5. Operators and onsite emergency response center staff lacked adequate procedures and training for accidents involving extended loss of all onsite AC and DC power, particularly procedures and training for managing water levels and pressures in reactors and their containments and hydrogen generated during reactor core degradation.
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