Click for next page ( 96


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 95
4 FUKUSHIMA DAIICHI NUCLEAR ACCIDENT The focus of this chapter is on the March 11, 2011, accident at the Fukushima Daiichi nuclear plant: the accident timeline, key events during the accident, actions taken to bring the plant’s reactors to cold shutdown, and challenges faced in taking those actions. This chapter has two objectives: 1. To address the first charge of the statement of task for this study (see Sidebar 1.1 in Chapter 1) on the “Causes of the Fukushima nuclear accident, particularly with respect to the performance of safety systems and operator response following the earthquake and tsunami.” 2. To provide information and analysis to support the committee-identified lessons learned in Chapter 5. It is not the committee’s intention to place blame for the accident or to find fault with how personnel at the Fukushima Daiichi plant responded to the earthquake and tsunami. With the benefit of hindsight, it is easy to second guess the decisions and actions taken during the accident. In reviewing the accident response, the committee came to appreciate the overwhelming challenges that plant personnel faced in responding to the accident. Some of those challenges are described in the next section of this chapter. Indeed, the conditions at the Fukushima Daiichi plant following the earthquake and tsunami would have challenged any nuclear plant operator. 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 lessons learned. The following reports, papers, and presentations were particularly useful for these purposes:  Post-accident investigation reports by Japanese and U.S. organizations, especially ANS (2012), EPRI (2012a), INPO (2011, 2012), Investigation Committee1 (2011, 2012), 1 Investigation Committee on the Accident at Fukushima Nuclear Power Stations of Tokyo Electric Power Company was established by the Japanese Government by Cabinet Decision on May 24, 2011. The committee was chaired by Dr. Yotaro Hatamura, professor emeritus of the University of Tokyo and professor at Kogakuin University. The committee published an interim report in 2011 and a final report in 2012. Prepublication Copy 4-1

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident NAIIC2 (2012), and TEPCO (2011a,b; 2012b; 2013). The Investigation Committee (2011, 2012) and TEPCO (2011a,b; 2012b) reports provide detailed documentation of the decisions and actions taken during the accident as well as key thought process behind those actions.  Technical papers on the accident, most notably EPRI (2013), Gauntt et al. (2012a,b), Levy (2012), and Phillips et al. (2012).  Slides from technical presentations by Japanese researchers at International Atomic Energy Agency conferences in 20123 and 2014,4 technical workshops in Japan, and other international meetings (e.g., Probabilistic Safety Assessment & Management 2013).  Discussions with Japanese technical experts at the committee’s November 2012 meeting in Tokyo, Japan.  Site visits to the Fukushima Daiichi, Fukushima Daini, and Onagawa nuclear plants in November 2012.  Discussions with U.S. technical experts at the committee’s meetings in the United States. Appendix B identifies the technical experts who participated at committee meetings in the Japan and the United States. It is important to acknowledge that there are information gaps and uncertainties about some details of the accident progression. The accident timeline presented in this chapter represents the committee’s best collective technical judgments informed by the information sources cited above. This chapter is organized into five sections. The first section provides a timeline for the accident. Additional details on the timeline are provided in Appendix C. The second section describes some of the challenges in responding to the accident. The third section describes key accident events and responses by plant personnel. 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. 4.1 TIMELINE FOR FUKUSHIMA DAIICHI ACCIDENT Table 4.1 provides a committee-constructed summary timeline for the accident in Units 1, 2, and 3 at the Fukushima Daiichi nuclear plant. A more detailed description of this timeline is provided in Appendix C. A simplified timeline of key events is depicted graphically in Figure 4.1. 2 The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) was established by the National Diet of Japan on October 30, 2011. The commission was chaired by Dr. Kiyoshi Kurokawa, academic fellow, National Graduate Institute for Policy Studies. The commission published its report in 2012. 3 International Experts’ Meeting on Reactor and Spent Fuel Safety in the Light of the Accident at the Fukushima Daiichi Nuclear Power Plant. March 19-22, 2012. IAEA, Vienna, Austria. Information available at http://www- pub.iaea.org/iaeameetings/43900/International-Experts-Meeting-on-Reactor-and-Spent-Fuel-Safety-in-the-Light-of- the-Accident-at-the-Fukushima-Daiichi-Nuclear-Power-Plant . 4 International Experts’ Meeting on Severe Accident Management in the Light of the Accident at the Fukushima Daiichi Nuclear Plant. March 17-20, 2014. IAEA, Vienna, Austria. Information available at http://www- pub.iaea.org/iaeameetings/46832/International-Experts-Meeting-on-Severe-Accident-Management-in-the-Light-of- the-Accident-at-the-Fukushima-Daiichi-Nuclear-Power-Plant. Prepublication Copy 4-2

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident The committee’s timeline was developed from previously published accident accounts, primarily ANS (2012), INPO (2011), Investigation Committee (2011, 2012), and TEPCO (2011a,b; 2012b, 2013). The committee gathered additional information through discussions with Japanese and U.S. technical experts to better understand some details of the timeline. The zero point of the timeline is the afternoon of March 11, 2011, just before the Great East Japan Earthquake struck Japan. Chapter 3 of this report describes the status of the six reactor units at the Fukushima Daiichi nuclear plant at this time:  Units 1, 2, and 3 were operating at licensed power level.  Unit 4 was in an outage for replacement of the reactor core shroud. 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. Fuel remained in their cores and the reactors were being actively cooled. The Unit 5 containment was open and the primary system was undergoing pressure testing; because the reactor was at elevated pressure it was not strictly in cold shutdown. The earthquake initiated the following chain of events at the plant (Table 4.1):  The reactors in Units 1-3 shutdown automatically (scrammed) as designed when high seismic accelerations (i.e., ground shaking) were detected in the units.  Offsite AC power to the site was lost because of the collapse of one transmission tower and severe damage to equipment in a substation as a result of ground shaking.  Following offsite AC power loss, the Main Steam Isolation Valves (MSIVs) in Units 1-3 closed automatically to isolate the reactors, limiting the potential loss of coolant, release of radioactivity, and the rate of reactor vessel cooldown.  Within about a minute of offsite AC power loss, the onsite emergency diesel generators automatically started and were connected to the power distribution system as designed to supply onsite emergency AC power to reactor safety systems. Normal reactor cool-down and decay heat-removal functions were in place and operating at the plant when the tsunami wave arrived starting about 41 minutes after the earthquake (Table 4.1). The tsunami flooded portions of the plant site (see Chapter 3), damaging pumps, electrical distribution panels, batteries,5 and emergency diesel generators. Units 1, 2, 3, 4, and 5 lost AC power within 5 minutes after the tsunami and Units 1, 2 and 4 lost DC power shortly thereafter. Unit 3 lost AC power but did not lose DC power immediately after the tsunami because its power distribution panels and backup battery were not damaged by flooding. Once power was lost the units’ control rooms lost lighting, indicators, instrument readouts, and controls. Although there were intermittent signs of power on some indicators in Units 1 and 2, reliable DC power was only available by connecting arrays of scavenged vehicle batteries to selected systems and instrumentation in the control rooms. Vehicle batteries also had to be employed in Unit 3 to operate critical systems after the installed backup battery was depleted (see Section 4.3.2 for details). 5 As noted in Chapter 2, nuclear plants have large backup batteries (or banks of batteries) to supply DC power to operate and monitor critical monitoring equipment and safety systems. Prepublication Copy 4-3

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident An emergency diesel generator at Unit 6 survived the tsunami because it was air-cooled and was located above flood level. It continued to supply emergency AC power to Unit 6 and was used to supply power to Unit 5 through a cross tie that had been installed during the evening and early morning following the earthquake (see Section 4.3.4 of this chapter for additional details). The cross tie was prepositioned prior to March 11 but installation was not started until after the tsunami and was not completed until 05:00 on March 12. Three tsunami warnings were issued by the Japan Meteorological Agency following the earthquake6:  Warning 1, indicating a major tsunami with 3 m wave amplitude for Fukushima Prefecture, was issued at +3 min (14:49). This warning was based on an initial analysis of earthquake strong-motion data.  Warning 2, indicating a major tsunami with 6 m wave amplitude for the Fukushima Prefecture, was issued at +29 minutes (15:15). This warning was based on observed tsunami amplitudes at tsunami meters and tide gauges.  Warning 3, indicating a major tsunami with 10 m or greater wave amplitude for the Fukushima Prefecture, was issued at +44 min (15:30), again based on observed tsunami amplitudes at tsunami meters and tide gauges. According to Investigation Committee (2011), the site superintendent at the Fukushima Daiichi plant (Mr. Masao Yoshida) learned about the first two tsunami warnings from TV news reports. 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. The third tsunami warning came after the first tsunami wave had already arrived at the Fukushima Daiichi plant (see Table 4.1). The tsunami warnings affected the site superintendent’s thinking about accident management because he was concerned that the tsunami might damage seawater pumps. Just before the earthquake occurred there were about 6400 personnel, including 750 employees of the plant owner-operator (TEPCO), on site (TEPCO, 2012b, p. 163). Many TEPCO and contractor workers left the plant on their own on March 11. Those who could not leave were evacuated to the seismic isolated building. TEPCO (2012b, p.166) estimates that an additional 300-400 people were evacuated in buses from March 12-14 and some additional unknown number of people self-evacuated during that time. By March 15 there were about 700 people left onsite (TEPCO, 2012b, p. 102). These included people who had no direct role in the emergency response. Appendix D describes the organization of personnel at the plant at the time of the accident. Ninety seven personnel were working in the main control rooms at the time of the earthquake. These personnel performed initial actions following the earthquake and tsunami. Additional personnel arrived to support control room staff in the following hours and days. Staffing reinforcements were dispatched to Fukushima Daiichi by TEPCO following the earthquake and tsunami to support restoration work. They started arriving on March 11 and arrivals continued over the next several days, averaging approximately 400 additional personnel 6 Information on tsunami warning is from a presentation by Osamu Kamigaichi, Japan Meteorological Agency, at the February 2012 meeting of the Intergovernmental Oceanographic Commission. This presentation is available at http://ioc-tsunami.org/index.php?option=com_oe&task=viewDocumentRecord&docID=8619. Prepublication Copy 4-4

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident on site each day. These included the “primarily 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] and its surroundings, and procurement team that provided material support” (TEPCO, 2012b, p. 303). In addition, in accordance with prior agreements, personnel from other utilities arrived to provide support starting on March 13. Early on March 15, 650 personnel temporarily evacuated to Fukushima Daini following a hydrogen explosion in Unit 4, leaving approximately 70 workers required for station monitoring and restoration activities (TEPCO, 2012b, p. 166). Some of the personnel that had evacuated to the Fukushima Daini plant returned by noon on March 15. These included operators responsible for monitoring data from the main control rooms, the health physics team responsible for performing radiation-level measurements in the field and for access control to the seismic isolated building, and the security guidance team responsible for controlling station access (TEPCO, 2012b, p. 166). The earthquake and tsunami resulted in three fatalities at TEPCO’s plants: two fatalities occurred at Fukushima Daiichi and one at Fukushima Daini. 4.2 CHALLENGES FOR RESPONDING TO THE ACCIDENT The Fukushima Daiichi accident occurred in the midst of a regional disaster involving the largest loss of life and civil disruption in Japan since WWII. The accident is historically unique in this regard. The earthquake and tsunami overwhelmed offsite emergency response efforts (see Chapter 6) and added greatly to the challenges of responding to the accident at the plant. Japanese investigations of the accident (Investigation Committee, 2011, 2012; NAIIC, 2012) concluded that the Fukushima Daiichi nuclear plant’s owner-operator (TEPCO) was not adequately prepared for an earthquake and tsunami of this magnitude. The plant lacked survivable onsite power supply, water pumping, and communications equipment. 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. Personnel involved in the accident response had to improvise, a fact highlighted by Investigation Committee (2011, p. 110-111): “The shift team7 used lights with portable batteries and LED flashlights to read the event-based and state-based "Emergency Operating Procedure." However, the content of the material could not be applied directly to the actual events taking place. The team members also checked the "Emergency Operating Procedure" for accident management (AM) to identify the operating procedure necessary to control Units 1 and 2. However, the "Emergency Operating Procedure" for AM contained only internal events as causal events for AM and did not consider external events such as an earthquake or tsunami as causal events. There was no 7 The shift team comprised the personnel in the control room of each reactor unit. See Appendix D. Prepublication Copy 4-5

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident reference taking into account the events where all AC and DC power sources would be lost. In addition, the descriptions of the standards were written on the assumption that the state of the plants can be monitored by the control panel indicators and measuring instruments in the main control room and that the control panel could be manipulated. As a result, the shift team was forced to predict the reactor state according to a limited amount of information and take such procedures [that] operators think best on the spot instead of following the instructions described in the standard manuals.” Staff in the onsite ERC was stunned to learn of the complete failure of power in three of the reactor units. Their reaction is described in Investigation Committee (2011, p. 108-109): “The NPS [nuclear power station] ERC8 received reports from the three main control rooms that the nuclear reactors were successively losing their power supplies and Units 1, 2 and 4 in particular had lost all of their power sources. Everyone at the NPS ERC was lost for words at the ongoing unpredictable and devastated state.” “Site Superintendent Yoshida understood that a situation that far exceeded any expected major accident had actually taken place. He could not think of anything on the spot and so decided to implement the procedure stipulated by the law.” Plant personnel confronted many challenges in responding to the earthquake and tsunami:  Flooding in the turbine buildings and lower portions of Units 1 and 2 rendered reactor control and safety systems inaccessible or unusable.  Damage to the site from the tsunami made roads impassable and generally hindered personnel access.  Loss of instrumentation readouts in the Unit 1-2 control rooms and loss of the safety parameter display systems9 in the Unit 1-3 control rooms and the onsite ERC and off-site center (OFC) made it impossible to obtain timely information about the condition of the Unit 1-3 reactors and Unit 1-4 spent fuel pools. Control room personnel reported basic reactor parameters to the onsite ERC using fixed-line telephones. These data were manually recorded on whiteboards to facilitate the sharing of information within the ERC.  Loss of lighting made it difficult to work, forcing control room and field personnel to use flashlights.  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. Field personnel wore standard anti- 8 This report uses the term “onsite ERC” to refer to this facility. 9 The safety parameter display system provides detailed real-time plant parameter and component status information. Prepublication Copy 4-6

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident contamination suits and self-contained breathing apparatus, which made their work and communications even more difficult. At one point during the accident the Unit 1 reactor operators had to don full face masks with charcoal filters, anti-contamination coveralls, and at times had to move to the Unit 2 side of the control room and crouch down to avoid excessive radiation exposure.  The lack of food, working toilets, and relief personnel during the early stages of the accident as well as the extended length of the accident response added greatly to personnel fatigue and distress. Plant personnel who responded to the accident exhibited a strong degree of self-sacrifice: Many suffered personal losses (homes destroyed or damaged, family members displaced or lost) but continued to work, in some cases for weeks following the tsunami. Personnel volunteered to enter high radiation zones and many received exposures well over permissible levels. The OFC, located in Okuma about 5 km southeast of the plant, did not function as intended following the tsunami. It was never fully staffed because of access difficulties owing to transportation system damage and traffic congestion. Additionally, all of its telecommunications circuits except for a satellite connection were inoperable.10 The OFC had to be evacuated on March 14 because of elevated radiation levels following the hydrogen explosion in the Unit 3 reactor building.11 The coordination activities that would normally be performed at the OFC were conducted at the TEPCO headquarters ERC, which was located in Tokyo (Appendix D), and at Japanese government offices. This reduced the effectiveness of communications between the onsite ERC, TEPCO, and local and national government agencies (INPO, 2011). According to NAIIC (2012), 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. 4.3 KEY EVENTS AND RESPONSE ACTIONS The following sections describe some of the major events during the accident and key response actions by plant personnel. These descriptions are not intended to be comprehensive; rather, they are intended to illuminate the factors that prevented a more successful response to the accident. These factors informed the committee’s finding on the causes of the accident (see Section 4.5 in this chapter) and discussions of lessons learned (see Chapter 5). Investigation Committee (2011, 2012) and TEPCO (2011a,b; 2012b) served as the main sources of information for the descriptions in the following sections. 10 Personnel in the OFC were unable to use the videoconferencing system, the Emergency Response Support System (ERSS), the System for Prediction of Environmental Emergency Dose Information (SPEEDI), email, Internet, or ordinary telephone/fax lines. 11 The OFC was not equipped with filtered ventilation for removing radioactive material even though it was intended for use in nuclear emergencies. Prepublication Copy 4-7

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident 4.3.1 Unit 1 Reactor Following the earthquake and scram of the Unit 1 reactor, its two isolation condensers (Figure 4.2-4.3) started automatically as designed (see Section 2.2 in Chapter 2). Following established operating procedures, the Unit 1 operators12 used these isolation condensers to control reactor pressure and cooldown rate. 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. It was inoperable after the tsunami because the operator had closed off the return line valve (valve MO- 3B in Figure 4.3) before the tsunami and was unable to open it afterward due to the lack of AC and DC power (Investigation Committee, 2011, p. 117; TEPCO, 2012b, p. 195). Subsequently, the tsunami flooded the Unit 1 emergency diesel generators and power panels (Figure 4.2), cutting off all AC and DC power to the unit. With no power for instrumentation or controls, the Unit 1 operators lost the ability to monitor plant indicators from the control room. Most critically, they were unable to check the status of the isolation condenser valves14 or to actuate them from the control room. Attempts to check the status of the valves in the field were unsuccessful because of access limitations and high radiation fields. Attempts to start up the high-pressure coolant injection system (Figure 4.2) also were unsuccessful due to the loss of DC power. The loss of AC and DC power in Unit 1 caused its isolation condenser to shut down because of failsafe control logic (this logic is described later in this section). As a consequence, Unit 1 essentially lost all cooling function. However, operators and onsite ERC staff did not understand at first that the isolation condenser had stopped functioning because plant indicators and controls were not available. In fact, the Unit 1 operators initially assumed that the isolation condenser was working. The staff in the onsite ERC and the site superintendent could not determine if the isolation condenser was functioning due to the failure of the safety parameter display systems and lack of definite information from the Unit 1 operators. Site Superintendent Yoshida was sufficiently concerned that he immediately reported to Tokyo that there was a failure of the emergency core cooling systems for Units 1 and 2 (Investigation Committee, 2011, p. 114). The onsite ERC began to take proactive actions to restore the Unit 1 monitoring systems and establish alternative water injection sources. 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 alternate water injection strategies: water injection via the diesel driven fire protection system (this system is depicted in Figure 4.2), a mitigation strategy specified in the plant’s accident 12 The committee uses the following terms to describe TEPCO and contractor staff involved in the response to the accident at the Fukushima Daiichi nuclear plant. The term operator refers to personnel stationed in the main control rooms at the plant. The term ERC staff refers to personnel stationed in the onsite or headquarters ERCs. The more general term plant personnel is used when the locations of personnel at the plant are not specified or important. 13 That is, maintain reactor pressure between 6-7 MPa and a cool-down rate of 55ºC (100ºF) per hour. 14 That is, to determine whether the valves were open or closed. Prepublication Copy 4-8

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident management procedures, and water injection through the fire protection system using fire engines, a strategy not specified in those procedures. Around 18:00 on March 11 some DC power was restored in Unit 1. Operators discovered that the isolation condenser valves outside of containment (i.e., valves MO-2A and MO-3A in isolation condenser A; see Figure 4.3) were closed. The fact that valve MO-2A read closed, when it normally should be open (see Section 2.2 in Chapter 2), caused operators to suspect that all of the isolation condenser valves had closed after loss of AC and DC power. At 18:18 operators decided to open valves MO-2A and MO-3A on the possible chance that the valves in containment (MO-1A, and MO-4A) had not fully closed. At this point the operators inferred that the isolation condenser was functioning; this inference was based on indirect audible (i.e., steam generation was heard) and visual (i.e., a steam plume was observed) cues. The operators informed the onsite ERC that the isolation condenser was functioning. However, operators closed the condensate return valve (valve MO- 3A in Figure 4.3) shortly thereafter (at 18:25). The reason for this action is unclear15 and the onsite ERC was not informed that it had been taken.16 By around 18:30 on March 11 the Unit 1 operators became convinced that the isolation condenser was not functioning. They recognized then that water injection into the reactor was the only option available to cool it. Preparations for injecting water into the Unit 1 reactor using the diesel driven fire protection system (Figure 4.2) had already been underway for over an hour; these preparations were completed by 20:50. However, the reactor pressure vessel had to be depressurized first (by opening the safety relief valves; see Figure 4.2) before low-pressure water from the fire protection system could be injected. The operators asked the onsite ERC to provide batteries so that the safety relief valves could be opened from the control room. However, the ERC team member who received this request did not understand its urgency, possibly because the ERC believed that the isolation condenser was still operating normally. 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 top of active fuel,17 which was likely not the case18), caused the onsite and headquarters ERCs to continue to believe that the isolation condenser was operating. By about 22:00 on March 11, rising radiation levels were observed in the reactor, drywell and turbine buildings, suggesting that fuel degradation and core damage were occurring.19 By 23:50 the site superintendent and other onsite ERC personnel fully understood that the isolation condenser was not operating. At approximately midnight on March 12, the Unit 1 operators began preparations for venting the containment (Figure 4.2). Operators consulted piping and instrumentation diagrams, valve drawings, and accident management procedures. These procedures assumed that power 15 Investigation Committee (2011) and TEPCO (2012b, 2013) discuss possible reasons for this action. The reasons are not relevant to the present discussion so are not described here. 16 Valve MO-3A was opened again at 21:30. 17 Top of active fuel, usually denoted TAF, is the uppermost point in a fuel rod that contains uranium fuel. It serves as the reference point for water level readings in the reactor. 18 Reactor pressure vessel level sensors likely provided misleading values due to sensor degradation. 19 TEPCO (2013, p. 11) suggests that water levels in the Unit 1 reactor dropped to the top of active fuel at about 18:10 on March 11 and that core damage was initiated at about 18:50. Prepublication Copy 4-9

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident would be available for remote valve control; consequently, they were not applicable to the then- current situation in Unit 1. The operators needed to develop (in real time) a plan for venting the containment by manual valve operation. This required study of the layout and configuration of the vent valves to determine which valves needed to be opened, their locations, and whether and how they could be opened manually. Operators confronted a number of additional obstacles for venting containment. These included a need to perform dry runs to keep field work time as short as possible (because of high radiation levels); the need to gather equipment (fireproof clothing, personal air supply, flashlights, full face masks); and the need to perform the work in shifts (three teams of two people) because the reactor building was pitch dark and radiation levels were high.20 Team 1 completed its assigned task but teams 2 and 3 had to turn back because of high radiation levels. Venting was eventually performed from the control room after a compressor was procured and installed to enable remote operation of the large air-operated suppression chamber vent valve (see Figure 4.2). Because of these delays venting did not begin until 14:30 on March 12 when containment pressure had reached over 0.75 MPa (110 psig), almost twice the design value of 0.43 MPa (63 psig). By 02:45 on March 12 the pressure in the reactor pressure vessel was determined to be near containment pressure21; fresh water 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. At 15:36 on March 12 a hydrogen explosion occurred on the refueling floor of the Unit 1 reactor building outside of containment. Further discussion of hydrogen generation and the explosion in Unit 1 is provided in Section 4.3.5. 4.3.1.1 Discussion The isolation condenser in Unit 1 most likely lost its ability to effectively cool the reactor when AC and DC power were lost.23 However, it wasn’t until approximately three hours later (at 18:30) that operators in the Unit 1 control room fully understood that the isolation condenser was not functioning effectively. It took the onsite ERC staff even longer—until about 23:50—to fully understand this fact. In hindsight, shutdown of the isolation condenser was an unanticipated side effect of the design of the failsafe control logic circuit that operates the isolation condenser valves. This circuit is powered by instrumentation DC. If this power is lost the logic circuit acts as if there 20 The three teams consisted of shift supervisors, deputy managers, and older workers. Younger workers were not permitted to participate because of the danger involved even though they volunteered to do so. 21 It is not clear whether depressurization occurred because of damage to the reactor pressure vessel, a pipe break, or safety relief valves that had stuck open due to thermal fatigue failure. 22 Only a fraction of the water injected using the fire truck pumps appears to have reached the reactor. Water may have been lost from leaky fire hoses, open valves, and branches in the piping system that diverted water. See TEPCO (2013, Attachment 1-4) for additional details. 23 TEPCO has concluded that the valves on the System A isolation condenser did not close fully because some water was lost from the Train A tank; it was measured to be 65 percent full in a post-accident inspection, a decline from the previous, and normal, level of 80 percent. However, as noted by TEPCO (2012b, p.197), since a substantial amount or water remained in the shell-side of the isolation condenser, the amount of heat removal during the accident must have been limited. Investigation Committee (2011, p.121) also supports this observation. Prepublication Copy 4-10

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident were a pipe break in the isolation condenser system and commands all four of its valves to close (see Figure 4.3). Whether the valves actually close, however, depends on the timing of power loss to three circuits:  instrumentation DC, which powers the logic circuit;  125V DC, which opens and closes the two valves outside containment24  AC, which opens and closes the two valves inside containment, as well as the time required to close the valves (20-30 s) once the actuation signals are received by them (Investigation Committee, 2011, p. 118). The two valves inside containment (i.e., valves MO-1A and MO-4A in Figure 4.3) are of greatest concern for operator control of the isolation condenser because they are not physically accessible. Consequently, once closed, without AC power they cannot be reopened by operators. Based on currently available information (see Footnote 23), it appears that the two valves inside containment received enough AC power to close most of the way, indicating that instrumentation DC power failed first (Craig Sawyer, General Electric (retired), written communication, January 14, 2014). However, the status of the valves inside containment will not be known for certain until they can be inspected, which will require physical entry into containment. Communications difficulties between operators and onsite ERC may have delayed recovery efforts. As noted previously, they did not communicate effectively about the operation of the isolation condenser. 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. Indeed, the site superintendent and onsite ERC initiated actions to identify alternative water injection means early in the accident.25 However, the severe conditions at the plant apparently prevented a faster response. There is some suggestion of lack of clarity in roles and responsibilities within the onsite ERC, particularly with respect to allocating responsibilities for responding to situations that are not covered by accident management procedures. This led to delays, for example, in developing and implementing the procedure for using fire engines to inject water into the reactor pressure vessel through the fire protection system. Preparations for this procedure (e.g., verifying the availability of fire engines, locating water discharge ports, positioning the fire engines, and laying fire hoses) did not get underway until dawn on March 12. 4.3.2 Unit 3 Reactor Unit 3 did not lose DC power immediately after the tsunami. Consequently, until its batteries became depleted, operators were able to monitor plant indicators from the control room, including reactor pressure and water levels. They were also able to activate, monitor, and control 24 If 125V DC is available it can be routed through inverters to produce AC power to operate the valves. However, such power was not available in this case because of flooding. 25 The site superintendent directed the onsite ERC staff to develop plans for alternative water injection as early as 17:12 on March 11. Prepublication Copy 4-11

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident SIDEBAR 4.2 Accident Response at Fukushima Daini TEPCO’s Fukushima Daini nuclear plant (see Chapter 3) sustained severe damage from the March 11, 2011, earthquake and tsunami. However, operators were able to bring the plant’s four reactors to cold shutdown by the morning of March 15. Their actions illustrate the successful application of emergency operating and accident management procedures in response to a severe external event. The earthquake shut down two of the three available offsite AC power lines to the plant (another line was shut down for inspection at the time of the earthquake). Flooding from the tsunami damaged power distribution systems and pumps for the emergency core cooling and residual heat removal systems in the Unit 1, 2, and 4 reactors. However, AC power from one offsite power line and onsite DC power remained available following the earthquake and tsunami. Consequently, operators were able to maintain instrument and control room command over critical plant systems. 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 pressures were less than 1 MPa eight hours after the tsunami. Cooling was then transitioned seamlessly to low-pressure water injection with an alternate water supply (the make-up water condensate system) by midnight of March 11. The water levels in the reactors were maintained at or near the “L8” level, over 5 m above the top of active fuel, during the cool-down phase. Drywell and suppression chamber sprays were used to control containment pressures to less than 0.4 MPa until power was restored to the residual heat removal systems on the morning of March 14. Operators were able to quickly and successfully execute several critical tasks that operators at Fukushima Daiichi attempted but could not complete. These included lining up vent valves, arranging alternate water supplies, controlling reactor core isolation cooling systems, and, most important for recovering the residual heat removal system, laying and connecting alternate power cables and replacing damaged motors, all carried out by hand or by using crane trucks. Operators took some actions (e.g., lining up vent valves) in anticipation that the accident might become more severe; however, existing emergency operating procedures were adequate for bringing the reactors to cold shutdown. Only one ad hoc measure suggested by the onsite ERC—water injection into the suppression chamber using an alternate water source—was employed (TEPCO, 2012b, p. 54). Although operators at Fukushima Daini faced some of the same challenges as those at Fukushima Daiichi—most notably onsite access difficulties due to tsunami-related flooding and damage and earthquake aftershocks—there were some key differences: flooding at the Fukushima Daini plant was not as severe; AC and DC power were continuously available in functioning control rooms; and onsite response efforts were not hindered by debris and radioactive contamination from hydrogen explosions. 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. According to TEPCO (2012b, p. 55): “During the accident, the decision-making procedure where the Shift Supervisor made determinations and the ERC at the power station made verifications was generally adhered to. This allowed operational manipulations to be implemented in a timely manner according to plant conditions and also was effective in allowing the ERC at the power station to fulfill its function of keeping a big-picture perspective to maintain oversight of response strategies and to manage equipment restoration activities.” Comparing the responses at the Fukushima Daiichi and Daini plants, where operators presumably Prepublication Copy 4-36

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident received the same levels of training, it is clear that the loss of all AC and DC power at Fukushima Daiichi precipitated a series of cascading failures that simply overwhelmed operators. In a sense, the events at Fukushima Daiichi represent a “cliff edge” in accident management capabilities. TEPCO anticipated and trained its operators for the situations they encountered at Fukushima Daini and the response was effective. TEPCO never anticipated nor trained its operators for the events at Fukushima Daiichi; the response was ineffective and the consequences were disastrous. Prepublication Copy 4-37

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident TABLE 4.1 Timeline of Key Events in Units 1-3 at the Fukushima Daiichi Nuclear Plant Event/Condition Unit 1 Unit 2 Unit 3 Prior to earthquake Operating at rated power level Earthquake T=0 (3/11/11 @ 14:46) Reactor Scram MSIVs close Loss of offsite AC power Emergency diesel generators (EDGs) start Tsunami warnings 14:49 (+3 min): 3 m (Fukushima 15:15 (+29 min): 6m Prefecture) and 15:30 (+44 min): >10m estimated wave heights Tsunami arrival times +41 m/+50-+51 m (1st/2nd waves) (15:27/15:36-15:37) Loss of onsite AC AC lost at +51 m AC lost at +55 m AC lost at ~ +51 m power (EDGs) and (15:37) (15:41) (15:37) DC power (batteries) DC lost at + 60 m DC lost at +60 m DC available until (15:46) (15:46) ~+36 hours Isolation Condenser Failed on loss of AC NA NA (IC) and DC power Performance Reactor Core Isolation NA Real-time status ~20 h of running time; Cooling (RCIC) uncertain; evidence of failed w/o restart at performance ~70 h running time +20 h High Pressure Coolant Unavailable due to Unavailable due to ~16 hr of running Injection (HPCI) loss of DC power loss of DC power time beginning at +20 performance hr Reactor pressure Depressurized due to Depressurized at Depressurization vessel assumed RPV failure +75.2 h and +78.3 h occurred at ~+42 h depressurization at +12 h Time of max +11.7 h ~+80 h ~+42 h containment pressure (0.84 MPa/0.43 MPa) (~0.75 MPa/0.38 (0.64 MPa/0.38 MPa) (Max containment MPa) pressure/design pressure) Estimated time of +4 h to +7 h +75 h to +85 h +36 h to +40 h core damage First indication of offsite release of +8.2 to +14.1 h radioactive materials Containment venting +9.7 h/~+24 h +26.7 h/not successful +29.5 h/+42 h Prepublication Copy 4-38

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident preparation/success Hydrogen explosion +24.8 h None +68.2 h Initial injection of +15.0/+28.8 h None/+77.2 h +42.6/+46.4 h fresh/seawater Restoration of offsite March 20 March 20 March 22 AC power NOTES: ADS = automatic depressurization system; 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; SRV = safety relief valve. Prepublication Copy 4-39

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident TABLE 4.2 Key Results for Accident Progression Simulations in Unit 1 Event Time after Notes earthquake (+ h) Core exposure (TAF) +2.5-+3 Core damage begins +4 Core damage timing is nominal and based on Sandia MECLOR analysis (Gauntt et al., 2012) Core fully uncovered +4.5-+5 MSL ruptures +6.5 Considered by Sandia Melcor analysis only (Gauntt et al., 2012) RPV damage +9-+11 RPV melt through +14 Probably occurred at +13 h, could have been as late as +16 h Containment leaks +3-+6 Depends strongly on assumed failure modes Hydrogen generated 900 kg; amount depends on extent of core concrete (kg) interaction Containment venting +23.7 Known from actions of operators and pressure records Explosion +24.8 Known from both seismic and video recordings NOTES: MSL = main steam line; RPV = reactor pressure vessel; TAF = top of active fuel. SOURCE: Estimates based on MELCOR and MAAP simulations by EPRI (2013), Gauntt et al. (2012b), TEPCO (2012a), and Yamanaka (2012). Prepublication Copy 4-40

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident TABLE 4.3 Spent Fuel Storage at the Fukushima Daiichi Nuclear Plant on March 11, 2011 Storage location Spent fuel (assembliesa) Fresh fuel (assemblies) Unit 1 292 100 Unit 2 587 28 Unit 3 514 52 Unit 4 1331 204 Unit 5 946 48 Unit 6 876 64 Common pool 6375 0 Cask storage building 408 0 a A BWR fuel assembly contains about 170-185 kg of uranium. SOURCE: TEPCO (2012b, p. 299) Prepublication Copy 4-41

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident FIGURE 4.1 Graphical depictions of accident time lines for Units 1-3 at the Fukushima Daiichi plant. The key events shown in the timelines are described in the text. Prepublication Copy 4-42

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident FIGURE 4.2 Schematic illustration of major safety systems in Unit 1 of the Fukushima Daiichi plant. SOURCE: Courtesy of TEPCO. Prepublication Copy 4-43

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident FIGURE 4.3 Schematic of the isolation condenser systems for Fukushima Unit 1. The unit contains two systems, labelled “A” and “B.” Motor-operated (MO) Valves are indicated by connected triangles. Black indicates valve closed during normal operations; white indicates valve open during normal operation. The valves inside of primary containment are operated by AC power. The valves outside of containment operate with DC power. A fuller description of isolation condenser operation is provided in Chapter 2. SOURCE: Government of Japan, 2011a, Figure IV-2-4. Prepublication Copy 4-44

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident FIGURE 4.4 Schematic illustration of major safety systems in Units 2 & 3 of the Fukushima Daiichi plant. SOURCE: Courtesy of TEPCO. Prepublication Copy 4-45

OCR for page 95
Chapter 4: Fukushima Daiichi Nuclear Accident Figure to be provided in final version of report. FIGURE 4.5 Photos showing damage to reactor buildings at the Fukushima Daiichi plant from hydrogen explosions. Upper row (L to R) Unit 1, Unit 3 and Unit 4 exteriors. Lower row: (L) close up of Unit 1 steel structure remaining above refueling level. (R) Interior of Unit 4. Prepublication Copy 4-46