Lessons Learned from the Fukushima Nuclear Accident for Improving Safety of U.S. Nuclear Plants (2014) / Chapter Skim
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5 Lessons Learned: Plant Operations and Safety Regulations
5 Lessons Learned: Plant Operations and Safety Regulations
Pages 153-195
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From page 153... ...
The focus of this chapter is on nuclear plant safety systems, operations, and regulations. Chapter 6 focuses on offsite nuclear emergency planning and emergency management, whereas Chapter 7 focuses on the nuclear safety culture.
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From page 154... ...
5.1 NUCLEAR PLANT SYSTEMS, PROCEDURES, AND TRAINING FINDING 5.1: Nuclear plant operators and regulators in the United States and other countries have identified and are taking useful actions to upgrade nuclear plant systems, operating procedures, and operator training in response to the Fukushima Daiichi accident. In the United States, these actions include the nuclear industry's FLEX (diverse and flexible coping strategies)
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From page 155... ...
Moreover, the committee had neither the time nor resources to carry out in-depth reviews of these initiatives, which in some cases would have required plant-by-plant examinations. 5.1.1 Nuclear Plant Systems RECOMMENDATION 5.1A: As the nuclear industry and its regula tor implement the actions referenced in Finding 5.1, they should give specific attention to improving plant systems in order to enable effective responses to beyond-design-basis events, including, when necessary, developing and implementing ad hoc1 responses to deal with unan ticipated complexities.
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From page 156... ...
5.1.1.1 DC Power for Instrumentation and Safety System Control As noted in Chapter 4, the loss of DC power at the Fukushima Daiichi plant severely impacted operators' ability to monitor the status of reactor pressure, temperature, and water level and operate critical safety equipment. A lesson that emerges from this accident is that high priority must be given to protecting DC batteries and power distribution systems at nuclear plants so that they remain functional during beyond-design-basis events.
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From page 157... ...
nuclear plants might need to be retrofitted and/or relocated to protect them during beyond-design-basis events. The specific actions required, if any, will be plant specific.
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From page 158... ...
The committee further judges that the existing thermal-hydraulics knowledge base can be leveraged to create aids for generating real-time estimates of key thermodynamic parameters and liquid level in the reactor pressure vessel and provide real-time support for response planning. 5.1.1.3 Decay-Heat Removal, Reactor Depressurization, and Containment Venting Systems The loss of AC and DC power at the Fukushima Daiichi plant severely impacted operators' ability to remove decay heat from the Unit 1-3 reactors and depressurize reactor press174ure vessels and vent containments, both to restore cooling to the core and to prevent leakage of fission products.
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From page 159... ...
2. Ad hoc low-pressure water injection systems were not effective for cooling the Unit 1-3 reactors because of difficulties in depressurizing reactor pressure vessels and venting containments.
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Core damage can occur if low-pressure injection does not restore water levels in a timely fashion.5 Consequently, reactor operators must have well-defined strategies and capabilities for depressurizing reactor pressure vessels and venting containments in a timely manner under loss-of-power conditions. Additionally, there must be a low-pressure heat removal capability that is independent of electrical power.
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From page 161... ...
FLEX would be greatly enhanced if it focused on preventing core damage as well as on mitigating damage severity should it occur. 5.1.1.4 Instrumentation for Monitoring Critical Thermodynamic Parameters The loss of AC and DC power in Units 1 and 2 at the Fukushima Daiichi plant shut down key monitoring instrumentation for the reactor pressure vessel, drywell, and suppression chamber (see Chapter 4)
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From page 162... ...
Additional monitoring instrumentation was added to U.S. nuclear plants as a result of this analysis: for example, reactor pressure indications, a wider range of reactor core temperature indications, and more robust temperature sensors.
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Nuclear plants with Mark I and Mark II containments worldwide are equipped with nitrogen inerting systems to maintain reduced oxygen concentrations in containment (see Appendix G)
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However, the Fukushima Daiichi accident demonstrated that the mere presence of containment vents12 does not eliminate hydrogen explosion hazards during severe accidents. Indeed, the effectiveness of these vents in limiting hydrogen releases in the buildings will depend on their operability under severe accident conditions (e.g., under loss of DC power and compressed air, as happened at Fukushima Daiichi)
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From page 165... ...
The reduction of physical protection at the plant increases its vulnerability to attacks from external forces or determined insiders. Additionally, the voluminous amount of information published about the accident provides potential adversaries with data about critical plant systems, their interdependencies, and key personnel; this information could be used to plan and carry out attacks on other nuclear plants.
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From page 166... ...
nuclear plants and USNRC headquarters to enable direct and automatic electronic transmission of critical plant parameters during emergencies. The task force report notes that Having data provided directly from automated sources at the site also gives confidence to government authorities and the public that the plant operator is not filtering the details of an evolving accident.
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The committee judges that peer review will also enhance the transparency, credibility, and public confidence in actions taken by industry and its regulator (USNRC) to implement lessons learned from the Fukushima nuclear accident.
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From page 168... ...
Strengthening and better integrating emergency procedures, extensive damage mitigation guidelines, and severe accident manage ment guidelines, in particular for • Coping with the complete loss of AC and DC power for extended periods, • Depressurizing reactor pressure vessels and venting contain ments when DC power and installed plant air supplies (i.e., compressed air and gas) are unavailable, • Injecting low-pressure water when plant power is unavailable, • Transitioning between reactor pressure vessel depressur ization and low-pressure water injection while maintaining sufficient water levels to protect the core from damage, • Preventing and mitigating the effects of large hydrogen explosions on cooling systems and containments, and • Maintaining cold shutdown in reactors that are undergoing maintenance outages when critical safety systems have been disabled.
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From page 169... ...
nuclear plants and these associated emergency response facilities need to be reassessed to ensure that critical personnel functions, including communication and coordination functions, can be supported in complex emergencies, particularly emergencies involving multiple reactor units (at multiunit sites) , and/or require 24-hour operations with shift turnovers.
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From page 170... ...
Off-normal events involving the loss-of-offsite AC power are within the design basis for nuclear plants. Operators are trained to respond to such events using EOPs and other plant procedures such as abnormal operating procedures and alarm response procedures.
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Depressurizing reactor pressure vessels and containments when DC power and installed plant air supplies are unavailable; 3. Injecting low-pressure water when plant power is unavailable; 4.
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Extensive testing of the integrated procedures will also be required at each nuclear plant. The nuclear industry could develop accident management advisory tools to assist in the development of SAMG, assess their effectiveness and
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FLEX strategies at individual nuclear plants might need to be augmented to provide the resources required to implement revised SAMG. For example: • Coping with power loss will likely require the availability of portable batteries, emergency generators, and prepared power cables.
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nuclear plants have accredited training programs that are conducted annually for a range of maintenance, engineering, technical personnel, and operators.
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reinforce fundamental understanding of nuclear plant system design and operation -- this includes having a full grasp of the capabilities of all plant equipment (not just so-called "safety critical" equipment) and how it can be marshaled in emergency situations; and (ii)
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. The analyses in those papers suggested that • Intact reactor buildings could play a significant role in mitigating the consequences of severe accidents in BWRs.
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events -- can produce severe accidents at nuclear plants that damage reactor cores and stored spent fuel. Such accidents can result in the generation and combustion of hydrogen within the plant and release of radioactive material to the offsite envi ronment.
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, and Limerick nuclear plants (Philadelphia Electric Company, 1981)
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. It evaluated severe accident risks at five nuclear plants.
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of currently installed seismic and flooding protection features at U.S. nuclear plants and identify, correct, and report any degraded conditions.
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PRAs in use at existing U.S. nuclear plants would need to be enhanced to make them useful for assessing beyond-design-basis external events such as occurred at the Fukushima Daiichi plant; in particular, they would need to 1.
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This includes a consideration of situational challenges (e.g., unavailability of or misleading sensor indications and lack of relevant procedural guidance) that are likely to arise in severe accidents, and the individual, team, and organizational decision-making processes that are 20 The isolation condenser failure in Unit 1 at the Fukushima Daiichi plant is an example of such an interaction.
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likely to influence performance under time-pressured, high-stress conditions. Additional discussion of human performance during severe accidents is provided in Appendix J
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This race had different outcomes in Unit 1 and Unit 2: In Unit 1, the isolation condenser's AC-operated valves inside containment were effectively closed before the power failed; in Unit 2, in contrast, the valves for the reactor core isolation cooling system remained open. These different outcomes were appar ently determined by small differences in the timing and sequence of power failures resulting from the flooding of multiple power sources and distribution systems.
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From page 185... ...
A sequence of valves was used to connect the fire protec tion plumbing to the reactor pressure vessel using components of the condensate makeup water system. Unfortunately, the valves leading to the condensate storage tank were open, diverting water flow from the reactor and reducing the effective ness of core cooling.
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186 LESSONS LEARNED FROM THE FUKUSHIMA NUCLEAR ACCIDENT SIDEBAR 5.4 East Coast Tsunamis Although tsunamis in the Atlantic Ocean Basin do not occur with the frequency of those in the Pacific and Indian Ocean Basins, the potential for tsunami gen eration is high in some locations. One such location is the eastern margin of the United States.
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LESSONS LEARNED: PLANT OPERATIONS AND SAFETY REGULATIONS 187 racks ated c elong FIGURE S5.2 (A) High-resolution image of the continental shelf and slope offshore of Virginia and North Carolina showing the Albemarle-Currituck slide and several large canyons.
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• PRAs are already being used to assess and mitigate internal hazards at nuclear plants and to establish maintenance and test protocols. Consequently, plant licensees are familiar with their use.
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As shown in Appendix L, the likely costs for the Fukushima Daiichi nuclear accident exceed the estimated costs for the hypothetical accident at the Peach Bottom plant by a factor of about 33. a Section 50.109 states that "The Commission shall always require the backfitting of a facility if it determines that such regulatory action is necessary to ensure that the facility provides adequate protection to the health and safety of the public and is in accord with the common defense and security." Dr.
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It is essential that the USNRC fully account for the costs of severe nuclear accidents when making backfit decisions. An opportunity exists to use the accident progression at the Fukushima Daiichi nuclear plant to validate and improve severe accident system models (e.g., MAAP and MELCOR; see Chapter 4)
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From page 191... ...
. It has been recognized since the 1950s that risks to the public from the operation of nuclear power plants are dominated by accidents involving core damage and radioactive material releases.26 Nuclear plants were 26 Radioactive material releases from spent fuel pools that lose their water inventories have also been suggested as a source of risk to the public (see, e.g., Alvarez et al., 2003; NRC, 2004b)
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From page 192... ...
nuclear plants were designed, licensed, and built under these different siting and design criteria. It was recognized in the 1960s that accident likelihoods (i.e., the probability that a postulated accident would occur)
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This analysis (USNRC, 1975) provided a standard methodology as well as a benchmark for future studies; it also reaffirmed the conclusion that severe accidents involving core melts and radioactive material releases dominated risks to the public from nuclear plants.
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This accident occurred in a Soviet-era reactor having an unstable design31 and was initiated by a series of inappropriate operator actions. The accident resulted in major offsite radioactive material releases with acute fatalities and long-term health effects (see Chapter 6)
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On the other hand, expansion has continued steadily in spite of resistance in some quarters of the USNRC and industry. The committee judges that the broader use and expanded scope of modern risk concepts in nuclear reactor safety regulations could improve safety and lead to better policy decisions.
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