Key Facts
- Safety is the nuclear energy industry’s highest priority, and it is standard practice for the industry and federal regulators to review events that occur both domestically and abroad to glean information that can help make our facilities even safer. A serious, multiple-reactor accident at the Fukushima Daiichi Nuclear Power Plant in Japan in 2011 prompted a comprehensive review of safety measures at U.S. facilities and multiple assessments to analyze and apply lessons learned from the accident.
- The U.S. Nuclear Regulatory Commission (NRC) issued new requirements and requested detailed information in several areas related to the accident in Japan. Implementation of actions to meet these requirements has been completed at all U.S. nuclear power facilities. Collectively, these post-Fukushima actions represent an investment of more than $4 billion across the industry.
- The most important lesson from the accident in Japan is that a nuclear power facility must be prepared to handle a catastrophic natural event simultaneously affecting all reactors on the site, regardless of the cause. The industry has developed a diverse, flexible approach called FLEX. Building on existing safety systems, this approach addresses the major problem encountered at Fukushima—the loss of power to equipment needed to maintain reactor fuel cooling—by stationing another layer of backup equipment at each facility site and opening national rapid response centers in Phoenix, Arizona, and Memphis, Tennessee.
What Happened at Fukushima?
The March 2011 accident at the Fukushima Daiichi nuclear power facility in Japan occurred after a series of earthquakes and at least seven tsunami waves struck the six-reactor facility. While the seismic ground motion at the site was very close to the plant’s design parameters, the highest tsunami wave—estimated at 45 feet—was much larger than the tsunami analysis reflected in the plant’s design.
Reactors 1, 2 and 3 were operating when the earthquake struck and shut down as designed. Reactors 4, 5 and 6 already were shut down for refueling and maintenance. The earthquake did not cause damage that prevented the operation of key safety systems. However, it eliminated off-site power supplies, which caused the plant’s on-site emergency diesel generators to start automatically. At this point, all six reactors were safely shut down and kept cool by emergency powered safety systems.
About 40 minutes later, a series of large tsunami waves struck the site. These waves flooded and disabled all but one of the emergency generators—which continued to power safety systems for reactors 5 and 6. At reactors 1, 2 and 3, “steam-driven and battery-powered safety systems … worked for several hours (and more than a day in some cases),” according to the NRC. However, “those systems eventually failed and all three reactors overheated, melting their cores to some degree.”
How Did the U.S. Nuclear Industry Respond?
Within days of the accident, reviews of safety equipment and procedures were conducted at all U.S. nuclear power facilities to verify that safety measures were in place to cope with severe unexpected events. The NRC also established a task force to review safety requirements in light of the accident at Fukushima. The task force confirmed that there were no safety concerns requiring immediate action, recommended that plant operations and licensing activities continue, and made a series of recommendations to enhance safety. In 2012, the NRC issued three orders to power reactor facility licensees:
- Implement mitigation strategies to maintain reactor and used fuel pool cooling and containment integrity, following a severe event that exceeds design parameters.
- Install a second tier of reliable used fuel pool level instrumentation.
- Install readily accessible hardened vents for heat removal and pressure control in boiling water reactors with Mark I and II containments like the reactors at Fukushima. Thirty-one U.S. reactors have this type of containment. This order was modified the following year by another order that had more refined requirements for reliable, hardened containment vents capable of operation under severe accident conditions.
The NRC also required energy companies to provide detailed information on earthquake and flooding hazards and staffing and communications needs during a severe natural event or emergency affecting all reactors on a site.
FLEX: Protecting Against the Unexpected
The principal lesson from Fukushima is that a nuclear power facility must be prepared to handle a catastrophic natural event simultaneously affecting all reactors on the site. Operators should also assume that access to the site could be hindered by debris and damage to local area infrastructure. Building on safety features added after the 9/11 terrorist attacks, the industry developed a diverse and flexible coping capability called FLEX to implement the NRC's mitigation strategies order.
After the 2001 terrorist attacks, U.S. nuclear power companies took steps to safeguard plants against the explosions and fires that would result from the impact of a large aircraft. Because it is not possible to predict exactly what plant equipment would be affected, the industry focused on what would be needed to cool the reactor and used fuel storage pool if the installed safety systems were not available. Based on this assessment, companies purchased portable equipment—such as generators, battery packs, pumps and battery chargers—that could be stored away from the plant and used to respond, regardless of the location of the aircraft impact.
FLEX expands on this approach with multiple sets of additional portable equipment at each nuclear power facility to provide backup power and inject cooling water into the reactor and the used fuel storage pool. The backup equipment has preinstalled and standardized connection points, making it easy for operators to place the equipment into service even in challenging conditions.
The industry also established national rapid response centers with the same portable equipment in Phoenix, Arizona, and Memphis, Tennessee. This equipment can be dispatched to a site if needed and and arrive within 24 hours of the request. If necessary to overcome a loss of access to the site, deliveries can be made using the airlift capabilities of the U.S. Department of Defense. The Institute of Nuclear Power Operations has upgraded its emergency response center to better facilitate the sharing of equipment and expertise whenever and wherever it is needed.
Used Fuel Storage Pool Instrumentation and Containment Vent Orders
In response to the second NRC order, nuclear power facilities installed an additional tier of instrumentation to monitor the water level in used fuel storage pools. These instruments will allow operators to monitor changes in pool water levels, even under extreme conditions, so they can ensure that the appropriate amount of water is covering the fuel.
Those plants affected by the third NRC order installed readily accessible hardened vents.
Flooding and Earthquakes
Flooding Safety
In response to the NRC’s request for detailed information, companies that operate nuclear power plants have re-evaluated the flooding hazard at their sites.
All nuclear power plant sites performed assessments and analyzed the potential consequences of floods at their facilities. Most sites were able to show that their key safety equipment is located in areas where even extremely rare floods couldn’t reach. For other sites, the evaluations ensured that any vital equipment that could be challenged by floods was protected by the site’s response strategy. This included analyzing and improving the reliability of flood protection equipment and barriers as well as validating the time available for site personnel to complete response actions.
Furthermore, all sites were required to show that they could still implement FLEX mitigating strategies during severe flooding events. The end result was a comprehensive response plan that would to protect sites against even the most rare flood events, ensuring reactors can be safely shutdown and maintain long-term cooling.
Earthquake Safety
Seismic experts have re-evaluated earthquake protection at nuclear power plants using the latest available data and methodologies. By a letter dated March 12, 2012 (ADAMS Accession No. ML12053A340), the NRC issued a request for information to all power reactor licensees and holders of construction permits in active or deferred status, under Title 10 of the Code of Federal Regulations (10 CFR), Section 50.54(f). Enclosure 1 to the 50.54(f) letter requested that licensees reevaluate seismic hazards for their sites using “present-day” methods and regulatory guidance used by the NRC staff when reviewing applications for early site permits and combined licenses.
Based on their assessment of the reevaluated hazard results, the NRC issued a final determination on Oct. 27, 2015 that required 20 sites to perform a more detailed evaluation of their plants’ ability to withstand stronger ground motion, known as a Seismic Probabilistic Risk Assessment (SPRA). The NRC also included a revised schedule for completion of these assessments and stated that, “the NRC staff continues to have reasonable assurance that plants can operate safely while these risk evaluations are completed.”
Since March 12, 2012, all operating power reactor licensees have completed the required seismic reviews. These reevaluations used modern techniques and information to determine the seismic hazard applicable to each site. The NRC staff reviewed each licensees’ submittals and evaluations and determined that no additional regulatory actions were needed to address seismic hazards at operating power reactor sites.