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 reactor accident in Japan in March 2011 prompted a rapid review of safety measures at U.S. facilities and led to a longer-term effort to analyze and apply lessons learned.
The U.S. Nuclear Regulatory Commission issued new requirements and requested detailed information in several areas relevant to the accident in Japan. The industry is taking steps to implement the new requirements and to develop the information requested by the agency.
The most important lesson from the accident in Japan is that a nuclear energy facility must be prepared to handle catastrophic events simultaneously at multiple reactors, regardless of the cause. To address these challenges, the industry has developed a diverse, flexible mitigation approach called “FLEX.” Building on existing safety systems, this strategy addresses the major problem encountered at Fukushima—the loss of power to maintain effective cooling—by stationing another layer of backup equipment at facility sites and regional depots.
Tsunami Leads to Reactor Accident in Japan
The March 2011 accident at the Fukushima Daiichi nuclear energy facility in Japan occurred after a series of earthquakes and at least seven tsunami waves struck the 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 worse than anticipated.
Three of the six reactors at the site were operating at the time. They shut down as designed when the earthquake struck. The earthquake itself did not cause damage that could prevent the operation of key safety systems, but the tsunami knocked out power supplies needed to keep the reactors cool. Without power for cooling, the nuclear fuel overheated and began to melt.
Within days of the accident, reviews of safety equipment and procedures were under way at all U.S. nuclear energy facilities. All U.S. reactors completed safety inspections and walk-downs in March 2011 to ensure that facilities and equipment are protected in the event of extreme natural hazards. All action items from the inspections have been completed.
Additional detailed inspections are under way to re-evaluate earthquake and flooding risks. The industry is confirming the ability of structures and equipment to withstand extreme weather events and enable safe shutdown of the reactor. If necessary, U.S. companies will add protection or mitigation capability. The industry also is updating seismic models based on new information and will make additional modifications to U.S. reactors, if needed, based on future evaluations.
The NRC also initiated inspections after the accident in Japan to verify that nuclear energy facilities are well-protected in the event of a natural disaster. In addition, the NRC established a task force to review safety requirements in light of the accident at Fukushima. The task force confirmed there are no safety concerns requiring immediate action, recommended that plant operations and licensing activities in the U.S. should continue and made a series of recommendations.
In early 2012, the NRC issued three orders and a request for detailed information. The orders require:
Strategies to maintain adequate reactor and spent fuel pool cooling and containment integrity in a severe event that exceeds design parameters.
A second tier of reliable spent fuel pool instrumentation.
Reliable and readily accessible hardened vents for heat removal and pressure control in boiling water reactors with Mark I and II containments like the ones at Fukushima. Of U.S. reactors, 31 have this containment design.
The NRC also is requiring licensees to provide detailed information on:
Additional company inspections to verify measures for protecting against seismic and flooding hazards.
Updated evaluations of the hazards associated with earthquakes and flooding, including instances where the hazard may significantly exceed previous expectations.
Staffing and communications needs in the event of an emergency affecting multiple reactors at a site.
A Diverse and Flexible Approach to Protect Against the Unexpected
Video: The FLEX Solution to Nuclear Safety
The principal lesson from Fukushima is that a nuclear energy facility must be prepared to handle catastrophic events simultaneously at multiple reactors, regardless of the cause. Operators also should assume that access around the site could be blocked by debris. Building on safety features added after 9/11, the industry developed a “diverse and flexible coping capability,” or FLEX.
After the 2001 terrorist attacks, U.S. nuclear energy facility licensees took steps to safeguard plants against a large fire or explosion that could disable vital equipment. Because it was not possible to predict exactly which equipment would be affected, the industry focused on what would be needed to keep the reactor cool if the usual safety systems were not available. Companies purchased portable equipment—such as generators, battery packs, pumps and battery chargers—that could be stored and used to respond, regardless of the location of an explosion, aircraft impact or massive fire.
FLEX uses multiple sets of additional portable equipment to provide backup power and inject cooling water into the reactor and the used fuel pool. This equipment is stored at strategic locations.
National Network for Equipment Sharing
The industry also is developing two regional response centers—one in Phoenix and one in Memphis—that will serve as dispatch points for additional equipment and resources. In addition, INPO has upgraded its emergency response center to better facilitate the sharing of equipment and expertise whenever and wherever it is needed.
Overview of Industry Actions Since Fukushima
The industry has taken several steps to enhance safety at nuclear energy facilities based on lessons from the accident at Fukushima. These actions include:
Acquiring or ordering backup safety equipment, including diesel generators, pumps and emergency response vehicles. Nearly 1,500 pieces of equipment have been acquired or ordered.
Enhancing the ability of nuclear energy facilities to remain safe even if there is an extended loss of electric power. The loss of all power sources at Fukushima Daiichi prevented the use of backup cooling systems for the reactors.
Developing strategies to mitigate external events beyond the design envelope for nuclear energy facilities.
Implementing improvements to ensure accessible and reliable hardened vents for Mark I and Mark II boiling water reactor containments, thus removing heat and maintaining of pressure control during an extended loss of off-site power.
Improving plants’ ability to monitor water level and temperature in storage pools for used nuclear fuel during an extended loss of electric power.
Assessing the staff needed to respond to a large-scale natural event at multiple reactors at a site and to implement strategies contained in the emergency plan.
Assessing communications and equipment used during an emergency to ensure that power is maintained during a large-scale natural event.