Catastrophic damage at the Fukushima Dai-ichi nuclear facility in Japan provided the world's nuclear community with valuable lessons.
The challenges faced by operators during the 2011 earthquake and subsequent tsunami were beyond any previously faced at a commercial nuclear reactor.
The disaster prompted the U.S. Nuclear Regulatory Commission (NRC) to take action, requiring new strategies for mitigating problems beyond reactor design capabilities for U.S. nuclear facilities.
Those strategies include beyond-design backup measures for loss of power during natural disasters, ensuring reliable hardened containment vents and enhancing spent fuel pool (SFP) instrumentation.
All of these were major problems in the Fukushima disaster, says Bob Arteaga, Electrical Department manager in the Nuclear Group at Burns & McDonnell.
Operators nationwide have until the end of 2016 to upgrade facilities that are already designed to handle the worst natural disasters.
"Even with all of the safeguards designed and built based on worst-case scenarios, the Fukushima plant couldn't sustain operations amid the water from the tsunami — it took everything with it," says Roman Estrada, the design engineering manager at Nebraska Public Power District's (NPPD) Cooper Nuclear Station in Brownville, Neb.
His plant is working with Burns & McDonnell to upgrade the Cooper facility that sits on the Missouri River about two hours north of Kansas City, Mo.
Following the Fukushima disaster, an NRC task force conducted a systematic review of its regulations and processes then developed a comprehensive set of safety enhancement recommendations.
The regulatory commission approved the recommendations and ordered all U.S. facilities to comply with the enhancements based on lessons learned at Fukushima.
"In the face of beyond-design-basis external events, the goal is to stretch safety measures to respond specifically to an extended loss of power, worst-case seismic events, severe flooding and loss of access to the ultimate heat sink, which provides an unlimited supply of water to nuclear reactors to cool vital systems and containment," says W. Larry Stendebach, senior client manager for the Burns & McDonnell Nuclear Group.
The purpose of these measures is to mitigate fuel damage in the reactor and SFP of an affected nuclear facility.
One of the main requirements is for plants to fulfill key safety functions for core cooling, containment integrity and the SFP. Plants must now implement a three-phase approach for mitigating such beyond-design-basis events.
The first phase requires the use of installed equipment and resources to maintain or restore core cooling, containment and SFP cooling. The transition phase requires plant operators to provide portable, on-site equipment and consumables to maintain or restore these functions until they can be handled with off-site equipment. In the final phase, operators must secure resources necessary to sustain the functions indefinitely.
Carrying Out Safety Enhancements
The U.S. is the world's largest producer of nuclear power, accounting for more than 30 percent of nuclear-generated electricity worldwide.
Cooper Nuclear Station is among 104 nuclear reactors nationwide installing systems to meet the latest NRC requirements. Cooper staff are building an arsenal of beyond-design resources such as fire trucks, pumps, diesel engines and an on-site helicopter pad for equipment delivery from a regional resource center in Phoenix.
Burns & McDonnell, the plant's engineer of choice, is handling several projects, including designing and building new SFP instrumentation and brainstorming ideas for handling improbable catastrophes.
"Their team is essentially an outreach of me and our team here," says NPPD's Estrada. "We have full confidence in their quality engineers, who are responsive, committed and knowledgeable about every facet of our plant."
Burns & McDonnell has 4,500 professionals, including a dedicated unit with nuclear specialties and experience.
"They carry a lot of experience and credibility and have the ability to go right after a problem and provide innovative ideas and solutions," says John Patterson, director of nuclear projects at the Callaway Energy Center in Fulton, Mo. Ameren, Callaway's owner, has hired the Burns & McDonnell team for various nuclear-related projects.
The group has built strong working relationships and a reputation for proficiency in nuclear projects.
"Our clients appreciate our depth of understanding of nuclear power - the plant, the procedures, the requirements and the day-to-day needs of operating a nuclear facility," Stendebach says. "Our people have lived it, working directly for and in plants around the country."
Putting Safeguards in Place
At the Cooper facility, Burns & McDonnell is designing improvements to the SFP instrumentation that currently monitors normal and slightly off-normal conditions. During the Fukushima event, remote readings of SFP water levels were not available, and plant operators could not get close enough to verify water levels because of high radiation.
"We have to be able to read it when it's been shaken up and the radiation levels are off the scale," Estrada says. "That's the level of worst-case scenario we are preparing for."
Planned improvements include remote indication capabilities with a backup channel for monitoring. The system's components also have internal and external batteries and a portable DC power source. Operators can remotely control where the equipment draws power.
"Another safeguard will be a hardened pipe vent system to release hydrogen," Arteaga says. "While all systems have designed-in ventilation, the NRC wants to see them streamlined, preventing the buildup of hydrogen in the reactor vessel, which is what led to the hydrogen explosions at Fukushima."
The upgraded ventilation is more robust, allowing for venting on multiple occasions in two places and allowing it to be triggered remotely if necessary.
Flooding was another key issue at Fukushima, and it's a realistic potential threat for the Cooper plant and a neighboring plant, Fort Calhoun Station.
In 2013, the Nebraska plants were surrounded by floodwaters from the Missouri River after record water releases from dams normally used to prevent flooding. While neither plant was at risk for a Fukushima-type disaster, these plants must now prepare strategies to mitigate a scenario worse than what actually occurred: complete submergence of the plant due to cascading dam failures.
"A concern for us would be what if a dam breaks upriver," Estrada says. "What would we do if we were totally underwater? Could we put a barge on the river and run power off it? Could we put something on the roof?"
These are among the many options Burns & McDonnell is analyzing for the Cooper plant and providing viable options to handle the improbable what-if.
"All of these safeguards give the NRC confidence that licensed plants are maintaining the highest level of public health and safety as well as common defense and security," Arteaga says.
What Happened at Fukushima?
On March 11, 2011, a magnitude 9.0 earthquake struck off the coast of Japan. The quake triggered a tsunami packing 45-foot waves that crashed ashore east of the Oshika Peninsula of Tohoku and traveled nearly 10 miles inland. More than 15,000 people died as a result of the natural disaster.
Following the earthquake, Tokyo Electric Power Co.'s nuclear reactors shut down as designed and safety systems kicked in to provide cooling and long-term protection for the nuclear reactors.
But the tsunami was beyond anything imaginable — three times greater than the facility's design basis. Massive uncontrollable flooding crippled even the backup diesel generators, leading to a complete loss of power.
With severe damage, a loss of off-site power and the inability to generate power on-site, operators used the installed plant batteries to control the plant systems. But they were not designed for long-term, continuous operation without recharging.
As the batteries began to fail, operators lost the ability to control reactor cooling. Repositioning the valves to manually control cooling wasn't feasible because high radiation levels prevented access.
As cooling systems malfunctioned, three reactors experienced melting of the reactor fuel and subsequent hydrogen explosions. The explosions severely damaged the units, blowing the walls and roofs off the reactor buildings. After the explosion, thousands of people were evacuated from the surrounding area to prevent radiation exposure. No one died in the nuclear incident and subsequent radiation exposure.
Japanese regulators ordered the facility to be decommissioned, as Fukushima became the largest nuclear incident since the 1986 disaster in Chernobyl, Ukraine.
Timeline of Events
March 11, 2011
A magnitude 9.0 earthquake strikes off the coast of Japan, generating a 45-foot tsunami that submerges the Fukushima Dai-ichi nuclear power plant.
March 18, 2011
NRC shares details of Fukushima incident with U.S. nuclear power plants and reminds operators of post-9/11 requirements for additional emergency equipment.
April 1, 2011
NRC forms task force to examine lessons learned from the Fukushima incident.
April 29, 2011
NRC inspectors begin examining severe accident management procedures and training at U.S. nuclear power plants.
May 11, 2011
NRC requires nuclear power plants to provide information on post-9/11 emergency equipment, as well as how the plants ensure strategies to use the equipment remain effective over time.
May 20, 2011
NRC reports all U.S. nuclear power plants have appropriate post-9/11 emergency equipment and procedures in place.
July 12, 2011
The NRC's task force issues a report on lessons learned from Fukushima. The task force determined U.S. plants are operating safely yet recommended 12 strategies for enhancing reactor safety.
Sept. 9, 2011
Commission hears report recommending the immediate implementation of six of the 12 task force strategies for U.S. nuclear power plants to enhance safety.
Oct. 3, 2011
NRC staff proposes three-phase approach for prioritizing the task force recommendations.
Oct. 18, 2011
Commission approves proposal for implementing recommendations by the end of 2016.
Dec. 15, 2011
Commission approves proposed three-phase approach to prioritizing the implementation of recommendations.
March 12, 2012
The first regulatory requirements for the nation's 104 reactors are set, based on lessons learned at the Fukushima Dai-ichi facility. NRC issues three orders requiring nuclear plants to implement safety enhancements related to mitigation strategies for loss of power during natural disasters, ensuring reliable hardened containment vents and enhancing spent fuel pool instrumentation.