How EPRI helped Japan’s nuclear industry navigate an extended shutdown, modernize for the future, and generate valuable lessons for the rest of the world
The second in a series of three articles about EPRI’s long-term commitment to the Japanese nuclear industry post-Fukushima examines the days and weeks after the 2011 tsunami.
There are many ways to describe nuclear power plants. Collectively, they are the second-largest provider of carbon-free electricity globally and the largest in the United States, France, and Japan prior to March 11, 2011. Individually, they are marvels of engineering and physics, where vast structures of steel and reinforced concrete safely contain controlled atomic reactions that convert heat into electricity.
Nuclear plants are also designed for continuous operation, typically ceasing power production for only a month or two each year to refuel and perform maintenance. But the tsunami triggered by the March 11, 2011, earthquake was anything but typical. Flooding inundated the Fukushima Daiichi site, knocking out power and cooling to three reactors and ultimately causing them to melt down, even though all operating units had safely shut down after the earthquake. In response, Japan shut down its entire fleet of 54 reactors for inspections, safety tests, and a complete overhaul of the nation’s regulatory framework.
The first reactors restarted in 2015, and more than a dozen are back in operation today, with many others working to secure the necessary approvals to begin generating electricity again. The long, unexpected shutdown of Japan’s nuclear fleet highlighted a fundamental reality and a challenge. “If you think about it, a nuclear power plant is a giant metal structure to contain water,” said Dan Wells, director of EPRI’s Nuclear Sector Fuels and Chemistry research department. “They’re not designed to sit there for 10 years and do nothing.”
While the metallic components and other materials used in nuclear power plants are designed to be robust and withstand the harshest conditions, confidently approaching a pause in operations of unknown duration requires clarity about the possible impact on components and materials.
“Water chemistry is a key piece of managing the corrosion of materials during extended periods of inactivity,” Wells said. “What does the science tell us about these materials that are going to sit there for an extended period? And what do we know from actual experience from plants around the world that did shut down for extended periods of time?”
Keeping Assets Alive and Safe
These are exactly the kinds of questions EPRI helped answer for Japan’s nuclear industry after the Fukushima accident, when the country’s reactors went offline. “We gathered all the available operating experience and materials research and shared it with the Japanese to say, OK, here’s what we have seen before and here’s what our recommendation is for how to manage these assets while they’re idle,” Wells said.
Though the plants were no longer generating electricity, they weren’t abandoned either. Fuel remained in spent fuel pools and required continuous cooling, and control rooms remained staffed.
This support built on EPRI’s immediate response to the tsunami, chronicled in the first story in this series. This story, the second in a three-part series, explores EPRI’s role in transitioning reactors from being active generators to an extended period of carefully managed shutdown, known as layup, and how EPRI supported plant modernization initiatives during that time. The final story in the series will explore how EPRI has helped with the restart and life extension of reactors.
Grappling with how to maintain material and component integrity during an extended layup was critical. But it was by no means the only issue Japanese nuclear power plant operators faced. For example, one non-technical challenge centered on the status of operating licenses during the layup. “Japanese regulations were such that time was being accrued towards the plants’ operating licenses, even though they weren’t operating,” said Nathan Palm, a program manager who leads EPRI’s Boiling Water Reactor Vessel and Internals Project.
In response, EPRI prepared a technical report explaining that under the expected conditions, aging degradation should not occur while the plants were shut down with effective layup controls and that there was a reasonable basis for extending the plants’ licenses by the amount of time equal to the shutdown period. Japanese regulations were ultimately revised to allow operating licenses to be extended to account for the generation time lost during the shutdown. The learnings from this research and experience could be leveraged in other countries or instances where the restart of shutdown plants is being considered.
A Rare Opportunity
At the time, the answer to whether and how the reactors would be used in the future was unclear, due to both public concerns about safety and pure economics.
The economic pressures of competing with other electricity sources was not new. “Nuclear plants have high labor costs compared to renewables and gas-fired power plants, even though their fuel costs are a rounding error compared to a gas plant,” said Rob Austin, who leads EPRI’s Nuclear Plant Modernization and Artificial Intelligence (AI) initiatives and contributed to EPRI’s post-Fukushima work. “Anything to reduce labor, while at the same time preserving safety and reliability, needed to be looked at.”
That economic uncertainty shaped how utilities thought about the decade ahead. On the one hand, an extended layup period represented an unprecedented opportunity to modernize aging reactors. “They found themselves in a very unique situation where if they got to start their plants again, it would be in a decade. It’s like a 10-year outage,” said Randy Stark, director of materials R&D at EPRI. “It’s a chance to do all the things they never could do before because they had at most a 30-day or 60-day outage.”
On the other hand, investing significant amounts of money to modernize a plant that might never come back online didn’t make much sense. On a more practical level, plant operators inevitably struggle to prioritize which modernization initiatives to pursue because there are so many potential improvements.
Building a Modernization Program
EPRI worked with utilities across the Japanese fleet to identify and prioritize modernization opportunities during the shutdown period. At Chubu Electric’s Hamaoka Nuclear Power Station on the Pacific Coast between Tokyo and Nagoya, for example, modernization projects that would drive efficiency were elevated. “They were spending a lot of money getting the plant ready to restart, including building a huge seawall to protect against tsunamis,” Austin said. “And obviously, during that time, they were not making any money because they were not generating electricity. So, they want to be as efficient as possible when they do get to restart.”
At Hamaoka, EPRI and Chubu chose to start with a single pilot project that plant staff could see working firsthand, with the intention of expanding the program from there. The modernization strategy was tailored specifically to Chubu’s circumstances—including its financial constraints, regulatory environment, staffing, and business goals—rather than applied as a generic template. The process was designed to integrate with Chubu’s existing procedures for project review, approval, and funding, so that modernization could continue as an ongoing program.
The modernization process EPRI brought to Hamaoka began with this basic question: of all the things that could be improved at this plant, which ones are actually worth doing, and in what order? To answer that, EPRI drew on its library of technology assessments and business case models and supplemented that foundation of knowledge with interviews of Hamaoka engineers and plant staff.
The result was an initial brainstorm of roughly 200 improvements, which is far too many projects to pursue. That list was then put through a structured screening process that applied even greater location-specific rigor to evaluating each idea. Each possible improvement was assessed against two questions: could it actually be applied at the plant, and did it address one of the operational priorities Chubu Electric had identified? Those priorities centered on reducing operations and maintenance costs, improving efficiency ahead of restart, and, where possible, transitioning from time-based to condition-based maintenance schedules informed by real-time equipment monitoring.
This reduced the list to 83 potential improvements. From there, EPRI worked with plant and utility staff to determine how applicable each improvement was to Hamaoka’s specific situation, the plant’s interest in pursuing it, and what the improvement would realistically deliver. Each idea was then scored across six factors: implementation cost, expected savings, projected payback, regulatory readiness, technological maturity, and the level of expertise required to execute it.
Chubu’s constraints shaped which ideas rose to the top. Funding for improvements was limited by the costs of shutdown and restart preparation. Japanese regulations at the time did not permit online maintenance of safety-related components, which shaped which improvements were practical to pursue. And because Chubu did not reduce its staff during layup, the efficiency gains needed to be translated into redeploying workers to higher-priority tasks rather than reducing headcount. After evaluating all 83 candidate ideas against those realities, eight emerged as worthy of a thorough business case analysis, with the goal of identifying a pilot project to begin.
Selecting the final pilot project wasn’t just about financial analysis. Chubu’s senior leadership wanted a project that plant staff could see working with their own eyes, something tangible enough to build confidence in the modernization program before pursuing broader implementation.
That pointed toward online condition monitoring for the plant’s medium-voltage motors, the workhorses that drive the pumps and systems moving water and coolant through the facility. Under the traditional approach, workers conduct periodic manual inspection rounds to check on equipment health. But problems can develop and worsen between those scheduled visits.
By installing sensors that continuously monitor motor performance, the plant could detect potential issues before they caused failures, avoiding the unplanned shutdowns and power reductions that an unexpected motor failure can trigger. The business case analysis projected a payback period of under four years, and the monitoring program could be started during the shutdown and expanded incrementally after restart, with each additional piece of monitored equipment delivering more value at relatively low additional cost.
Preparing to Restart
As plant restarts began moving from hypothetical to possible, the nature of EPRI’s support also shifted. Instead of focusing on protecting plant assets, the priority became bringing them back safely.
EPRI developed guidance on managing water chemistry during the transition from layup conditions to full operation. A particular concern during startup is the intrusion of chlorides and sulfates into plant systems. If not carefully controlled, these contaminants can compromise the integrity of components and piping. EPRI’s guidance provided specific recommendations for flushing systems and managing chemistry during a startup sequence to prevent excursions that could damage fuel or create radiation exposure challenges for plant workers. The approach proved its worth. After more than 13 years of shutdown, Tohoku Electric Power Company’s Onagawa Unit 2 and Chūgoku Electric Power Company’s Shimane Unit 2 became the first Japanese boiling water reactors to return to service in 2024. Neither reported significant corrosion issues, validating the layup and chemistry management work that had been done.
EPRI experts also took on an independent review role, evaluating the startup plans Japanese utilities had developed to ensure they were consistent with EPRI guidance. That review surfaced a consequential finding. Some plants had struggled to maintain optimum chemistry during the long shutdown period. For those plants, proceeding with a standard startup sequence was not sufficient. Recovery plans had to be developed first, drawing on additional EPRI guidance to address the specific conditions those plants were in and reduce the risk of chemistry problems or fuel integrity issues during restart.
Lessons for the World
The work EPRI did with Japan’s nuclear industry during the decade-plus shutdown addressed problems that were, at the time, largely unique to Japan. No other country had ever shut down an entire national nuclear fleet simultaneously and faced the challenge of managing, modernizing, and eventually restarting dozens of reactors after an extended pause. The body of knowledge that the work produced, however, now belongs to and benefits the whole industry. That knowledge was built over years of close collaboration between EPRI and Japanese utilities, a relationship that deepened after the Fukushima accident.
EPRI’s 2025 plant restart guidelines draw directly on the Japanese experience and provide technical, regulatory, and administrative guidance for any utility considering a restart. The layup strategies Japan developed, the chemistry controls that proved effective, and the independent review process that caught gaps before they became problems are now codified in guidance available to plants around the world.
The relevance is not theoretical. Plants in the United States that spent years on a path toward decommissioning are now asking the same fundamental questions Japan spent a decade answering. “There are plants coming back online in the United States after years of thinking they were going to decommission,” Stark said. “They are reaching out to the Japanese, saying, ‘You were down for an extended period of time.’ What did you do? What were the lessons learned?”
What Japan has had to learn from tragedy and disruption, the rest of the world is now seeking out by choice. More plants are working through the approval process to restart, and the conversation about extending operating licenses well beyond their original terms is ongoing. How EPRI is supporting that effort, and what it means for nuclear power’s future in Japan and around the world, will be the subject of the final story in this series.
EPRI Technical Experts:
Randy Stark, Robert Austin, Dan Wells, Nathan Palm
For more information, contact techexpert@eprijournal.com.
Additional Resources
- The Right Research at the Right Time (First in the series of three EPRI Journal articles)
- Nuclear Power Plant Modernization-Strategy Development and Implementation Process Update
- Application of EPRI Plant Modernization Strategy Development at Chubu: Showcasing the Value of Establishing a Plant Modernization Program (Requires EPRI login)
- Plant Restart Guidelines: Preserving and Restoring Nuclear Power Facilities
Banner Photo Credit: Hamaoka / Chubu Electric Power Co.
