Cultivating Expertise in Risk and Probability

EPRI Trains Nuclear Power Industry on How to Identify and Assess Risks That May Affect Plant Operations

By Chris Warren

When a nuclear power plant gets licensed, the regulator typically requires owners to demonstrate the plant’s ability to withstand various design-basis accidents, such as a loss of coolant as a result of a pipe break or a loss of offsite power supply for critical equipment. Such events can create situations in which the plant is unable to maintain the nuclear fuel in a safe condition.

In the United States, these requirements are the foundation for the safe operation of the nation’s nearly 100 reactors. It’s an approach that begins during the design phase, long before a plant begins generating electricity. And it’s an approach embraced by car and airplane manufacturers and other industries that require high levels of safety in engineering and construction.

“To establish engineering specifications for equipment, you need to consider the worst-case conditions and then establish the requirements for the equipment,” said Kelli Voelsing, EPRI program manager for risk and safety management. “The design-basis accident approach does that and does it very well. It results in a very robust design.”

Even the most robust designs, however, involve some level of risk when they encounter real-world conditions. To bolster the safety of the nuclear fleet, the U.S. industry in the 2000s supplemented its existing design-basis approach with a process known as probabilistic risk assessment, or PRA, to more effectively understand and incorporate risk into plant design and operation. EPRI has played a critical role in this effort.

“With PRA, we look at the entire range of things that could go wrong instead of just worst-case conditions,” said Voelsing. “This draws on enormous amounts of data. For U.S. nuclear reactors, which have operated about 30 years on average, we have thousands of years of operational data. This data shows us, for example, the probability that an important backup pump will not start or will fail at random when it is needed to move water in response to an event at a plant.”

PRA can enable nuclear power plant operators to identify and effectively respond to the most likely risks as well as provide sufficient protection for unlikely events. For example, one risk might be the loss of a plant’s offsite power supply as a result of severe weather (such as a hurricane). Once operators identify and assess this risk, they might decide to shut down a plant in advance of extreme weather or prepare backup power sources to keep the plant safe.

PRA models can also provide insights about how scheduled equipment maintenance impacts the risk to a plant and about which human actions are most important to reduce risk. “Armed with these insights, operators can focus their training on the most important actions,” said Voelsing.

Recognizing the need for consistent, effective PRA training, EPRI developed a course for nuclear plant operators in 2007. The six-week course, Education of Risk Practitioners, provides the knowledge to put PRA into action. It begins with an introduction to PRA and its major elements:

• Nuclear plant system analysis
• Initiating event analysis
• Event sequence analysis
• Success criteria
• Data analysis and statistics
• Human reliability analysis
• Quantification of risk and interpretation of results

Subsequent course modules provide detailed instruction about these elements, and students build their own PRA models based on a simplified plant. “They draw conclusions about the probabilities of their systems failing and about the combination of equipment failures that presents the highest risk,” said Voelsing. “Students learn from each other by working together. They present their results to the class and discuss their ideas about how to model complex processes.”

Over the past decade, the course has been taught to students from all U.S. nuclear utilities, as well as to participants from Japan, Korea, Canada, Mexico, Brazil, the United Kingdom, Spain, South Africa, and the United Arab Emirates.

The Fukushima Daiichi nuclear accident in 2011 provides an example of a series of events—an earthquake and subsequent tsunami—beyond those considered in the plant’s design process. Since the accident, Japanese policymakers, regulators, and nuclear utilities have recognized the benefits that PRA can provide. In a 2017 strategic plan establishing how risk can inform decision making, the 11 Japanese nuclear utilities put it this way: “As a result of the Fukushima Daiichi accident, Japanese utilities have decided to face squarely the risks of nuclear power generation and started to improve their risk assessment and management capability.”

To implement a risk-informed approach and realize the benefits of PRA, all Japanese nuclear utilities decided to send staff to EPRI’s training. Since 2014, EPRI has led four 6-week courses in Japan, and more than 140 utility and regulatory staff have completed the training. In addition, all Japanese nuclear utilities have joined EPRI’s Risk and Safety Management Program and have been using the program’s research results to inform their post-Fukushima safety efforts.

Expertise in PRA will likely become even more important in Japan. The Japanese regulator and the Federation of Electric Power Companies, an industry trade group, have expressed their intent to adopt a risk-informed approach by 2020.

“Our company has enhanced its risk-informed approach for plant operations and maintenance, and my vision is to engage every part of our nuclear business to implement this approach,” said Mr. Masayuki Yamamoto, Deputy Chief Nuclear Officer of Tokyo Electric Power Company (TEPCO) and General Manager of the Nuclear Asset Management department. “EPRI has been supporting our efforts through PRA courses and workshops, and I expect continued collaboration with EPRI to expand the benefits of a risk-informed approach.”

Artwork by Craig Diskowski/Edge Design

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