SAFETY & SECURITY | MULTI-UNIT PSAS
Respect your neighbours
In the future there are likely to be more sites that host multiple nuclear reactors. A
discussion at the NRC’s Regulatory Information Conference covered some of the issues that raises, in a session on multi-unit Probabilistic Safety Assessments (PSAs)
Multiple units potentially interact, sharing systems like central services where coupling could come into play. Source: Akkuyu
WHAT ARE THE SAFETY IMPLICATIONS of having more than one reactor on a single site to operate and maintain? Multi- unit sites are not new, but they may have been treated as if the units are entirely separate from a safety viewpoint. The prospect of sites with shared services across a group of small reactors, or mixed sites, have led safety organisations to develop Probabilistic Risk Assessments (PRAs) – more commonly only known as Probabilistic Safety Assessments (PSAs) in Europe – specifically for multi-unit sites. In a session titled ‘Multi-Unit Risk: Are We Ready for This?’ at the most recent US Nuclear Regulatory Commission (NRC) Annual Regulatory Information Conference, Jonathan Evans, Chief of the Probabilistic Risk Assessment Branch, for Nuclear Regulatory Research (RES) at the NRC chaired the session, noting that since Fukushima, multi-unit risk has received increasing attention. “It continues to shape how we think about site level safety and future reactor deployment,” said Evans. This session explored the topic of multi-unit (MU) risk for both the existing fleet of light water reactors and advanced reactor designs. Dr. Shahen Poghosyan, Senior Nuclear Safety Officer at
the International Atomic Energy Agency (IAEA) noted that PSA in general plays an important role in the IAEA framework and safety standards and is considered a powerful tool. Together with deterministic safety analysis it allows a holistic
38 | May 2026 |
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understanding of the risk profile for nuclear installations and thus risk-informed decision-making. However, on digging deeper into the safety assessment, “we still have one reactor at the time mindset,” he said, despite the fact that nearly 80% of the sites worldwide are multi-unit sites. The Electric Power Research Institute (EPRI) has been
involved in developing and applying MU PRA guidance for both US and international nuclear power plants. Outlining the current approach and status for developing an MU PRA standard, Mark Wishart, senior technical lead at EPRI, said “it’s not about taking units’ [PRAs] and adding them, but about them interacting”. He said there are “shared systems” like central services or even a containment and “coupling comes into play”. In the future, risk assessment also may have to consider other types of coupling, such as co-generation (with heat customers) or hydrogen production. However, Wishart said the there was “no substantial difference… the [existing] framework is generally applicable” to enable risks to be assessed for each unit before considering the multi- unit implications. He highlighted takeaways from a pilot with Canada – which of course has decades of experience of multi-unit sites, including those that share key equipment such as steam generators, and has used multi-site considerations for PRAs from the start. European experience of multi-
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