Almost two years following an unusual fish incident at the largest nuclear power facility in Canada that posed a challenge to reactor safety, recently acquired documents shed light on the exceptional nature of the event. This occurrence, involving millions of gizzard shad overwhelming the cooling systems at the Bruce Nuclear Generating Station near Tiverton, Ont., led to the shutdown of one reactor and a power reduction in another, with dead fish washing up along Lake Huron for several days.
Revealed through over 1,200 pages of federal and provincial records obtained by CBC News, the environmental anomaly with the cold-sensitive gizzard shad tested the capabilities of Canada’s largest nuclear power plant in unforeseen ways. Despite Bruce Power, the plant’s operator, declining a broadcast interview, the company later issued a written statement without directly addressing CBC News’s inquiries.
Bruce Power’s internal investigation identified the event as “a novel environmental event,” while Canada’s nuclear regulator described it as an unforeseen and uncharacterized influx of gizzard shad that overwhelmed the plant’s fish exclusion defenses. This event, which transpired in February 2025, put reactor safety to the test, according to a company investigation conducted in June of the same year.
Experts in the field, such as David Novog, a professor at McMaster University specializing in Nuclear Safety, highlighted the significance of challenges to reactor safety, emphasizing that any reduction in the effectiveness of defense mechanisms could necessitate plant shutdowns. Douglas Boreham, a former principal scientist and manager at Bruce Power, echoed this sentiment, indicating that while the reactor’s operations were impacted, there was no compromise to reactor safety as the systems responded as designed.
Furthermore, a report from the Canadian Nuclear Safety Commission (CNSC) outlined the potential ramifications of the fish die-off incident that disrupted the cooling systems serving two operating reactors, resulting in the shutdown of Unit 2 and power reduction at Unit 1. The large-scale event, which saw an estimated 8.1 million gizzard shad congregating near the facility, was attributed to severe winter conditions pushing water temperatures lower, causing cold shock among the fish.
The aftermath of the fish event saw between 3.8 million and 4.95 million gizzard shad becoming trapped and perishing at Bruce A, a significant increase from previous records. The substantial number of fish necessitated the use of a crane to remove the carcasses, highlighting the magnitude of the incident. Reactor shutdowns, such as the one experienced at Bruce Power, have broader implications on the electricity grid, necessitating careful planning and coordination for a safe and efficient restart process.
Instances of environmental conditions disrupting nuclear plants’ cooling systems have been observed in various regions, underscoring the need for adaptation to changing climates. Bruce Power’s post-incident actions have focused on enhancing monitoring capabilities, implementing mitigation measures, and investing in advanced technologies to prevent future disruptions. As nuclear plants continue to play a crucial role in providing low-carbon electricity, the importance of proactive measures to safeguard against environmental challenges cannot be understated.