The Canadian Nuclear Safety Commission announced that the Bruce Nuclear Generating Station in Kincardine, Ont., can continue using pressure tubes that have degraded past previous regulatory limits, which the station violated in 2021.
The removal of a requirement in Bruce Power’s operating licence represents a final step in dramatic changes to how the CNSC regulates pressure tubes. It was issued by former president Rumina Velshi on the last day of her five-year term, and by fellow commissioner Marcel Lacroix, an engineering consultant and professor at the University of Sherbrooke.
The commissioners found that since the prior regulatory limit had already been exceeded, the licence requirement “is no longer applicable and thus can be removed from the licence.” They wrote that they were satisfied with how the industry had responded, and that they were confident the CNSC’s oversight “maintains high standards of safety while responding to new developments in a reasoned and science-based manner.”
Pressure tubes – often referred to as the heart of Candus, the reactor design found in Canada’s nuclear power plants – deteriorate as they age. They gradually accumulate deuterium, which can embrittle them. Should a tube rupture, coolant would spill out, which could potentially trigger a serious accident.
Deuterium levels are measured in parts per million of hydrogen equivalent concentration. Issued in 2018, the Bruce station’s operating licence allowed tubes to operate up to a maximum of 120 ppm, a level deemed adequate to ensure fracture toughness. Once tubes exceeded that limit, the utility would have to prove they could operate safely above that level.
Officials understood it was a hard limit. “Your document’s quite clear, we shall not operate above 120,” observed Sandor Demeter, then a commissioner, according to a CNSC transcript of the 2018 licensing hearings.
But Bruce Power knew tubes in some of its reactors would exceed that limit prior to being overhauled, and asked the CNSC to raise it to 160 ppm. CNSC officials signalled they might acquiesce, but said more data was needed; Ms. Velshi put the utility on notice that she regarded this as a significant undertaking.
“Just so we understand, no authorization will be given for you to proceed until you can demonstrate that you can get to the 160 parts per million,” she told company officials at a 2018 hearing. “And I assume that you’re ready to accept if you can’t demonstrate it, it’s not going to happen.”
But things turned out differently.
In July, 2021, Bruce Power revealed it had found tubes with concentrations as high as 212 ppm. The cause remains unknown, but Bruce Power has said on its website that “there is no impact from the findings on the safety of its units.” It asked the commission to strike the 120 ppm limit in October, 2022.
A handful of opponents argued there was no rationale for doing so, particularly since the cause hadn’t been diagnosed. But the nuclear industry strongly supported the move; the Canadian Nuclear Association pointed out that the CNSC had already allowed Bruce Units 3, 4, 5, 7 and 8 to operate tubes beyond the limit. Ontario Power Generation, which operates ten Candu reactors and owns the Bruce units (Bruce Power leases them), also supported it.
CNSC staff calculated that the risk of severe core damage from a possible tube failure was negligible for up to three years while Bruce Power continued studying the problem. Research appears to have been a clinching factor; one internal document released under the Access to Information Act reveals CNSC staffers had “adopted the principle that a licensee proposal to decrease conservatism (in some safety factor, for example) is not necessarily a bad thing – provided it is supported by adequate R&D, and a commitment to continue the research.”
Further support came from the External Advisory Committee on Pressure Tubes. Published last month, its final report stated that it agreed with “virtually all the conclusions presented by the licensees and by the CNSC staff.”
Advisers to nuclear regulator bolster industry position on deteriorating pressure tubes
Eliminating the 120 ppm limit should be of considerable commercial value to Candu operators in Ontario and New Brunswick. In 2018, Glen McDougall, a specialist in the CNSC’s operational engineering assessment division, explained that the old rules eventually made it prohibitively expensive for utilities to continue operating aging reactors.
“What you tend to get is a situation where it becomes economically unviable to operate certain pressure tubes, because their fracture toughness is low enough that they simply cannot operate the reactor in such a way to demonstrate that safety margins are present.”
Matthew McClearn