External experts looking into the unexpectedly rapid deterioration of key components in nuclear power reactors have concluded that the plants can operate safely, although they say it’s unclear what caused the problem in the first place.
The External Advisory Committee on Pressure Tubes was convened in the summer of 2021 to help the Canadian Nuclear Safety Commission after the discovery of unexpectedly high levels of deuterium (an isotope of hydrogen) in pressure tubes at the Bruce Nuclear Generating Station, violating the terms of its operating license.
After two years, the committee agreed with all the conclusions previously arrived at by industry representatives and commission staff.
In a final report dated July 4 and published without fanfare last month, the committee declared that while “a great deal of work” remains to be done to understand the problem’s cause, “enough has been done by the licensees to definitively provide assurance that the plants can operate safely.”
The committee included Mark Daymond, a professor at Queen’s University’s engineering and physics departments, and Paul Spekkens, a former employee of Ontario Power Generation. (Its chair, John Luxat of McMaster University, withdrew for health reasons in January.) Their final report is a boon for Bruce Power, which has an application before the CNSC seeking to remove limits in its operating license that its pressure tubes violated more than two years ago.
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 pick up deuterium, which in high concentrations can embrittle them. If ruptured, coolant would spill out, which could trigger a range of scenarios from a relatively minor incident that’s contained by the plant’s safety systems, to a catastrophe in which fuel overheats.
Deuterium levels are measured in parts per million of hydrogen equivalent concentration, or Heq for short. The Bruce Station’s license, issued by the CNSC in 2018, allowed pressure tubes to continue operating up to a maximum level of 120 parts per million (PPM) Heq, a limit “judged to be adequate to assure the required fracture toughness.” To continue operating tubes beyond that limit, the utility was required to provide evidence it was safe to do so.
But in July, 2021, Bruce Power revealed that it had discovered Heq levels as high as 212 PPM in tubes. That prompted a frenzy of concern within the CNSC and across licensees, as the nuclear industry sought to grapple with a problem that could affect all Candus. Eventually, however, the CNSC set aside its former requirements and demanded that Bruce Power conduct research into the problem.
Last October, Bruce Power asked the CNSC to strike the 120 PPM limit from its license. Its application received strong support from across the nuclear industry, including from the Canadian Nuclear Association, which pointed out that the CNSC had already allowed Bruce Units 3, 4, 5, 7 and 8 to operate with tubes exceeding the limit.
Ontario Power Generation, which operates 10 Candu reactors and also owns the Bruce units (Bruce Power leases them), also supported the change, as did the CANDU Owners Group, a non-profit that represents all Candu operators in Canada and abroad.
The External Advisory Committee’s report criticized nuclear industry participants for characterizing discoveries associated with the high Heq values as unsurprising. It pointed out high levels were discovered at both ends of pressure tubes.
“The fact is that nowhere had it been suggested ahead of time that anomalously high Heq values might exist at the top of pressure tubes near the inlet and outlet rolled joints,” the report noted. “This was a double surprise.”
Mark Winfield, a professor at York University who focuses on energy and environment policy issues, said the report’s main findings didn’t square up with concerns expressed by the committee about the quality of the information it received from industry representatives throughout the process.
“Basically what they’re describing is a lack of rigour, below what the accepted standard within the industry would be,” Prof. Winfield said.
“You really shouldn’t be making definitive statements that everything is okay, if you don’t have a full understanding of what is actually going on,” he said. “This is a fairly central issue of vulnerability around Candus. And the implications of a pressure tube failure are quite serious.”
The Saugeen Ojibway Nation, a body representing the Chippewas of Saugeen First Nation and Chippewas of Nawash Unceded First Nation, issued a statement to the CNSC earlier this year that it regarded the existing licensing conditions as crucial for safety. Bruce Station lies within its territory.
“The industry’s inability to meet this safety threshold is not a justification for removing the condition” from the license, it stated in a written submission.
“Allowing the Bruce [Station] to operate above the established safety limits to provide industry with the time to complete additional research to demonstrate it is safe to continue to do so is not sound or logical.”
CNSC spokesperson Braeson Holland said it expects to issue its decision on Bruce Power’s application “in the coming weeks.”
As research continues into deteriorating pressure tubes, most of the nation’s 19 reactors in service are in extended operation, meaning they are operating beyond their 30-year design lives. Ontario’s Bruce and Darlington stations are in the midst of extensive refurbishments, which include complete replacement of old pressure tubes. The provincial government is now considering whether to refurbish four reactors at Pickering station, its oldest operating nuclear plant. Hydro-Québec is considering restarting its Gentilly-2 reactor, which was shut down in 2012.