Unexpectedly high levels of hydrogen in pressure tubes at a nuclear power plant in Ontario have renewed questions about how long Canada’s aging CANDU reactors can continue to operate safely.
At a meeting before the Canadian Nuclear Safety Commission (CNSC) on Friday, officials confirmed more pressure tubes at the Bruce Nuclear Generating Station on the eastern shore of Lake Huron were found to contain hydrogen concentrations in excess of regulatory limits, and above what the industry expected. Bruce Power disclosed in July that it had discovered two tubes that exceeded regulatory limits.
The cause has not been determined, nor is it clear how many other pressure tubes in Canadian reactors might also be affected. Bruce Power did not answer questions from The Globe late Friday on how many additional tubes were found to violate the station’s licensing requirements.
Pressure tubes are six-metre rods that contain fuel bundles of uranium, and are considered the major life-limiting component in CANDUs, the reactors in all of Canada’s nuclear power plants. Tubes containing high hydrogen concentrations are more vulnerable to fracturing if they have pre-existing cracks. If one ruptures, coolant could be lost, which could trigger a range of scenarios from a relatively minor incident contained by the plant’s safety systems to a catastrophe in which fuel overheats.
The issue is particularly a concern when the reactor is below normal operating temperatures, such as during shutdown or startup.
Alexandre Viktorov, director-general of the CNSC’s power reactor regulation directorate, said CNSC staffers were satisfied with the industry’s response, which included increased inspection of pressure tubes and reviewing known cracks. “Continued reactor operation does not pose unreasonable risk,” he added.
The CNSC has imposed various regulations concerning pressure tubes, including a maximum permissible hydrogen concentration of 120 parts per million. Until now, the industry has relied on a combination of inspections and predictive modelling to estimate when tubes would reach that limit. The levels at the Bruce station indicate those predictive models have failed.
The Bruce station has eight reactors, each containing 480 pressure tubes; the offending tubes were found in Units 3 and 6, neither of which is operating. One tube in Unit 6 exhibited readings of 211 parts per million, approaching double the regulatory limit, and far above Bruce Power’s prediction of 100 ppm. Bruce Power officials said all of the elevated readings were discovered in the same region, close to one end of the tube.
“Based on hundreds of inspections, reinspections, and recently carried out enhanced inspection activities in Unit 3, we can affirm that there are no flaws in this region,” said Chris Mudrick, the company’s chief nuclear officer. “Our pressure tube integrity remains strong.”
Frank Greening, a retired Ontario Power Generation chemist who worked in the company’s pressure tube group for the last decade of his career, warned that it’s possible the rate of hydrogen pickup may have accelerated in older tubes in the past few years.
“And if that’s true, then the rate at which it’s going in is scary,” he said.
He added: “There is something happening that’s quite serious. And they’re saying ‘We don’t know how or why it’s happening.’ It’s pathetic. I can’t accept that.”
A CNSC commissioner, Marcel Lacroix, said during the meeting that industry officials had not satisfactorily explained what might be causing the problem.
“There are many potential reasons for this phenomenon, and we weren’t able to nail [it] down exactly,” Dr. Viktorov said.
In late July, the CNSC imposed new requirements on Bruce Power and two other utilities operating CANDU reactors (Ontario Power Generation and New Brunswick Power). Before restarting reactors, the utilities would need to carry out additional inspections to “demonstrate a high degree of confidence” that the tubes remained under the regulatory limit for hydrogen concentration, or show that no flaws existed in the specific area of the reactor’s tubes where the high levels at Bruce Station were discovered.
In a presentation to the CNSC on Friday, OPG officials said pressure tubes at the company’s Darlington and Pickering stations “are not experiencing similar levels of degradation” seen at Bruce station, and that its own tubes were within licensing limits “with considerable margins.” New Brunswick Power said the tubes at its Point Lepreau station were “relatively young.”
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