Canada has four operating nuclear power plants, clockwise from top left: the Pickering, Darlington, and Bruce Power facilities in Ontario, and the Point Lepreau Power Plant in New Brunswick. The Globe and Mail, The Canadian Press, Handouts
In the early summer of 2021, Canada’s nuclear safety regulator received troubling news.
Inspections have revealed that two pressure pipes from different reactors at Canada’s largest nuclear power plant, the Bruce Nuclear Power Station, have deteriorated much faster than expected. This meant that the operator of the station, Bruce Power, had breached the terms of his license to operate. The revelation put the Canadian Nuclear Safety Commission in a difficult position. How were its leaders to respond?
Pressure tubes are commonly described as the heart of the CANDU reactor, the Canadian nuclear reactor design. The tubes contain bundles of uranium fuel and heavy water that serves as a coolant.
Each of Canada’s 19 operating CANDU reactors – including the eight in Bruce – contains several hundred pressure tubes. They worsen with age, gradually increasing their tendency to fracture. So the industry has developed sophisticated systems to monitor this deterioration and mathematical models to predict when pipes will no longer be usable. CNSC officials have assured outsiders that this approach is systematic and thorough.
But news from Bruce Power revealed that the system had broken down. In an email to colleagues written shortly after the discovery, Vali Tavasoli, director of the CNSC’s Operational Engineering Evaluation Division, noted that the regulator already knew that pressure tubes absorb deuterium (an isotope of hydrogen) faster as they near the end of their lives, making them fragile and more prone to failure.
“But the rate of increase was not expected to be so great,” he wrote.
Inquiries from elected officials and the media soon followed. It was a delicate issue: more than half of Canada’s CANDUs had already exceeded their original 30-year design life.
Canada’s aging reactor fleet
Canada’s fleet of CANDU reactors were built between the 1960s and early 1990s; of the 19 still in service, more than half have aged beyond their estimated 30-year design life and are considered “extended service”.
THE CANADIAN REACTORS BY STATUTE
Pressure pipes are currently being replaced
Matt MacLearn and John Sopinski/The Globe and Mail,
Source: cnsc
Canada’s aging reactor fleet
Canada’s fleet of CANDU reactors were built between the 1960s and early 1990s; of the 19 still in service, more than half have aged beyond their estimated 30-year design life and are considered “extended service”.
THE CANADIAN REACTORS BY STATUTE
Pressure pipes are currently being replaced
Matt MacLearn and John Sopinski/The Globe and Mail,
Source: cnsc
Canada’s aging reactor fleet
Canada’s fleet of CANDU reactors were built between the 1960s and early 1990s; of the 19 still in service, more than half have aged beyond their estimated 30-year design life and are considered “extended service”.
THE CANADIAN REACTORS BY STATUTE
Pressure pipes are currently being replaced
Matt McLearn and John Sopinski/The Globe and Mail, Source: cnsc
A federal election campaign was underway. Top-level involvement required: CNSC President Rumina Velshi was put in charge of the team responding to regulatory violations.
Documents obtained by The Globe and Mail under the federal Freedom of Information Act reveal how the CNSC, faced with these serious violations, effectively dropped its previous demands. The episode shows that regulatory constraints on Canada’s nuclear industry can be remarkably elastic when it comes to facilitating the continued operation of Canada’s aging reactor fleet.
“They’re breaking their own rules,” Frank Greening, who worked in Ontario Power Generation’s pressure pipe group for a decade before retiring in 2000, said in an interview.
“Everyone has always understood that these restrictions … exist for a reason,” he added. “There was a hard border for years and years and years. And now all of a sudden they’re saying it’s not really that important. And that worries me.”
Ever since the 1983 accident at the Pickering Nuclear Generating Station, in which a pressure pipe in the Unit 2 reactor began to leak, Canada’s nuclear industry and its regulator have tried to deal with the problem, with mixed results. Frank Gunn/The Canadian Press
Congenital disease of CANDU
On August 1, 1983, alarms went off in the control room of the Pickering Nuclear Generating Station. A pressure pipe at the Unit 2 reactor there has begun leaking 17 kilograms of heavy water per second. Operators safely shut down the reactor.
Laboratory tests found several round blisters in the metal surface of the pipe, through which a 20 mm crack had formed. The culprit: deuterium levels at the outlet end of the tube were more than three times higher than expected.
At the heart of the CANDU reactors,
birth defect
Pressure tubes are key components of CANDU reactors that contain fuel bundles and coolant. Subjected to intense radiation, heat and pressure, they deteriorate over time. In particular, they accumulate deuterium (an isotope of hydrogen), which makes them more vulnerable to destruction.
CANDU reactor shells, also known as calandria, contain several hundred fuel channels
The fuel lines contain pressure pipes
The pressure pipes contain bundles of fuel rods and heavy water coolant
Fuel rods made of zirconium alloy are filled with uranium pellets.
Side view of a pressure tube inside the calandria
Approximate degradation sites
Note: Charts are not to scale.
MURAT YUXELIR / THE GLOBE AND THE POST, SOURCE:
CANADIAN NUCLEAR SAFETY COMMISSION
In the hearts of CANDU reactors, a birth defect
Pressure tubes are key components of CANDU reactors that contain fuel bundles and coolant. Subjected to intense radiation, heat and pressure, they deteriorate over time. In particular, they accumulate deuterium (an isotope of hydrogen), which makes them more vulnerable to destruction.
CANDU reactor shells, also known as calandria, contain several hundred fuel channels
The fuel lines contain pressure pipes
The pressure pipes contain bundles of fuel rods and heavy water coolant
Fuel rods made of zirconium alloy are filled with uranium pellets.
Side view of a pressure tube inside the calandria
Approximate degradation sites
Note: Charts are not to scale.
MURAT YUXELIR / THE GLOBE AND THE POST, SOURCE:
CANADIAN NUCLEAR SAFETY COMMISSION
In the hearts of CANDU reactors, a birth defect
Pressure tubes are key components of CANDU reactors that contain fuel bundles and coolant. Subjected to intense radiation, heat and pressure, they deteriorate over time. In particular, they accumulate deuterium (an isotope of hydrogen), which makes them more vulnerable to destruction.
CANDU reactor shells, also known as calandria, contain several hundred fuel channels
The fuel lines contain pressure pipes
The pressure pipes contain bundles of fuel rods and heavy water coolant
Fuel rods made of zirconium alloy are filled with uranium pellets.
Side view of a pressure tube inside the calandria
Approximate locations
of degradation
Note: Charts are not to scale.
MURAT YÜKSELIR / GLOBE AND MAIL SOURCE: CANADIAN NUCLEAR SAFETY COMMISSION
Subsequent inspections also found blisters at the outlet ends of other pipes.
The Canadian nuclear industry ran into an inherent flaw in CANDU’s design. Subjected to high temperatures, pressure and intense radiation, the pressure tubes absorb deuterium over time.
The 1983 incident prompted a review. Ontario Hydro (the predecessor company of Ontario Power Generation) chose to replace all the pipes in Pickering Units 1 and 2 with new ones made of a more damage-resistant material – an expensive undertaking. All tubes currently operating in CANDU reactors are made using a more flexible material.
Ever since the Pickering accident, Canada’s nuclear industry and its regulator have been trying to deal with the problem, with mixed results.
Deuterium exposure is measured in parts per million (ppm) concentration of hydrogen equivalent. When the CNSC renewed Bruce Station’s license in 2018, it insisted that if readings from any of the station’s tubes exceeded 120 ppm, Bruce Power would have to either shut down the reactor or demonstrate that its tubes were safe to operate at concentrations of hydrogen above this level.
In the summer of 2021, when Bruce Power discovered the unexpected deterioration, it was in the midst of an overhaul of its Unit 6 reactor at Bruce Station, which is located in Kincardine, Ont. Removed the tubes and sent some for lab testing. It turned out that one of the oldest pipes exceeded the 120 ppm limit.
Around the same time, the company discovered that another pipe, that of Unit 3, was also ambushed. Both violations were found in the same segment of each of the six-meter pipes, near the outlet end.
The picture became more alarming as the scale of the excess became apparent. Hydrogen levels in the unit 6 tube were as high as 212 ppm, while unit 3 tube readings were 131 ppm.
Although CNSC officials accepted that the violations…
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