Canada’s nuclear safety regulator has ordered all of the country’s CANDU nuclear operators to check their pressure tubes for fitness after Bruce Power reported elevated readings in two of its nuclear reactors.
Operating while pressure tubes with hydrogen equivalent (Heq) concentrations in excess of 120 parts per million violated terms of Bruce Power’s operating licence, as it would for all other licencees.
Bruce Power reported on July 5 a Unit 6 pressure tube sample contained 211 ppm hydrogen equivalent and on July 8 a Unit 3 sample contained 131 ppm, the Canadian Nuclear Safety Commission said in response to Sun Times questions.
No other operators have reported results above 120 ppm so far, the CNSC said.
The pressure tubes contain nuclear fuel bundles which must remain separate from the heavy water coolant surrounding them. But hydrogen in the heavy water, or deuterium, alters the tubes, weakening them. Reactors contain hundreds of pressure tubes. Operators are required to inspect them regularly.
At normal operating temperature, Heq levels have “no impact on fracture toughness” but there is an “unlikely” possibility of a problem during a reactor restart, a CNSC memo on its website said about the Bruce Power readings.
Both units are shut down and no restart may occur without CNSC approval. While shut down, “there is no safety concern arising from the measured hydrogen concentration in their pressure tubes,” CNSC ‘s memo said.
Bruce Power’s Unit 3 is shut down and receiving scheduled inspection and maintenance and Unit 6 is undergoing major component replacement, including all its pressure tubes.
In the event of a pressure tube rupture, the nuclear plant has adequate safe shutdown provisions “to prevent an accident and mitigate their consequences . . . ,” the CNSC memo said.
“It is our view that the area where the results were found is not of concern from a pressure tube fitness for service perspective,” Bruce Power spokesman John Peevers said Tuesday, speaking of the elevated readings reported to the CNSC.
But CNSC staff said they think these sample results “put into question the accuracy” of Bruce Power’s predictive model because it predicted all Bruce A and B pressure tubes should have Heq values below 120 ppm.
At the same time, CNSC said by email Tuesday that the 120 ppm licence condition doesn’t represent a safety limit, only what’s been demonstrated to be safe under existing modeling.
“Although a licence non-compliance is serious, it does not mean that safety of the operating units is compromised. The issue will be presented to the Commission in the coming weeks for consideration.”
In CNSC’s view, Bruce Power did not knowingly operate these units with pressure tubes with an Heq above 120 ppm, the memo said.
CNSC staff are investigating whether some new phenomenon specific to these units or their pressure tubes is responsible for the higher readings.
They’re seeing if past tube inspections investigated locations identical or similar to those sampled in Units 3 and 6; evaluating the adequacy of Bruce Power’s model, calculations and inputs and are considering regulatory actions, the memo said.
The remaining six generating stations at Bruce Power, near Kincardine along the Lake Huron shoreline, continue to operate and were generating about 25 per cent of the province’s electricity Tuesday.
Bruce Power said in a statement July 13 about the pressure tube issue: “We completed an immediate review of this following our rigorous processes and concluded there was no impact on the safety of the units. All six units that are currently operating have recently undergone similar inspections and demonstrated fitness for service.”
CNSC said it is evaluating that conclusion.
During a 2018 licence renewal hearing, commission staff said Bruce Power’s current pressure tube fracture toughness models did not support the safety case to operate any pressure tube with an Heq in excess of 120 ppm.
The commission issued the licence but with a condition that before reaching 120 Heq, Bruce Power must demonstrate the pressure tubes could be operated safely beyond that level. All CANDU operators have the same operating threshold.
Bruce Power was required to submit a fracture toughness model for review and acceptance “and no unit is authorized to operate above the 120 ppm Heq limit.”
Bruce Power’s fracture toughness model was submitted for review earlier this year and is under review, CNSC said Tuesday.
Bruce Power’s Peevers said testing over the past 18 months found no exceedances in the other six reactor units at the Bruce site. More inspections will be done during planned outages. Heq concentrations can only be assessed when the unit is shut down.
Pressure tubes in Bruce generating station A’s units 1 and 2 were all replaced during refurbishment in 2012 and Bruce B’s Unit 6’s pressure tubes are being replaced currently, he said. Bruce A’s units 3 and 4 entered service in the late ’70s, were shut down in the late ’90s, then restarted in 2003-04, after maintenance and inspections.
Units 5, 7 and 8 at Bruce B entered service in the ’80s and will all have their pressure tubes replaced over the next 10 to 12 years, while undergoing maintenance and inspections during planned outages in the meantime.
Frank Saunders, formerly the vice-president of nuclear oversight and regulatory affairs at Bruce Power, said in a Sun Times interview in 2014 that the company had learned the design longevity estimates made some 30 years earlier for the pressure tubes turned out to not be accurate, based on long-term observations.
“Basically, we measure how strong the tubes are,” Saunders said. “So the tubes are strong and they’re not showing the weaknesses that may have been forecast and they’re operating fine.”
He said they’re predicted to be safe “for several years yet.”
At that time, the CNSC granted temporary permission to run Bruce B reactors 5 and 6 beyond the assumed design life of their pressure tubes. At that time, Bruce Power was permitted to operate units 5 and 6 for about five more years.
“Bruce Power has clearly demonstrated that these units will be safely operated” beyond the initial projected life expectancy of their pressure tubes, the commission decided.
The 2014 report also said this pressure tube brittleness only becomes an issue during heat-up and cool-down phases.