In all, 10 defective welds were found on the fuel bundle, a collection of processed uranium rods resembling the barrel of a Gatling gun about a half-metre long.
The rods contain pellets of uranium dioxide - used to generate electricity by heating water into steam to drive turbines.
The defective bundle was discovered at the Bruce Power plant, north of Tiverton, Ont., in June 2007.
Minutes of the Nuclear Safety Commission's meeting, held Dec. 5 and 6, 2007 and obtained by Canwest News Service, show the commission asking if similar incidents had ever occurred. "CNSC staff responded that weld failures have occurred over the last 10 to 15 years, but that this event of 10 weld failures on one bundle is unprecedented," the document says.
The information was contained in a report labelled: "Update on containment isolation due to a defective fuel bundle at Bruce B Nuclear Generating Station."
Aurele Gervais, a spokesman for CNSC, said last week in an e-mail response to Canwest News Service that CNSC considers it a minor problem since only a few fuel bundles were found to have faulty welds.
"Based on measurements of radiation levels, many of these welds did not fail until after the bundle was removed from the reactor," he said. "It is not unusual for the temperature change involved in removal to cause poor welds to completely fail."
Gervais also clarified the timeline of the discovery. Although the December commission document says the faulty fuel bundle was discovered in September 2007, it actually occurred four months earlier.
"There were two (defective) fuel bundles removed from Bruce B in 2007. In the first bundle removed in June 2007, there were 10 weld failures observed after the fuel was removed," he said. "The second bundle, removed in October 2007, had one weld failure."
The minutes reveal the defective bundles, each weighing about 23 kilograms, were manufactured only for Bruce Power at Zircatec Precision Industries Inc., in Port Hope, Ont. As a result, "Bruce Power is monitoring all bundles produced since November 2005," the minutes say.
Gervais said 104 fuel bundles, all manufactured on the same day as the failed bundles, were removed from the Bruce B reactors.
"No further fuel failures have been found since October 2007," he said, adding examination of the bundles is continuing at Atomic Energy of Canada Ltd.'s Chalk River Laboratories.
The minutes of the December 2007 meeting show Zircatec told the commission it had identified two possible causes: "water ingress into fuel elements, and a potential mix-up of pellet type at one particular unit process."
Gervais said the cause of the failures has not been positively identified but is assumed to be the same since the two bundles were manufactured on the same day using the same welding machine.
Bruce Power said there was no increased risk to the public or workers as a result of the problem bundles.
Gervais said Bruce Power reacted properly to the discovery of the failures and the bundles were quickly removed from the reactor.
"The fuel performance since the discovery of the initial failure has been good, indicating that there is not a wider problem," he said, noting that Bruce B is equipped with online detection systems that can immediately identify even pin hole leaks in a fuel bundle.
The minutes show Zircatec told the commission quality assurance was the responsibility of contract personnel employed by Bruce Power. Bruce Power said until it knew the cause of the defects it could not say if inspection should have caught them at the Zircatec plant.
Gervais added Bruce Power has since improved its management of fuel bundles and it has required Zircatec to strengthen quality control to prevent recurrences.
According to its website, Bruce Power provides about one-quarter of Ontario's electricity.
Located approximately 250 kilometres northwest of Toronto, it operates six Candu reactor units, with a combined 4,700 megawatts of electricity, and is in the process of restarting two others. Once restarted, those units will boost total output to more than 6,200 megawatts.