OTTAWA - Canada's nuclear-safety watchdog is worried tubes carrying radioactive water in nuclear power plants could rupture and shut down reactors.
The Canadian Nuclear Safety Commission plans to simulate breaches inside giant boilers to better understand what would happen if the pipes burst.
The boilers -- called steam generators -- play a crucial role in electricity production at nuclear-power plants.
The generators use heat from the nuclear reactor core to convert water into steam. The steam is then fed into a turbine to generate electricity.
In that way, nuclear power plants aren't all that different from coal-fired stations, said Steve Aplin, an energy policy consultant with Ottawa-based HDP Group.
"They all do the same thing. They boil water, make steam and drive the turbine," he said.
Steam generators in Candu reactors are shaped like light bulbs filled with thousands of tubes. Pressurized heavy-water flows through the tubes to cool the reactor.
The tubes are held in place by supports to keep them from vibrating. But the tubes can rub against the supports and wear away over time.
"Such wear reduces the tube wall thickness and, therefore, the tube's margin of safety against failure," says a safety commission tender issued Monday.
"Since steam generator tubes represent the interface between the primary side coolant and secondary side fluid, it is clearly essential that such failures be avoided."
In other words, a tube rupture would spill radioactive water into the non-radioactive water inside the steam generator.
If that happens, "the water in the (steam generator) will very suddenly flash to vapour causing a substantial increase in the pressure drop," the tender says.
The reactor would then need to be shut down so technicians could find the leak in a towering tangle of tubes weighing hundreds of tonnes.
John Dyke, a retired engineer with Babcock and Wilcox Canada Ltd., helped design the steam generators at the Pickering A nuclear station near Toronto and says it's a complex repair job.
"You have to shut down the reactor, find out where the leak is, and then go and plug each of the tube ends -- one at the top, one at the bottom," Dyke said.
The study will help the safety commission respond if pipes burst inside steam generators at Candu facilities. The commission apparently hasn't done this kind of planning before.
"It appears that no such experimental study for Candu steam generators has previously been carried out," the tender says.
No one from the safety commission was immediately available to comment.
A different kind of leak at a different kind of reactor downed Atomic Energy of Canada Ltd.'s research facility at Chalk River, Ont., last month.
AECL shut down the National Research Universal reactor on May 15 after finding a heavy-water leak at the base of the reactor vessel.
The Chalk River reactor produces more than a third of the world's supply of medical isotopes used to detect cancer and heart ailments.
