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Nuclear safety FAQ
Emergency power supply at heart of plant shutdown
Last Updated December 12, 2007
CBC News
New questions about nuclear safety have been raised following the shutdown of the National Research Universal reactor at Chalk River, Ont., and the federal government's subsequent decision to restart the plant to address a shortage of radioactive isotopes needed in hospitals.
At the heart of the issue is an almost unanswerable question: When it comes to nuclear power, how safe is safe enough? Here we look at the specifics of the Chalk River shutdown, the upgrades that the Nuclear Safety Commission is asking for and what atomic energy has done in response.
Why was the Chalk River plant closed?
Commission president Linda Keen said that on Aug. 1, 2006, the commission granted a new licence to the 50-year-old NRU reactor in Chalk River, in part based on assurances from the plant's operator, Atomic Energy of Canada Ltd., that it had completed seven safety requirements.
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But during a safety inspection on Nov. 19, the commission staff discovered some of these requirements hadn't been carried out. In response, Atomic Energy of Canada voluntarily shut down the plant to address the problems.
At issue was the installation of a dedicated and emergency power system, or EPS, capable of withstanding earthquakes and other natural disasters. The inspectors found two pumps that push through heavy water as part of the reactor's cooling system were not connected to the upgraded EPS.
How important are the cooling pumps to reactor safety?
Cooling is an important part of safety with nuclear reactors; the accidental shutdown of crucial cooling water for the radioactive core of the reactor at Three Mile Island in Middletown, Penn., led to the overheating and partial meltdown at the plant in 1979.
Keen said the connections of these heavy water pumps to the emergency power supply were needed "in order to avoid a fuel failure resulting in potential radioactive releases into the environment."
John Luxat is a University of McMaster engineering physics professor who serves as a research chair in nuclear safety analysis through two research partnerships funded equally by government and industry. He said the emergency power backups act as the last line of defence for reactors.
In a typical CANDU reactor, two computers control the reactor's key instruments, including overseeing its pressure and temperature, but a number of safety systems operate independently in case those computers fail. One of these safety systems is the emergency core cooling system.
Should their reactor's turbines fail, Luxat said power plants typically rely on power from the electrical grid the rest of the country draws power from. Only if this failed as well would the emergency power system be required, he said. These battery backups would generally supply power to essential systems, including cooling.
How is the problem being fixed?
Atomic Energy of Canada said on Tuesday it had since connected one of the two pumps and is performing the final tests before declaring it ready. But the company said the second pump wouldn't be connected until Dec. 20 at the earliest.
Instead, it proposed to the federal government to start the reactor at full power with only one of the pumps connected to the emergency power supply. After 120 days, the reactor would then be taken off line and the second pump connected.
Keen said to the House of Commons that the commission would need to review the safety case for operating the plant with only one pump connected to the new EPS, but government decided to go ahead with the recommendation of Atomic Energy of Canada in order to meet the radioisotope shortage.
Why is earthquake proofing the power supply important?
Luxat said no one can make any facility 100 per cent resistant to earthquakes, but nuclear facilities do tests to ensure they are highly resistant.
Power supplies are tested on "shaker tables" designed to produce the expected vibrations of an earthquake, said Luxat.
Earlier this year in Japan, the Kashiwazaki-Kariwa nuclear plant, the world's biggest, got a first-hand test on its seismic resistance when a powerful earthquake caused 1,200 litres of water with trace amounts of radiation to leak into the Sea of Japan.
The earthquake, which killed 10 people and injured dozens more, was more powerful than the plant's seismic protection was meant to handle, said Luxat. But given the severity of the quake, he said, the plant "survived the event well." Tokyo Electric Power Co. said the amount leaked was "one-billionth of Japan's legal limit."
While Ontario may not be as prone to earthquakes as Japan, Luxat said the measures are still important.
"Just because a power supply hasn't been earthquake proofed doesn't mean it will fail; it just means it hasn't been demonstrated that it won't fail," said Luxat. "Ideally, what you want to see is demonstrated confidence over expected confidence."
