SCIENCE

Nuclear jitters

A radioactive spill shuts down a reactor

MARK NICHOLS December 26 1994
SCIENCE

Nuclear jitters

A radioactive spill shuts down a reactor

MARK NICHOLS December 26 1994

Nuclear jitters

A radioactive spill shuts down a reactor

MARK NICHOLS

SCIENCE

As a snowstorm swirled around the domed buildings housing eight nuclear reactors on Dec. 10, workers erected barriers on the roads leading to the plant in Pickering, Ont. They bore an ominous message: “Radiation Incident Area—Emergency Traffic Only.” The reason: about 30 minutes earlier, a ruptured air-pressure tube in the network of pipes girdling the station’s No. 2 reactor triggered a chain of dangerous events—a mere 30 km east of Toronto. As about 140 tons of heavy water laced with radioactive tritium cascaded onto the floor of the reactor building, the plant’s emergency core cooling system went into action to bathe the reactor core in more water—and prevent a potentially disastrous buildup of heat in the core. It was the first time that the emergency cooling system has ever been activated on one of the Canadianbuilt CANDU reactors. “The smoking gun is on the table,” said Dave Martin, spokesman for Durham Nuclear Awareness, a local organization that is campaigning for stricter safety measures at the Pickering plant. “We’ve seen a very serious accident here.”

■ Pickering’s No. 2 reactor; chain of events (right): valve No. 1 opens accidentally, causing pressure slump; as pressure is restored, valve No. 2 opens and the pipe cracks, spilling heavy water.

Officials of Ontario Hydro, which uses the Pickering plant to produce electricity, downplayed the event. Although the tritium spill caused radioactivity levels within the reactor building to shoot up, they said, no radiation was released to the air outside. And no one was inside the remote-controlled plant at the time. But the incident stirred renewed concern over the Pickering plant’s aging reactors—four of them are more than 20 years old. In Ottawa last week, a previously scheduled licence renewal hearing by the federal Atomic Energy Control Board (AECB) listened to an account of the events at Pickering. Then, the agency—which usually makes up its mind on the spot—reserved its decision, probably until this week, on whether to allow the Pickering plant to continue operating.

The reactor accident focused attention on the risky nature of nuclear power at a time when U.S. officials are weighing a proposal to use some of Canada’s 22 CANDU reactors to dispose of surplus U.S. and Russian weaponsgrade plutonium by burning it as fuel. The proposal, put forward by Ontario Hydro and Atomic Energy of Canada Ltd. (AECL), is one of several plans that the U.S. department of energy is considering to get rid of an estimated 55 tons of plutonium left over from the Cold War era. John Macpherson, an AECL spokesman, said that, unlike most other types of reactors, CANDUs could easily and safely use plutonium as a fuel. But Steve Shallhom, Ottawa-based campaign director for Greenpeace Canada, called the plan “a harebrained scheme” that would involve major environmental risks and the problems of transporting weapons-grade plutonium.

The trouble at Pickering’s Unit No. 2 began in the system that circulates deuterium oxide, or heavy water, through the reactor, transferring heat generated by the nuclear fuel rods in the reactor’s core to a boiler system. The boilers produce steam, which a generator converts into electricity. Valves are located in the system so that, when necessary, heavy water can be bled away to relieve pressure. Normally, the valves are kept closed by air pressure.

But at about 5:27 p.m. on Dec. 10, a half-inch copper airpressure tube leading to a relief valve either broke or became disconnected. As the valve popped open and heavy water surged out, pressure dropped in the system. Sensors promptly detected the pressure change, and the control system automatically began shutting down the reactor. Then, a new problem flared: as pumps poured new supplies of heavy water into the system, the pressure rose again—and opened another relief valve in the area of a piece of equipment called the bleed condenser. As heavy water flowed into the relief pipe, the pipe fractured—and heavy water poured onto the floor of the reactor building’s boiler room. ¿lAs pressure plummeted once again, sensors triggered the emergency cooling system to flood the reactor core with ordinary water and prevent dangerous overheating.

It was the kind of event, said Pierre Charlebois, director of the plant’s nuclear division, “that gets your blood flowing.” Charlebois admitted that it is unusual for two things to go wrong in quick succession— “that’s why we’ll be investigating what happened very carefully.” During the incident, he added, the reactor’s core never came close to being exposed—one of the most dangerous situations that can arise in a nuclear plant. And no one is sure what would happen if the core of a CANDU were deprived of coolant and an uncontrolled nuclear reaction began. But some experts think that the uranium-oxide rods that fuel the reactor would superheat to a point where an explosion would blow the dome off the reactor building, sending radioactivity into the surrounding atmosphere. About 1.5 million people live within a 30-km radius of the Pickering reactors.

Tom Adams of the Toronto-based environmental group Energy Probe said that the incident underscored the fact that, unlike Ontario Hydro’s other 16 nuclear reactors, the four oldest Pickering units do not have a second, backup shutdown system. Pickering’s Unit No. 2 has had other accidents in the past: in 1983, a ruptured pressure tube resulted in a heavy water spill that cost $800 million to clean up. While the AECB pondered Pickering’s future, Ontario Hydro officials predicted that it would take at least a month before Unit No. 2 is back in operating condition—and to figure out how, in the space of a few horrifying minutes, so many things went wrong.