NEWPORT, Mich. - The source of Fermi II's leak has been traced to a gasket on one of 14 air cooling units inside the nuclear plant's containment structure.
"If not inoperable, it compromised its effectiveness," John Austerberry, a Detroit Edison Co. spokesman, said of the affected chiller.
The gasket and the cooling unit will be repaired before the plant is allowed to resume operation, Jan Strasma, a Nuclear Regulatory Commission spokesman, said.
The utility expects the fix to be made within days, Mr. Austerberry said.
Cooling units inside nuclear containment vessels act like air conditioners. They help moderate air temperature and pressure inside those vessels.
When nuclear plants are operating, the temperature of their reactors is usually 400 to 600 degrees depending upon whether their reactors are based on a boiling-water or pressurized-water design.
At many plants with boiling-water reactors, the air temperature inside containment is 120 degrees or more even with the chillers working, David Lochbaum, a Union of Concerned Scientists nuclear safety engineer, said.
That's why the sudden disabling of one chiller can have an affect on temperature, pressure, and other conditions within containment vessels, Mr. Lochbaum said.
He said that initially he had been taken aback by the first notice that Detroit Edison gave the NRC when the leak began late Monday afternoon.
Unknown leakage at a rate of about 30 gallons a minute was reported to be in progress inside the radioactive containment vessel as well as a slight increase in air pressure.
The leak rate eventually exceeded 50 gallons a minute, requiring the utility to notify emergency personnel throughout Michigan that the plant had crossed over to an alert status.
Increasing air pressure could signal leakage of hot reactor coolant, Mr. Lochbaum said.
That coolant is both hot and radioactive because it's circulated through the nuclear reactor.
Not knowing what kind of leak was in progress, Detroit Edison operators took the conservative approach - one lauded by the NRC - by manually shutting down the plant immediately, rather than waiting to diagnose the problem.
Eventually the utility was able to rule out reactor coolant by chemically matching a sample of water drawn from the containment sump with nonradioactive water from a secondary cooling system. That system passes through containment without going through the reactor and is used to cool equipment other than the reactor.
Mr. Lochbaum agreed the actions taken by Detroit Edison were appropriate. Reactor coolant could be ruled out because it doesn't have the corrosion inhibitor found in the secondary cooling loop, he said.
He said the pressure increase likely would not have occurred under those circumstances unless leakage from the secondary cooling system caused a problem with one of the coolers, which is just what happened. The secondary cooling system feeds water into the coolers.
The leak involved about 20,000 gallons of water. The water has been collected and will be redistributed into the secondary cooling loop, Mr. Austerberry has said.
- Tom Henry