Byron Nuclear Generating Station uses Westinghouse (owned by Japan's Toshiba)'s pressurized water reactors.
From Chicagobusiness.com citing AP (1/30/2012):
Exelon shuts Byron nuclear plant unit after power loss
(AP) — A nuclear reactor at a northern Illinois plant shut down Monday after losing power, and steam was being vented to reduce pressure, according to officials from Exelon Nuclear and federal regulators.
Unit 2 at Byron Generating Station shut down around 10:18 a.m., after losing power from an off-site source, Exelon officials said. Diesel generators began supplying power to the plant equipment and operators began releasing steam from the non-nuclear side of the plant to help cool the reactor, officials said.
Even though the turbine is not turning to produce electricity, "you still need to cool the equipment." said U.S. Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng. Releasing steam helps "take away some of that energy still being produced by nuclear reaction but that doesn't have anywhere to go now."
The steam contains low levels of radioactive tritium, but the levels are safe for workers and the public, federal and plant officials said.
Unit 1 was operating normally while engineers investigate why Unit 2 lost power, which comes into the plant from the outside power grid, Mitlyng said. Smoke was seen from an onsite station transformer, she said, but no evidence of a fire was found when the plant's fire brigade responded.
Exelon spokesman Paul Dempsey said there is "no reason we can pinpoint right now" for the power loss.
Mitlyng said Nuclear Regulatory Commission inspectors were in the control room at Byron and in constant contact with the agency's incident response center in Lisle, Ill.
Byron Generating Station is in Ogle County, about 95 miles northwest of Chicago.
In March 2008, federal officials said they were investigating a problem with electrical transformers at the plant after outside power to a unit was interrupted.
In an unrelated issue last April, the commission said it was conducting special inspections of backup water pumps at the Byron and Braidwood generating stations after the agency's inspectors raised concerns about whether the pumps would be able to cool the reactors if the normal system wasn't working. The plants' operator, Exelon Corp., initially said the pumps would work but later concluded they wouldn't.