BYRON, Ill. — 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, about 95 miles northwest of Chicago, shut down at 10:18 a.m., after losing power, Exelon officials said. Diesel generators began supplying power to the plant, and operators began releasing steam to cool the reactor from the part of the plant where turbines are producing electricity, not from within the nuclear reactor itself, officials said.

( Robert Ray, File / Associated Press ) - FILE - In this March 16, 2011 photo, steam escapes from Exelon Corp.’s nuclear plant in Byron, Ill. A nuclear reactor the plant shut down Monday, Jan. 30, 2012 after losing power, and steam was being vented to reduce pressure, according to officials from Exelon Nuclear and federal regulators.

The steam contains low levels of tritium, a radioactive form of hydrogen, but federal and plant officials insisted the levels were safe for workers and the public.

The U.S. Nuclear Regulatory Commission declared the incident an “unusual event,” the lowest of four levels of emergency. Commission officials also said the release of tritium was expected.

Exelon Nuclear officials believe a failed piece of equipment at a switchyard caused the shutdown. The switchyard is similar to a large substation that delivers power to the plant from the electrical grid and that takes power from the plant to the electrical grid. Officials were still investigating the equipment failure.

Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng said officials can’t yet calculate how much tritium is being released.