A reactor at a nuclear plant in northern Illinois has shut down after losing power. Exelon Nuclear says steam is being vented at its Byron Generating Station to reduce pressure. Plant operators say the unit is safe. They declared an unusual event at 10:18 a.m. Monday, when Unit 2 shut down after losing power from an off-site source. They say the facility's generators are providing power to equipment. Officials say Byron Station is designed to vent steam in the event of a power loss. They say the steam contains low levels of tritium, a radioactive isotope, but not at unsafe levels. Officials say engineers are investigating why the unit lost offsite power. Unit 1 continues to operate normally. Byron Generating Station is about 95 miles northwest of Chicago.