Unit 2 at Exelon's La Salle nuclear plant in Illinois tripped offline Tuesday when a safety system activated unexpectedly.
At 5:34 p.m., the reactor protection system shut the unit down automatically when main steam isolation valves closed, according to a report filed with the Nuclear Regulatory Commission. Part of the nuclear steam supply shutoff system caused the MSIVs to close, although no leak was reported. Valves also isolated the reactor water cleanup system during the event, according to the NRC. The shutdown proceeded normally, and Exelon said in a release that it is investigating the cause of the closed MSIVs.
Located 75 miles southwest of Chicago, La Salle uses two General Electric boiling water reactors completed in the mid-1980s to generate about 2,200 megawatts. Unit 2 is a GE type 5 BWR with Mark II containment. According to the NRC, it remained offline as of Wednesday morning.