A loss of offsite power Monday morning caused one of the reactors at Exelon's Byron Generating Station to automatically shut down.
Exelon spokesman Paul Dempsey said an insulator failed in the station's switchyard shortly after 10 a.m., causing the loss of power.
The Unit 2 reactor then automatically shut down, as it is designed to do, he said.
"Our emergency system worked as planned," said Dempsey. "All systems responded as designed."
According to a U.S. Nuclear Regulatory Commission (NRC) press release, an Unusual Event was declared at 10:18 a.m. on Jan. 30.
An Unusual Event is the least serious of the four levels in the NRC's emergency classification system.
By late Tuesday morning the plant's crew had repaired the failed insulator, Dempsey said.
"We hope to have the offsite power restored by this evening," he said. "Once the operators restore the offsite power to Unit 2, they will work through procedures and then terminate the Unusual Event."
Once that is accomplished, Unit 2 can be put back into service, Dempsey said, but he does not know when that will happen.
"There's no timeline to return Unit 2 into service," he said.
Until then no steam will rise from north cooling tower.
Unit 1 was unaffected by the incident and continues to produce electricity at full power.
Dempsey said the incident posed no danger at any time to the public or plant employees.
He said Exelon officials notified all the appropriate local, state, and federal officials of the Unusual Event.
Ogle County Emergency Management Agency Coordinator Candace Humphrey said she and Sheriff Michael Harn were in constant contact with plant officials Monday and that all emergency procedures were followed.
"We did not staff the EOC (Emergency Operations Center) because the plant assured us they had it under control," she said. "At no time was the public, first responders, or workers at the plant in danger."
The NRC press release said smoke seen coming from an auxiliary building shortly after the incident began was due to a relief valve allowing steam from a generator to escape.
The plant's fire brigade determined that no fire had occurred.
The Byron Fire Department, as well as Mt. Morris, Leaf River, Forreston, and Oregon Fire Departments, were called to stand by at the scene as a precautionary measure.
Dempsey said low levels of tritium were released in the steam from Unit 2 through safety relief valves designed for that purpose.
Tritium is a naturally-occurring isotope of hydrogen and contains weak levels of radioactivity, he said.
"It's naturally occurring, but more common in production at a nuclear plant," Dempsey said. "It quickly evaporates and poses no danger."
The steam from Unit 2 was released until 3 a.m. Tuesday as part of the process to cool the reactor.
According to a press release from the Illinois Emergency Management Agency (IEMA), environmental monitoring personnel collected samples around the nuclear plant on Tuesday to verify the level of tritium in the release of steam.
"While we don't expect to find harmful levels of tritium from the steam release at Byron, I believe it's prudent to collect these samples and verify what levels are present," said IEMA Director Jonathon Monken.
He said results will be available in a few days from the samples taken from water and vegetation around the plant.
Dempsey said an automatic shutdown previously occurred at Byron station approximately seven years ago, but he could not recall the details of what happened.
"It's not an uncommon event in the industry," he said.
Vinde Wells contributed to this story.