Monticello Nuclear Generating Plant workers are still trying to find the cause of the scram that caused an emergency shutdown of the plant, officials said.
The Nuclear Regulatory Commission (NRC) said its on-site inspectors at the nuclear plant are monitoring the situation.
The 600-megawatt plant shut down automatically at 11:12 p.m. last Saturday when safety systems detected low oil pressure in its turbines, according to Xcel Energy, which owns and operates the plant.
There was no radiation release and no danger to the public or plant workers, said Xcel Energy spokeswoman Mary Sandok.
Xcel officials said the outage will not be lengthy but declined to specify how long.
The single-reactor plant, located about 40 miles northwest of Minneapolis, generates enough electricity to power nearly 500,000 homes.
The NRC’s resident inspectors at the plant are monitoring the emergency shutdown to identify the cause, NRC spokeswoman Viktoria Mitlyng said.
The NRC also is trying to locate a part missing from a device used to monitor the level of nuclear reaction inside the reactor, Mitlyng said.
The monitor, which is highly radioactive, was out of service so they could replace it. It typically stores in the spent fuel reactor pool for safekeeping until workers can ship it to a disposal site, Mitlyng said.
There is no indication the missing part is outside of the pool — it simply was not in its designated location, so workers will search the entire pool, Mitlyng said.
This is the third non-scheduled outage reported at the nuclear power plant this year.
“While we are still investigating the cause of the weekend outage, the causes of the first two were unrelated,” Sandok said.
On Oct. 21, the plant had a 10-day outage when a cable failure caused a transformer to lock out, interrupting nonsafety-related electricity from reaching the plant from the grid.
In June, Xcel shut down the plant to replace a faulty safety valve installed on the plant’s main steam line just a few months previously during a scheduled refueling outage.