Waterford — Millstone Power Station Unit 2 was taken out of service Sunday after staff identified that in-core detectors — devices placed in the reactor core to measure and map power levels — were not working properly, the Nuclear Regulatory Commission and Millstone said Monday.

Millstone recently completed a refueling and maintenance outage, during which all of the plant's in-core detectors were replaced, according to NRC spokesman Neil Sheehan. The detectors are miniature fission chambers that provide continuous information in the reactor core, Sheehan said.

Sheehan noted the reactor "was at about 20-percent power and coming out of the outage when it was determined the detectors were not operating properly and a shutdown was necessary yesterday afternoon."

Millstone spokesman Ken Holt said staff identified some of the detectors were not working properly during testing of the in-core instrumentation at Unit 2.

"There is an ongoing effort to resolve the issue," Holt said.

As of Monday at 2 p.m., Holt said the unit remained "in a safe, shutdown condition."

Sheehan said NRC resident inspectors assigned to Millstone will follow up on Dominion Energy's troubleshooting efforts and repairs, and will track plans to restart the unit.