SAN CLEMENTE (CBS) — A partial shutdown of the San Onofre nuclear power plant continued Thursday after sensors detected a possible leak in one of the reactor units.

An alarm alerted station personnel to the presence of radioactivity in the ancillary structure, and they immediately began shutting down the reactor, according to Nuclear Regulatory Commission spokesman Victor Dricks.

Gil Alexander, a spokesman for facility operator Southern California Edison, said that radioactive water was passing through a crack inside the generators at a rate of 80 to 100 gallons per day when Edison began the shutdown early Tuesday evening.

It is unclear if any of it was released into the atmosphere, and if any did, it would have been a “very, very low level of radioactivity” that would be “barely measurable,” according to the NRC.

As of Wednesday night, Southern California Edison, which operates the facility, said that sensitive monitoring instruments continued to show no change in radiation levels that would be detectable off-site.

Crews will fully evaluate the cause of the mishap and the steps required to repair it before resuming operations, according to Edison.

Once the problem is resolved, it will likely take several days for the reactor to be restarted, the Rosemead-based utility advised.

At the time of the accident, the other reactor at the plant already was powered down for routine maintenance, refueling and technology upgrades.