The San Onofre nuclear generating plant in San Diego County, Calif, is pictured in this March 14, 2011 file photo.
A worker was leaning over to retrieve a flashlight when he lost his balance and fell into a reactor pool at the San Onofre nuclear power plant last week but he did not receive a significant dose of radiation, Southern California Edison officials said on Friday.
The worker was wearing a life preserver when he fell into a pool more than 20 feet deep that holds water that circulates through the reactor core.
He received 5 millirems of radiation, Gil Alexander, a spokesman for plant operator Southern California Edison, told the North County Times.
That's not considered a major dose and he went back to work the same day. By comparison, a chest X-ray provides about a 4-millirem dose.
The worker fell into the pool Jan. 27, five days before officials reported an "extremely small" amount of radiation could have escaped from the plant after a water leak prompted operators to shut down the reactor.
Alarms alerted station personnel to the leak at the power plant at about 6 p.m. Tuesday, according to the U.S. Nuclear Regulatory Commission.
Because the building into which the gas leaked is not airtight, it is possible that a small amount of radioactivity escaped into the environment, officials said. But he said the levels would likely be immeasurable against existing levels in the atmosphere.
The leak occurred in the part of the facility, located off the I-5 just south of San Clemente, which houses thousands of tubes carrying radioactive water, officials said.
There are radiation detectors throughout the plant and none measured any amount of radioactivity, said Alexander.
The investigation into what caused Tuesday's leak continues. An evacuation was not required.
Officials were waiting for the reactor to cool before crews were sent in to analyze and fix the leak.
Once the problem is resolved, it will likely take several days for the reactor to be restarted, officials said.
This was not the first time a leak scare has occurred at the San Onofre plant. In November, a level 1 alert was issued at the plants, but the appropriate alarms did not go off.
The Associated Press contributed to this report.
More content from msnbc.com and NBC News