CRYSTAL RIVER, Fla. (AP) _ The Crystal River nuclear power plant is expected to be out of service until mid-December following a leak of 830 gallons of radioactive water, Florida Power Corp. officials said.

Federal inspectors said the water from the reactor cooling system was contained inside the plant Saturday and posed no danger to the public.

Utility officials said repairs started Monday.

The 830 gallons, part of 85,000 gallons of water in the plant's cooling system, was channeled into a backup cooling system, said Mark Jacobs, spokesman for the St. Petersburg-based utility which serves more than 1 million people in a 32-county area of north and west central Florida.

Thomas Stetka, an inspector for the Nuclear Regulatory Commission, said the leak may have been caused by a cracked weld in part of the cooling system.

The leak occurred in a ''letdown cooler'' where radioactive water is cooled to allow for chemical testing and purification, he said.

''There's a very high temperature difference across these instruments. I suspect it's not a hole, but a crack, perhaps in the welding.''

Operators noticed the loss of cooling water Friday night while warming up the reactor to restart it following an eight-day outage caused by an electrical short.

Early Saturday, the leak became serious enough that the company had to shut the reactor down again.

The plant was out of service for the first six months this year after one of four reactor cooling water pumps failed.

It was not the first time that letdown coolers have caused problems at Crystal River. Jacobs said an incident in late 1982 ''bore some similarity.''

According to Joe Gilliland of the NRC, a letdown tube ruptured Nov. 25, 1982. But he said other problems complicated that incident which didn't occur this time.