SEOUL - South Korea has resumed operation of the country's longest-running nuclear reactor, Gori-1, the government said Monday, amid fears of summer power shortage in the country.

Knowledge Economy Minister Hong Suk-woo told a press conference that the Gori-1 reactor went back into operation earlier in the day and is expected to produce electricity at 100 percent of its capacity starting as early as Friday.

Hong, who is in charge of the country's energy policies, explained that the decision came as the public understanding of the safety of the Gori-1 reactor has been enhanced.

The 578-megawatt reactor, located in the southern port city of Busan, had been shut down since March after the reactor briefly lost power in February during a safety check and the power failure was covered up by nuclear officials for more than a month.

The Nuclear Safety and Security Commission approved a restart of the reactor early last month, but the government had delayed resuming its operation in the face of protests from residents living near the reactor.

Meanwhile, the country's electricity reserve levels fell below three million kilowatts on Monday, causing the Korea Electric Power Corp. to issue a power shortage alert.

South Korea currently has 23 operational reactors, supplying about one-third of its electricity demand.

The Gori-1 reactor was built in 1977 and its initial 30-year operational lifespan ran out in 2007. Its operation was extended by 10 more years with some technological innovations.