The figures were found by simulating a radioactive leak at the Gori-1 nuclear reactor in the southeastern city of Busan, according to the report's publishers, the Korean Federation for Environmental Movement (KFEM) and the No Nukes Busan Citizen Countermeasure Commission.
The experiment assumed the leakage to be as large as during the 1986 Chernobyl nuclear disaster, with all citizens remaining in place prior to evacuation.
The number of casualties was estimated at 47,580 immediate deaths and about 850,000 more eventual deaths caused by cancer from exposure to radiation, the report said.
It also said most of the economic damage would have to be covered by the government, as the reactor's operator would only be able to pay 50 billion won in compensation.
South Korea currently has 21 commercial reactors and has plans to build 12 more by 2022.
The issue of nuclear safety has become a growing public concern in South Korea following last year's nuclear disaster at Fukushima and recent revelations that officials tried to cover up a brief power outage at the Gori-1 reactor in early February.
One of Gori's reactors temporarily lost power during a safety inspection, prompting regulators to launch a probe into its operations. The power cut did not lead to any accidents, but the plant authorities came under fire for belatedly reporting the incident to the state safety commission.
Soon after, a dozen officials at Gori and its subcontractors were convicted on charges of defrauding the nuclear plant by refurbishing used valve and other parts.