Japan's estimate of dismantling the Fukushima nuclear plant is ballooning far beyond the utility's estimate of 2 trillion yen ($19 billion).
A government study released Tuesday found decommissioning the Fukushima Dai-ichi plant already has cost 80 billion yen ($770 million) over the last three years.
The plant suffered multiple reactor meltdowns due to damage from the March 2011 earthquake and tsunami.
The ministry overseeing nuclear power said the decommissioning costs will continue at several hundreds of billions of yen (billions of dollars) a year.
Tokyo Electric Power Co., the utility that operated and is now decommissioning Fukushima Dai-ichi, has said decommissioning will take several decades.
Even if it were to take 30 years at an estimated annual cost at 300 billion yen ($3 billion), both conservative projections, the cost would be nearly 1 billion yen or $100 billion.
TEPCO spokesman Shinichi Nakakuki declined comment on the government projection, but he acknowledged TEPCO was still trying to determine what exactly the decommissioning effort might involve.
"It is difficult to calculate the entire cost for the decommissioning," he said, adding that the 2 trillion yen figure had so far taken into account the effort to remove the nuclear debris, taking the example of Three Mile Island in the U.S., as well as costs and equipment needed to keep the reactors stable.
The study did not distinguish between costs borne by the government and borne by TEPCO, which received a government bailout.
Japan has been struggling to clean up parts of the no-go zone to put the worst nuclear catastrophe since Chernobyl behind it.
The government has estimated that decontaminating the areas around the Fukushima plant, including removing radiated topsoil, buildings and trees, will cost at least 2.5 trillion yen ($24 billion).
But experts have been warning that such estimates may be too optimistic. The nuclear disaster in Fukushima displaced about 150,000 people.