The Ministry of Economy, Trade and Industry formally admitted Monday to drawing up an estimate in 1994 saying the cost of burying spent nuclear fuel is far lower than the cost of recycling it.
The ministry had said earlier that there was no cost estimate other than that for reprocessing.
"It was regrettable that we did not know about the existence of such a document and we want to apologize for giving the wrong reply to the Diet," said Hideji Sugiyama, vice minister of economy, trade and industry.
He said METI did not intend to conceal the data.
The ministry is investigating why it was unaware of the data's existence, he said.
The estimate by an advisory panel to the Natural Resources and Energy Agency in February 1994 put the cost of burying spent nuclear fuel at 0.348 yen per kilowatt-hour of power generation, compared with 1.336 yen for reprocessing spent nuclear fuel.
During a committee session in the House of Councilors in March, Kazumasa Kusaka, then director general of the agency, said there was no cost estimate other than for reprocessing.