Based on Japan's new Prime Minister Yoshihiko Noda's policy of not building new nuclear power plants and decommissioning aged ones, "it would be zero," Hachiro told reporters when asked whether the number of nuclear plants in the country would be reduced.
Noda replaced Naoto Kan, who stepped down a week ago amid criticism over his handling of the aftermath of the March 11 earthquake, tsunami and the Fukushima nuclear power plant disaster.
Noda has suggested Japan will eventually phase out nuclear power generation in the resource-poor nation, Asia's second-largest economy.
Hachiro said it would be "difficult" for Japan to go ahead with plans to build nuclear plants on which construction hasn't started.
"Public opinion is generally united in reducing (nuclear plants), instead of increasing them," he said.
Under Japan's previous energy plan, prior to the Fukushima disaster, nuclear power had been set to meet more than half of demand by 2030, up from about one-third.
Only 12 of Japan's 54 commercial power plant reactors are in operation following the shutdown last week of Kyushu Electric Power Co.'s second reactor at its Sendai nuclear power plant in Kagoshima prefecture for a three-month scheduled inspection.
Regarding the resumption of reactors idled for regular checkups, Hachiro said the prime minister has approved a plan to seek safety assessments from the International Atomic Energy Agency as part of the government-imposed nuclear stress tests introduced in July.
But if none of Japan's reactors get the go-ahead to restart in the coming months, Japan could lose its nuclear output completely next April or May, Platts news service says, because of a Japanese regulation that requires nuclear power plants to perform scheduled maintenance on reactors every 13 months.
In an interview with Japanese newspaper Tokyo Shimbun published Tuesday, Kan said, "I thought nuclear plants were safe as they were supported by Japan's technology but I changed my mind after the experience of the March 11 disaster."
"When I think of safety not being outweighed by risk, the answer is not to rely on nuclear," Kan said.
Immediately before his resignation, Kan had pushed through a feed-in tariff law creating preferential pricing for large-scale renewable energy projects.
Renewables, including hydropower, account for about 10 percent of Japan's energy mix.