TOKYO, Aug 24 (Reuters) - Japan will restart more idled nuclear plants and look at developing next-generation reactors, Prime Minister Fumio Kishida said on Wednesday, setting the stage for a major policy shift on nuclear energy a decade after the Fukushima disaster.

The comments from Kishida - who also said the government would look at extending the lifespan of existing reactors - highlight how the Ukraine crisis and soaring energy costs have forced both a change in public opinion and a policy rethink toward nuclear power.

Japan has kept most of its nuclear plants idled in the decade since a massive earthquake and tsunami in 2011 triggered a nuclear meltdown at the Fukushima Daiichi power plant. Quake-prone Japan also said it would build no new reactors, so a change in that policy would be a stark turnaround.

Kishida told reporters he had instructed officials to come up with concrete measures by the year-end, including “gaining the understanding of the public” on sustainable energy and nuclear power.

Government officials met on Wednesday to hammer out a plan for so-called “green transformation” aimed at retooling the world’s third-largest economy to meet environmental goals. Nuclear energy, which was deeply opposed by the public after the Fukushima crisis, is now seen by some in government as a component of such green transformation.

Public opinion has also shifted, as fuel prices have risen and an early and hot summer spurred calls for energy-saving.

“It is the first step towards the normalization of Japan’s energy policy,” said Jun Arima, a project professor at the University of Tokyo’s graduate school of public policy.

Japan needs nuclear power because its grid is not connected to neighboring countries, nor is it able to boost the output of domestic fossil fuels, he said.

Last month the government said it hoped to restart more nuclear reactors in time to avert any power crunch over the winter.

As of late July, Japan had seven operating reactors, with three others offline due to maintenance. Many others are still going through a relicensing process under stricter safety standards imposed after Fukushima.

Kishida also said the government would look at extending the lifespan of existing reactors. Local media earlier reported this could be done by not including the time reactors remained offline - years in some cases - when calculating their operating time.

Under current regulations, Japan decommissions plants after a predetermined period, which in many cases is 60 years.