Nearly a year after Japan's Fukushima nuclear disaster, then-premier Naoto Kan is haunted by the spectre of an even bigger crisis forcing tens of millions of people to flee Tokyo and threatening the nation's existence.

Having experienced the 3/11 nuclear disaster, I changed my way of thinking. The biggest factor was how at one point, we faced a situation where there was a chance that people might not be able to live in the capital zone including Tokyo and would have to evacuate, Kan told Reuters in an interview on Friday.

If things had reached that level, not only would the public have had to face hardships but Japan's very existence would have been in peril.

That was the fear that convinced Kan, in office for less than a year when the March 11 triple disaster struck, to declare the need for Japan to end its reliance on atomic power and promote renewable sources of energy such solar that have long taken a back seat in the resource-poor country's energy mix.

While many technological measures can be taken to secure safety at nuclear power plants, such measures on their own cannot cover great risks, said Kan, sitting in front of a calligraphy scroll inscribed with the ancient Chinese proverb Be Brave, But Not Reckless.

I came to think that the safest way is to build a society that does not have to depend on nuclear power plants, and that this is possible.

Kan, 65, stepped down last September after seeing his ratings sink below 20 percent over perceived policy flip-flops and dissatisfaction with his handling of the Fukushima crisis as well as what critics saw as rash and hasty decisions.

Kan's defenders, though, say a key cause of his downfall was his call to wean Japan from nuclear power -- a stance popular with the public but opposed by many including politically powerful utilities.

Kan, a fiery former civic activist known for his short temper, stressed during an hour-long interview that his priority now is promoting renewable energy rather than political manoeuvring, and steered clear of any criticism of his successor, Yoshihiko Noda.

After the March 11 accident, three reactors at the Fukushima plant suffered meltdowns and radiation spewed widely through eastern Japan, forcing tens of thousands of residents to evacuate from near the plant and contaminating food and water. The reactors were stabilised by December.

POLITICIANS' FAILURE

The Fukushima crisis destroyed the myth that atomic energy is safe, cheap and clean and prompted Japan to scrap a plan to boost its share of electricity demand to more than half by 2030.

The government is now crafting a new mid-term programme.

But critics have questioned whether Noda, a former finance minister keen on fiscal reform, is really committed to reducing reliance on atomic energy and introducing reforms that would help give renewable energy a bigger share.

It is not true that things have not moved forward (under Noda) ... There have been dramatic changes, Kan said.

The Noda administration is proceeding basically with what I was thinking at that time (of the accident) although there are various forms of resistance such as from some businesses. Noda and the nuclear minister are steadily proceeding with reform.

He did, however, take a swipe at politicians on both sides of the aisle for bickering rather than cooperating in the wake of the accident.

The Japanese people have acted calmly and were patient in their response to the accident, he said. But as for whether politicians fully cooperated at the time of the accident and thereafter, unfortunately that's not the case.

He stopped short of floating a target date for when Japan should exit nuclear power entirely, although he noted that reality on the ground was moving ahead of policy.

All but three of Japan's 54 reactors are off-line mostly for maintenance, and the government has yet to persuade wary local authorities that it is safe to resume operations.

I believe that most people will seek a society that does not rely on nuclear power if we can show them specifically that (non-nuclear) energy can realistically be supplied, Kan said.

Japan can strengthen its energy supply system outside of nuclear and fossil fuel while moving forward with energy conservation ... It is possible for Japan to become the model of a society that does not rely on nuclear power.

Many had hoped the triple disasters would be the tipping point that pushed Japan to make vital reforms to escape decades of economic stagnation. Kan said it was too soon to judge, but what keeps him motivated is a desire to fix what's wrong.

If I think something is out of kilter, then I think about it and make a proposal. Instead of just criticising, I try to find an alternative, he said.

I guess you could say that's the source of my energy.

(Additional reporting by Rie Ishiguro; Editing by Edwina Gibbs and Jonathan Thatcher)