It reads like a dispatch from the pages of Godzilla, who was a fictional metaphor for nuclear weapons, wreaking destruction with a blast of radiation from his mouth: the world's most populated metropolis is threatened with fallout from an out-of-control nuclear power plant. But such was the real-life situation facing greater Tokyo, a city of 35 million people, after last year's triple meltdown at the Fukushima Daiichi complex 250km away. And for many, the unlikely hero of the capital's most compelling drama since World War II is the-then Prime Minister, Naoto Kan.

Widely criticised for his handling of the March disaster, Mr Kan has nevertheless been hailed for his decision to confront plant operator Tokyo Electric Power Co (Tepco) and stop it abandoning its six reactors and seven lethal nuclear fuel pools. "It is for history to evaluate what took place, but as Prime Minister I just did what I could," he said in his first interview with a British newspaper since stepping down last year after turning against nuclear power.

He feared what the-then Cabinet spokesman Yukio Edano called a "demonic chain reaction," meaning Daiichi could trigger meltdowns elsewhere. "If that happened, it was only logical to conclude that we would also lose Tokyo itself," Mr Edano said earlier this year. Mr Kan said he was haunted by an image of a deserted, uninhabitable greater Tokyo if the crisis spun out of control. "I assumed it would have brought devastating destruction in East Japan."

The key moment in the crisis came at about 3am on 15 March, four days after a huge earthquake and tsunami struck, when Mr Kan was woken from a nap in the Prime Minister's residence in central Tokyo. Exhausted and still wearing his rumpled utility suit, Mr Kan heard startling news from the government's economy minister, Banri Kaieda: Tepco officials and 700 workers were preparing to abandon the Daiichi complex.

Mr Kan told his driver to take him to the utility's headquarters about a kilometre away. Shoulders hunched, he stormed into a conference room to find it crowded with 200 people. "Why on earth are there so many people here?" he yelled. "Important decisions should be made by five or six people." He trained his fire on Chairman Tsunehiko Katsumata and the firm's top executives.

There was "absolutely no way" the workers could pull out. Abandoning the plant would create a catastrophe "twice or three times worse than Chernobyl," he said, referring to the 1986 Ukraine nuclear disaster. If people had to die, they would die. Workers over 60 should go to the plant. "I'm over 60 myself and it makes no difference if we get cancer 20 or 30 years down the line," he said, adding he would go to Daiichi if he had to. It was no longer a question of only Japan. The disaster would hit the world.

The Prime Minister left behind him a room of exhausted, chastened Tepco executives. Later they would deny there was any plan to completely abandon Daiichi. Sakae Muto, a vice president, later said that the utility only intended to withdraw "some of the workers."

Today, Mr Kan says: "Reactor 1 had exploded on March 12th; Reactor 3 on March 14th; Reactor 2 looked very dangerous. In that situation, it's not surprising that some people would conclude that there was nothing else they could do and that it was time to pull out.

"But there is no way that could happen. In a chemical plant, once everything burns, the plant sort of returns to normal. Nuclear reactors don't return to normal... It wasn't like fleeing a fire that you could come back to when it had died down."

The verdict on Mr Kan's confrontation came last month. Speaking on behalf of an independent panel of 30 professors, journalists and lawyers who spent six months probing the accident, lawyer Akihisa Shiozaki characterised the overall response to the crisis as "crude, reactionary, but lucky."

Nobody was "even remotely prepared", he said. "A small group of politicians out of panic and distrust became increasingly involved in the technical management of the on-site accident. Unfortunately, this political intervention was rarely effective, triggered confusion, and at times was extremely dangerous." Mr Kan's move to confront Tepco and stop the pullout was critical. "The worst would have happened... Fukushima would totally have gone out of control."

A report commissioned by Mr Kan during the first week of the crisis and delivered two weeks later backed his fears of a worst-case scenario, warning that compulsory or voluntary evacuation orders would have to be issued to residents living within 250km (155 miles) of the Daiichi plant if the situation spiralled out of control. The warning was kept secret until this year.

"It was providence that the crisis was not worse," Mr Kan says. "We were being pushed back by this unseen enemy and on the very brink of disaster on March 15-17. But finally we began to fight back."

Mr Kan quit last August after urging Japan to scrap plans to build 14 more nuclear reactors. There is speculation he was hounded from power by Japan's powerful nuclear industry and the conservative media, a charge he denies.

His successor, Yoshihiko Noda, has pledged to get idle nuclear plants working again before the sweltering summer. All but one of Japan's 54 reactors are offline for post-Fukushima safety checks.

Mr Edano, who is now Japan's Economy Minister, predicted this week that Japan's entire nuclear capacity will be shut down from 5 May, when Hokkaido Electric Power Co takes the No 3 Reactor of its Tomari plant offline for regular inspections.

And the plant that caused last year's crisis? Mr Kan says he remains worried about the state of the Daiichi complex.

"Even after a year, we're not certain about the situation there," Mr Kan says, citing Tepco's discovery last month that only 60cm of water was inside the containment vessel of Reactor 2, far below the three to six metres expected. The ex-Prime Minister said it is difficult to conclude yet that the plant is under control.

From activist to PM: The rise of Naoto Kan

Born in 1947, Naoto Kan was a founder of the Democratic Party. In 2010, less than a year after the party swept the Liberal Democratic Party from power, he took over from the unpopular Yukio Hatoyama to become Japan's fifth Prime Minister in three years.

At the helm when last year's earthquake and tsunami triggered the Fukushima Daiichi nuclear crisis, he was eventually forced out of office after just over a year in the job.