A worst-case scenario sketched out by the Japanese government foresaw the end of Tokyo in a chain of nuclear explosions that would mean evacuating the city, an independent panel said Tuesday.

Plans were drawn up for the mass withdrawal from the capital as at least one senior minister fretted that meltdowns at Fukushima might spark crises at reactors all along the coast and engulf the city of 13 million people.

The revelations came in a 400-page report published by a panel of experts who were given free rein to probe the events surrounding the world's worst nuclear crisis in a generation.

"I had this demonic scenario in my head" that nuclear reactors could break down one after another, then chief cabinet secretary Yukio Edano told the panel.

"If that happens Tokyo will be finished," he said, according to the report.

The panel said as the situation on Japan's tsunami-wrecked coast worsened, Fukushima operator Tokyo Electric Power (TEPCO) had wanted to abandon the plant and evacuate its workers.

But the utility, which refused to co-operate with the study, was ordered to keep men on site by then prime minister Naoto Kan.

Experts concluded that if the premier had not stuck to his guns, Fukushima would have spiralled further out of control, with catastrophic consequences.

"When the prime minister's office was aware of the risk the country may not survive (the crisis)...TEPCO's president (Masataka) Shimizu....frantically called" to tell the premier he wanted his staff to leave the crippled nuclear reactor, panel head Koichi Kitazawa told a news conference.

Kitazawa said Kan threatened to break up the powerful utility if the company insisted on pulling its men out.

He said Kan's refusal to bow to TEPCO's demand had averted a worse crisis.

Kan told Shimizu: "It's impossible. If you withdraw staff, TEPCO will be demolished," according to Kitazawa.

"Consequently, it's Mr Kan's biggest contribution that the Fukushima 50 remained at the site," added Kitazawa, referring to dozens of operatives who worked to contain the accident and were feted as heroes.

Respected academics, engineers and journalists were drafted in by the Rebuild Japan Initiative Foundation after public demands for an independent probe into the meltdowns at Fukushima in the aftermath of the monster tsunami of March 2011.

The six-member panel led a team that interviewed more than 300 people central to the disaster response and was given access to data and documents used in the days and weeks after disaster struck.

The panel said Kan had instructed experts to draft a plan to evacuate a huge swathe of the country, based on the worst case scenario.

Planners worked on the assumption that if the nuclear crisis were to worsen "it is possible that a compulsory evacuation zone will spread to 170 kilometres (105 miles)... and a voluntary evacuation zone will spread to 250 kilometres and beyond".

Tokyo lies around 220 kilometres from the stricken plant.

The panel found that while some of Kan's actions in the aftermath of the disaster had been helpful, a tendency to micro-manage events had hampered the emergency response.

"Direct interventions" by Kan and his aides that ignored the distinctions in roles between politicians and bureaucrats "proved ineffective" in defusing the widening crisis, it said.

Those in Kan's office spent a lot of time trying to understand the minutiae of the situation, which meant they tried to intervene in the day-to-day detail in a way that was not helpful.

The report said the delay in the use of seawater as a coolant for overheating reactors was a prime example and came about because the prime minister's office had insisted on the use of freshwater.

Experts later said the use of seawater -- which was available in plentiful supply -- had probably averted a worse disaster.