Three former executives at the operator of Japan's Fukushima nuclear plant went on trial Friday, the only people ever to face a criminal court in connection with the 2011 meltdowns that left swathes of countryside uninhabitable.

Ex-Tokyo Electric Power (Tepco) chairman Tsunehisa Katsumata, 77, and former vice presidents Sakae Muto, 66, and Ichiro Takekuro, 71, all pleaded not guilty to charges of professional negligence resulting in death and injury, over six years after the worst atomic accident in a generation.

Katsumata told a three-judge panel hearing the case that it was impossible for him to have foreseen the risk of the towering waves that pummelled Japan's northeast coast and swamped reactors in March 2011.

"I apologise for the tremendous trouble to the residents in the area and around the country because of the serious accident that caused the release of radioactive materials," Katsumata said in a barely audible voice, as he bowed.

But "I believe I don't have a criminal responsibility in the case", he said inside the packed courtroom.

The indictments are the first -- and only -- charges stemming from the tsunami-sparked reactor meltdowns at the plant that set off the worst atomic crisis since Chernobyl in 1986.

Legal proceedings in Japan can sometimes move at a glacial pace and the trial is reportedly expected to last more than a year.

If convicted, the men face up to five years in prison or a penalty of up to one million yen ($9,000).

Prosecutors had twice refused to press charges against the men, citing insufficient evidence and little chance of conviction.

But a judicial review panel composed of ordinary citizens ruled in 2015 that the trio should be put on trial, which compelled prosecutors to press on with the case under Japanese law.

- 'Held accountable' -

"Since the accident, nobody has been held responsible nor has it been made clear why it happened," Ruiko Muto, who heads the group that pushed for the trial, told AFP.

Evacuee Yoshiko Furukawa was outside the Tokyo courthouse Friday with dozens of others who fled their homes after the accident. 

Thousands have still not returned, largely due to lingering radiation fears.

"What I lost on March 11, 2011 was a normal life," said Furukawa, who now lives in a different city.

"As someone who knew nothing (about the risks of an accident), I'm now in my seventh year as an evacuee."

The prosecutor told the court Friday he would try to prove that the three defendants were able to foresee the risk of a huge tsunami and failed to take necessary safety steps.

The trio were present at safety meetings where experts warned of the anticipated height of a tsunami off the Fukushima coast, he said, with some 230 documents, including e-mails, to be presented as evidence against the men.

The executives also had access to data and studies pointing to the risk of a tsunami exceeding 10 metres (32 feet) in the area that could trigger power loss and severe accidents, he added. 

"If they had fulfilled their responsibility to safety, the accident would have never occurred," the prosecutor said.

An earlier report by a government panel said Tepco -- which is facing massive compensation and cleanup costs -- simulated the impact of a tsunami on the plant in 2008 and concluded that a wave of up to 15.7 metres could hit after a magnitude-8.3 quake.

Waves as high as 14 metres swamped the reactors' cooling systems in March 2011 after a 9.0 magnitude tremor.

Although the quake-tsunami disaster left some 18,500 people dead or missing, the Fukushima accident itself is not officially recorded as having directly killed anyone.

- 'Obedience' -

The charges against the three are linked to the deaths of more than 40 hospitalised patients who were hastily evacuated from the Fukushima area and later died.

A 2015 report by the International Atomic Energy Agency said a misguided faith in the safety of atomic power was a key factor in the accident, pointing to weaknesses in disaster preparedness and unclear responsibilities among regulators.

A parliamentary report a year after the disaster said Fukushima was a man-made crisis caused by Japan's culture of "reflexive obedience".

source: AFP