Former chairman of Tokyo Electric Power (Tepco) Tsunehisa Katsumata and former vice presidents Sakae Muto and Ichiro Takekuro are charged with professional negligence resulting in death and injury, and have pleaded not guilty.

They are the only charges to have stemmed from the tsunami-sparked reactor meltdowns at the plant that set off the worst atomic crisis since Chernobyl in 1986.

Attorneys, who are exceptionally acting as prosecutors in the trial, said the three executives were aware of data indicating the nuclear plant risked being hit by a tsunami with waves exceeding 15 metres (52 feet) -- enough to trigger power loss and cause severe accidents.

"They should have halted operations at the nuclear plant" until the company finished anti-tsunami measures, including construction of a breakwater, the prosecutors told Tokyo District Court, according to Jiji Press.

Katsumata, 78, has said during the trial he could not have predicted the towering waves that pummelled Japan's northeast coast and swamped reactors in March 2011.

The disaster forced tens of thousands to evacuate their homes near the plant. Many are still living in other parts of Japan, unable or unwilling to go back home as fears over radiation persist.

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

Prosecutors had twice refused to press charges, 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 special trial in which designated attorneys accuse defendants and demand a penalty.

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.

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

si/tom/rma

TEPCO - TOKYO ELECTRIC POWER