The operator of Japan's crippled Fukushima nuclear plant said Wednesday it will decommission two reactors at the troubled site that escaped major physical damage from the 2011 tsunami.

Tokyo Electric Power Co (TEPCO) said its management board decided permanently to shut down reactors 5 and 6 at the Fukushima Daiichi power plant.

The plant had a total of six reactors when a record 9.0-magnitude earthquake struck on March 11, 2011 and unleashed a killer tsunami that swamped emergency cooling systems.

The natural disasters wrecked Fukushima's reactors 1 through 4, with three of them suffering meltdowns and spewing radioactive material over a vast farming region.

Reactors 5 and 6 were offline at the time for routine inspections and escaped major damage.

Politicians and the public, from the prime minister down, have long pushed TEPCO to promise that they would not restart reactors at Fukushima.

The utility officially made the decision after accounting rules were changed so that it would not have to book massive losses associated with the decommissioning of the reactors.

"We are studying if we could use the facilities for research purposes," as the firm examines ways to dismantle heavily damaged reactors 1 through 4, said TEPCO spokesman Koichiro Shiraki.

TEPCO and the government are expected to spend some four decades cleaning up and dismantling the four units at the Fukushima plant, the site of the worst nuclear accident in a generation.