Japanese nuclear reactor operator Tepco mishandled the Fukushima nuclear crisis and tried to cover up the extent of the accident, Greenpeace Germany claims.

The environmental group released a study into the accident that lashes out at Tepco for its emergency response and information campaign after an earthquake and a tsunami damaged the plant's four reactors on March 11.

For several weeks, Tepco released what Greenpeace called "managed data" on the situation inside the damaged reactors, vowing that things were critical but stable. That was a lie, said John Large, of Large and Associates, a British nuclear power consultancy group that compiled the study.

"Within five hours of the tsunami striking, Tepco must have known that the cores were melting down," Large said at a news conference in Berlin.

The extreme temperature increases that were publicly available in the first 24 hours after the accident clearly indicated this, he added.

It wasn't until this past Tuesday that Tepco admitted that Reactors 1, 2 and 3 had melted down in the days after the tsunami. "That was a complete turnaround," from previous statements, Large said.

A few days earlier, contrary to what had been released before, the company had admitted that the reactors, believed to be seismic-proof, were severely damaged by the earthquake.

Yet it wasn't just misinformation, Greenpeace said.

In the days and weeks after the accident, Tepco engineers made several mistakes that exacerbated the crisis, Large said. The biggest one was dumping massive amounts of sea water onto the reactors at a time when the pressure vessel was already broken.

"They knew that the water would contaminate the ground nearby and flow into the Pacific," Large said. "Even today, the radiological consequences aren't revealed fully."

All this should be enough to drop nuclear power generation, said Heinz Smital, a nuclear expert with Greenpeace.

"No one lies without a reason," Smital said in a statement. "The excuses and coverups by Tepco and the repeated downplaying of the incident have only one goal: To make people believe that nuclear energy is, even after accidents such as Fukushima, still controllable. But this is the biggest error you could make."

The Greenpeace study was released a few days before German Chancellor Angela Merkel is to receive a report by an ethics commission on nuclear power, a paper that could influence her decision when to phase out nuclear power in Europe's largest economy.

Greenpeace said it would hand its own study to the ethics commission and added that it should only strengthen the case for abandoning nuclear power as soon as possible.

Merkel is currently deciding when to shut down the country's 17 reactors and she is doing so against a backdrop of large anti-nuclear demonstrations that have popped up across Germany.

"After Fukushima, it's clear that you can't keep the old boiling-water reactors online," Smital said.

Given that the earthquake damaged the Japanese reactors that were to withstand such a tremble, the nuclear power plants situated in earthquake areas should be shut down as well, he added.

"Like Japan, Germany isn't prepared for a worst case scenario," Smital said.

Meanwhile in Japan, the situation remains critical.

Some 90 tons of molten nuclear material remain inside the reactors, Large said, likely to be released into the atmosphere slowly or discharged via a massive and quick "superheat steam explosion."

Large said he doesn't trust Tepco in managing the situation.

"The operators are running around like headless chicken," Large said. "It's reaction rather than pro-action."

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