The operator of Japan’s stricken nuclear plant appears to have downplayed dangers the site was vulnerable to a massive tsunami.

Despite Tepco insisting the scale of the March 11 disaster was unforeseeable, a 2007 report warned there was a roughly 10 percent chance Fukushima’s defences could be overrun by a massive tidal wave.

The revelations come as engineers at the complex continue their desperate battle to stop radiation leaking from the site.

The discovery of plutonium in soil around the plant has also caused further anxiety. Tepco says the levels found aren’t harmful to humans, although its presence could indicate a breach in some of the reactors’ containment mechanisms.

The find has also heaped further pressure on Japan’s beleaguered Prime Minister Naoto Kan. He has been fiercely criticised by opposition MPs over his handling of the disaster. Kan apologised for flying over the atomic site a day after the quake, which media reports said had delayed operations to cool the reactors.

His statement followed the discovery yesterday of highly radioactive water outside the plant for the first time, adding to concerns that the contamination could seep further into the environment.

Hundreds of evacuees, who lived around the now crippled plant, still find themselves sheltered in a stadium some 70 kilometres. The latest news means any hopes that they could go home soon appear unlikely.