I'm sure it'll all be fine. I mean, I'm sure they'd tell us if it was going to be a problem!

A senior adviser to the operator of the wrecked Fukushima Daiichi nuclear power plant has told the firm that it may have no choice but to eventually dump hundreds of thousands of tonnes of contaminated water into the Pacific Ocean.

Speaking to reporters who were on a rare visit to the plant on the eve of the third anniversary of the March 2011 earthquake, tsunami and nuclear disaster, Dale Klein said Tokyo Electric Power [Tepco] had yet to reassure the public over the handling of water leaks that continue to frustrate efforts to clean up the site.

"The one issue that keeps me awake at night is Tepco's long-term strategy for water management," said Klein, a former chairman of the US nuclear regulatory commission who now leads Tepco's nuclear reform committee.

"Storing massive amounts of water on-site is not sustainable. A controlled release is much safer than keeping the water on-site.

"Tepco is making progress on water management but I'm not satisfied yet. It's frustrating that the company takes four or five steps forward, then two back. And every time you have a leakage it contributes to a lack of trust. There's room for improvement on all fronts."

Tepco's failure to manage the buildup of contaminated water came to light last summer, when it admitted that at least 300 tonnes of tainted water were leaking into the sea every day.

That revelation was followed by a string of incidents involving spills from poorly assembled storage tanks, prompting the government to commit about $500m (£300m) into measures to contain the water.

They include the construction of an underground frozen wall to prevent groundwater mixing with contaminated coolant water, which becomes tainted after coming into contact with melted nuclear fuel deep inside the damaged reactors.