TIM PALMER: The former chief nuclear regulator in the United States has delivered a damning verdict on the ability of Japanese authorities to stop contaminated groundwater from the crippled Fukushima nuclear plant from flowing into the sea.

Asked about comments by the Japanese prime minister that the situation at Fukushima was under control, Gregory Jaczko replied that the surging groundwater "was beyond human control".

Speaking to foreign journalists in Tokyo, Dr Jaczko warned that a planned underground ice wall around the site would also fail to stop the water becoming contaminated.

North Asia correspondent Mark Willacy reports from Tokyo.

MARK WILLACY: It was a last ditch guarantee from a prime minister fighting to win the Olympics for his country.

SHINZO ABE: Let me assure you the situation is under control.

MARK WILLACY: That was Japanese prime minister Shinzo Abe assuring the International Olympic Committee earlier this month that everything at the Fukushima nuclear plant was in hand. And the IOC seemed convinced. Hours later it awarded the 2020 Games to Tokyo.

Meanwhile, at the Fukushima nuclear plant, the equivalent of 130 Olympic swimming pools of highly contaminated water was sitting in storage pits and tanks at the site. One tank had even sprung a toxic leak.

But of more serious concern was and still is the water underneath the plant. It's estimated that at least 300 tonnes of contaminated groundwater flows under the Fukushima complex every day before seeping into the sea.

Speaking to foreign journalists in Tokyo, the former chairman of the United States Nuclear Regulatory Commission Gregory Jaczko was asked if the groundwater situation at Fukushima really was under control.

GREGORY JACZKO: What was unleashed was a force beyond human control. What you can do is try and mitigate that but you can't really control it. You cannot control groundwater.

MARK WILLACY: In a bid to stop this groundwater becoming contaminated the operator of the Fukushima nuclear plant, TEPCO, plans to build a giant underground wall of frozen soil stretching nearly three kilometres around the shattered reactor buildings. It's a massive project, costing nearly half a billion dollars.

But Washington's former chief nuclear regulator Gregory Jaczko seems to think it could all be a colossal waste of time and money.

GREGORY JACZKO: Whatever system you build is going, groundwater will find a way around it and into it and affect it. Any if you have homes, you probably have had leaks in your homes. I mean you, water is a terribly potent entity in that regard.

TIM PALMER: The former chief nuclear regulator from the United States, Dr Gregory Jaczko.