Workers are using a milky white dye as they frantically try to trace the path of highly radioactive water gushing from a tsunami-damaged Japanese nuclear plant and leaking into the ocean.

Over the weekend, workers at the Fukushima Daiichi plant found a crack in a maintenance pit, the latest confirmation that radioactivity continues to spill into the environment.

The leak underscores the chicken-and-egg nature of the containment efforts at the stricken plant: the radioactive risk will be high as long as the cooling system is out, but the cooling system won't work until the radioactive water leaks are stopped long enough for workers to start it.

'We must keep putting water into the reactors to cool to prevent further fuel damage, even though we know that there is a side-effect, which is the leakage.' — Hidehiko Nishiyama, Nuclear Safety and Industrial Agency

The plant's operators also deliberately dumped 10,000 tonnes of tainted water into the ocean Monday to make space for more highly radioactive water. The dumped water is 500 times the legal limit for radioactivity.

Efforts to plug the leak took an unconventional turn Monday as officials tried to plug a suspected leak with concrete and then a mixture of polymer, sawdust and shredded newspaper, closer to where they believed the source was.

The failure of these efforts appeared to signal that officials were targeting the wrong channel to the maintenance pit. The white dye has also yet to hit the ocean in any clear way.

"There could be other possible passages that the water may be travelling," Hidehiko Nishiyama, a spokesman for the Nuclear Safety and Industrial Agency, said. "We must watch carefully and contain it as quickly as possible."

Years of work ahead

Government officials conceded Sunday that it will likely be several months before the cooling systems are completely restored.

And even after that happens, there will be years of work ahead to clean up the area around the complex and figure out what to do with it.

Danger minimal: experts

Radioactive water escaping from the devastated Fukushima Daiichi nuclear plant shouldn't be a big problem for sea animals or the people who might eat them, experts in the United States say.

The ocean strongly dilutes the concentrations of radioactive materials. One expert said readings have shown a 1,000-fold drop by the time the materials got about 30 kilometres offshore.

Experts said the discharges might pose some hazard for sea creatures that live very close to the plant, but that those farther away should not be at significant risk.

However, a government spokesman said Monday the sheer volume of contamination is becoming a concern. "Even if they say the contamination will be diluted in the ocean, the longer this continues, the more radioactive particles will be released and the greater the impact on the ocean," Chief Cabinet Secretary Yukio Edano said.

Until all the pools of contaminated water are pumped to storage tanks and the cooling system restored, the makeshift methods of pumping water into the reactors and allowing it to gush out wherever it can are the only ways to bring down temperatures and pressure in the reactor cores, where fuel rods continue to produce massive amounts of heat even though nuclear reactions have stopped.

"We must keep putting water into the reactors to cool to prevent further fuel damage, even though we know that there is a side-effect, which is the leakage," Nishiyama said.

"We want to get rid of the stagnant water and decontaminate the place so that we can return to our primary task to restore the sustainable cooling capacity as quickly as possible."

That makeshift system also complicates plant operator Tokyo Electric Power Co.'s other goal: containing the spread of radiation.

Radioactivity has spewed from the plant since March 11, when a 9.0 magnitude earthquake spawned a massive tsunami that decimated large swaths of Japan's northeastern coast. Up to 25,000 people are believed to have died in the disaster, and tens of thousands lost their homes. Thousands more were forced to flee a 20-kilometre radius around the plant because of the radiation.

Crack in maintenance pit

Over the weekend, a 20-centimetre-long crack was discovered in a maintenance pit, sending a stream of water into the sea. The area is normally blocked off by a seawall, but a crack was also discovered in that outer barrier Monday.

A man is screened for radiation before entering an evacuation centre in Fukushima on Monday.A man is screened for radiation before entering an evacuation centre in Fukushima on Monday. Damir Sagolj/Reuters The operator said Monday it is ordering fencing that is typically used to contain oil spills. The screens are not designed to trap radioactivity but might curtail the flow of water and reduce the spread of contamination, said TEPCO manager Teruaki Kobayashi.

It was not clear when they would arrive.

Before restoring the cooling system, workers must rid the plant of the pools of radioactive water that have collected under each of the three troubled reactors' turbine buildings and spilled into various trenches around the complex. TEPCO has proposed pumping it into tankers, barges and is now considering sending it to a storage facility on site.

Work on those problems continues to make progress, even as workers try to stop the latest leak, Nishiyama said.

"We have to apply stopgap measures to day-to-day problems, like the pit water leakage, but we are continuing on our effort to achieve the goal," he said.

With files from The Associated Press