Japan's prime minister, Naoto Kan, will tell workers at the Fukushima Daiichi nuclear power plant to "fight with the conviction that you absolutely cannot lose", as highly radioactive water continued to seep into the sea.
The discovery by officials of a 20cm crack in the wall of a concrete pit could explain the continued flow of contaminated water that has jeopardised the operation to calm the reactors and raised fears about radiation levels in the sea and nearby soil.
Tokyo Electric Power (Tepco) said that it would pour concrete into the pit, where a radiation reading of 1,000 millisieverts per hour has been recorded, to try to seal the crack.
Workers have taken samples of the water in the pit and seawater to determine the level of contamination. Experts said that while the leak was a cause for concern, radiation would be quickly diluted in the ocean.
"With radiation levels rising in seawater next to the plant we have been trying to confirm why that's happening, and [the crack] could be one source," said Hidehiko Nishiyama, a spokesman for Japan's nuclear and industrial safety agency.
The plant, 240km (150 miles) north of Tokyo, will continue to leak radiation until four of its six reactors have been reconnected to cooling systems that were knocked out by the 11 March earthquake and tsunami. An artificial "floating island" is being towed to the plant to store contaminated water, samples of which have shown radiation levels 4,000 times the legal limit. The vast tanker could store about 10,000 tonnes, Tepco said; an estimated 13,000 tonnes of contaminated water has collected beneath some of the reactors.
"We are trying to employ as many measures as possible to regain control," a Tepco official said, adding that he had "high hopes" for the storage vessel.
Radiation levels in the plant and its vicinity have reached such high levels that Tepco is looking to hire special workers prepared to enter contaminated areas to perform essential tasks before rushing out to avoid prolonged exposure. In return for their bravery it is reported that the "jumpers" are being offered up to £3,000 a shift.
Kan has been to visit an evacuation centre in the coastal town of Rikuzentakata, which was engulfed by the tsunami. Most of its 23,000 residents were killed or injured. He later entered the 20km zone around the plant from which 70,000 people have been evacuated. He told Tepco workers, troops and firefighters : ''I want you to fight with the conviction that you absolutely cannot lose this battle."
Police said that more than 11,800 people had been confirmed dead in the disaster, while more than 15,540 people remained missing. More than 165,000 people are living in shelters.