Japan hopes to stop pumping radioactive water into the sea today, which should help ease concerns in neighbouring China and South Korea over the spread of radiation from the worst nuclear crisis since Chernobyl.
But problems in restoring cooling systems at Japan's crippled nuclear plant, hit by a tsunami on March 11, mean more contaminated water may eventually be pumped into the sea if the complex again runs out of storage capacity.
Japan is struggling to regain control of the Fukushima Daiichi nuclear plant after a massive earthquake and tsunami devastated its northeast on March 11, and is facing a major humanitarian and economic crisis.
"There are still numerous aftershocks and there is no room for complacency regarding the situation (at Fukushima Daiichi)," Japan's Deputy Cabinet Secretary Tetsuro Fukuyama said.
Plant operator Tokyo Electric Power Co (TEPCO) said it was continuing to inject nitrogen into reactors to prevent another hydrogen explosion which would spread highly radioactive material into the air.
China and South Korea have criticised Japan's handling of the nuclear crisis, with Seoul calling it incompetent, reflecting growing international unease over the month-long atomic disaster and the spread of radiation.
Japanese voting in local elections on Sunday are expected to vent their anger over Prime Minister Naoto Kan's handling of the nuclear crisis, further weakening him and bolstering opponents who will try force his resignation once the crisis ends.
The unpopular Kan was already under pressure to step down before the worst disaster to hit Japan since World War Two, but analysts say he is unlikely to be dumped during the nuclear crisis, which is set to drag on for months.
Operator Tokyo Electric Power Co (TEPCO) apologised on Saturday over the crisis.
"I would like to apologise from my heart over the worries and troubles we are causing for society due to the release of radiological materials into the atmosphere and seawater," Sakae Muto, a TEPCO vice president, told a news conference.
Radiation from Japan spread around the entire northern hemisphere in the first two weeks of the nuclear crisis, according to the Vienna-based Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organisation.
Japan's economy, the world's third largest, is reeling from the triple disaster and several countries have banned or restricted food imports after detecting radiation.
China, one of Japan's major trading partners, has banned imports of farm products from 12 areas.
More critically, the nuclear crisis and power shortages have disrupted Japan's manufacturing and electronics global supply chains, hitting computer and automakers in particular.
Power blackouts and restrictions, factory shutdowns, and a sharp drop in tourists have hit the world's most indebted nation, which is facing a damages bill as high as $300 billion, making it by far the world's costliest natural disaster.
The government had called for restraint from Japanese to help the recovery effort, but families and friends were out in force for traditional "hanami" cherry blossom viewing parties, although some were toning down the usual alcohol-induced revelry in deference to the disaster victims.
"It's quieter than usual. There are lots of people but they're a bit subdued," said one middle-aged woman, strolling beneath the delicate pink blossoms. "The blossoms are in full bloom for us and we should appreciate them," she said.
An unmanned drone helicopter is scheduled to fly over four reactors to video damage and gauge radiation in areas where workers are unable to safely enter. Remote-controlled trucks will be used to remove some of the radioactive rubble.
Efforts to regain control of six reactors hit by the 15 metre high tsunami, which caused partial meltdowns to some reactor cores after fuel rods were overheated, has been hindered by 60,000 tonnes of radioactive water.
Workers used seawater to cool fuel rods and the tsunami left low-level radioactive water, which has been pumped back into the sea to make room for much more contaminated water from reactors.
"Some of the highly radioactive water will be moved within the plant. But a second and third solution needs to be discovered as water is being pumped in constantly, increasing the total amount," Fukuyama told local television on Sunday.
Japan's Nuclear Industry and Safety Authority said efforts to restore cooling systems were not making clear progress.
"We may be able to use (electric) systems that are currently functioning for cooling, and that may speed up the cooling restoration. But there is no concrete and clear option," said Hidehiko Nishiyama, a deputy director-general at the authority.
"It is one step forward, one step backwards."