Some workers at its No 2 reactor, the site of yesterday's second blast, were evacuated, but officials sought to reassure the public that the explosions posed no immediate health threat, and that the potential reactor meltdown that has been feared would still be averted. The latest flurry of incidents has nonetheless added to an already tense mood and raised concerns about the ability of the authorities to take a proactive grip of a fast-moving situation.

In an indication of the concern about the possible threat from radiation, US authorities relocated vessels from the 7th Fleet operating in Japanese waters off the coast from Fukushima after detecting low-level radiation on 17 crew members who have been taking part in relief operations. In Sendai, the nearest large city to the epicentre of the earthquake, news of the nuclear setbacks came with fresh warnings about further possible tremors and another tsunami wave. Yesterday saw a number of after-shocks, including one that measured 6.2 on the Richter scale.

On the city's wind-whipped streets – where people scavenged for food and supplies from one emptied convenience store to the next – anxiety was universal. "We thought our building was going to fall. Yes, we are worried; we are getting away," said one woman, Tia, who added that she and her partner were leaving for Hokkaido island.

For the authorities – still trying to reach many villages and towns which were destroyed by the quake and tsunami, while also helping hundreds of thousands of people now forced from their homes – the greatest concern is what is playing out in a dark piece of theatre at the badly damaged Fukushima nuclear complex, located 150 miles north of Tokyo.

The 40-year-old complex has now seen three hydrogen explosions – one at each of its reactors – in the past four days, with plumes of smoke visible in the sky for miles around.

Yesterday afternoon, the Jiji news agency said fuel rods at the complex's No 2 reactor had been entirely exposed for around two and a half hours and a fuel rod meltdown – which could cause a major release of radioactive material – could not be ruled out. The plant operator confirmed that there was little water left in the reactor. Then last night it was reported that the rods, which had been at least partially covered by sea water in an effort to cool them, were once again exposed. Although the latest hydrogen blast, inside reactor No 2, is not the full-blown catastrophe feared, the plant operator said that radiation levels in the air around the reactor had risen fourfold after the explosion.

There have been previous reports of possible partial meltdowns of the rods at the No 1 and No 3 reactors, though experts said it was impossible to verify such claims at this stage. "Although we cannot directly check it, it's highly likely to be happening," said the Chief Cabinet Secretary, Yukio Edano.

Most nuclear experts have so far rejected fears of a nuclear disaster and radioactivity leak on the scale of Chernobyl, in 1986, or the 1979 catastrophe at Three Mile Island in Pennsylvania. Crucially, officials in Japan said the thick walls around the radioactive cores of the damaged reactors appeared to be intact after the earlier hydrogen blasts.

Mr Edano said that the internal containment apparatus which surrounds the No 3 reactor was intact, even if the outer building appeared to have been destroyed by the blast. Yet in the aftermath of the explosion, the government told those still present within the 13-mile evacuation zone to stay indoors. It also revealed that 11 people had been injured by the blast.

Yesterday it was reported that another 80,000 people had been evacuated from the disaster area, adding to around 450,000 who have already been told to leave their homes. While some people are staying with friends and relatives, many are making do in a network of emergency shelters that have been erected. In Sendai, large numbers of citizens have been bedding down on the floors of the headquarters of the municipal authority.

Along with trying to deal with the aftermath of the quake, which was yesterday upgraded from 8.9 to 9.0 on the Richter scale, making it one and a half times greater in magnitude than previously thought, many people are also searching for friends and relatives of whom there has been no word since last Friday. "My girlfriend's mother is missing. She is from Matsushima [a cluster of islands off the coast from Sendai] and we are looking for her," said Yuchiro Hori, who sells advertising space in a magazine. "We are looking for information on the internet and on the phone because we cannot go there – it's very dangerous."