Desperate attempts are being made to cool a Japanese nuclear power plant crippled by the country's huge earthquake and tsunami - but indications are that one of its uranium reactors has at least partially melted.

And last night, a top Government official warned that a meltdown in the third of the plant's three reactors was "highly possible".

Radioactive caesium, one of the elements released when overheating damages the core of a nuclear reactor, was detected around the Fukushima Daiichi power station, 270km northeast of Tokyo.

This indicated that a meltdown, caused by a nuclear reaction running out of control, had affected the fuel rods in the power station's No 1 reactor, although possibly only to a limited extent.

The prospect of a reactor meltdown overshadowed the wider tsunami disaster and led to 170,000 residents being evacuated from within a 20km radius of the nuclear plant.

A further 30,000 were being evacuated from within 10km around the nearby Fukushima Daini nuclear power plant.

Government officials revealed plans to distribute iodine tablets, a treatment for radiation poisoning, to locals.

Fukushima Daiichi, run by the Tokyo Electric Power Company, is one of 17 nuclear power plants in Japan, the world's third-largest user of nuclear power.

The Japanese Government played down fears of radiation leakage, although nuclear agency spokesman Shinji Kinjo acknowledged there were still fears of a meltdown.

University of Auckland nuclear physicist Dr David Krofcheck said yesterday that the detection of "finger-print" radiation from caesium indicated some sort of core melting in the reactor.

Fears of nuclear poisoning were raised when a huge explosion rocked the plant on Saturday, following damage to the No 1 reactor on Friday.

A pall of white smoke rose over the plant and four workers were injured.

Chief Cabinet Secretary Yukio Edano gave the warning of a meltdown in the No 3 reactor.

Asked whether a partial meltdown had occurred, he said that "because it's inside the reactor, we cannot directly check it".

But officials were "taking measures" on the assumption" that it had.

Mr Edano said mechanical problems with the pump delayed the injection of cooling water into the reactor, leaving the fuel rods briefly exposed and raising the risk of a nuclear meltdown and an explosion.

Before the first explosion, operators had detected eight times the normal radiation levels outside the plant and 1000 times normal inside the No 1 reactor's control room.

Mr Edano said radiation around the plant and pressure in the damaged reactor had decreased after the blast.

The explosion was caused by a "hydrogen bubble", vented from the reactor core, reacting with oxygen in the air or water.

This blew apart the structure housing the reactor, but not the reactor's containment chamber, a 15cm-thick stainless-steel vessel that envelopes the metal-encased uranium fuel rods and the control rods.

The Fukushima Daiichi accident is rated at level four - "accident with local consequences" - on the seven-point scale of the International Atomic Energy Agency, making it the sixth most serious nuclear accident recorded.

Friday's earthquake was the seventh most powerful on record, and the resulting tsunami was the most destructive wave since the 2004 Boxing Day tragedy.

The earthquake and tsunami severed the electricity supply at the power plant, crippling its cooling system. The diesel generator back-up supply did not kick in properly at Unit 1.

Without power, and possibly because of tsunami damage to water pumps, the cooling system was inoperable.

To prevent the stainless-steel container exploding under increasing heat and pressure, some of the steam build-up was vented.

But the temperature in the core apparently kept rising, overheating the zirconium casings of the fuel rods. Once the zirconium reached 1200C, it reacted with the water, releasing zirconium oxide and hydrogen.

Authorities have resorted to injecting seawater and boron into the reactor core in an attempt to control the heating, which continues because of the presence of the radioactive byproducts, despite the nuclear reaction of the uranium fuel having been shut down.

Experts consider this a desperate measure and expect it to continue non-stop for days.

If the temperature inside the reactor continues to rise and reaches roughly 2200C, the uranium fuel pellets will start to melt.

From there, melted fuel will eat through the bottom of the reactor vessel, then the floor of the damaged containment building.

At that point, the uranium and radioactive byproducts of the nuclear reactions will start escaping into the environment.

At some point, the walls of the reactor vessel will melt into a lava-like pile, slump into any remaining water on the floor, and could cause an explosion much bigger than the one caused by the hydrogen, enhancing the spread of radioactive contaminants.

American nuclear expert Peter Bradford said that if the reactor core became exposed to the external environment, officials would probably start pouring cement and sand over the entire facility, as was done at Chernobyl (see graphic on A3).

In Auckland, Dr Krofcheck said that if the Fukushima Daiichi accident became a meltdown and released large amounts of radiation, "I'm sure it would not be a major problem for New Zealand. Most of it would be confined to the Northern Hemisphere, and most certainly, Japan itself."

This was because there was little mixing of winds between the Northern and Southern Hemispheres, he said.

Japan's nuclear industry has a tarnished image. Britain's Observer News Service reports that in 2002, the president of Japan's largest power utility was forced to resign after he and other senior officials were suspected of falsifying plant safety records.

The Sunday Telegraph said the Tokyo Electric Power Company had had to stop operations at five of its reactors, including the one damaged in the latest earthquake, for safety reasons.

"We know that they have lied in the past," one local man told the paper. "They may be wanting to stop panic now."