Japanese nuclear crisis deepens

Efforts to make Japan's quake and tsunami-hit nuclear plants safe and halt their radioactive fallout have taken a "very dangerous" turn, experts say.

They say the threat to human health however, is still confined within the exclusion zone around the stricken plants, in the absence of a major explosion that could send large amounts of radioactive material into the atmosphere.

However, the operation to cool the cores of the damaged plants has taken a backward step with problems emerging at a fourth plant.

"The critical thing in this situation at the moment is that (Fukushima Daiichi nuclear plants) unit 1, 2 and 3 are under seawater injection," said Dr John Price, a former member of the Safety Policy Unit of UK's National Nuclear Corporation and former Professor at Monash University, now private consultant.

"That means they are now pumping seawater into those three units. If that keeps up those units are stable.

"And we go to unit 4 and the statement is much more obscure, the water injection was stopped."

It is believed that the problem at unit 4 is based not on cooling the shut-down reactor itself, but that a nearby storage pool, which holds spent but still highly radioactive fuel rods, was breached and so water kept draining out and exposing parts of the rods to the air.

"That's a very dangerous situation because the storage pond is in the building, but not within the containment vessel for the reactor, so any radiation released from there can go directly into the atmosphere," said Peter Burns, former chief executive of the Australian Radiation Protection and Nuclear Safety Agency.

The exposed fuel rods pose a much greater radiation risk to emergency crews working inside, and nearby the unit 4 sections, Burns said, but he added the risk to the Japanese populace outside the exclusion zone still remained "extremely small".

How do we protect ourselves from radiation?

Authorities are advising people within 30km of the nuclear plant to stay indoors, close windows and turn off air-conditioning systems, which will help stop the movement of air carrying radioactive particles into their homes.

Refraining from eating food or drinking water which may have been exposed to outside air is also being suggested. If it is necessary to go outside, people are advised to wear facemasks and an extra outer layer of clothes which can be disposed of before re-entering their homes, leaving radioactive particles outside.

What is the risk outside Japan?

While the reactor cores at the Fukushima power plants remain intact, there is no serious contamination risk outside Japan or any immediate danger in other countries. Currently, the serious effects can be confined to the 20km radius around the plant.

This will only change if one or more of the reactors melt down, which may cause plumes of radioactive dust to move offshore. This could ultimately affect countries around the North Pacific, including Russia, China, Canada and the US.