EFFORTS to make Japan’s quake and tsunami-hit nuclear plants safe and halt their radioactive fallout have taken a “very dangerous” turn, Australian experts say.

They say the threat to human health 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, work to cool the cores of the damaged plants has taken a backwards 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 sea water injection,” said Dr John Price, a former member of the Safety Policy Unit of the UK’s National Nuclear Corporation and former Professor at Monash University, now private consultant.

“That means they are now pumping sea water 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 understood 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 Mr Peter Burns, former chief executive of the Australian Radiation Protection and Nuclear Safety Agency.

“It will make it very difficult for operators to go in there and work because the radiation will get to very high levels and that’s why they are trying extreme measures … water bombing with helicopters.”

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

It was the venting of radioactive steam that was the major risk at the site, he said, and the plants had been spared a major explosion involving the more hazardous fission material inside the reactors or in spent fuel rods.

“The health risk to all people in the quake zone, and particularly if you consider beyond the 20km evacuation zone and beyond the 30km exclusion zone where people are being told to shelter, the risk is extremely small at the moment,” Mr Burns said.  “There have been no reports of actual large releases (of radiation) that has gone back inland.”

Professor Stephen Lincoln, an environmental chemist at the University of Adelaide, said the steam probably contained nitrogen 16, which is radioactive with a half-life of 10 minutes.

“This means that after 10 minutes the radioactivity is down to a half, after 20 minutes a quarter, after 30 minutes one eighth … and so on (leading to) no long-term environmental consequences,” Prof Lincoln said.

The fuel rods contain uranium 235 and possibly plutonium 239 and they could decay completely when exposed to air, allowing the “escape of dangerous radioactive fission products such as iodine 131 (half-life 8 days), cesium 137 (half-life 30 years) and strontium 90 (half-life 29 years)”.  The iodine accumulates in the thyroid where it can cause cancer, but this risk can be addressed by taking tablets.

Cesium accumulates in the body’s soft tissues while strontium accumulates in the teeth and bones, where it can cause cancer.

These cannot be easily removed from the body once exposed.