A NEW explosion and fuel rod exposure at Japan's stricken Fukushima No.1 nuclear power plant have engineers racing to avert a reactor meltdown.

What does the latest explosion mean for the reactor?

The blast at Unit 2 this morning AEDT follows hydrogen explosions at Unit 1 on Saturday and Unit 3 on Sunday. Water levels have dropped precipitously inside Unit 2, twice leaving the uranium fuel rods completely exposed and raising the threat of a meltdown. The plant's owner, Tokyo Electric Power Company (TEPO), said the explosion occurred near the suppression pool in the reactor's containment vessel. The company had just minutes before revealed that the containment vessel had been damaged and warned that there could possibly be serious radiation leaks. It later said the fact that the radiation level had not jumped, indicated the vessel had not sustained damage such as a hole. There is a fire burning at Unit 4 at the plant, officials confirmed, however a Japanese minister said there were no fuel rods contained in that location. There are six units at the plant. Radiation levels have risen considerably, Japanese Prime Minister Naoto Kan says. Mr Kan has told people within 30km of the plant to stay indoors.



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How much damage has been done?

No one knows how much damage has been done to the fuel rods.

Has there been a meltdown?

Officials have called the situation a partial meltdown, or "fuel-rod meltdown", because they have detected minute quantities of radioactive cesium and iodine - by-products of the nuclear fission that powers the reactor - outside the plant. This suggests that some of the metal casing enclosing the reactors' uranium fuel has melted. There is no indication that the uranium fuel has melted or indication of a "China Syndrome", where the fuel melts, gathers below the reactor, melts everything in its way and bores a path deep into the Earth.

How does a reactor shut down and what happens in a meltdown?

The operating reactors at Fukushima Daiichi power station automatically shut down during the earthquake. But after subsequent cooling failures, two of them went into partial meltdown. More at The New York Times here.

Is a nuclear reactor meltdown a catastrophic event?

According to the American Nuclear Society, not necessarily. It says that nuclear reactors are built with redundant safety systems and that even if the fuel in the reactor melts, "the reactor's containment systems are designed to prevent the spread of radioactivity into the environment. Should an event like this occur, containing the radioactive materials could actually be considered a 'success'" given that plant was not designed to withstand the combined forced of an earthquake and tsunami.

What is the likelihood of a nuclear explosion?

Zero. A nuclear bomb and a nuclear reactor are different things.

What type of radioactive material has escaped?

There are reports of radioactive isotopes of caesium and iodine in the vicinity of the plant. Experts say it's like radioactive isotopes of nitrogen and argon have escaped as well. There is no evidence that any uranium or plutonium has escaped.

What harm do these radioactive materials cause?

Radioactive iodine could be harmful to those living near the plant but can be treated with iodine tablets. Radioactive caesium, uranium and plutonium are all harmful but radioactive nitrogen decays within seconds of its release and argon poses no threat to health.

How did the radioactive materials escape?

When steam in the power plants built up to dangerous levels, small amounts were vented.

How long will any contamination last?

Radioactive iodine decays quite quickly. Most will have disappeared within a month. Radioactive caesium does not last long in the body - most has gone within a year. However, it lingers in the environment and can continue to present a risk.

What happens if there is a nuclear fallout?



Authorities have a several-pronged strategy for shielding civilians if there is an explosive breach of the reactor, but the three main weapons against contamination are: evacuation, confinement and iodine.

Evacuation: About 200,000 people have already been evacuated from the residential areas around Fukushima.

Confinement: If evacuation to a safer area is not an option, then the best strategy is confinement. This means taking shelter in an enclosed space, preferably a basement room with doors and windows sealed tight with plastic sheets and adhesive tape. This is to prevent radioactive dust from entering the lungs and the digestive tract.

Those in the danger area should discard their clothes and shoes and shower off any contact between the fallout and the skin. They should avoid scrubbing the skin, nail-biting, smoking and sucking or licking their fingers.

Iodine: In a nuclear alert, authorities hand out iodine pills to prevent cancers of the thyroid, which is a particular risk for babies, young children, teenagers and expectant or breastfeeding mothers.

The goal is to saturate the thyroid with "healthy iodine", shielding it from radioactive iodine, said Mr Gourmelon.

Prompt action is essential. The iodine should preferably be taken an hour before a known fallout incident. Japanese guidelines say the pills should be distributed when the likely absorbed dose of radioactivity is 100 milligray.

What are the health effects of radiation exposure?

Those exposed to even moderate amounts of radiation can expect to suffer sickness and vomiting within the first few hours, followed by diarrhoea, headaches and fever.

More serious damage - such as widespread and potentially fatal damage to internal organs - may become apparent after a few weeks.

These symptoms become immediately apparent after exposure to high levels of radiation.

After the 1986 Chernobyl nuclear disaster there were some cases of thyroid cancer and large numbers of birth defects.

How do you treat radiation sickness?

First, by gently washing off any contact between the fallout and the skin - scrubbing increases the risk of particles penetrating the skin.

Second, taking special drugs to increase white blood-cell production with drugs to counter bone marrow damage and to reduce the risk of further infections due to immune-system damage.

There are specific drugs that treat organ damage.

On what scale is the Fukushima crisis?

Andre-Claude Lacoste, head of France's Nuclear Safety Authority, says Fukushima is "worse than Three Mile Island but not as great as Chernobyl". Japan's nuclear safety agency had rated the damaged nuclear plant at four on an International Nuclear Event Scale of zero to seven. By comparison, the 1979 Three Mile Island accident in the US was a five while the 1986 Chernobyl disaster in the Ukraine was a seven. However, Mr Lacoste said: "We have the feeling that we are at least more than level five and probably at level six. I say this after speaking to my Japanese counterparts."

Is there a danger of Fukushima becoming a Chernobyl-like disaster?

The combined impact of the earthquake and tsunami has been many times greater than any disaster envisaged at the Japanese plan but the Soviet reactor was a poor design and the technicians involved had much less understanding of the proper processes to follow. Yukiya Amano, the head of the UN's atomic watchdog, the International Atomic Energy Agency, has said that "the possibility that the development of this accident into one like Chernobyl is very unlikely".

Could Japan have done more to prevent the disaster?

The mistakes at Fukushima were made long before Friday's earthquake. According to The New York Times, the diesel generators intended to provide back-up power for the pumps that cooled the reactor core failed because they were built behind a sea wall on low-lying coastal ground. The tsunami overwhelmed the barrier. Once the generators went out, the back-up batteries could not last for long and, as in Ukraine and Pennsylvania, the water level in the reactor started to fall.

According to the Japanese nuclear safety agency, at one stage half the length of each 3.6m long fuel rod in the Fukushima No 1 plant was out of the water. Even after a nuclear chain reaction is stopped, a reactor continues to emit heat, which has to be cooled by water - the ocean was the only available source to Japanese engineers.

To reduce the temperature of the reactor core, TEPCO started injecting sea water (reportedly using fire engines) mixed with the chemical element boron, which can squelch a nuclear reaction. In the process TEPCO accepted that the corrosive sea water will permanently disable the plant's core.

In all, there are six units within the Fukushima No 1 plant . The reactors for Units 1, 2, and 6 were supplied by General Electric, those for Units 3 and 5 by Toshiba, and Unit 4 by Hitachi. All six reactors were designed by General Electric.

Doesn't Japan have a record of understating damage at nuclear power plants?

The Guardian website reports that the nuclear safety experts believe that the official information being issued about the Fukushima nuclear accident follows a pattern of secrecy and cover-ups employed in other nuclear accidents.

John Large, an independent nuclear engineer who has worked for the UK government, told the site: "The actions of the Japanese government are completely contrary to their words. They have evacuated 180,000 people but say there is no radiation. They are certain to have readings but we are being told nothing."

The site also posted diplomatic cable obtained by WikiLeaks that states "a high-profile Japanese politician told US diplomats that the Japanese government department responsible for nuclear energy - has been covering up nuclear accidents and obscuring the true costs and problems associated with the nuclear industry.

A senior nuclear industry executive in the US told The New York Times that Japan's power industry managers are "basically in a full-scale panic. They're in total disarray, they don't know what to do."

Tilman Ruff, associate professor in the University of Melbourne's Nossal Institute for Global Health and chairman of the international campaign to abolish nuclear weapons, says companies running nuclear power plants in Japan have covered up potentially dangerous problems in the past.

Leaving aside the latest accident, Professor Ruff says the increased threat of nuclear proliferation and problems disposing of waste are good reasons to avoid using nuclear power. "In terms of power generation . . . it's way too dangerous to be a safe or sustainable way to boil water," he says.

What went wrong at Chernobyl?

The four reactors at the Chernobyl power station produced electricity by heating water to steam that turned a generator turbine. Soviet reactors were badly designed and the reactor did not have the big containment walls common in Western reactors.

The reactor was designed to slow down if it overheated but an experiment to test this went badly wrong. The emergency shutdown procedure failed, the reactor fuel ruptured and a violent explosion of steam and gas blew the 1000-tonne roof off the reactor building.

That was followed by a second explosion. Temperatures reached over 2000C and the fuel rods melted. The graphite covering of the reactor caught fire and burned for 10 days, spewing vast amounts of radioactive material into the air.

In all, 57 people died as an immediate result of the explosions.

Nuclear regulators estimate that about 4000 people, most of them children, developed thyroid cancer as a result of their exposure to radiation and, by the end of 2002, 15 of them had died.

Doctors estimate a further 4000 people in the areas with highest radiation levels could eventually die of cancer caused by radiation exposure. About 6.8 million other people living farther from the explosion received a much lower dose of radiation and it has been estimated 5000 of them could die from illness caused by that exposure.

What happened at Three Mile Island?

On March 28, 1979, feedwater pumps that cooled the reactor on Three Mile Island in the US failed and the reactor automatically shut down. This led to a pressure increase that triggered a safety valve automatically opening. But the valve, designed to close when the pressure dropped, failed. A second system failure meant plant operators thought everything was working properly.

Cooling water continued to pour out of the valve, causing the core of the reactor to overheat. Alarms sounded across the system, alerting operators to contradictory problems, but with no evidence that the temperature of the nuclear core was compromised they reduced the coolant flow to it, making the situation worse.

Without the required cooling, the tubes holding nuclear pellets ruptured and the fuel itself began to melt, resulting in a severe core meltdown. This could have led to a breach of the walls containing the reactor and a large release of radiation into the atmosphere, but the defences held.

This first stage of the crisis occurred across 12 hours or so and, although there was a small release of radioactive gas in the late morning, by evening the core had cooled and the reactor seemed stable.

However, on March 30, repair measures led to a second problem: radioactive gases from the reactor cooling system had built up in a tank in the auxiliary building and plant operators were pumping it to tanks designed to allow it to decay. But the compressors pumping the gas leaked and radiation was released into the atmosphere. The news generated panic in Harrisburg, Pennsylvania, the closest city to the plant.

The crisis finally concluded at the end of April when the reactor core cooled and was shut down.

There were no fatalities and the World Nuclear Association claims the amount of radiation released was a fraction of that used in chest X-rays.