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There have been a number of explosions and fires at the Fukushima Daiichi nuclear power plant in Japan, following Friday's earthquake and tsunami.
Four of the plant's six reactors have been in trouble. How great a danger do these problems pose for people in Japan and further afield?
Has there been a leakage of radioactive material?
Yes. Harmful levels of radiation have caused at least one temporary evacuation of staff at the power plant. Higher than normal, but harmless, levels of radiation have been registered in Tokyo, 140 miles (220km) away.
How much radioactive material has been released?
The World Health Organization's representative in China says there is no evidence of any significant international spread of radiation.
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 would be natural for radioactive isotopes of nitrogen and argon to 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 young people living near the plant. After the 1986 Chernobyl nuclear disaster there were some cases of thyroid cancer as a result. However, people who are promptly issued with iodine tablets ought to be safe. Radioactive caesium accumulates in soft tissue, while plutonium accumulates in the bone and liver. Radioactive nitrogen decays within seconds of its release, and argon poses no threat to health.
How did the radioactive materials escape?
In at least two ways. Some is known to have escaped as a result of steam and gas released from overheating reactors. There has also been a release from the fourth reactor's fuel storage pond, which was damaged in an explosion, and caught fire.
Could radioactive materials have escaped by any other means?
The authorities have pumped seawater into three reactors. This water is likely to have been contaminated by its passage through the reactor, but it is currently unclear whether any of it has been released into the environment.
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 problem for many years.
Has there been a meltdown?
The term "meltdown" is used in a variety of ways. Some of the metal encasing fuel rods have been damaged by heat, and may have partially melted (a "fuel-rod meltdown"). However, there is as yet no indication that the uranium fuel itself has melted. Still less is there any indication of a "China Syndrome" where the fuel melts, gathers below the reactor and resumes a chain reaction, that enables it to melt everything in its way, and bore a path deep into the earth. If there were to be a serious meltdown, the Japanese reactor is supposed to be able to handle it, preventing the China Syndrome from taking place.
Could there be a Chernobyl-like disaster?
Experts say this is highly unlikely. The chain reaction at all Fukushima reactors has ceased. The explosions that have occurred have mostly taken place outside the steel and concrete containment vessels enclosing the reactors. At Chernobyl an explosion exposed the core of the reactor to the air, and a fire raged for days sending its contents in a plume up into the atmosphere. At Fukushima the explosions have damaged mainly the roof and walls erected around the containment vessels - though it is feared the steel and concrete containment vessel of reactor number two has been damaged. Even if a reactor at Fukushima were to explode - according to the UK government's chief scientific adviser - it would send radioactive material only 500m into the air (rather than 9,000m) and the fallout would be concentrated within 20km or 30km of the site.
Could there be a nuclear explosion?
The explosions so far have been caused by hydrogen released from the reactors. At Chernobyl there may have been a nuclear explosion in the reactor, but this has not been confirmed.
What caused the hydrogen release from the reactor?
At high temperatures, steam can separate into hydrogen and oxygen in the presence of zirconium, the metal used for encasing the reactor fuel. This mixture is highly explosive.
How do iodine tablets work?
If the body has all the iodine it needs, it will not absorb further iodine. The tablets fill the body up with non-radioactive iodine, which prevent it absorbing radioactive iodine from contaminated milk, or other food sources.
What kind of radiation levels have been recorded at Fukushima?
Levels as high as 400 millisieverts per hour have been registered at the plant itself. A couple of hours exposed to this dose-level could cause radiation sickness. However, for long periods since the crisis began, the level has been at 10 millisieverts per hour or lower. (A spinal X-ray delivers roughly one millisievert of radiation, a CT scan of the abdomen and pelvis has an effective dose of 15 millisieverts.) On Monday morning the level was as low as 0.02 millisieverts per hour - only a few times more intense than the level of radiation experienced on a passenger jet flying at 40,000 feet.
Is any level of exposure to radiation safe?
In some parts of the world, natural background radiation is significantly higher than others - for example in Cornwall, in south-west England. And yet people live in Cornwall, and many others gladly visit the area. Similarly, every international air flight exposes passengers to higher than normal levels of radiation - and yet people still fly, and cabin crews spend large amounts of time exposed to this radiation. Patients in hospitals regularly undergo X-rays. Scientists dispute whether any level of exposure to radiation is entirely safe, but exposure to some level of radiation - whether at normal background levels or higher - is a fact of life.