More than 100,000 people have been evacuated from homes near Fukushima
There have been two explosions at the Fukushima Daiichi nuclear power plant in Japan, following Friday's earthquake and tsunami, and a third reactor is reportedly at risk of fuel-rod meltdown.
How great a danger do these problems pose for people in Japan and further afield?
Has there been a leakage of radioactive material?
Yes. Local government officials in Fukushima say 190 people have been exposed to some radiation. An American warship, the USS Ronald Reagan, has detected low levels of radiation at a distance of 100 miles (161km) from the Fukushima plant.
How much radioactive material has escaped?
The Japanese authorities say only low levels of radiation have been detected outside the plant. The International Atomic Energy Agency has described it as a level four event on the International Nuclear and Radiological Event Scale (INES), which is used for an accident "with local consequences". No abnormal levels of radiation have been detected in Russia.
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 were promptly issued with iodine tablets ought to be safe. Radioactive caesium, uranium and plutonium are harmful, but do not target any particular organ of the body. Radioactive nitrogen decays within seconds of its release, and argon poses no threat to health.
How did the radioactive materials escape?
There have been problems with cooling systems, causing the reactors to overheat. Production of steam has caused pressure to build up inside the reactor, so small amounts of steam have been deliberately released. Experts say that the presence in the steam of caesium and iodine - which are among the by-products of nuclear fission - suggests that the metal casing of some of the fuel rods has melted or broken. But the uranium fuel itself has a very high melting point so it is less likely to have melted, let alone vapourised.
Could radioactive materials have escaped by any other means?
The authorities have pumped sea water into three reactors. This water will be 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. As noted above, the reported detection of radioactive caesium and iodine may indicate that some of the metal casing enclosing the reactors' uranium fuel has 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. Reports suggest that underneath the reactor, within the outer containment vessel, there is a concrete basin designed to capture and disperse any molten fuel.
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 taken place outside the steel and concrete containment vessels enclosing the reactors, which apparently remain solid. 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 - caused by hydrogen and oxygen vented from the reactor - have damaged only the roof and walls erected around the containment vessels.
Could there be a nuclear explosion?
No. A nuclear bomb and a nuclear reactor are different things.
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 from the atmosphere. The tablets fill the body up with non-radioactive iodine, which prevent it absorbing the radioactive iodine.
What kind of radiation levels have been recorded at Fukushima?
The Kyodo news agency reports that a radiation level of 1,557 microsieverts per hour was registered on Sunday. At this level, one hour's exposure is roughly equivalent to one chest X-ray. Later measurements included 750 microsieverts per hour at 0200 on Monday, and 20 microsieverts per hour at 1145. The last of these measurements is not much to worry about - on a long-haul flight passengers are exposed to about five microsieverts per hour. Furthermore, moving away from the source of radiation, measurements would quickly tail off. Five or 10km away from the plant, the radiation level would be significantly lower.
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.
How do Fukushima's problems affect the rest of the world?
It depends on how much radiation is released. At present, the IAEA says the effects are of a "local" nature.