Latest from the BBC on the Fukushima nuclear power plant disaster:
Timeline: Nuclear plant accidents
Nuclear power may be considered by some as a "viable" fuel source for the future, but anti-nuclear campaigners point to the industry's safety record around the world as a reason for caution.
The BBC News website lists a selection of accidents at nuclear plants over the years:
29 September 1957(INES Level 6)
Mayak or Kyshtym nuclear complex (Soviet Union): A fault in the cooling system at the nuclear complex, near Chelyabinsk, results in a chemical explosion and the release of an estimated 70 to 80 tonnes of radioactive materials into the air. Thousands of people are exposed to radiation and thousands more are evacuated from their homes. It is categorised as Level 6 on the seven-point International Nuclear Events Scale (INES).
7 October 1957(INES Level 5)
Windscale nuclear reactor (UK): A fire in the graphite-cooled reactor, in Cumbria, results in a limited release of radioactivity (INES Level 5). The sale of milk from nearby farms is banned for a month. The reactor is unsalvageable and buried in concrete. A second reactor on the site is also shut down and the site decontaminated. Subsequently part of the site is renamed Sellafield and new nuclear reactors are built.
3 January 1961
Idaho National Engineering Laboratory (USA). A steam explosion in reactor SL-1 during preparation for start-up destroys the small US Army experimental reactor and kills three operators.
29 March 1979(INES Level 5)
Three Mile Island nuclear power plant, Pennsylvania (US). A cooling malfunction causes a partial meltdown in one reactor, resulting in a limited release of radioactivity (INES Level 5).
The site's first reactor (TMI One) on the Susquehanna river was closed for refuelling. The second was at full capacity when two malfunctions occurred: first there was a release of radioactive water, then radioactive gas was detected on the perimeter. No deaths or injuries were reported.
It is considered the United States' worst nuclear accident and led to major safety changes in the industry.
26 April 1986 (INES Level 7)
Chernobyl nuclear power plant (Soviet Union). One of four reactors explodes after an experiment at the power plant (INES Level 7). The resulting fire burns for nine days and at least 100 times more radiation than the atom bombs dropped on Nagasaki and Hiroshima is released into the air. Radioactive deposits are found in nearly every country in the northern hemisphere.
Two people die in the explosion and another 47 from Acute Radiation Sickness. Thousands of extra cancer deaths are expected as a resulted of the disaster. A huge cover, known as the New Safe Confinement, is due to be erected over the existing sarcophagus covering the site some time after 2008.
6 April 1993
Severesk, formerly Tomsk-7 (Soviet Union). A tank at a uranium and plutonium factory inside the plant explodes, resulting in radioactivity being dispersed into the atmosphere contaminating an area of over 120 sq km (INES Level 4). A number of villages are evacuated and left permanently uninhabitable.
30 September 1999
The Tokaimura accident shook confidence in the industry in Japan
Tokaimura nuclear fuel processing facility (Japan). Workers break safety regulations by mixing dangerously large amounts of treated uranium in metal buckets, setting off a nuclear reaction (INES Level 4). Two of the workers later die from their injuries, and more than 40 others are treated for exposure to high levels of radiation.
9 August 2004
Mihama nuclear power plant (Japan). Five people die in an accident at the plant in the Fukui province (INES Level 1). Seven people are also injured when hot water and steam leaks from a broken pipe.
Officials insist that no radiation leaked from the plant, and there is no danger to the surrounding area.
11 March 2011(INES Level 5)
Fukushima nuclear power plant
By Richard Black
Environment correspondent, BBC News
As it was almost bound to do at some point, Japan's nuclear safety agency has uprated its assessment of the Fukushima power station incident from a level four to a level five.
These are categories on the International Nuclear and Radiological Events Scale (INES), which runs from zero (nothing happened, essentially) to seven, a "major accident".
So far, Chernobyl is the only seven-rated incident in nuclear history.
Level five is defined as an "accident with wider consequences".
So what is the worst-case scenario for those "wider consequences" at Fukushima?
What clues are there either from that level five rating, or from the situation on the ground, as to how things might transpire - whether it will in the end prove to have been a disaster or a distraction from the serious and widespread impact of the tsunami?
"The worst-case scenario would be where you have the fission products in stored canisters or in the reactors being released," said Professor Malcolm Sperrin, director of medical physics and clinical engineering at Royal Berkshire Hospital, UK.
"Radiation levels would then be very high around the plant, which is not to say they'd reach the general public.
"And we're definitely not in the situation where we're going to see another Chernobyl - that possibility has long gone."
Read more here.
Latest News:
Workers are close to restoring power to cooling systems at a quake-hit Japanese nuclear power plant, officials say.
Engineers connected a cable through which they hope to supply electricity to part of the Fukushima Daiichi plant.
The earthquake and tsunami it triggered crippled the plant's cooling systems, and some radiation has leaked.
At the stricken plant, firefighters have continued to spray water at the dangerously overheated fuel rods, in a desperate attempt to avert a meltdown.
Engineers hope that restoring power will allow them to restart pumps to continue the cooling process.
Japan's nuclear safety agency earlier hoped that electricity would be restored on Saturday but later revised its projection.
"If no problem is found at the facility today, the power will resume as early as tomorrow [Sunday], " agency spokesman Fumiaki Hayakawa is quoted by the AFP news agency as saying.
However, an official from the power company which runs the plant told AFP: "Although we are doing our best, unfortunately we cannot say when electricity will be restored."
Given the scale of the damage, it is not certain the cooling systems will work even if power is restored. Workers are also boring holes in roofs at the plant to prevent a potential gas explosion.
Timeline: Nuclear plant accidents
Nuclear power may be considered by some as a "viable" fuel source for the future, but anti-nuclear campaigners point to the industry's safety record around the world as a reason for caution.
The BBC News website lists a selection of accidents at nuclear plants over the years:
29 September 1957(INES Level 6)
Mayak or Kyshtym nuclear complex (Soviet Union): A fault in the cooling system at the nuclear complex, near Chelyabinsk, results in a chemical explosion and the release of an estimated 70 to 80 tonnes of radioactive materials into the air. Thousands of people are exposed to radiation and thousands more are evacuated from their homes. It is categorised as Level 6 on the seven-point International Nuclear Events Scale (INES).
7 October 1957(INES Level 5)
Windscale nuclear reactor (UK): A fire in the graphite-cooled reactor, in Cumbria, results in a limited release of radioactivity (INES Level 5). The sale of milk from nearby farms is banned for a month. The reactor is unsalvageable and buried in concrete. A second reactor on the site is also shut down and the site decontaminated. Subsequently part of the site is renamed Sellafield and new nuclear reactors are built.
3 January 1961
Idaho National Engineering Laboratory (USA). A steam explosion in reactor SL-1 during preparation for start-up destroys the small US Army experimental reactor and kills three operators.
29 March 1979(INES Level 5)
Three Mile Island nuclear power plant, Pennsylvania (US). A cooling malfunction causes a partial meltdown in one reactor, resulting in a limited release of radioactivity (INES Level 5).
The site's first reactor (TMI One) on the Susquehanna river was closed for refuelling. The second was at full capacity when two malfunctions occurred: first there was a release of radioactive water, then radioactive gas was detected on the perimeter. No deaths or injuries were reported.
It is considered the United States' worst nuclear accident and led to major safety changes in the industry.
26 April 1986 (INES Level 7)
Chernobyl nuclear power plant (Soviet Union). One of four reactors explodes after an experiment at the power plant (INES Level 7). The resulting fire burns for nine days and at least 100 times more radiation than the atom bombs dropped on Nagasaki and Hiroshima is released into the air. Radioactive deposits are found in nearly every country in the northern hemisphere.
Two people die in the explosion and another 47 from Acute Radiation Sickness. Thousands of extra cancer deaths are expected as a resulted of the disaster. A huge cover, known as the New Safe Confinement, is due to be erected over the existing sarcophagus covering the site some time after 2008.
6 April 1993
Severesk, formerly Tomsk-7 (Soviet Union). A tank at a uranium and plutonium factory inside the plant explodes, resulting in radioactivity being dispersed into the atmosphere contaminating an area of over 120 sq km (INES Level 4). A number of villages are evacuated and left permanently uninhabitable.
30 September 1999
The Tokaimura accident shook confidence in the industry in Japan
Tokaimura nuclear fuel processing facility (Japan). Workers break safety regulations by mixing dangerously large amounts of treated uranium in metal buckets, setting off a nuclear reaction (INES Level 4). Two of the workers later die from their injuries, and more than 40 others are treated for exposure to high levels of radiation.
9 August 2004
Mihama nuclear power plant (Japan). Five people die in an accident at the plant in the Fukui province (INES Level 1). Seven people are also injured when hot water and steam leaks from a broken pipe.
Officials insist that no radiation leaked from the plant, and there is no danger to the surrounding area.
11 March 2011(INES Level 5)
Fukushima nuclear power plant
By Richard Black
Environment correspondent, BBC News
As it was almost bound to do at some point, Japan's nuclear safety agency has uprated its assessment of the Fukushima power station incident from a level four to a level five.
These are categories on the International Nuclear and Radiological Events Scale (INES), which runs from zero (nothing happened, essentially) to seven, a "major accident".
So far, Chernobyl is the only seven-rated incident in nuclear history.
Level five is defined as an "accident with wider consequences".
So what is the worst-case scenario for those "wider consequences" at Fukushima?
What clues are there either from that level five rating, or from the situation on the ground, as to how things might transpire - whether it will in the end prove to have been a disaster or a distraction from the serious and widespread impact of the tsunami?
"The worst-case scenario would be where you have the fission products in stored canisters or in the reactors being released," said Professor Malcolm Sperrin, director of medical physics and clinical engineering at Royal Berkshire Hospital, UK.
"Radiation levels would then be very high around the plant, which is not to say they'd reach the general public.
"And we're definitely not in the situation where we're going to see another Chernobyl - that possibility has long gone."
Read more here.
Latest News:
Workers are close to restoring power to cooling systems at a quake-hit Japanese nuclear power plant, officials say.
Engineers connected a cable through which they hope to supply electricity to part of the Fukushima Daiichi plant.
The earthquake and tsunami it triggered crippled the plant's cooling systems, and some radiation has leaked.
At the stricken plant, firefighters have continued to spray water at the dangerously overheated fuel rods, in a desperate attempt to avert a meltdown.
Engineers hope that restoring power will allow them to restart pumps to continue the cooling process.
Japan's nuclear safety agency earlier hoped that electricity would be restored on Saturday but later revised its projection.
"If no problem is found at the facility today, the power will resume as early as tomorrow [Sunday], " agency spokesman Fumiaki Hayakawa is quoted by the AFP news agency as saying.
However, an official from the power company which runs the plant told AFP: "Although we are doing our best, unfortunately we cannot say when electricity will be restored."
Given the scale of the damage, it is not certain the cooling systems will work even if power is restored. Workers are also boring holes in roofs at the plant to prevent a potential gas explosion.
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