1920 GMT, 13 March 2011
Three days after a magnitude 9.0 earthquake and ensuing tsunami struck Japan, killing an estimated 10,000 people and leaving many more destitute, the country is still struggling to avert nuclear disaster, with problems reported at four separate nuclear power plants.
The Tokyo Electric Power Company (Tepco) is continuing attempts to cool down two reactors at the Fukushima-Daiichi plant 240km north-east of Tokyo, where a dramatic explosion destroyed the roof of the building housing reactor No. 1 on Saturday. Seawater mixed with boric acid has been introduced to reactors Nos. 1 and 3 in an attempt to cool the reactors' cores and kill the nuclear fission reaction more quickly.
It's not clear how much progress has been made, although nuclear power experts canvassed by Reuters were cautiously optimistic that the situation was being brought under control. The Japanese government has acknowledged that fuel roads at one or both reactors may not have been fully submerged for a time, and may have melted or become deformed as a result, but that would fall short of a complete meltdown and does not necessarily constitute a risk to the public unless the situation worsens.
Japan's nuclear safety agency also faces an emergency at Tokai nuclear power station, 120km from Tokyo in Ibaraki Prefecture, where one of two cooling systems has stopped. But the Japan Atomic Power Company, which operates the plant, says that the remaining systems are working effectively and the reactor core is cooling smoothly.
At a third nuclear plant, in Onagawa, Miyagi Prefecture, an initial report of elevated radiation levels led to a low-level emergency being declared, but Tohoku Electric, the company that runs the Onagawa plant, said the cooling systems at all three reactors are functioning properly. The BBC reported that the increase in radiation was brief, with one possibility being that it originated at the Fukushima plant.
The site causing greatest concern is reactor No. 3 at Fukushima-Daiichi, whose plutonium-uranium fuel mix poses a greater radiological risk than that of reactor no. 1. Chief Cabinet Secretary Yukio Edano said at 20.00 local time on Sunday that water levels within the pressure vessel could no longer be confirmed to be increasing and that there was a "high possibility" that a valve used to vent steam was malfunctioning. Earlier in the day, Tepco had warned that an explosion like that at reactor No. 1 was possible.
A state of emergency has also been declared at the nearby Fukushima-Daini plant, where preparations to vent steam to reduce pressure have been drawn up but have not yet been implemented. More than 200,000 people have been evacuated from the vicinity of the two nuclear plants, although the Japanese government continues to stress that the radiation known to have leaked thus far poses little risk to human health.
The difficulties at the nuclear power plants, as well as other power generation facilities, mean that rotating power outages will be imposed across Japan as of Monday.
1300 GMT, 13 March 2011
Rowan Hooper, news editorAs fears grow of an explosion at the number 3 reactor in the Fukushima-Daiichi nuclear power plant, Japanese prime minister Naoto Kan said in a press conference that the disaster was the worst since the atomic bombing of Hiroshima and Nagasaki in the second world war.
Unlike the uranium-based Fukushima number 1, the number 3 reactor uses a mixture of plutonium oxide and uranium oxide. In the event of a meltdown, plutonium is considered more dangerous than uranium alone because of its increased volatility and its reactive, "neutronic" effects.
Michael Bluck, a nuclear engineer at Imperial College London, told New Scientist that plutonium is used because it increases the efficiency of power generation. "It improves the burn up, so you get more energy out of the fuel than if you just use uranium dioxide (UO2). Plutonium dioxide enhances burn up in normal situations in a controlled reactor, so it may result in even greater heat generation in the event of a meltdown than is the case with UO2 alone. It's why plutonium is used in nuclear weapons, because it is more reactive and produces more energy."
Bluck notes that plutonium is produced anyway in a "UO2 only" reactor, as part of the fission process. Adding plutonium at the beginning just gives us more.
The danger is that in the event of a meltdown there would be even greater generation of heat, with the additional demands of cooling. Metallic plutonium is a serious fire hazard, Bluck added, further complicating the situation.
To prevent the catastrophe of a meltdown, boric acid - a water solution containing boron - is being pumped into the number 3 reactor. Boron is used because it captures neutrons and reduces the risk of a fission chain reaction. It is being pumped with sea water into the reactor.
Japan generates about a third of its electricity from nuclear power. In order to conserve energy during this crisis, Kan warned that there will be rotating power outages across Japan.
"There have been quite a few nuclear power plants affected by this earthquake," he said. "We have no prospect of restoring electricity supply within the next few days therefore there is a good possibility that we will remain without electricity - and there may be a possibility of large scale blackouts. These will affect people's lives and industrial activities. And we have to avoid large scale unexpected blackouts, so from tomorrow in the area covered by Tepco I have asked them to apply rotating outages."
Meanwhile the Japan Meterological Agency warned that there is a 70 per cent chance of a magnitude-7 aftershock striking the country in the next three days. It said there is a 50 per cent risk over the three subsequent days.
0030 GMT, 13 March 2011
Sumit Paul-Choudhury, editor, newscientist.comCooling systems have failed at a second reactor at the stricken Fukushima Daiichi nuclear power plant, where a massive blast ripped through a reactor building on Saturday. The Tokyo Electric Power Company, which operates the plant, said it was preparing to release mildly radioactive steam to reduce pressure in the plant's No. 3 reactor.
The move follows the decision to use seawater mixed with boric acid to cool the No.1 reactor, whose core container remained intact despite a dramatic explosion which destroyed the walls and ceiling of the reactor building. The expectation is that the boron will kill the nuclear reaction while the corrosive seawater will cool the core and render the reactor unusable. Tepco described the reactor as "stable" late on Saturday and the Japanese government said radiation levels around the plant had decreased.
Around 200,000 people have now been evacuated from the vicinity of the Fukushima Daiichi plant and the nearby Fukushima Daini plant, where a state of emergency has also been declared and plans to vent steam have also been prepared. Up to 160 people may have been exposed to radiation, and those arriving at evacuation centres are being scanned and given iodine supplements. Iodine is used to mitigate radiation sickness.
1740 GMT, 12 March 2011
Roger Highfield, magazine editor, and Yuriko Nagano, contributor, TokyoA turning point in the efforts to avert a meltdown at Fukushima Daiichi nuclear power station came in the wake of the blast that destroyed the exterior walls of the crippled reactor.
The emergency began when the magnitude 8.9 earthquake which rocked the region on 11 March put the 439 MWe Boiling Water Reactor into shutdown mode.
Even after shut down, however, a reactor still requires cooling. Diesel generators initially supplied cooling water but they failed about an hour after the quake as a result of the tsunami, prompting fears of a meltdown. The pressure in Fukushima 1 started to rise, as the cooling water covering the core boiled into steam. Malcolm Grimston, an associate fellow at Chatham House in London, said that the fuel began to overheat.
At around 1500 ºC, the zirconium metal cladding the uranium fuel would react with the steam to form hydrogen.
If any of the fuel rods have been compromised, there would be evidence of a small amount of other radioisotopes called fission fragments (specifically radio-caesium and radio-iodine), according to Paddy Regan of Surrey University.
Regan added that while the intergrity of the pressure vessel is secure, the vast majority of the fission fragments and radioactive fuel material is safely contained within the pressure vessel and should not escape.
However, the pressure in the steel vessel would have increased inexorably.
The Tokyo Electric Power Company, Tepco, had the flexibility to use pressure release valves to vent some steam, even though it was mildly contaminated, because it had taken the precaution of evacuating the local population within a 12-mile radius. Grimston described this as "extraordinary forward planning".
The steam was released from the pressure vessel into the surrounding building and this was consistent with reports that radiation levels had soared to around 1000 times the background level. Officials also said they had detected caesium, an indication that some fuel was already damaged.
The blast occurred at 3:36 PM local time after a large aftershock shook the plant, though Grimston said that it was not clear the two were connected.
The shock wave that can be clearly seen in video of the blast suggests a point ignition source detonated the released hydrogen when it came into contact with oxygen in the air, he said.
Four workers were injured, according to Atsushi Sugimoto of Tepco.
"At this point, we don't know how much radiation has escaped," said Shinji Kinjo of the Nuclear and Industrial Safety Agency. "Should the situation change, the evacuation zone could become larger."
Yukio Edano, Japan's chief cabinet secretary, said the cause of the explosion was a mixture of hydrogen, from steam escaping the core, and oxygen from the surrounding air.
He added that the pressure vessel was unaffected and the incident would not be a cause for a large amount of radiation to leak.
Although the concrete cladding disintegrated in a spectacular fashion, Grimston said that the fact that the metal frame of the building was left intact suggests that the explosion was not as violent as it looked.
Because the plant went into operation in 1971 and is due for decommissioning, the decision was taken by Tepco to flood it with seawater containing boric acid to kill the nuclear reaction.
This began just after 2 pm UK time and would take up to 10 hours.
The use of corrosive seawater would render the reactor unusable but would ensure that the risk of a meltdown had been averted, said Grimston.
He said that, if the information he had received was accurate, it looked a "textbook example" of how to deal with a nuclear emergency.
Tepco said Fukushima was stable but remained sketchy on key details.
More measures are under way to protect the local population. "The authorities also say they are making preparations to distribute iodine to residents," said the International Atomic Energy Agency.
Meanwhile, an official at Japan's nuclear safety agency rated the incident a 4, according to the International Nuclear and Radiological Event Scale. Three Mile Island was rated a 5, while Chernobyl was rated 7 on the 1 to 7 scale.
The Kyodo news agency reported that some 10,000 people in the town on Minamisanrikucho, in Miyagi prefecture, are missing in the wake of yesterday's tsunami.
1230 GMT, 12 March 2011
Jeremy Webb, editor-in-chief and Rowan Hooper, news editorDetails are emerging of the explosion at the nuclear power plant in Fukushima on the east coast of Japan, 240 kilometres north of Tokyo.
The blast blew off the outer concrete shell of a building housing one of the six reactors at the Fukushima Daiichi (number 1) nuclear power station, leaving behind a skeleton of metalwork. Four workers are reported to have been injured at the site and radiation is leaking into the environment. Japanese authorities have extended the evacuation zone around the plant to 20 kilometres.
Chief Cabinet Secretary Yukio Edano told a press briefing that the pressure vessel that houses the radioactive core of the plant is intact, and that a large amount of radiation leakage is not expected. He said that radiation is remaining at a low level. The Japanese news agency Kyodo earlier reported levels of 1050 micro Sieverts - within Japanese national safety levels - around the explosion at the Fukushima 1 reactor building.
Edano announced that the Tokyo Electric Power Company (TEPCO), which runs the Fukushima facility, will be allowed to use sea water to cool the reactor down.
The cause of the explosion is still unclear, but suggestions include a build up steam released from the reactor cooling system or by the ignition of hydrogen gas. That hydrogen could have been liberated by water "cracking" in the ultra high temperatures in the reactor.
Trouble at Daiichi began on March 11, when the earthquake struck offshore, northeast of Fukushima. The plant tripped out immediately, as it's designed to do, shutting down the chain reaction in the core.
The reactors at the Daiichi station are boiling water reactors built by US company GE in the 1960s. Water passes up through the core, turning into steam, which powers the turbines to generate electricity. The steam is then cooled and pumped back into the core.
When the reactor trips out, water needs to keep circulating to remove residual heat in the core. But, according to TEPCO, an hour after the earthquake, the diesel engines running the cooling system failed. This led to evaporation of water in the core and a build up of steam in the pressure vessel.
TEPCO managed this by releasing the steam from the pressure vessel into the large surrounding building. This appears to be the building that has exploded.
If heat continues to build up in the core, there is a possibility that it could melt, as happened in the Three Mile Island accident in the US in 1979.
1130 GMT, 12 March 2011
Paul Marks, senior technology reporterA massive explosion has ripped through a nuclear power plant in the city of Fukushima on the east coast of Japan, raising fears of a radioactive meltdown. Four workers are believed to have been injured in the blast, which has caused major structural damage. Radioactive caesium and iodine isotopes, by-products of nuclear fission, have been detected in the vicinity.
A state of nuclear emergency was declared yesterday at the Fukushima-Daiichi plant, 240 kilometres north of Tokyo, as its operator, the Tokyo Electric Power Company (Tepco), struggled to contain rising temperatures and pressures in the core of two reactors whose cooling systems failed after Friday's magnitude 9.0 earthquake shook Japan and sent tsunami waves across the Pacific. Tepco has also reported problems at the nearby at the neighbouring Fukushima Daini plant, meaning that a total of five nuclear reactors are now covered by the state of emergency.
It is not yet clear what has been destroyed, but Japan's public broadcaster NHK is reporting that the walls to reactor number 1 at the Fukushima Daiichi plant - also known as Fukushima I - have been blown apart. It is not yet known if the reactor's containment vessel was affected. Nor is the cause of the explosion yet known, although commentators for the BBC and for Reuters suggested it was more likely to be chemical in nature than nuclear.
Updates from the Tokyo Electric Power Company over the few hours preceding the explosion indicated an inexorable build up of pressure in a number of the reactor containment buildings at both Fukushima sites.
All six of Fukushima 1's reactors are shut down - reactors 1, 2 and 3 were closed for precautionary reasons as the quake struck, while reactors 4, 5 and 6 had already been switched off for inspections. At 1pm local time on 12 March, TEPCO reported that pressure was increasing in the containment vessel of reactor 1 and that it was taking steps to vent the pressure at the direction of the national government. At the same time, water was being introduced in a bid to cool the core - but that creates steam and adds to the pressure.
It appears from the explosion that the TEPCO lost its battle to keep the lid on the pressure on that reactor. If the temperature is still rising the core could melt into an uncontrollable radioactive-particle-ejecting mass - a "meltdown".
At the nearby Fukushima II plant an alarm suggested that one of the control rods used to quench the fission reaction had not been fully inserted - perhaps meaning that fission could continue. The alarm was later called off and Tepco said that other control rods had been confirmed as fully inserted, but the pressure and temperature nonetheless increased enough for another radioactive steam venting operation to be prepared.
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