1.27pm: We've just launched a fresh live blog which will continue to follow the events in Japan – please click here.

This blog will no longer be updated, comments will close in 30 minutes. Thanks for reading.

1.23pm: It's time for another summary, after which this now rather long live blog will be replaced with a new version here, to be launched imminently.

• Authorities are struggling to control the situation at the Fukushima Daiichi nuclear plant, where the situation in reactors No 3 and No 4 remains fast-moving and perilous.
Reactor 3 is a particular worry – Japan's government said today that there a "possibility" that the reactor's primary containment vessel for radiation had been damaged after steam was seen rising into the air. Attempts to drop water from helicopters was abandoned due to radiation levels. The latest mooted plan involves a police water cannon truck.
Staff dousing reactors with seawater were forced to evacuate part of the building for a period this morning after radiation levels surged. These later receded and staff returned.
French government ministers have urged the country's nationals in Tokyo to leave Japan or else head south in case radiation is blown onto the capital, saying Japanese authorities had "visibly lost control".

• The country faces an increasingly desperate humanitarian crisis caused by the direct effects of Friday's huge earthquake and resultant tsunami, one made worse by freezing weather.
The official death toll has now hit 4,255 deaths, with 8,194 people registered as unaccounted for. Survivors, many of them homeless, are struggling with a wave of cold weather forecast to last well into this week, with night time temperatures dipping to -5C in some places.

• Japan's emperor, Akihito, has made a rare TV appearance to express his condolences to his people and his worry at the nuclear situation.

• Following its precipitous plunge yesterday, Tokyo's stock market bounced back nearly 6%. Japan's central bank has injected a further $40bn into the financial markets.

12.47pm: My colleague James Randerson, formerly the paper's science correspondent, has sent me a link to an interesting Q&A on the Australian Science Media Centre website. This is the most interesting part, with Peter Burns, former chief executive of the Australian Radiation Protection and Nuclear Safety:

How is the current Japanese situation different to Chernobyl?
Chernobyl did not have a containment vessel and a large fraction of the inventory of volatile fission produces was released. Releases to date have been minor. If there is a major meltdown and the containment vessels are breached to some degree it is still likely that much of the radioactive material would be contained."

Current reports state that the radiation levels at the plant reached 400 millisieverts per hour (mSv/h). What does this number mean, is it comparable to other sources of radiation (x-ray etc)? How is it comparable to the radiation released at Chernobyl or 3 Mile
Island?
Current international recommendations allow for planned exposures up to 100mSv when dealing with accidents. At 400mSv this would occur in 15 minutes, so 400 mSv/h is a high exposure rate. A dose of 400mSv is comparable to the dose from 50 -100 CT Scans.

How dangerous are these levels for the general population in the surrounding area and further afield?
Doses from the plant fall off quickly. With a 20km exclusion zone in place doses to the public would be low in comparison to those from natural background radiation.

12.41pm: The office of Japan's prime minister has just launched a new English-language Twitter feed. As one of the initial tweets notes, above, it's mainly aimed at updates about the quake aftermath.

12.31pm: My colleague Suzanne Goldenberg, our US environment correspondent, has been speaking to scientists about the implications of a fire in the pools storing spent atomic fuel at the Fukushima plant. Her full story is here, but in brief it is – as you might expect – a gloomy prospect. One nuclear expert, David Lochbaum, told her:

If the spent fuel pool is on fire, the chances of radioactivity getting to the public are very much higher.

12.21pm (9.21pm JST): A reader has emailed to point out, quite correctly, that I got my radiation units mixed up in my 10.19am update. To clarify, the new limit to which staff can be exposed is 250 millisieverts, not 250 millisieverts per hour. Thus, with readings in some parts of the plant hitting 400 millisieverts per hour yesterday, staff would be able to work there for just over 35 minutes.

12.13pm (9.13pm JST): Here's a link to our latest gallery of photos from the quake aftermath. A number of them show the appalling conditions faced by survivors and rescue teams amid snow and freezing weather. It's worth reiterating yet again that for all the drama of the Fukushima nuclear crisis, hundreds of thousands of people remain homeless.

12.02pm (9.02pm JST): China has become the latest nation to re-think its nuclear plans following the situation in Japan. The State Council in Beijing announced tonight that it will suspend approval of new nuclear projects until new safety rules are introduced, the Wall Street Journal reports.

Until now China had been eagerly trying to build nuclear power stations as fast as possible, trying to reduce its traditional reliance on old and polluting coal-fired facilities. The nation currently has 25 nuclear plants under construction, with 13 already running, the WSJ calculates.

On Monday Germany announced the temporary closure of its two oldest nuclear power stations and suspended plans to extend the life of remaining plants. Switzerland has also put on hold plans to build and replace nuclear plants.

11.49am (8.49pm JST): It's very long, but this is a useful piece of background about the current situation at the Fukushima Daiichi nuclear plant. This Reuters backgrounder lists the situation at each of the six reactors at the Fukushima No 1 plant, ranked according to the level of risk. There are four further reactors in the No 2 plant, but these are all in successful cold shutdown. The Kyodo news agency has been running its own similar guide - the latest version is here.

REACTOR No 3:
What is happening?
Tepco said on Wednesday that resolving problems at this reactor was the top priority because it had the highest radiation levels. This reactor is the only one that includes plutonium in its fuel mix.
The operator has been pumping sea water into the reactor to prevent overheating. The building housing the reactor was hit by an explosion on Monday.
An attempt by a military helicopter to drop water on the reactor failed on Wednesday probably because radiation levels were too high, Kyodo reported. The Japan nuclear agency had said earlier in the day that the pumping of sea water was proceeding smoothly.
Tepco said the spent fuel pool may have heated up, producing steam. The temperature has risen to around 60C from the usual 30C-40C but the change was not critical, it said.
What are the risks?
The International Atomic Energy Agency (IAEA) said on Tuesday that the primary containment vessel, the first line of defence against a radiation leak, appeared intact.
However, government spokesman Yukio Edano said on Wednesday there is a "possibility" the vessel had been damaged, Kyodo reported.
If that is the case, authorities will be worried that radiation may leak through the first containment wall into the secondary containment building.
The spent fuel pools present a radiation risk if the spent fuel is exposed to the atmosphere. When a rod is exposed to the air, zirconium metal on the rods will catch fire, which could release radiation contained in the fuel, said Arnie Gundersen, a 39-year veteran of the nuclear industry who is now chief engineer at Fairwinds Associates Inc.

REACTOR No 4:
What is happening?
TV on Wednesday showed smoke or steam rising from the facility after flames were seen earlier. The reactor had been shut down for maintenance when the earthquake and tsunami struck.
On Tuesday, a pool where spent fuel is stored caught fire and caused an explosion. Japan's nuclear safety agency says the blast punctured two holes around 8-metres square in the wall of the outer building of the reactor.
Tepco has said it may pour water through the two holes within two or three days to cool spent nuclear fuel that is inside. Workers cannot prepare to pour water into the pool sooner because of high radiation levels, Kyodo said.
Instead, Tepco plans to bulldoze a road to the reactor building so water-pump trucks can approach and hose water inside, said Kazuya Aoki, a director of Japan's Nuclear and Industrial Safety Agency.
What are the risks?
Exposure of spent fuel to the atmosphere is serious because there is more radiation in the spent fuel than in the reactor, said Gundersen. The spent fuel pool is not inside a containment facility either.
"They need to keep water in those pools because the roof over the building housing the pools is already damaged and radiation will escape," he said.
The pools contain racks that hold spent fuel taken from the reactor. Operators need to constantly add water to the pool to keep the fuel submerged so that radiation cannot escape.
Exposing the spent fuel to the atmosphere will release radiation.

REACTOR No 2:
What is happening?
An explosion rocked the plant on Tuesday, damaging a suppression pool, into which steam is vented from the reactor to relieve pressure. The roof of the reactor building is damaged, Jiji news agency reported.
Tepco said on Tuesday the fuel rods were fully exposed. Kyodo reported an estimated 33% of the nuclear fuel rods have been damaged at the No 2 reactor.
However, on Wednesday, Japan's nuclear agency said the pumping of sea water into the reactor was proceeding smoothly.
What are the risks?
When fuel rods are no longer covered in coolant they can heat up and start to melt, raising the risk of a radiation leak.
The suppression pool is part of the primary containment vessel, which is designed to prevent a leak, but the IAEA said the blast "may have affected the integrity of its primary containment vessel."
Still, beyond the primary containment vessel is the containment building, which is also designed to prevent radiation from escaping.

REACTOR No 1:
What is happening?
An explosion occurred at the reactor on Saturday. Kyodo reported on Wednesday an estimated 70% of the nuclear fuel rods have been damaged.
Authorities are pumping sea water into the reactor to prevent overheating, and pressure levels were stable, Edano said on Tuesday.
The Japan nuclear agency said on Wednesday the pumping was proceeding smoothly.
-What are the risks?
The IAEA said on Tuesday the primary containment vessel appeared intact. If the fuel rods in the reactor are not covered by coolant, they can heat up and start to melt.

REACTOR No 5:
What is happening?
The reactor had been shut down for maintenance at the time of the quake and tsunami.
Tepco said on Wednesday water was being poured into the reactor and that temperatures in the spent fuel pool were rising slightly.
What is the risk?
Reactor 5 and reactor 6 are seen less at risk than reactors 1 to 4.

REACTOR No 6:
What is happening?
Tepco said on Wednesday water was being poured into the reactor and that temperatures in the spent fuel pool were rising slightly.

11.41am (8.41pm JST): NHK has some more details of the latest comments by government spokesman Yukio Edano. He has again insisted that radiation levels even within the 30km zone around the Fukushima plant remain completely safe. While people within the area have been urged to stay indoors, this does not mean people should stay away – notably those delivering supplies, Edano said.

Here also is an EPA photo showing a woman evacuated from her home near the plant being checked for radiation.

Medical personnel check a woman evacuated from her home near the Fukushima 1 nuclear plant for radiation exposure in Hitachi Photograph: Asahi Shimbun/EPA

-

11.38am (8.38pm JST): A reader, seaninsendai, has been providing updates on the situation in the devastated north-eastern region:

there is a huge response in the way of aid. local people have been using "old rice" reserves to provide many of the evacuees onigiri (rice balls). seriously, the response from local people is just amazing and completely humbling
it's just been announced on the news now that more roads in tohoku are being reopened to provide further aid to the most devastated areas, but there is still very little petrol. many houses are still without electricity and gas, indeed it's been predicted that gas won't return in tohoku for another month.

11.33am (8.33pm JST): The BBC has a story which I imagine will gain a lot of prominence in the UK – a group of UK rescue workers say they have had to leave Japan because they could not get the necessary paperwork completed by the British embassy in Tokyo. It continues:

The International Rescue Corps said they were not given permission to work in Japan because it would have made the embassy legally responsible for them.
A spokesman for the team said it was "gut-wrenching" to be stopped from helping by "your own country".
The British ambassador to Japan said they helped them as much as they could.

11.28am (8.28pm JST): Further to my various updates about the cold, above is a Reuters photo of a man in Otsuchi trying to keep warm at an emergency shelter.

11.21am (8.11pm JST): Yet another plan has emerged to somehow cool the spent nuclear fuel pool at the No 4 reactor, after efforts failed to use helicopters dropping seawater from the air.

Authorities now hope to use a police water cannon truck to fire water into the pool, NHK television said. The operation could begin tonight.

10.58am (7.58pm JST): It's worth reiterating in connection to the last update that France is not a country averse to nuclear power – fully 75% of its own electric power comes from the source. France's nuclear safety authority has already rated the Fukushima at six on its one-to-seven scale of nuclear accidents, below only Chernobyl.

10.47am: 10.46am (7.46pm JST): France has urged its nationals in Tokyo to leave the country, or at least head towards southern Japan.

This is a significant development: the first time a nation has explicitly said it does not believe Japanese reassurances about the safety of the Fukushima plant, about 150 miles from the capital. The comments by French ministers are very strong.

The industry minister, Eric Besson, told BFM television:

Let's not beat about the bush. They have visibly lost the essential of control (of the situation). That is our analysis, in any case, it's not what they are saying.

The environment minister, Nathalie Kosciusko-Morizet, said the latest news about the nuclear situation "does not lead to optimism":

We recommend that all French citizens who do not have a good reason to stay in Tokyo either take a plane or, if they absolutely insist on staying, head south.

While this is not an official evacuation order, Air France has already increased capacity on its Tokyo to Paris services.

10.31am (7.31pm JST): Japan's defence minister has called up reservists from the country's ministry, the Self Defence Force, to help with relief efforts, Jiji Press says. Already more than 5,000 reservists have indicated their willingness to help, it adds.

10.19am (7.19pm JST): A few varied updates from the Japanese media:

• Japan's government has more than doubled the amount of radiation that staff at the Fukushima plant can legally be exposed to. They can now face up to 250 millisieverts per hour in cases of emergency, NHK says, noting that this is below the general international standard of 500 millisieverts.

• The central government is mulling a system of special reconstruction grants for quake-hit areas, Kyodo news reports.

• Elsewhere, some aspects of life are resuming. Toyota is to re-start the production of auto parts tomorrow at its plants. Meanwhile, a planned football friendly against New Zealand - coincidentally, another recently quake-hit nation – will go ahead on 25 March in Shizuoka, south-west of Tokyo.

10.08am (7.08pm JST): To reinforce my earlier points about the freezing weather, the picture above, from AP today, shows firefighters searching for survivors beneath a collapsed road flyover in Otsuchi, on Japan's north-east coast. More or less the entire town was destroyed on Friday; thousands of people are existing in the snow with no home or possessions.

10.04am (7.04pm JST): A quick clarification to my 9.51am update – it seems it was Japan's military who announced that the helicopters could not drop water due to high radiation levels.

10.00am (7pm JST): The operators of the Fukushima's Daiichi plant, Tokyo Electric Power, or Tepco, are also holding a press conference. Water is now being poured into reactors No 5 and No 6, they say, according to Reuters.

9.51am (6.51pm JST): The update about the radiation levels preventing helicopters dropping water (9.21am) appears to come from another media briefing by the government's chief spokesman, Yukio Edano. At the same time he has stressed that there is no immediate risk to health around the plant, and that the hope is instead to inject water into the stricken reactor four from ground level.

9.37am (6.37pm JST): It's time now for a summary:

• The situation at the Fukushima Daiichi nuclear plant remains extremely perilous and seemingly beyond the control of authorities. Staff at the plant were moved to safer areas for a period this morning after radiation levels rose.
A fire broke out at the No 3 reactor, sending a plume of light grey smoke – possibly radioactive steam – above the plant. Helicopters carried containers of water above the complex but were unable to dump them, reportedly due to the high radiation levels.

• The country faces an increasingly desperate humanitarian crisis caused by the direct effects of Friday's huge earthquake a resultant tsunami, one made worse by freezing weather.
The official death toll has now hit 3,676, with 7,845 people registered as unaccounted for. Survivors, many of them homeless, are struggling with a wave of cold weather forecast to last well into this week, with night time temperatures dipping to -5C in some places.

• Japan's emperor, Akihito, has made a rare TV appearance to express his condolences to his people and his worry at the nuclear situation.

• Following its precipitous plunge yesterday, Tokyo's stock market bounced back nearly 6%. Japan's central bank has injected a further $40bn into the financial markets.

9.21am (6.21pm JST): Kyodo news has a slightly worrying update, in a breaking news item on its website – the military helicopters seen over the Fukushima nuclear plant (8.06am) have been unable to drop any water "due to high radiation".

8.53am (5.53pm JST): Above is a TV screengrab, via Reuters, of Emperor Akihito's address to the nation. Here's also a couple more quotes from the 77-year-old who, let's remember, is by tradition a more remote and detached figure than most heads of states, even compared to his fellow monarchs:

I hope from the bottom of my heart that the people will, hand in hand, treat each other with compassion and overcome these difficult times... I am deeply concerned about the nuclear situation because it is unpredictable.

8.40am (5.40pm JST): Tania Branigan has been speaking again to Professor David Hinde, head of the department of nuclear physics at the Australian National University. He says that the status of the spent fuel pools at the Fukushima plant is very concerning. Water levels are reportedly dropping in unit 4, and the temperature of the pools in 5 and 6 are now rising.

It is clearly a serious situation now because there is no containment for those spent fuel pools… My feeling is that they are probably a more serious issue now than the reactors, [where] there's at least a degree of containment remaining.

Spent fuel rods are strongly radioactive and the water above them shields against that radiation so as long as the water level is sufficiently high – you can walk up to the edge of the pool and pour a bucket of water in. Once it is even close to the top of the rods the levels are too high to approach the pool, which is clearly what has happened in unit 4.

It's been suggested that the Japanese Self Defence Force could use helicopters to drop water onto the plant (see 8.06am). But even if the military is willing to risk exposing pilots to radiation, Hinde says this is far easier said than done:

My estimates suggest they might need 50 tonnes an hour of water. You could do that easily with a large bore hosepipe but if you are doing it with helicopters it is a lot more difficult – and harder to get the water into the pool. I very much hope they rig up a temporary pipe works in 5 and 6 to pump water in remotely to avoid reaching this situation.

He said the ideal situation would be to re-establish the cooling system, which seems to have been knocked out by the tsunami.

8.31am: (5.31pm JST) It's worth noting, too, that large areas of Japan are still being rattled by strong aftershocks. The Chiba region in the east of the country experienced a 6.0 magnitude quake at lunchtime, Jiji Press reports.

8.24am (5.24pm JST): Reuters have out together a useful timeline of how the nuclear emergency has unfolded since Friday. It's a bit long to paste into the blog, but you can read it here. What struck me was the number of times various authorities have assured everyone that the situation is under control.

8.16am (5.16pm JST): Amid the understandable focus on the nuclear drama, it's important not to forget the sheer scale of human tragedy already brought about by Friday's quake and tsunami. This is currently being increased by the freezing weather, mentioned by Tania already (5.57am). To reinforce that point, here's a screengrab (above) from the BBC of the next couple of days of forecast weather for Sendai.

8.06am (5.06pm JST): Japan's military is flying helicopters above the Fukushima plant carrying seawater in giant containers, to be dropped into the number three reactor. Here's a screengrab (above) from NHK's coverage of the operation. The helicopters are having to pass over the site repeatedly to avoid excessive radiation contamination and no water has yet been dropped.

7.55am (4.55pm JST): This is Peter Walker, taking over from Tania. AP has a first translated quote from the emperor: ""We don't know the number of victims, but I pray that every single person can be saved."

7.49am (4.49pm JST): The emperor has urged people not to give up hope after a disaster "unprecedented in scale", but says he is "deeply worried". He has acknowledged that the problems at the nuclear reactors are unpredictable. And yes, apparently these are his first public comments since the earthquake and tsunami.

7.43am (4.43pm JST): My colleague Justin McCurry says that Emperor Akihito is now addressing the nation on television; we think this is his first public appearance since the earthquake.

7.39am (4.39pm JST): A few people have been asking in the comments about the fact that radiation levels we have quoted in the last couple of days seem to differ so wildly. A few points to make here:

Firstly, we are entirely dependent on the figures that Tepco and the Japanese government release. They are giving individual readings rather than issuing a set of data that covers a given group of locations. (Ie, they are not giving us readings for different reactor buildings/around the plant/by the main gate every couple of hours).

Secondly, it is clear that readings are fluctuating greatly over time and from place to place. For instance, NHK has just mentioned a reading of 6.4 millisieverts per hour at one location at 10am which had dropped to 2 millisieverts within nine minutes. Similarly, although we saw a very alarming measurement of 400 milliseiverts per hour at one point yesterday, other areas close by had far lower readings.

Thirdly, the figures at the gate clearly don't reflect the risks faced by all the workers inside the plant, who are of course in the greatest danger. They do, however, offer some indication of the broader risk so should be seen in that light.

All that said, please let us know if you spot a specific error.

7.10am (4.10pm JST): A bit more detail on the spent fuel pool in reactor 4: NHK says its temperature has reached 89C, double the usual level. It has also confirmed that the temperatures in units 5 and 6 are higher than usual. All three units were off-line when the quake and tsunami hit, undergoing routine servicing.

6.45am (3.45pm JST): If you're looking for a round-up of events at Fukushima No 1 plant this morning, my colleague Justin McCurry in Japan has a story here.

6.40am (3.40pm JST): Better news on the economic front, at least: stocks have rebounded, with the benchmark Nikkei 225 closing 5.68% up. According to the Financial Times, yesterday's slump - which followed a bad day on Monday - was the Nikkei 225's third largest daily drop in its history.

6.30am (3.30pm JST): As miwa points out in the comments below, Sir John Beddington - the UK's chief scientific adviser - has briefed British expatriates that the Japanese government's measures (a 20km exclusion zone round the plant and a warning to stay indoors if 20-30km away) are "entirely proportionate and appropriate to the risk". More information is available at the British embassy's website.

China is evacuating its nationals from the north-east - ie closer to the stricken facility - citing its concerns about the plant and France has recommended that its citizens leave Tokyo. The Austrian embassy announced yesterday that it was moving to Osaka due to radiation fears.

But like many more countries, the UK is not recommending that nationals leave the capital. Radiation levels there are slightly higher than normal, but nowhere near a level of concern, say experts.

6.12am (3.12pm JST): A useful summary of what we know about the status of the reactors at Fukushima No 1 plant, from Kyodo news agency:

-- Reactor No. 1 - Cooling failure, partial melting of core, vapor vented, building damaged Saturday by hydrogen explosion, seawater being pumped in.

-- Reactor No. 2 - Cooling failure, seawater being pumped in, fuel rods fully exposed temporarily, vapor vented, building damaged Monday by blast at Reactor No. 3, damage to containment vessel on Tuesday, potential meltdown feared.

-- Reactor No. 3 - Cooling failure, partial melting of core feared, vapor vented, seawater being pumped in, building damaged Monday by hydrogen explosion, high-level radiation measured nearby on Tuesday, plume of smoke observed Wednesday, damage to containment vessel likely.

-- Reactor No. 4 - Under maintenance when quake struck, fire Tuesday possibly caused by hydrogen explosion at pool holding spent fuel rods, pool water level not observed, fire observed Wednesday at building housing reactor, no water poured in to cool pool.

-- Reactor No. 5, No. 6 - Under maintenance when quake struck, temperature slightly rising in spent fuel pool.

6.07am (3.07pm JST): Broadcaster NHK reports problems with monitoring posts around the Fukushima plant, making it harder to keep track of the spread of radioactivity. They seem to have been affected by the power outages. It explains:

The system, called SPEEDI, predicts how radioactive substances will spread in case of radiation leakage from nuclear power plants, based on measurements taken at various locations, prevailing winds and other weather conditions.

SPEEDI data are intended to be used to draw up evacuation plans for residents around power plants in case of accidents.

Bear in mind that inhabitants within a 20km radius of the plant have already been evacuated, and those between 20km and 30km away told to stay indoors.

A boy (R) waits for boiled water to cook instant noodle outside a shelter in Sendai, in Miyagi prefecture on March 16, 2011. Photograph: Str/AFP/Getty Images

Many survivors are still packed into emergency shelters in the disaster zone, where food and fuel supplies are said to be running low.

5.57am (2.57pm JST): There's a lot of concern about the conditions survivors face. NHK are reporting snow in Miyagi prefecture and the Japanese meteorological agency predicts snow in Iwate today with heavier falls tomorrow. Across the disaster zone, temperatures will drop as low as -5C overnight.

The government's chief spokesman, Yukio Edano, has asked the public not to panic-buy fuel, warning:
''The supply situation for fuel such as gasoline, heavy oil and light oil is worsening in the quake-hit areas."

The defence ministry has announced it is sharing some of its reserves, but needs to hold some back to avoid disrupting the rescue operations.

Private enterprises are offering help but Kyodo reports that the Japan Business Federation's chairman said the government needs a stronger command centre to channel assistance.

''The lack of a strong control tower is likely to have been dragging delivery of relief goods,'' the chief of Japan's most influential business lobby, known as Nippon Keidanren, told reporters, while urging the government to disclose accurate information.

''Though companies are trying to send relief supplies, they cannot secure fuel for returning,'' Yonekura said, stressing that gasoline stations along expressways and supply roads are in need of swift supply of gasoline.

5.46am (2.46pm JST): More from Fukushima: Reuters is reporting that authorities plan to bulldoze an emergency route to crippled reactor No.4 to allow access for fire trucks, although it appears this morning's blaze is now out. The unit was not operating when the earthquake hit but was storing spent fuel rods.

Kyodo says that workers the facility have been unable to pour water into the pool containing the spent fuel rods because of high radiation levels. Tepco, the plant's operators, are considering spraying the reactor with boric acid from overhead, warning: ''The possibility of recriticality is not zero".

Kyodo has just flashed up a statement that winds are preventing Self Defence Force helicopters from dousing it with water, citing an unnamed minister.

5.24am (2.24pm JST): A colleague points out that Couchsurfing has set up a site for people displaced by the disasters. People are offering space in Hiroshima, Osaka, Yokohama and numerous other cities.

5.21am (2.21pm JST): Seoul's Yonhap news agency is reporting that South Korea will send part of its reserve of boric acid - used to slow down fission reactions - to its neighbour. Japan has used up most of its own stockpile to try to cool down the reactor cores.

5.15am (2.15pm JST): A quick update on casualties from the earthquake and tsunami: the national police agency is saying that 3,676 deaths are confirmed and 7,845 people are registered as unaccounted for. But Japanese media have pointed out that many bodies have been found but not identified. The Guardian's Jonathan Watts has reported from Ishinomaki on how authorities are struggling to deal with the dead and are now contemplating mass burials.

This is Tania Branigan in Beijing taking over from Richard

4.50am: Japan's health ministry announced the rise in the legal radiation exposure limit because workers could no longer get close enough to the Fukushima Daiichi No 4 reactor for urgent attempts to repair the reactor's cooling system.

The ministry raised the level two and a half times, to 250 millseiverts.

Earlier, Kyodo news agency said 730 out of a total of 800 workers had been evacuated from the site.

"The workers cannot carry out even minimal work at the plant now," chief cabinet secretary Yukio Edano told a press conference. "Because of the radiation risk we are on standby."

4.36am: To make sense of all this, it appears that reactor no 4 was badly damaged by the hydrogen explosions in nearby reactors, and it is possible that the unit's water level has disappeared entirely.

Tepco said that water in a pool storing the spent fuel rods may be boiling and that its level may have dropped, exposing the rods. The government ordered the firm to inject water into the pool ''as soon as possible to avert a major nuclear disaster.''

Due to high radiation levels at the reactor, workers have been unable to pour water into the troubled pool.

4.13am: More bad news: Japan's Nuclear and Industrial Safety Agency said the radiation level at the Fukushima Daiichi nuclear power plant reached 10 millisievert per hour at one point on Wednesday morning, possibly due to the damage at its No 2 reactor the day before, the government's nuclear safety agency said.

The maximum level was measured at the plant's front gate at 10:40am. It fell to 6.4 millisievert at 10:45am and to 2.3 millisievert at 10:54am but rose again to about 3.4 millisievert at 11am according to Kyodo

In this screenshot from Japan's NHK television, the No 4 unit of the Fukushima Daiichi complex is seen damaged on 15 March. Photograph: AP

4am: Japan's nuclear nightmare continues, as the situation at the damaged Fukushima Daiichi plant remains grave and the latest developments within the crippled reactors show a further deterioration.

Here is a round-up of the latest news:

• Japan's chief cabinet secretary Yukio Edano announced that the containment vessel of the Fukushima Daiichi No 3 reactor is thought to have been severely damaged and may have leaked radioactive steam, emitting high-level radiation.

• Japan's nuclear safety agency required Fukushima plant workers to evacuate the site due to high radiation levels, but the government later upped the legal radiation exposure level, allowing work to continue inside the plant.

• Smoke was seen rising from the No 3 reactor from the early hours of Wednesday morning.

• The No 4 reactor remains a source of major concern, with gaping holes in the reactor's outer building and another fire within the reactor was spotted at 5.45am JST that may have lasted half an hour. The reactor's internal temperature rose to 84C as cooling remained difficult.

• Japan's central bank has injected a further $40bn into the financial markets, meaning that the Bank of Japan has added nearly $330bn in liquidity in the last three days. The Nikkei stock exchange has rebounded a little as manufacturing companies announce the resumption of production.

3.30am: Good morning. There is news just coming from Japan that a surge in radiation at the Fukushima power plant mean workers were unable to continue even minimal efforts to work there.

Chief Cabinet Secretary Yukio Edano has said that work on dousing reactors with water was disrupted when workers were forced to withdraw after radiation levels surged early in the day.

This follows reports that a white plume of smoke has been seen rising from the crippled nuclear plant's No 3 reactor.

Within hours of the Japanese tsunami, as we watched ships stranded in streets like beached whales, outrageous posts were appearing on the internet suggesting that the tragedy was a revenge for that nation's continuing whale hunt. Some even fantasised that the terrible wave had been caused by the whales themselves. A persistent web hoax proposed that the tsunami had launched a whale into a building. Modern myths are made of such sensational stuff; taste and decency are not the prevailing factors.

What happened last Friday, deep beneath the surface of the Pacific, was invisible to us; as invisible as the radiation that is now drifting southwards from Fukushima reactors. But others conclude, as they did in the aftermath of the 2004 tsunami, that whales and dolphins – cetaceans – acted as advance warnings of what was to come. Forty-eight hours before the Christchurch earthquake, for instance, more than 100 pilot whales were stranded on New Zealand's South Island. Then on 4 March, 50 melon-headed whales washed up on the eastern Kashima shore of Japan.

There is no scientific basis for such theories – although whales, like birds, probably do use the Earth's electromagnetic field for navigation, and such abrupt alterations in it may well cause them to strand. (Indeed, the Maori who first colonised New Zealand probably followed migrating whales there from Polynesia, who themselves were following electromagnetic lines).

And now, the insidious evil of contamination, in an island nation on which its manmade version was first visited. The Pacific has ever been the nuclear arena – one metaphorically haunted by the hunted whales – and artists and writers have long seen the whale, whether real or allegorical, as an augury of disaster. In the late 1940s and early 50s, the American artist Gilbert Wilson became obsessed with Herman Melville's novel, Moby-Dick. In 1952, Wilson wrote in the Bulletin of the Atomic Scientists that the "White Whale" had become – a century after Moby-Dick's publication – an augury of atomic conflict, and Captain Ahab's "insane pursuit of Moby-Dick into the Sea of Japan" analogous to America's "atrocious nuclear experiments and explosions in the same area".

Similarly, in his critical work, The Trying-Out of Moby-Dick, published in 1949, the literary critic Howard P Vincent considered that Moby-Dick, the mythical animal, was "ubiquitous in time and place. Yesterday he sank the Pequod; within the past two years he has breached five times; from a New Mexico desert, over Hiroshima and Nagasaki, and most recently, at Bikini atoll." Meanwhile, nuclear submarines – lubricated with sperm whale oil (since it does not freeze in extreme temperatures) – were moving in those same Pacific depths, their very shapes designed to replicate those of hydrodynamic whales.

The Pacific, which covers one third of the Earth, is the great unknown; even now, the last ocean to be explored. Yet it is the backyard of the world's most developed (Japan, the US) and developing (China) nation states. In his book Atlantic, Simon Winchester points out that the modern world began around the Mediterranean, which ceded the locus of power to the Atlantic with America's uprise. Now its future lies in the Pacific, an arena "discovered" by James Cook's voyages of the late 18th century and barely 200 years old in western history.

That yawning, freighted space between Japan and America seems so blank and so full of potential at the same time. From the bland two dimensions of the atlas, this expanse of blue appears entirely empty. Yet it is filled with life: with 25,000 islands, and "ultra-societies" of vast pods of sperm whales that associate in "nations" of their own, communicating in discreet dialects of clicks, uninterpreted by humans. What kind of disasters did they suffer during that terrible seismic shift last Friday? Like the thousands of animals that died in the oil spill in the Gulf of Mexico, their fate is unknown to us, as we understandably focus on the human suffering in Japan. But I would contend that all these things do have a deeper connection, for all of those tasteless internet claims. As Nietzsche wrote in Beyond Good and Evil: "He who fights with monsters should be careful lest he thereby become a monster. And if thou gaze long into an abyss, the abyss will also gaze into thee."

The catastrophe in Japan has sparked debate internationally on nuclear energy, but it is especially fierce in Germany. After Fukushima, it can no longer be viewed as a viable energy source for the future. German chancellor Angela Merkel must alter her pro-nuclear stance.

There are, of course, several arguments in favour of nuclear energy. In contrast to coal-fired power plants, atomic reactors produce little in the way of CO2 emissions – which is good news for the climate. In addition, the technology is helpful for regions which may not have natural gas reserves, for example. Nuclear energy means a certain degree of independence and a modicum of political autonomy when it comes to determining energy policy. Furthermore, energy produced from nuclear power plants tends to be cheap, making it popular with consumers.

But none of that counts – at least not any more. After the earthquake and tsunami in Japan followed by the ever-worsening stream of terrible news relating to the country's nuclear power facilities, even the last remaining advocates of the technology must realise that we can't go on like this. It is over. Done. Finished. Nuclear energy cannot be controlled by humans, no matter how good the arguments might be in its favour. The danger of disaster is real, and it can happen at any time – even in a super high-tech country such as Japan. And it could also happen here in Germany. A sense of security when it comes to atomic reactors is no longer possible. Not anywhere.

The earthquake in Japan is emerging as a decisive turning point in the history of nuclear technology. People learn from experience, and the lesson is clear: not everything that is technically possible is a good thing. New ways need to be found to cover the energy needs of a growing population. And people need to move faster on this issue than they have so far.

That is true worldwide and also, of course, in Germany. After the events of the weekend, anyone who tries to claim that it couldn't happen here looks ridiculous. In Japan there was a chain of unfortunate events: the devastating earthquake followed by the disastrous tsunami. That is true. But what is a disaster apart from a chain of unfortunate events? Be it plane crashes, car accidents or core meltdowns, something will always go wrong when people are involved. At some point this unfortunate chain of events will also hit us – or our neighbours, such as France, a nation which is so fond of nuclear energy. And what will happen then?

Merkel and her government want to review the safety standards at German nuclear power plants. By doing so, they hope to stifle the atomic energy debate. But why do the security measures need to be checked? Haven't we always being told that German nuclear power plants are the safest in the world?

Crisis meetings, security summits, special checks, those are the signs of a dying industry and the rearguard action of its political helpers. At some stage Germany will exit nuclear power – and sooner than advocates of the technology like to think. Old fears of a massive nuclear meltdown have resurfaced in Germany. Traditional nuclear critics within the Green party and the center-left Social Democrats will exploit these fears. A diehard pragmatist such as Merkel will certainly recognise this. And she will change her course. That much is certain.

But how quickly will Berlin be able to shift its tack on the issue? Is the country really moving quickly enough in its adoption of sustainable energies? Certainly not. The companies operating Germany's nuclear power plants are earning billions, but only a tiny fraction of those profits are being channelled into the expansion of the country's use of alternative energies. That must change.

Of course it would be nonsense to switch off all of Germany's power plants overnight just to appease the critics. But the development of sufficient quantities of renewable energies requires political resolve. And that resolve can only be generated once it becomes clear that Germany's nuclear power plants will soon be taken off the grid permanently. That was the logic behind the decision to phase out nuclear energy in Germany by 2022 – a policy established over a decade ago by Chancellor Gerhard Schröder's Social Democrats and their coalition partners, the Greens.

But that plan was reversed last autumn when Merkel's Christian Democrats, together with the business-friendly Free Democrats, put a halt to the phase-out and passed legislation extending the operating lives of Germany's nuclear power plants. Now it is clear: that decision was a mistake. And that mistake must now be corrected. No, it is no longer tempting to argue in favour of nuclear power. Not any more.