Nuclear meltdown - Fukushima Daiichi

[DukeLander]

Krško je u pitanju.

Krsko je tako odradjen? Bem ga davno sam ucio,vec jako brkam stvari... mada mislim i da su neke stambene sgrade tako odradjene u Zagrebu...

 

[dulsrules]

Meni nikako nije jasno kako su japanci inace poznati po zemljotresima i cunamijima izgradili nuklearku pored mora... to mi je nekako...drsko... pre ili kasnije bi se desilo ovo,zar ne?

Procitaj temu od pocetka.

 

[KAMIKAZE]

svaka cast! kakav je to narod. mi nikada necemo biti kao oni

Bili smo nekada i to na o-ruk :trust:

http://sr.wikipedia.org/sr-el/Пруга_Брчко-Бановићи

Sredinom marta 1946. godine, građevinsko odeljenje tadašnjeg ministarstva saobraćaja dobilo je nalog da pristupi izvođenju pripremnih radova. Elaborat o izgradnji bio je gotov za nepunih mesec dana. Na dan 1. aprila počele su pripreme i taj datum je Dan početka omladinskih radnih akcija. 1. maja, prve omladinske radne brigade otvorile su radove na pruzi Brčko-Banovići, dugoj 92 km. U toku izgradnje pruge iskopano je 1.361.680 km² zemlje i 134.460 km² kamena. Izgrađena su dva tunela u dužini 667 m i 22 mosta ukupne dužine 455 m.

Prvi voz ovom prugom prošao je 7. novembra 1946. godine.

CNN-a mi je vec preko glave :vomit:

Ne znam zasto je SBB izbacio BBC World iz osnovne ponude

 

[slobans]

mali off topic,pogledajte antenice u cernobilu

http://www.youtube.com/watch?v=zQfzUHeH4Eg&feature=related

 

[exste]

pa...to je to... nema vise pomoci..koliko vidim pitanje je trenutka kada ce objaviti da je iscurelo otislo ko zna gde...ostaje nejasno da li ce se sva 4 otopiti ili samo 2 kao i radijus nove zone...:S:

Ovo sa helikopterom i topovima je potez iz ocaja i posle svega iznenadjen sam nacinom kako su mislili da mogu da izvedu to hladjenje...da su krenuli sa zalivanjem betonom iz helikoptera mozda bi i postigli nesto..

 

[marko_n]

Chernobyl disaster

Nisam imao pojma da je nuklearka u Černobilu nastavila sa radom i posle incidenta i radila sve do 2000. godine?? :zgran: :trust:

Russia, Ukraine, and Belarus have been burdened with the continuing and substantial decontamination and health care costs of the Chernobyl accident. More than fifty deaths are directly attributed to the accident, all among the reactor staff and emergency workers. Estimates of the total number of deaths attributable to the accident vary enormously, from possibly 4,000 to close to a million.[4][5] Despite the accident, Ukraine continued to operate the remaining reactors at Chernobyl until 2000, when the last reactor at the site was closed down.

 

[yooyo]

Kada se desio zemljotres, NE je automatski zapocela proceduru gasenja reaktora. Posto tokom gasenja reaktora elektrana ne proizvodi struju, pokrenuli su se dizel generatori koji su napajali pumpe za destilovanu vodu. Medjutim, tsunami je ostetio dizel generatore i oni su prestali da rade. Tada se upalio jos jedan sistem zastite - akumulatori koji su napajali pumpe za vodu jos 8 sati, dok se nisu potrosili. Posle toga na lokaciji nije bilo nikakvog izvore energije. A da bi se jezgro ohladilo potrebno je 2-3 dana konstantnog hladjenja.

Sto se aktuelne situacije tice mislim da mediji preteruju i prave nepotreban hype. Vodena para koja izlazi iz jezgra je radioaktivna ali to nije kolicina koju je recimo ispustio Cernobil prilikom pozara. Prilicno je radioaktivno na samoj lokaciji.. tj iznad. Sa strane je povisen nivo radioaktivnosti, tj. na granici je opasnog. Radnici koji su ostali da spasavaju stvar nisu samoubice vec trenirani ljudi koji znaju sta rade.

Na granici od 20km od lokacije nivo radijacije je sasvim normalan.. osim ako se nesto nije nepredvidjeno desilo i da sam propustio.

Povisen nivo radijacije u Minhenu nema nikakve veze sa situacijom u Japanu. Vazdusne struje nemogu tako brzo da obidju planetu. Obzirom da vetar iznad NE duva ka jugoistoku i nosi radioaktivno zagadjenje ka Tihom Okeanu i Australiji (mada ce je promasiti) nema sansi da je taj vazduh dospeo iznad Nemacke.

Jedini fail koji ja primecujem je da su dizel generatori bili nisko na zemlji imesto da su i oni podignuti na visi nivo dalje od vode.

 

[Levčanin]

Chernobyl disaster

Nisam imao pojma da je nuklearka u Černobilu nastavila sa radom i posle incidenta i radila sve do 2000. godine?? :zgran: :trust:

Da tako je... mogli su oni još da šljakaju... a šta misliš zašto su reaktori isključeni u Bugarskoj NE kad su ušli u EU... isti razlog a to je rizik verovatno...

Jel čuo neko da su doveli struju u Fukušimu i uključili hlađenje reaktora? trebalo bi danas to da bude.

 

[yooyo]

Kada se desio zemljotres, NE je automatski zapocela proceduru gasenja reaktora. Posto tokom gasenja reaktora elektrana ne proizvodi struju, pokrenuli su se dizel generatori koji su napajali pumpe za destilovanu vodu. Medjutim, tsunami je ostetio dizel generatore i oni su prestali da rade. Tada se upalio jos jedan sistem zastite - akumulatori koji su napajali pumpe za vodu jos 8 sati, dok se nisu potrosili. Posle toga na lokaciji nije bilo nikakvog izvore energije. A da bi se jezgro ohladilo potrebno je 2-3 dana konstantnog hladjenja.

Sto se aktuelne situacije tice mislim da mediji preteruju i prave nepotreban hype. Vodena para koja izlazi iz jezgra je radioaktivna ali to nije kolicina koju je recimo ispustio Cernobil prilikom pozara. Prilicno je radioaktivno na samoj lokaciji.. tj iznad. Sa strane je povisen nivo radioaktivnosti, tj. na granici je opasnog. Radnici koji su ostali da spasavaju stvar nisu samoubice vec trenirani ljudi koji znaju sta rade.

Na granici od 20km od lokacije nivo radijacije je sasvim normalan.. osim ako se nesto nije nepredvidjeno desilo i da sam propustio.

Povisen nivo radijacije u Minhenu nema nikakve veze sa situacijom u Japanu. Vazdusne struje nemogu tako brzo da obidju planetu. Obzirom da vetar iznad NE duva ka jugoistoku i nosi radioaktivno zagadjenje ka Tihom Okeanu i Australiji (mada ce je promasiti) nema sansi da je taj vazduh dospeo iznad Nemacke.

Jedini fail koji ja primecujem je da su dizel generatori bili nisko na zemlji umesto da su i oni podignuti na visi nivo dalje od vode.

 

[Levčanin]

Chernobyl disaster

Nisam imao pojma da je nuklearka u Černobilu nastavila sa radom i posle incidenta i radila sve do 2000. godine?? :zgran: :trust:

Da tako je... mogli su oni još da šljakaju... a šta misliš zašto su reaktori isključeni u Bugarskoj NE kad su ušli u EU... isti razlog a to je rizik verovatno...

Jel čuo neko da su doveli struju u Fukušimu i uključili hlađenje reaktora? trebalo bi danas to da bude.

 

[bit]

Most of people campare Fukushima with Chernobyl, seeing the consequences of breakdown, such as radiation, evacuation of civilians etc.

I want to compare another aspect: behavior and actions of plant's management, personnel and rescue services. In my opinion, this is definitely NOT in favor of Fukushima.

In Chernobyl the sutuation was much worse: destroyed reactor with extremely high radiation level. But after few minutes after disaster people started real acttions, not caring about their lives and health much... Fire brigades, troops, civil defence and specialists were doing their job, and they succeeded. That was a real herouism, not Hollywood-style trouble shooting.

But in Fukushima there are weak attemts to do cooling and take care about the staff' health, but not about nation's one... Whey can not they arrange more pumping stations to cool the system? Why they waited until the hydrogen exploded, not releasing the pressure continuosly? Why they stop the works and evacuate the staff instead of rotating them within acceptable radiation limits? Thes is strange behavior, and I do not believe in miracle of Japanese technology if they can not effectively manage diffucult situations like this.

....

 

[DukeLander]

Jedini fail koji ja primecujem je da su dizel generatori bili nisko na zemlji umesto da su i oni podignuti na visi nivo dalje od vode.

E o tome ja pricam...

 

[kUdtiHaEX]

^^ Koja gomila nebuloza...

 

[R_O_N_I_N]

Da se ovo nije desilo pojma ne bih imao koliko u svetu i kod nas u stvari ima eksperata za nuklearnu energiju....prosto neverovatno....,)

 

[marko_n]

Posto tokom gasenja reaktora elektrana ne proizvodi struju, pokrenuli su se dizel generatori koji su napajali pumpe za destilovanu vodu. Medjutim, tsunami je ostetio dizel generatore i oni su prestali da rade

...

Jedini fail koji ja primecujem je da su dizel generatori bili nisko na zemlji umesto da su i oni podignuti na visi nivo dalje od vode.

Ili to, ili zapravno nije bilo dizela ili su sami agregati bili zapušteni. Ne verujem da se za UPS jedne nuklearke koriste generatori koji otkažu zbog vode, mada ko zna, možda je i tako.

Jel čuo neko da su doveli struju u Fukušimu i uključili hlađenje reaktora? trebalo bi danas to da bude.

TEPCO reče (HKN) da neće pre petka jer im ide užasno sporo zbog povišene radijacije i uslova rada.

 

[rAMA]

....

Upravo o ovome pričam, moraju slati, rotirati ljude koji će se baviti hlađenjem, diretno šmrkom ako treba, a istovremeno raditi na tome da se izradi neki sistem koji će taj posao obavljati dugotrajno, umesto toga oni povlače i helikoptere zbog "radiacije", ne može tako, mora se podneti žrtva, pogotovo ako postoji realna opasnost za nešto daleko gore. Nema tu mesta sentimentalnosti..

 

[BelThaZoR]

Kada se desio zemljotres, NE je automatski zapocela proceduru gasenja reaktora. Posto tokom gasenja reaktora elektrana ne proizvodi struju, pokrenuli su se dizel generatori koji su napajali pumpe za destilovanu vodu. Medjutim, tsunami je ostetio dizel generatore i oni su prestali da rade. Tada se upalio jos jedan sistem zastite - akumulatori koji su napajali pumpe za vodu jos 8 sati, dok se nisu potrosili. Posle toga na lokaciji nije bilo nikakvog izvore energije. A da bi se jezgro ohladilo potrebno je 2-3 dana konstantnog hladjenja.

Sto se aktuelne situacije tice mislim da mediji preteruju i prave nepotreban hype. Vodena para koja izlazi iz jezgra je radioaktivna ali to nije kolicina koju je recimo ispustio Cernobil prilikom pozara. Prilicno je radioaktivno na samoj lokaciji.. tj iznad. Sa strane je povisen nivo radioaktivnosti, tj. na granici je opasnog. Radnici koji su ostali da spasavaju stvar nisu samoubice vec trenirani ljudi koji znaju sta rade.

Na granici od 20km od lokacije nivo radijacije je sasvim normalan.. osim ako se nesto nije nepredvidjeno desilo i da sam propustio.

Povisen nivo radijacije u Minhenu nema nikakve veze sa situacijom u Japanu. Vazdusne struje nemogu tako brzo da obidju planetu. Obzirom da vetar iznad NE duva ka jugoistoku i nosi radioaktivno zagadjenje ka Tihom Okeanu i Australiji (mada ce je promasiti) nema sansi da je taj vazduh dospeo iznad Nemacke.

Jedini fail koji ja primecujem je da su dizel generatori bili nisko na zemlji umesto da su i oni podignuti na visi nivo dalje od vode.

+1

A sto se tice pitanja zasto nisu koristili Iljusine za gasenje umesto helikoptera odgovor je vrlo jednostavan. IL-76 je dobar za gasenje pozara na velikoj povrsini, ne postoji nacin da u letu sipa vodu direktno u reaktor, vise vode bi se prosulo okolo. Ne postoji avion (ne racunam VTOL letelice koje nemaju veliki kapacitet) koji moze da se krece dovoljno sporo da bi sav svoj tovar ispraznio u tako mali prostor.

 

[yooyo]

@marko_n:
Dizel generatori su zaista radili nekoliko minuta tj. do trenutka kada ih je pogodio tsunami. Sve Japanske NE se automatski gase prilikom zemljotresa. Njihov sistem za rano upozorenje je u stanju da predvidi zemljotres par minuta pre nego sto se zaista desi i tada se NE automatski gase.
Cak je i sama NE izdignuta na obali, ali ocito ne dovoljno za ovaj tsunami.
Cinjenica je da su NE (pa cak i ostale gradjevine u Japanu) izdrzale 10x veci zemljotres od predvidjenog govori da se oni nisu igrali sa bezbednoscu.

Takodje u slucaju potpunog topljenja jezgra, radioaktivna masa ce pasti u posebo pripremljen bunker ispod jezgra koji je napravljen od olova, betona i ko zna ceka sve tu ima i bice hermeticki zatvoren (zaliven). Trenutna opasnost je da se okolina ne ozraci previse da bi ljudi mogli da stanuju. Ponavljam, novi radioaktivnosti na 20km od NE je normalan.

@BelThaZoR:
Rizik od direktnog prosipanja tolike kolicine vode na reaktor je veliki.. Niko nezna u kakvo je stanju konstrukcija i da li bi zbog siline udara vode nesto popustilo i puklo.

 

[magny_cours]

Ponavljam, novi radioaktivnosti na 20km od NE je normalan.

Netacno.

14:48 - Visoka razina radijacije izmjerena je 30 kilometara dalje od oštećene nuklearke, izvijestila je NHK televizija citirajući ministra znanosti.

[9:38 a.m. ET Thursday, 10:38 p.m. Thursday in Tokyo] High levels of radiation have been detected 30 kilometers (18.6 miles) from the damaged Fukushima Daiichi nuclear plant, broadcaster NHK reports, citing Japan's Science Ministry. Exposure to those levels of radiation for six hours would be equivalent to the safe level of what a person can absorb in a year, according to the report.

 

[Levčanin]

TEPCO reče (HKN) da neće pre petka jer im ide užasno sporo zbog povišene radijacije i uslova rada.

Auf ako je to tačno onda je pitanje dal će izdržati reaktori dotle... Auuuuuu ako krenu da se tope, biće crno... i to nekoliko , ne jedan

 

[__zevs__]

Chernobyl disaster

Nisam imao pojma da je nuklearka u Černobilu nastavila sa radom i posle incidenta i radila sve do 2000. godine?? :zgran: :trust:

Zasto ne bi. To ozraceno podrucje je idealno za rad NE. Ako se nesto lose dogodi ionako niko tu ne zivi. Svakako bolje nego da su izgradili nove reaktore negde u zdravoj sredini.

 

[bit]

^^ Koja gomila nebuloza...

nebuloze ili ne, hlađenje nuklernog reaktora iz helikoptera koji 95% vode prospe na pogrešno mesto, je u najmanju ruku smešno (pa kada gase šumske požare pa ne uspevaju) i da prospe sve tamo gde treba, to su smešni pokušaji.Po vestima, 7 dana nisu u stanju da dovuku struju, ajde da se to događa u Mongoliji ili negde gde nema neke naročite elektro strukture.
same eksplozije su rasturile reaktore...

“All reactors were damaged in the earthquake. The containment vessels are breaking down and they will continue breaking down”, says Even. “The question now is: what will happen to the content of these reactors, these highly radioactive isotopes? Especially worrying is the presence of plutonium in these reactors. It is something which has not happened in Chernobyl – it did not contain plutonium, which is very, very poisonous. Contamination by plutonium is very difficult to handle. On the other hand, unlike what happened in Chernobyl, there is no graphite in the reactors that can burn and send the contamination into the air and hence disperse to a wide area. So, contamination is there. It is spreading slowly. Hopefully, it will be carried to the sea, but no one can guarantee that.”

http://rt.com/news/pessimism-fukishima-1-crisis/

 

[magny_cours]

Njemački profesor: Radnici koji su u nuklearki umrijet će za nekoliko tjedana!

Njemački profesor Edmund Lengfelder s minhenskog instituta za radiologiju izjavio je kako će radnici koji se nalaze u nuklearki Fukushima sigurno umrijeti, a prema njegovim riječima, neki imaju još samo par tjedana života. "Oni će se zaista žrtvovati. 50 posto onih koji su kontaminirani u Fukushimi, umrijet će za nekoliko tjedana.", izjavio je Lengfeder prenosi Bild.

http://www.index.hr/vijesti/clanak/...-s-istrosenim-gorivom-je-bez-vode/542758.aspx

 

[marko_n]

Auf ako je to tačno onda je pitanje dal će izdržati reaktori dotle... Auuuuuu ako krenu da se tope, biće crno... i to nekoliko , ne jedan

Upravo na HNK pričaju da je povezivanje struje (i kada se dogodi) tek jedan deo posla, a da onda treba nekako osposobiti pumpe.

Zasto ne bi. To ozraceno podrucje je idealno za rad NE. Ako se nesto lose dogodi ionako niko tu ne zivi. Svakako bolje nego da su izgradili nove reaktore negde u zdravoj sredini.

A posada? Ko je pristao da tamo živi & radi? Malo mi je to neverovatno, ali definitivno je tako bilo.

 

[P$ych0]

Ja ne znam ko je ovo snimio ali nece mu biti dobro :zgran:

http://www.youtube.com/watch?v=lBXqiw6EJUk&feature=player_embedded

 

[marko_n]

Ja ne znam ko je ovo snimio ali nece mu biti dobro :zgran:

http://www.youtube.com/watch?v=lBXqiw6EJUk&feature=player_embedded

Uh... ovo ne izgleda dobro. Uopšte ne ostavlja utisak da unutra bilo šta od kontrolne elektornike ili pumpi može da radi. Sve izgleda potpuno rastureno.

 

[Boris Lozac]

Moguce iz Cinukova koji su bacali vodu.. koliko bolesno izgleda

 

[Levčanin]

Fukušima nadrljala jer je bila najizloženija cunamiju lepo se vidi... Pitam se samo kako to nisu predvideli u projektovanju... i kako ih je baš sve što su hteli da urade poteralo naopako... to mi je nekako čudno. NE jednostavno nije projektovana da izdrži ovakav cunami... to je najveći fail što nasu razmišljali šta ako ... to AKO je bitno... Izgleda je su Japanci poslovično precizni nije bilo razmišljanja o najgorem mogućem scenariju...

 

[P$ych0]

Ako je ovo istinit snimak (60-70 km ZAPADNO dok vetar duva ISTOCNO) onda ne mogu ni zamisliti koliki je nivo radijacije na licu mesta

http://ireport.cnn.com/docs/DOC-573238

 

[IzNoGooD]

Ja ne znam ko je ovo snimio ali nece mu biti dobro :zgran:

http://www.youtube.com/watch?v=lBXqiw6EJUk&feature=player_embedded

sad bas gledam taj snimak na BBC i jurim ga negde na netu - hvala!

inace, bacite pogled na ovo:

Reactor 1: Explosion on 12 March, ripped off roof.

Reactor 2: Explosion on 15 March, some internal damage, most likely source of radiation spike that day.

Reactor 3: Explosion on 14 March, roof damaged. Fears damage may have led to radiation spike on 16 March - although later reports said it was unclear what caused the leap in levels.

Reactor 4: Under maintenance at time of quake. At least two fires reported since.

Reactors 5 and 6: Under maintenance at time of quake. Fears overheating may lead to explosions.

Reactors 4, 5 and 6 were shut down at the time of Friday's earthquake, with some or all of their fuel rods extracted and left in the cooling ponds that each reactor building has under its roof.

Once a reactor is turned off, radioactivity and heat generation in the rods die away quickly; down to 7% of the original power within a second of switch-off, 5% within a minute, 0.5% within a day.

Transferred to the cooling pond, allowing technicians to do routine maintenance on the reactor, the rods are supposed to sit quietly until the time comes for their re-insertion or their journey towards disposal.
MOX fuels in Fukushima reactor 1 pool Hot fuel rods in dry pools may be the source of increased radiation levels

The tops of the rods are supposed to be about 5m (16ft) below the water surface.

The water keeps them cool and also blocks radiation.

Over the last few days there have been reports suggesting water levels were low and the water "boiling"; and now the US Nuclear Regulatory Commission (NRC), which has a team of 11 experts advising in Japan, says the pool is completely dry.

This means the fuel rods are exposed to the air. Without water, they will get much hotter, allowing radioactive material to escape.
Continue reading the main story
“Start Quote

The moment when fuel rods are covered with water, the situation is basically stabilised”

End Quote Jasmina Vujic University of California

* Aerial view of the damage

More remarkably, the Tokyo Electric Power Company (Tepco), which owns the power station, has warned: "The possibility of re-criticality is not zero".

If you are in any doubt as to what this means, it is that in the company's view, it is possible that enough fissile uranium is present in the cooling pond in enough density to form a critical mass - meaning that a nuclear fission chain reaction could start.

The pool lies outside the containment chamber.

So if it happened, it would lead to the enhanced and sustained release of radioactive materials - though not to a nuclear explosion - with nothing to stop the radioactive particles escaping.
Water cannon

The first event in this chain appears to be that the level of water in the pond fell.

Why that should happen is not entirely clear.

Was the fuel so hot that it caused an unanticipated amount of evaporation? Did the earthquake somehow crack the building's structure, allowing water to leak out?

Did the two fires in the building have an impact? Visually, the building housing reactor 4 is the most damaged on the site - suggesting that reports of the two fires being relatively minor were wide of the mark.

Or, did technicians at some point take water from the pond for use in reactor 4's cooling system?

Click to play

Helicopters abandon nuclear plant water drop

There is nothing to say they did; but during the chaos of the weekend, with power systems and options disappearing before their eyes, it might have seemed like a good idea.

Whatever the reason, the rods became hot enough that steam reacted with the zirconium cladding around the fuel rods, generating hydrogen and causing an explosion.

The same thing happened earlier in reactor buildings 1, 2 and 3 - except that in building 4, the rods were in the fuel pond, not in a reactor.

The government ordered Tepco to put water back in the pool.

But either because of high radiation levels or broken pumps or some other reason, they could not.

Wednesday's plan to drop water in from a helicopter - a technique that is used to fight forest fires - had to be scrapped because of concerns about radiation affecting the pilots. Without the water, gamma-rays travel straight up into the air.

There are reports that the authorities have asked US military personnel to bring in water cannon, which would presumably be fired from the ground, aiming to shoot the water in through the broken roof.
'Rock and a hard place'

The NRC says that in the current dry state, radiation levels from the pond are probably "extremely high", creating a danger to workers at the plant.

Still, in principle this should not raise any possibility of resumed criticality.

According to Laurence Williams, professor of nuclear safety at the University of Central Lancashire, it could depend on how the rods are arranged in the water.
Man watching TVs in South Korea The incident has sparked fears of radioactivity in other Asian countries

"In some fuel ponds, they dose the water with boric acid at low levels," he told BBC News.

"In some systems they've re-racked the fuel assembly making it possible to put more rods in the pond than it was originally designed for, and then you might put extra sheets of boron in between."

Boron and boric acid mop up neutrons, the particles that sustain the chain reaction.

In this sort of reactor, water is a crucial component of the fission process.

It acts as a moderator - it reduces the speed of the neutrons, meaning they can be captured by uranium nuclei in the fuel rods, inducing them to split.

Without water, the neutrons travel too fast, and are not captured.

Professor Williams raised a scenario that may be unfolding in the cooling pond in building 4. It is just a possibility, because information is scanty; but here it is.

If the fuel rods are dry and hot, there could be damage to the cladding and the release of light radioactive nuclei.

To prevent that, you would want to inject water. But water on its own is a neutron moderator and would enhance the chances, however small, of criticality.

"You're caught between a rock and a hard place," he observed.

Now, Tepco is also talking about putting boric acid into the cooling pond of number 4 building.

How closely the rods were packed, whether any boron sheets were in place and if so whether they were damaged by one of the two fires in the building; these are among the many unanswered questions.
Core task

Meanwhile, the most important task remains to get enough water flowing into reactors 1, 2 and 3 to cool the cores.
Stop signs outside Chernobyl site Even at its worst, Fukushima will not be a replay of Chernobyl

Following Tuesday's apparent cracking of the suppression chamber in reactor 2 - a likely cause of the radioactivity spike then seen - there were concerns that the same thing might have occurred on Wednesday in reactor 3.

However, the latest reports from Japanese news sources suggest reactor 3's containment system is intact.

If that proves to be the case, the source of Wednesday's radiation spike remains a mystery.

It seems to have been big enough to force technicians to leave the plant.

Clearly, their presence is crucial to stabilising the reactors - and to control the situation in the dry pool in building 4. If they were forced away for long periods, the chances of containing the crisis would fall.

"If water is continuously pumped, they could stabilise the position, because the moment when fuel rods are covered with water, the situation is basically stabilised," observed Jasmina Vujic, professor of nuclear engineering at the University of California, Berkeley.

Tepco sounded a rare optimistic note by saying engineers would soon restore an electrical connection from the national grid - which should allow them to re-start water pumps, provided they have not been damaged by the tsunami or the hydrogen explosions.

While this work continues, so do the questions over the building 4 fuel pond.

Re-criticality seems an extraordinary thing to contemplate; but if it is not a real possibility, why was such an idea floated by the company itself?

The bigger picture, though, is still one of a serious local incident, with minor impacts outside the plant.

US Energy Secretary Steven Chu, however, suggested Fukushima was now more serious than the 1979 Three Mile Island incident in the US - and if contamination does spread outside the immediate area, that will prove to be the case.