Newsgroup: it.politica
Titolo: Fukushima 'stable' as nuclear meltdown fears recede

http://www.newscientist.com/blogs/shortsharpscience/2011/03/massive-explosion-rips-through.html
Roger Highfield, magazine editor, and Yuriko Nagano, contributor, Tokyo
A 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 degrees Celsius, 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 2pm UK time and would take up to ten 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.
-- 
Namib
"I should never have switched from Scotch to Martinis"
(ultime parole di Humphrey Bogart)

 7 on the 1 to 7 scale.
"It also asks a key question: whether the floor of the reactor chamber
has been broken through in the explosion. If so, Japanese authorities
will have to contend with radioactive materials seeping into the
ground and turning the terrain into a Chernobyl-esque zone of
alienation."
http://www.mediaite.com/tv/report-fukushima-japan-reactor-now-in-meltdown-after-explosion/

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Mica dico sia tempo di stappar champagne. Ma (forse) i peggiori timori, 
per quel poco che (dubitosamente: in fondo, le autorità hanno in parte 
le loro ragioni, a minimizzare, pur mantenendo un ragionevole allarme) 
si sa, sono *un po'* più lontani. Teniamo le dita incrociate, insomma.
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