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发帖数: 5516 | 1 http://www.bbc.co.uk/news/world-12723092 （附图）
The word "meltdown" goes to the heart of the big nuclear question - is
nuclear power safe?
The term is associated in the public mind with the two most notorious
accidents in recent memory - Three Mile Island, in the US, in 1979, and
Chernobyl, in Ukraine, seven years later.
You can think of the core of a Boiling Water Reactor (BWR), such as the ones
at Fukushima Daiichi, as a massive version of the electrical element you
may have in your kettle.
It sits there, immersed in water, getting very hot.
The water cools it, and also carries the heat away - usually as steam - so
it can be used to turn turbines and generate electricity.
If the water stops flowing, there is a problem. The core overheats and more
of the water turns to steam.
The steam generates huge pressures inside the reactor vessel - a big, sealed
container - and if the largely metal core gets too hot, it will just melt,
with some components perhaps catching fire.
In the worst-case scenario, the core melts through the bottom of the reactor
vessel and falls onto the floor of the containment vessel - an outer sealed
unit.
This is designed to prevent the molten reactor from penetrating any further.
Local damage in this case will be serious, but in principle there should be
no leakage of radioactive material into the outside world.
But the term "in principle" is the difficult one.
Continue reading the main story
“Start Quote
The job of keeping dangerous materials sealed in falls to the
containment vessel inside.”
End Quote
* Timeline: Japan power plant explosion
Reactors are designed to have "multiply redundant" safety features: if one
fails, another should contain the problem.
However, the fact that this does not always work is shown at Fukushima
Daiichi.
The earthquake meant the three functioning reactors shut down. But it also
removed the power that kept the vital water pumps running, sending cooling
water around the hot core.
Diesel generators were installed to provide power in such a situation. They
did cut in - but then they cut out again an hour later, for reasons that
have not yet been revealed.
In this case, redundancy did not work.
And the big fear within the anti-nuclear movement, as used in the film The
China Syndrome, is that the multiple containment of a molten core might not
work either, allowing highly radioactive and toxic metals to burrow into the
ground, with serious and long-lasting environmental impacts - total
meltdown.
Boiling water reactor system schematic diagram
However, the counter-argument from nuclear proponents is that the partial
meltdown at Three Mile Island did not cause any serious effects.
Yes, the core melted, but the containment systems held.
And at Chernobyl - a reactor design regarded in the West as inherently
unsafe, and which would not have been sanctioned in any non-Soviet bloc
nation - the environmental impacts occurred through explosive release of
material into the air, not from a melting reactor core.
To keep things in perspective, no nuclear accident has caused anything
approaching the 1,000 fatalities stemming from Friday's earthquake and
tsunami.
'Subcritical' reactors
Whether a partial meltdown is under way at Fukushima Daiichi is not yet
clear.
The most important factor is summed up in a bulletin from the Tokyo Electric
Power Company (Tepco) that owns the facility: "Control rods are fully
inserted (reactor is in subcritical status)."
Smoke billowing from Fukushima nuclear plant A large explosion was seen at
the plant with debris blown out from the building
Control rods shut off the nuclear reaction. Heat continues to be produced at
that stage through the decay of radioactive nuclei - but that process in
turn will begin to tail off.
Intriguingly, Ryohei Shiomi, an official at Japan's Nuclear Safety
Commission, is widely quoted as having said a meltdown was possible and that
officials were checking.
Meanwhile, a visually dramatic explosion in one of the reactor buildings has
at least severely damaged the external walls.
In principle, this should not cause leakage of radioactive material because
the building is just an outside shell; the job of keeping dangerous
materials sealed in falls to the the metal containment vessel inside.
Chief cabinet secretary Chief Yukio Edano confirmed this was the case,
saying: "The concrete building collapsed. We found out that the reactor
container inside didn't explode."
He attributed the explosion to a build-up of hydrogen, related in turn to
the cooling problem.
Under pressure
The only release of any radioactive material that we know about so far
concerns venting of the containment vessel.
When steam pressure builds up in the reactor vessel, it stops some of the
emergency cooling systems working, and so some of the steam is released into
the containment vessel.
Continue reading the main story
“Start Quote
The whole incident so far contains more questions than answers”
End Quote Richard Black
However, according to World Nuclear News, an industry newsletter, this
caused pressure in the containment vessel to rise to twice the intended
operating level, so the decision was taken to vent some of this into the
atmosphere.
In principle, this should contain only short-lived radioactive isotopes such
as nitrogen-16 produced through the water's exposure to the core. Venting
this would be likely to produce short-lived gamma-ray activity - which has,
reportedly, been detected.
One factor that has yet to be explained is the apparent detection of
radioactive isotopes of caesium.
This is produced during the nuclear reaction, and should be confined within
the reactor core.
If it has been detected outside the plant, that could imply that the core
has begun to disintegrate.
"If any of the fuel rods have been compromised, there would be evidence of a
small amount of radioisotopes in the atmosphere [such as] radio-caesium and
radio-iodine," says Paddy Regan, professor of nuclear physics at the UK's
University of Surrey.
"The amount that you measure would tell you to what degree the fuel rods
have been compromised."
It is an important question - but as yet, unanswered.
Cover-ups and questions
In fact, the whole incident so far contains more questions than answers.
Parallels with Three Mile Island and Chernobyl suggest that while some
answers will materialise soon, it may takes months, even years, for the full
picture to emerge.
How that happens depends in large part on the approach taken by Tepco and
Japan's nuclear authorities.
As with its counterparts in many other countries, Japan's nuclear industry
has not exactly been renowned for openness and transparency.
Tepco itself has been implicated in a series of cover-ups down the years.
In 2002, the chairman and four other executives resigned, suspected of
having falsified safety records at Tepco power stations.
Further examples of falsification were identified in 2006 and 2007.
In the longer term, Fukushima Daiichi raises several more very big questions
, inside and outside Japan.
Given that this is not the first time a Japanese nuclear station has been
hit by earthquake damage, is it wise to build such stations along the east
coast, given that such a seismically active zone lies just offshore?
And given that Three Mile Island effectively shut down the construction of
civilian nuclear reactors in the US for 30 years, what impact is Fukushima
Daiichi likely to have in an era when many countries, not least the UK, are
looking to re-enter the nuclear industry? |
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