'Possible meltdown' at Japanese nuclear plant

'Possible meltdown' at Japanese nuclear plant

Nuclear Physicist Walt Patterson told the BBC's Nik Gowing that even
after the plant was shutdown its core would need to be cooled



Japan's prime minister
has declared a "nuclear emergency" after a number of reactors shut down
in the wake of a massive earthquake hitting the country.
Eleven
reactors at four nuclear power stations automatically shut down, but
officials said one reactor's cooling system failed to operate correctly.
Under Japanese law, an emergency must be declared if a cooling system fails.
In total, the country has 55 reactors providing about one-third of the nation's electricity.
In
a statement, the Japan Atomic Industrial Forum released a statement
that said Prime Minister Naoto Kan had declared the emergency "in case
prompt action" had to be taken, but added that "no release of
radioactive material" had been detected



It added: "Nuclear and Industrial Safety
Agency (Nisa) of the [Ministry of Economy, Trade and Industry] set up
an emergency preparedness headquarter... in an effort to collect
information on any possible damage to the NPPs (nuclear power plants).
"Since
emergency diesel generators at the Fukushima-1 and -2 NPPs are out of
order, (energy company) TEPCO sent the emergency report to Nisa. There
is no report that radiation was detected out of the site."
The
reactor at the Fukushima Daiichi power station that triggered the
emergency alert was the 40-year-old Reactor 1, one of six on the site.
Reactors
1, 2 and 3 automatically shut down when the Magnitude 8.9 quake shook
the plant, while reactors 4, 5 and 6 were not in operation as they were
undergoing scheduled inspections.
The reactors are
Boiling Water Reactors (BWR), one of the most commonly-used designs,
and widely used throughout Japan's fleet of nuclear power stations.
Heat
is produced by a nuclear reaction in the core, causing the water to
boil, producing steam. The steam is directly used to drive a turbine,
after which it is cooled in a condenser and converted back to water.
The water is then pumped back into reactor core, completing the loop.
Local evacuation
A
statement by the power station's operator, Tokyo Electric Power
Company, pressure inside the reactor had risen after the cooling system
had been damaged by the quake.
About 3,000 residents
within a two-kilometre radius of the power station, located about 170
miles north-east of Tokyo, were told to leave their homes as part of
the emergency procedure.
Even when the reactor is shut
down and the nuclear fission is halted, an intense level of heat
remains and needs to dissipated, which is the role of the cooling
mechanisms.
Dr Richard Phillips from the University
of Leeds said that a reactor has to be rapidly cooled when it is
automatically switched off.
"One power station failed
to cool sufficiently but the stations are robust and there is no
expectation that any leaks will occur," he explained.
"Once checks have been undertaken the stations should be back online in a few days."
It
is understood that the earthquake cut electricity supplies to the power
station, and the back-up generators did not come into operation when
the outage occurred. As a result, not all of the cooling systems were
available.
World Nuclear Association (WNA) spokesman
Jeremy Gordon said the state of emergency was a legal requirement and
did not mean that there was a genuine case for concern.
"It
allows authorities to take additional measures," he told BBC News. "It
empowers officials in the local region, such as the fire service,
police etc to take the action they need to take, but at this stage it
is purely precautionary."
Under Japanese law, a nuclear
emergency must be declared if there is a release of radiation, if there
is a dangerous level of water in the reactor, or if the cooling
mechanisms fail.
"It is important to remember that for
a large reactor like that, it would have a number of diesel generators
that are supposed to start up automatically, when the plant is
disconnected from the grid," Mr Gordon said.


But it is not the case that you have just one
generator - the nuclear business is not like that, you never rely on
just one thing. You always back up your back-ups."
Mr
Gordon added: "It is hard to find country more experienced in
earthquakes than Japan, and they are also one of the most experienced
in nuclear power."
He said that the country has been
commercially operating nuclear power stations for 45 years, during
which time there have been a number of major earthquakes.
"The
most recent quake to affect a nuclear power station was in 2007, and it
hit the seven-reactor Kashiwazaki-Kariwa plant very hard as I think the
epicentre was very close by.
"It was shaken a lot
harder than anyone ever thought it would be, so it was an example of
how the over-engineering in nuclear power station design goes beyond
the super-conservative regulatory requirements."
But
Steve Thomas, professor of energy policy at Greenwich University, said
the reactors were only now just beginning to come back into operation.
"There
were things that should've held together but didn't, and it's taken
them years to get [the reactors] back in service," he told BBC News.
"I think it was a shock to the Japanese that the plants didn't hold up as well as they should've done."
'Earthquake proof'
Nuclear
engineer and fellow of the UK's Royal Academy of Engineering Dame Sue
Ion said Japan had extremely tight standards when it came to ensuring
nuclear power plants were earthquake-resistant.
"Authorities,
utilities and reactor vendors ensure that appropriate safety systems
are incorporated at the design stage and implemented in construction
and operation," she observed.
"Systems automatically
shut down when trigger points are reached to allow for relevant safety
inspections to take place before restart.
"Japan's
nuclear power stations are being shown to be robust against the threat
of earthquake: Safety systems have operated as they should."
The
Japan Atomic Industrial Forum said that it would continue to post
regular updates on its website to keep people informed about
developments at Fukushima.



http://www.bbc.co.uk/news/science-environment-12711707