The terrible events in Japan have caused a lot of confusion. Some
of our readers are scientists and they've recommended some source
material for us to produce a summary of what might have happened as a
result of the earthquake and damage caused by the Tsunami. We
don't know if the events happened in exactly the way described, but they
might have. We also don't know what modifications were made to the
1970's version reactors that are causing so much trouble. So here
is the summary. Hopefully, it will give you an idea of what is
going on with the disaster:
The Japanese power station consists of six boiling water reactors
These are old designs having come in to service a long time ago:
The core is a pressure vessel filled with a few
hundred fuel assemblies. Each fuel assembly has a couple of
The fuel rods are long tube like things made of
The zirconium tubes are filled with pellets of
enriched uranium. Some contain U-235 and some Plutonium-239.
It's said that some of the latter might have come from Russia as
fuel to be burned.
This fuel can be used for as much as a year and then
it needs to be replenished.
The whole process depends upon water. That's
why the plant is located so close to the sea. What is needed
to boil to drive the turbines and water is needed to cool the core
and in part control the powerful nuclear reaction.
When enough fuel is brought together under the right
circumstances, heat is generated that produces steam that in turn
rotate turbines (blade like affairs). This generates
electricity, which runs Japanese industry and homes.
Since water is heated it needs to eventually be
cooled and some of it disappears in steam and some is cooled and
then returns to be used again.
What Might Have Happened?
The giant 9.0 quake caused a huge wave to move at
500 miles per hour with the wave wall very thick, not like a Lake
Huron wave. The combination of the quake and the wave interrupted
electrical service, which in turn stopped the water from being
The reactor went into automatic shutdown and the
electrical standby generators were set to 'kick in', but they were
somehow damaged or flooded.
In the shut-down process special rods are inserted.
They are called control rods and they are designed to stop the power
As nuclear fuel burns, it produces some new products
that don't respond to the control rods effects. A small part
of this type of reactor's heat comes from these new by-products of
the reaction. Without cooling water, they begin to build up
Without the water circulation, heat begins to rise
slowly, but surely.
As the water's temperature rises to more than 1000
degrees, the zirconium coating melts and what water is left turns
into highly volatile hydrogen and oxygen as H20
Whatever happened next produced a gigantic
explosion, probably hydrogen ignition. Were the operators
trying to release the pressure.? This will come out sometime.
Were they availing themselves of the backup power from the batteries
or were the batteries working at all?
The operators, it is said, have been trying to
inject the reactors with boric acid. Boron is supposed to be
able to absorb neutrons.
Also compromised may be the spent fuel pools that
might have lost their cooling and circulating water
Again the key is cooling and that can come with water,
but how to inject it? Here is what has been tried to date:
Helicopters have tried dumping water from a safe
height, but this is dangerous and the dispersal it wide and the
effects don't seem to be good.
Water cannons have been tried, but they don't seem
to be able to get close enough.
They are hoping to restore some power and see if
they can restart the water circulation.
If all these fail, will they have to pour concrete
in to seal the reactors?
Simplified Schematic of a BWR
One major disadvantage of some Boiling
Water Reactors (BRWs) is that the same
loop serves as core moderator, steam source for the turbine and coolant
for the core. The moderator part is to slow the neutrons to
maintain the chain reaction
The bad part of these reactors is that an interruption
or break in this loop can really cause a lot of trouble.
In Pressurized Water Reactors
(PWRs), the water which passes over the reactor core to act as
moderator and coolant, does not flow to the turbine, but is contained in
a pressurized primary loop.
There are lot of variations of these reactors. The
CANDU reactors are made here in Canada and are reported to have many
safety features. They were originally designed by the predecessor
of Ontario Power Generation and GE of Canada. More about them
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