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Fukushima Crisis

Science

Boiling Water Reactor

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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:

Background

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 hundred rods.

  • The fuel rods are long tube like things made of zirconium alloy.

  • 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 circulated.

  • 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 producing reactions.

  • 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 heat too.

  • 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 breaks down. 

  • 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

What Now?

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 later.


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Friday, March 18, 2011