More on Spent fuel storage

http://www.nytimes.com/2011/03/18/world/asia/18spent.html?pagewanted=2&_r=1&ref=world

A great article – what the NY Times can be when it wants to…

Details it brings out (or things correcting misunderstandings on my part)

What is a nuclear fuel rod assembly?

Imagine those round dog treats that are shaped like an oatmeal box.  Now take a 13 foot long metal straw and fill it up with a string of pellets, made of about 4% Uranium 235 fuel and 96% inert Uranium 238.   A brand new fuel rod emits minimal radiation and you could hold it in your hand for days without any radiation consequences to you.

Now take 64 of those rods and put them into a frame, so each rod is held firmly in place and can be easily lifted in and out of the reactor core.   The storage tank at reactor #4 is essentially full – it has 1,479 of these fuel assemblies in storage.

A storage tank is like a 39 foot deep swimming pool.   The 13 foot high fuel rod assemblies sit on the bottom, covered by another 26 feet of water.   The water is light water and laced with boron.  It is kept cool by circulating water (that’s what is broken now).   That keeps things quiet and the 26 feet of water protects the workers from radiation when they need to go inside the reactor containment building.    Under normal circumstances, used fuel rods will cool down enough in 3 to 5 years that they need less cooling.   Storage pool #7 at Fukushima is holding the cooled down stored spent fuel.

Fukushima has so much spent fuel in its pools (as does America) because we have failed to develop a way to reprocess the fuel rods or build permanent storage sites (See: Yucca mountain) to get the spent fuel away from the the plants.  

Since 1982, US power companies have been paying about $500 million a year into a Nuclear Waste Fund to pay the costs for eventual permanent storage, but like all “trust” funds, it is being used instead to hand out free cheese to poor people and Harry Reed has shut down any idea of using Yucca Mountain.   Even if the DOE had the money, the fee being charged is nowhere near the anticipated actual disposal costs.  The US military also contributes – as of 2007, the “Trust fund” had $21.7 billion, which is sitting in an “account” over at the U.S. Treasury, the same as the Social Security Trust Fund.

When you take a fuel rod and put it inside a reactor where Nuclear Fission is happening, the neutrons hit the Uranium 235 and that creates more neutrons and heat.   The neutrons also hit the inert U238 and suddenly it is no longer inert.  It converts eventually into Plutonium 239 which is very radioactive for a very long time, but is also a good nuclear fuel as well, like U235.  Other radioactive stuff is also generated – Iodine, Cesium, and Strontium.   As long as the fission has stopped, no new radiative byproducts are created (although what you created already continues to decay into other “stuff”).  Iodine loses its radiation in a few weeks, but the others stay radioactive for many many years.

As the U235 gets “used up” (down to 1%), the rod is less and less able to keep the fission reaction going, and eventually has to be pulled out of the reactor.   At that point, a little over 1% of the fuel rod is Plutonium, which merrily likes to keep emitting neutrons even after the chain reaction has stopped (“Fissile”)

One of the ways to lose water in a storage tank is sloshing during a large earthquake.  Another is that a hole is created in the tank and it starts to leak out.  Another is that cooling is lost and the water starts to boil off.

Once you lose 26 feet of water, the fuel rods become exposed.  The fact that much of what was in Pool #4 was just put there while the plant is shut down means there is more really hot fuel in there than you might want.   Without water, the uranium and plutonium keeps generating heat that has nowhere to go.    That nice outer metal rod now catches on fire – the things that are gaseous inside the rod escape into the air (Cesium is the marker for loss of the integrity of the fuel rod).   Now you have a 13 foot tall stack of white hot pellets.  One source in the NY Times story holds out hope the fuel pellets will continue to stand up in a straight 13 foot high stack.

More likely, as the water level keeps dropping the fuel pellets melt and find their way to the bottom of the tank, greatly speeding up the boiling of the water.  Once all the water is gone, after a while, you just have a molten mess of spent fuel at the bottom with no neutron absorbers (ie Boron).  The 1% of plutonium is a natural neutron emitter, and could cause the chain reaction to resume (re-criticality), generating a huge amount of heat and radiation, but now outside of the radioactive shielding surrounding the reactor core.   Instead of a small amount of cooling, the pool of molten stuff would require 100s of thousands of gallons of water per hour to be cooled down, but without containment, you aren’t going to easily stop the neutrons from continuing to gain momentum in the chain reaction.