This is a good question im gong to try and answer it even though I know little on the subject:
It is my belief that once you start a reactor up and it operates for a while you can't just "shut it down".
You can stop it from producing energy (stop the turbines) by removing the fuel rods.... but you can't stop the reaction after the reactor has been "turned off".... the semi spent / spent fuel needs to be cooled for a long long time after their use.
IE: if you just pull the rods out the reactor will shut down but the rods will still start to heat up for a very very long time 100s?1000s? of years?
So the fuel would have to be stored somewhere that could cool them sufficiently to keep them safe.
If you don't cool the spent/semi spent fuel, the fuel will continue to heatup (maybe to melting point?) forming a liquid / uranium slag of molten radioactive deadliness.
Im not sure if the heat could ever push the material to the limit where it could "explode" in a nuclear bomb / atomic bomb style / size boom.... (it might for all I know).
I think there are counter measures in place at most nuclear plants to stop the reaction (by injecting some kind of metal into the core fuel supply that will nullify the reaction?) SCRAM systems I think its called, im not sure if that system can stop the reaction and make it so that the nuclear fuel no longer needs to be cooled.http://en.wikipedia.org/wiki/Scram
I think it has something to do with injecting the fuel rods into a metal casing that separates the fuel rods and prevents further fission from occurring.
Honestly, I think the best solution on the planet for spent fuel is to make giant containers with radiator fins filling them up sealing them and throwing them in the ocean (where the water will never boil off completely).
We simply don't have the infrastructure in place to keep that much material cold for that many years.
Whoever says nuclear power is "clean" is an idiot... it creates lot of deadly waste... the only difference is the waste is in the form of containers instead of CO2.
Its my honest opinion that the only reason we have nuclear power plants is for creating weapons grade uranium.
IE: the entire "nuclear power plant" idea since inception was essentially a "front" for the United States to create nuclear weapons grade material, we could of been operating Thorium reactors for power since 1960's but we didn't because we needed the uranium to degrade in reactors to create the isotopes of uranium needed for weapons.
Thats why everyone gets nervous when random nations say "we are developing nuclear power plants for peaceful purposes" its because we invented that lie, the only reason for nuclear power plants is to collect the semi spent fuel and turn it into weapons.... they are uranium factories, power to the grid is a "waste product", sorta like " we are making material for bombs, all this heat has to go somewhere, lets use it to generate electricity and claim its used for peace"... it gains public support and everyone is none the wiser.