How long does it take to cool nuclear fuel rods?

So nuclear fuel rods are dangerous when exposed because they are hot and emit radiation.


What is the science behind not being able to cool the fuel rods easily and not keeping them cool at room temperature without the need for water cooling?|||Your question parallels mine from a few minutes ago, namely why there's heat generated.





There are spontaneous fission events, which generate very energetic neutrons. These neutrons have very low collision cross section with the host nuclei, because of their very high velocity, so generally do not create another fission event. In order to have a sustained reaction, you either need to have a critical mass of the material (which means enough of it that the probability of collision is high enough to generate a chain reaction) OR you moderate the neutron velocity, like with water.





So, unless the rods themselves are near critical mass, the loss of water should shut the reaction off. However, this doesn't seem to be the case. Indeed, the plants have cooling ponds for the spent nuclear fuel, too.





The answer to your question, and therefore to mine, must be that the fuel rods themselves are near critical in mass and that they can generate enough internal neutron/nuclear collisions to keep a reaction going. Once it starts, the material heats, and the collision cross section for a neutron on a nucleus goes way up.





In terms of time, the answer is measured in the half-life of the fuel. You have to keep the rods cool for a decent length of time, at least until the neutron emission is back to "background" levels. You're left with stable fission products like lead and polonium.





Thanks for helping me figure out the answer to both of our questions!|||A decade. A VERY long decade.