Why do fuel rods have to be cooled for so long after being not actively used anymore?
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A used fuel rod contains the original fissile material (U-235), new fissile material (U-233, Pu-239), non-fissile materials (U-238), and the fission products produced after fissile material has undergone fission. Many of those fission products (Xenon-135 in particular) are neutron absorbers, and *poison* the fuel rod so it’s generation efficiency goes down.
Once the rod is out of the reactor, fission ceases – there isn’t enough neutron density to support it. However, the fission products are all radioactive, with varying half-lives – some are extremely short, and some can be thousands or millions of years. After a fission product undergoes a decay event, the resulting decay products may also be radioactive. This is called a decay series, and will continue until a stable or long half-life isotope is reached.
The result of all those decay events causes the fuel rod to heat up from the kinetic energy of the decay events. Absorption may not be complete with high-energy gamma rays and neutron emissions escaping the fuel rod (alpha and beta particles will almost be entirely contained within the fuel rod), so there is emitted radiation.
Storing the fuel rod in water both cools the rods and helps absorb the emitted radiation. As the short half-life isotopes decay to longer-lived isotopes, less heat and emitted radiation comes from the fuel rod, and it can be stored long-term or reprocessed to extract additional fissile material (U-235, U-233 and Pu-239).
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