Why are new or unused nuclear fuel rods safe to handle but spent fuel rods are extremely radioactive?
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The materials that power a nuclear reactor have pretty long half-lives. For example, uranium-235 has a half-life of hundreds of millions of years when left alone. So while some U-235 is decaying in a chunk of enriched uranium, it’s doing so pretty slowly. But in a reactor, a chain reaction artificially speeds up those decays. A large portion of the whole sample of U-235 decays over the course of months, a rate millions of times faster than its natural decay rate.
Unfortunately, while U-235 has a long half-life, its fission products – the pieces that are left over after a U-235 atom splits – largely do not. Like all heavy elements, U-235 has far more neutrons than protons. And most of those neutrons are still present in its fission products, since U-235 in a reactor gets split apart. But since those fission products are much lighter, they prefer a much higher proton-to-neutron ratio, which makes their neutron-rich forms quite unstable.
For example, one major fission product is [caesium-137](https://en.wikipedia.org/wiki/Caesium-137) (half life 30 years), which has four too many neutrons to be stable (natural caesium is caesium-133). That means that the number of decays happening in spent fuel – which is full of fission products – is counterintuitively much **greater** than it is in unspent fuel prior to the creation of a chain reaction in a reactor.
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