Why are new or unused nuclear fuel rods safe to handle but spent fuel rods are extremely radioactive?


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.

Nuclear fuel is nice because it doesn’t react until struck by a neutron. By controlling neutrons, we can turn the fuel ‘on’ and ‘off’.

If the isotopes were more naturally radioactive without the neutrons, they would be much rarer and much less effective as fuels.

However, these properties are relatively rare for heavy isotopes, and in fact most isotopes are radioactive. This means that the products of the reaction are mostly much more radioactive, with many of them decaying nearly instantly. Many of those that remain are still much more radioactive than the fuel naturally is.

The nuclar fuel is Uranium-235 and Uranium-238 oxide encased and metal and other material. The half-life of the U-238 is 4.5 billion years and U-235 is 700 million years. So it is not especially radioactive and you can touch it.


The half-life is the time it take half of all atoms to decay radioactively the longer it is the less radiation the material emmit


When you split the U-235 you initially get Barium-141 and Krypton-92. The Barium-141 half-life is 18 minutes and Krypton-92 1.8 seconds. They continue to decay to elements with a short half-life but it will not jut be a minute or second but a year.

You get elements like Strontium-90 with 28 years and Caesium-137 with 30 years.

So if you have the same amount of Caesium-137 as Uranium-235 the number of radiative decays per unit of time is 700 000 000/30 =23 333 333 so 23 million times more. So even if all U-235 is converter to the element the mount you get results in millions of time more radioactive decays per unit of time

So the danger comes from the elements produced in splitting uranium apart and the element they decay into that have half-lives that are a lot shorter so they emit more radiation