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