Why do fuel rods have to be cooled for so long after being not actively used anymore?
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Nuclear fuel consists of heavy atomic nuclei, such as uranium. During use, the heavy atoms split into lighter ones – for example, a uranium atom may split into caesium and rubidium.
The problem is that the composition of a uranium nucleus is different to the composition of smaller nuclei. Atomic nuclei are made from protons which are positively charged and neutrons which are not. The protons repel each other like crazy, but the neutrons act like a sort of glue. The more protons an atom have the more neutrons are needed to glue them together – but the amount of neutron glue you need goes up much faster than the protons.
The problem is that the split atoms have way too many neutrons for their new size. The neutrons aren’t actually glue, and this imbalance doesn’t work. These atoms tend to rearrange themselves into a better arrangement – the extra neutrons periodically convert into protons and electrons. This conversion process is called “beta decay” and is a form of radioactivity.
The process of beta decay releases a lot of energy as radiation. When the radiation hits another atom, that energy is converted into heat. For something like nuclear fuel rods, there is so much beta decay that, the heat production can be very large.
The process of beta decay continues until all the atoms have converted into forms which have an internal balance. This take a long time. The heat production continues for thousands of years, but gradually fades with time. It can be a serious problem for around 1-2 years (they have to be stored under water, because otherwise air wouldn’t be able to remove the heat, and the rods would overheat and be damaged), but after that the heat production has faded enough that they could be stored dry if necessary. Burying used fuel rods requires special care, because the ground can trap the heat, and even after burial they could overheat. Waiting 60-100 years before burying them means that the heat has faded enough that it should be much of a problem.
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