Why do fuel rods have to be cooled for so long after being not actively used anymore?
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They are still radioactive, throwing off neutrons and heat, for a long, long time. They are considered “spent” only because they are no longer radioactive *enough* to be an efficient heat source in the power plant. They aren’t good enough at generating steam to spin the turbines anymore.
Because while they don’t produce enough energy to be worth generating power from, they do still produce some heat as the fission fragments decay, and can melt if that heat isn’t removed.
They are still functional rods, just no longer producing enough radiation to be useful in the generation of electricity. They will continue to produce heat, however, for years to come.
The heat produced by nuclear fuel rods isn’t like heat produced by coal or natural gas. There isn’t something being *done to* them to get them to release heat… as with fossil fuels when you burn them.
The heat produced by nuclear fuel rods is because just them existing creates heat. They’re actively breaking down as atoms, and each time one atom breaks down it’s a little bit of heat, times bajillion breakdowns happening at once because there are **so** many atoms in the rods.
So, you need to keep them cool until enough breakdowns have happened over time that they aren’t making so much heat anymore.
They cool down over time because each time an atom breaks down, that’s one fewer atom making that heat. They’ll continue to be warm for decades, but I believe a few months or years gets them past the point of lighting themselves on fire out of nowhere.
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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.
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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Because they’re still actively generating a lot of heat and it takes a long time for that to stop. Nuclear reactors use radioactive fuel to generate heat which boils water. When the reactor is running, the fuel rods are actively undergoing nuclear fission, but even when the reactor is shut down and the fission has stopped, the fuel is still radioactive and is still emitting heat from that radioactive decay. That radioactive decay continues (and thus the heat it generated continues) until the isotopes responsible for that heat have decayed.
The moment the reactor is switched off the fuel rods will continue to produce heat at about 6% of full power level. Sounds like not so much but this amount of heat will easily melt up the core in a couple of hours if not cooled away constantly. This residual heat comes from the radioactive decay of the freshly produced isotopes of the spent nuclear fuel, the fission products. There is no known physical process to stop radioactive decay so we need to wait a couple of months to happen this the natural way.
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