Eli5 How does nuclear fuel get spent so fast?


With the half life of the radioactive metals used in the fuel rods being thousands of years, the fuel used in reacors, i would think, should last for similar amounts of time. How come nuclear plants go through large ammount of spent fuel that then has to be stored?

In: 23

You’re comparing natural decay with induced fission.

In ambient conditions, radioactive elements with unstable atomic structure are basically falling apart slowly. In turn, they only released a small amount of energy as they do so.

With fission, you speed up the process by shooting a bunch neutrons at the radioactive atoms so they fall apart much, much faster. So much faster that neutrons in the radioactive atoms explode out, hit other atoms and cause them to break apart too. If the effect is strong enough, you get a chain reaction that produces a lot of energy (but also causes all your radioactive fuel to “fall apart” faster).

Natural decay is a rickety building falling apart slowly over years or decades into rubble. Fission is when you topple that rickety building so that it hits the rickety building beside it, which then also tips over and hits another building, and another, etc. domino effect.

A fission reactor is actively smashing the fuel to pieces, in order to release the energy faster. There are [RTGs](https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator) that only use the normal decay as its energy source, but they produce far less energy.

They’re not the same reactions

Normal decay is random. A U-235 decays and then nothing else happens, there’s no chain reaction

In nuclear reactors the fuel is refined and set up in the reactor so it’ll be critical so each atom that splits causes one other atom to split. Now you’re not waiting for each one to randomly breakdown by instead trigging a chain reaction that works its way through the fuel fairly rapidly splitting the available atoms into smaller ones. If they need more power from the reactor they briefly adjust the control rods so each split triggers more than one other split to get up to a higher power level then reduce it back to 1-1 to hold at that level. The more power you need to pull from the reactor the faster you need to burn through the fuel

They also don’t burn through all the fuel of the fuel rod, but it starts building up byproducts that muck with the power you can get out of it so they have to swap it out. Unfortunately the byproducts are wayyy less stable than the starting uranium/plutonium so now the random decay is occurring at a much faster rate creating a lot more radiation but less useful kinetic energy(heat) from the process. The really angry byproducts are mostly cleared out after a year because they decay that rapidly

That’s like comparing the rate at which wood rots to the rate at which it *burns*. Radioactive decay is the spontaneous decomposition of unstable atoms. Nuclear fission inside a reactor is a chain-reaction which *causes* the atoms to split, harnessing the exothermic products of the reaction to heat water and drive aturbine.

The [U235 decay chain](https://www.chemistrylearner.com/uranium-235.html#:~:text=this%20radioactive%20metal.-,Uranium%2D235%20Radioactive%20Decay,decay%20energy%20of%204.679%20MeV.) goes like this:

>Uranium-235 →Thorium-231 → Protactinium-231 →Actinium-227 →Thorium-227 →Radium-223 →Radon-219 →Polonium-215 →Lead-211 →Bismuth-211 →Thallium-207→ Lead-207 (stable)

The fission products of a nuclear reactor are far less predictable, but include isotopes of Iodine, Caesium, Strontium, Xenon, and Barium. That’s because the neutrons which collide with the U235 nuclei crack them apart.

Nuclear fission works by a neutron (a tiny particle) hitting an atom of uranium-235, causing it to split into two new atoms, both of which are smaller.

These new atoms can’t be used in a nuclear reaction but still have a lot of mass (only a teeny amount is turned into energy) so the spent fuel rod still has a mass which is almost identical to the “new” rod, hence a lot of waste being produced after the reaction because these new atoms are quite unstable in their own right.