system
Given the following post, respond as if you're commenting on Reddit:
human
[ELI5] The situation in Fukushima, nuclear meltdown and radiation.

Just read an article about Fukushima in Japan and the scientists are saying "all of humanity will be threatened for thousands of years’ if the Fukushima unit 4 pool is not kept cool." What does this mean?
gpt
To understand this you have to understand what is going on in a reactor. So let's start there:

Everything is made up of atoms. An atom has 3 distinct components called protons, neutrons and electrons. The exact combination of these three things determine how an atom behaves and what element it is known as. Some combinations are unstable and will break down into smaller parts and in doing so release energy and the unused parts. This is nuclear fission. The energy released is of various types and some of it is what we feel as heat. We can use this heat to make steam from water and drive turbines which then generate electricity.

This rate of decay is normally fairly slow (for more on this, look up 'half life'), but there is a way in which it speeds up. In some elements (a specific combination of protons, neutrons and electrons remember), the unused parts of a broken down, or decayed, atom trigger other atoms to break down. This is a chain reaction, as one atom sets off another. If uncontrolled, you have a reaction that keeps growing and growing and providing more and more energy, also known as an explosion. If you want to generate power, this is a very bad thing indeed! So what is done is another material is added that will absorb the unwanted parts of the broken down atoms and stop them from triggering another atom to decay, these are the control rods.

This gives us a nuclear fission reaction which we can control and use to generate heat, which creates steam, which creates electricity.

Now the situation at Fukushima:

Fuskushima is a nuclear power station, which means it produces electricity using the method outlined above. In a large earthquake, the reactors of the nuclear power station were shut down by adding more of the control rods to absorb the unwanted parts of the reaction. This stops the chain reaction in part, but not entirely. The system is still very hot and radioactive (the breaking down and releasing of energy is still going on). Radiation being released into the environment is a very bad thing indeed, as it is very destructive and kills lots of things.

The first thing to remember is that the reactor doesn't go from hot to cold in a few hours, it takes much longer than that. Also, it has to be cooled to stop the heat building up and destroying the part that shields the reactor and stops the radiation getting out. So long as this heat is managed, there will be no meltdown. This means that, under current conditions, Fukushima cannot have a meltdown and is pretty much not going to.

The problem with Fukushima is that they are using water to cool the reactor (it's on the sea for a good reason!), but in the process of cooling the reactor the water becomes radioactive. Putting radioactive water back into the sea then affects the marine life and the food web in general. However, the amount of water required to cool the reactor is so much that storing it is proving impossible, not to mention the water flowing through the ground is also turning radioactive. The end result is that radioactive materials are being released into the world, which is a bad thing.

TLDR: Nuclear fission is the breaking down of atoms into smaller atoms, bits of atoms, and energy. This energy is used to heat water to drive steam turbines to make electricity. The bits of atoms collide with other atoms, making them decay and forming a chain reaction. Fukushima is a power station that uses this to generate power, and was damaged in an earthquake. The chain reaction was mainly stopped by using control rods, though the reactor is still very hot. Water is being used to cool the reactor stopping it from melting down, but is being turned radioactive in the process. Other water flowing through the site is also becoming radioactive. This radioactive water is being deliberately and undeliberately released into the sea, where it will cause problems over the years.