How are the cores of nuclear power plants constructed?

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I’m watching Chernobyl on HBO and they’re describing the different areas of the building after the explosion, some of which they say would kill you within days or weeks due to the high level of radiation. They also talk about how they can’t even use robots to clean the material up because the extreme radiation will shred the circuitry of the robots. So, how on earth are these things constructed in the first place?

In: Engineering

The core is built just like any other building, the radioactive fuel rods are only inserted after everything else is done and all the radiation shielding is in place protecting the workers.

After the explosion all the protection is obviously gone, and in addition to that fuel rods actually become significantly more radioactive and dangerous after being used in the reactor, uranium while not exactly safe for children to play with is actually not that dangerous unless you ingest it.

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There are many different designs of reactors and it’s too long to list to explain each one. What makes reactor cores dangerous is fuel that’s either partially irradiated in the core or spent fuel that’s no longer in the core. Different reactors use slightly different fuels, but the main component of the fuel is uranium. In most reactors, the fuel is composed of pellets stacked into [rods](https://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Fuel_Pellet.jpg/553px-Fuel_Pellet.jpg), and then the rods combined into [larger assemblies](https://www.nrc.gov/reading-rm/doc-collections/fact-sheets/fuel-assembly-new.gif). Prior to being placed in the reactor, the fuel is *relatively* safe, since it’s just uranium or some uranium compound. In fact, you can hold a chunk of uranium in your hand and be fine.

What makes it so dangerous is the fission process that turns the uranium into energy also, as a byproduct, creates many more radioactive isotopes that are far more dangerous than the original uranium fuel. This process also irradiates all of the non-fuel components of the core, so the core has have heavy shielding (usually several meters of steel and concrete) to keep the radiation inside. This also means that you need some mechanism to keep radiation from escaping while removing spent fuel and adding new fuel to the core.

So back to your question, the core itself is different each reactor design, but it’s still just made out of mostly normal construction materials concrete and steel. Fresh fuel is also mostly safe to handle. It’s partially or fully used fuel that’s dangerous. With adequate shielding, it’s not problem.

This has little bearing on Chernobyl but most reactors in the world are PWR or BWR (incl. Fukushima), which are basically big steel pressure vessels to hold water under pressure (PWRs under significantly higher pressure). There is a pretty neat [video](https://www.youtube.com/watch?v=91yVhrSZ5jQ) by russian industry on their production. They are so big that only a few facilities in the world can make them.

The core proper is just some geometrical arrangement of fuel assemblies and control rods placed [in the middle of the vessel](https://cdn.britannica.com/96/102096-004-4018C7BB.jpg). None of the components are significantly radioactive initially. Unused fuel can be held by hand without concern.

Once the reactor has been run though, there is significant radioactivity in the core even after shutdown. Either way, refueling is performed on an open reactor vessel using cranes that handle the fuel assemblies. The water in the vessel is sufficient to shield from radiation coming from the core, but if a used assembly is lifted out of the water, its radiation is dangerous to anyone immediately nearby. Usually the used assemblies are just placed into another pool of water that sits next to the reactors, using the crane.

fuel rods are only really dangerous after the fuel is spent / irradiated by use in a reactor. You can handle new fuel rods by hand with only minimal protection. They only have 3-20% of U235 in them (which mostly decays by low-penetrating alpha particle relase, unlike high energy, high penetration gamma radiation relase in a fission reaction). Spent fuel is full of fission byproduct isotopes that are usually high energy, short halflife – thus there is a LOT of radiation emission.

Chernobyl’s reactor 4 core was full of a) partially used fuel, b) its internal construction was already exposed to fission neutrons / gamma radiation from the process. So the graphite moderator, steel construction, etc was highly radioactive as well, c) primary loop cooling water was obviously radioactive (and so was the steam going up into the air)