How are nuclear cores, such as the ones used in reactors, created safely?

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Having recently watched the Chernobyl mini-series on HBO, one of the questions that keeps coming to my mind is this. If the nuclear material is so deadly that even people in full protective gear are only allowed to be in certain areas of the clean-up for 90 seconds at a time, and some areas were so contaminated that the radiation would destroy any electronics more complicated than a light switch, how do they build the things in the first place? Does the radioactive output become stronger over time?

In: Physics

The radioactive output becomes weaker over time, not stronger. The reason the Chernobyl mess is so highly radioactive is that it melted all the fuel rods together and burned through the floor, resulting in a huge blob of radioactive molten metal that eventually hardened but still remained uncontained.

When making fuel rods, they are made isolated from other fuel rods to prevent a chain reaction and are made and stored in areas designed to absorb the radiation to contain it from causing much damage.

The reactor building itself it just made of steel and concrete like any other structure. It’s not radioactive at all before you put fuel in it, so there’s no issue there. Reactors run on uranium. Prior to being placed in the reactor, unused uranium fuel is actually pretty safe. You could hold a chunk in your hand and be fine as long as you don’t eat it or inhale any dust from it. It’s what happens to it while it’s inside the reactor that’s the problem. When the uranium atoms are split to make energy, it makes new, smaller atoms. It’s these atoms are far more dangerous than uranium.

tl;dr: new, unused fuel is pretty safe. It’s the fission process that generates energy that creates far more deadly waste from partially or fully spent fuel.