Question: What was the reason for the Chernobyl disaster?

James Smallcombe answered on 6 Oct 2020:

The reactor had a design flaw and due to politics at the time, there was a lot of secrecy so the engineers running the reactor probably didn’t have all the information they should have done.

In a nuclear reactor the reaction rate has to be very carefully controlled to keep the reactor going, but not to go too fast, have a chain reaction and go boom. This is done my controlling the number of neutrons bouncing around inside the reactor and making the nuclear fission happen. The two main factors are slowing (moderating) neutrons to make more fission and absorbing (removing) neutrons to make less fission.

One material, often water, is used to surround the reactor to slow the neutrons down and make it easier for fission to happen (this type of material is called a moderator). Rods made of a different material that remove neutrons are then moved in and out of the reactor to control the reaction. Although the rods are the main thing absorbing neutrons other bits of the reactor, like the water, do as well.

In the Chernobyl disaster they were trying to turn the reactor up so they took the control rods out, meaning more neutrons and more heat. The water started to boil and bubble and the bubbles meant fewer neutrons being absorbed, so more total neutrons, too much fission, too much heat. To stop the reaction the control rods were put back in, but as they moved in they increased the moderation (slowing) of neutrons at a critical point, which increased the reaction rate and caused it to go supercritical.

We design reactors now in ways that mean these problems cant happen. In modern reactors everything is designed to make the reaction stop if things aren’t perfect.