[ok_dad]

The Chernobyl disaster was partly because the design was graphite moderated, which does not have the safety that water does since it’s not self regulating due to the GPs explanation about that above. When the reactor started to go supercritical, it was reinforced by the moderator working better to create more neutrons, the opposite of what you’d want.

[cyberax]

> supercritical

All reactors are technically supercritical. Chernobyl reactor became _prompt_ _critical_.

Normally, a small amount (just around 0.2%) of fission neutrons are emitted within a 1-3 seconds after a fission event. They are called "delayed neutrons", and this small percent of delayed neutrons is what pushes a reactor over the criticality threshold.

Since these neutrons are delayed, it gives enough time for control systems and natural feedback mechanisms to keep the reaction rate steady.

If you push your reactor past the delayed neutrons so that there are enough of prompt neutrons to sustain the criticality, you're screwed. The reaction rate can double within microseconds, far too fast for anything macroscopic to react. So within less than a millisecond your reactor can overheat, until the nuclear fuel becomes too hot to fission because its atoms move too fast (usually somewhere around 1000C).

And then it'll be followed by some extreme thermodynamics and chemistry: steam explosion, water-zirconium reaction, graphite moderator fire, etc.

Here's a research reactor that does prompt criticality excursions in controlled conditions: https://www.youtube.com/watch?v=pa0Fmcv83nw (with a countdown!)

[cyberax]

It's a tad more complicated. Light water is both a good neutron moderator _and_ a good neutron absorber.

If you vaporize the water, thus reducing its density, it reduces both the neutron absorption, and it reduces the moderation efficiency. But crucially, the moderating efficiency matters much more in regular reactors, so the overall reactor power will drop.

In a graphite-moderated reactor, water's moderating efficiency might not matter much. So if you vaporize the water, there's going to be less neutron absorption, but there's still going to be plenty of graphite moderator to help neutrons to slow down. So the reactor power will _increase_ unless compensated by other means, and this can result in a self-reinforcing loop (see: Chernobyl).

BTW, the neutron absorption is the reason it's very hard (though not impossible) to make water-cooled breeder reactors that produce more nuclear fuel than they consume.

After the Chernobyl disaster, several remaining RBMK reactors were made safer by enlarging the cooling channels. This increased the amount of water present in the core, thus increasing the dependency on water's moderating effect, greatly reducing the positive void coefficient. It couldn't be completely eliminated, but it was reduced to a level where it can't result in prompt criticality anymore.

[tiffanyg]

Excellent point - you are 100% correct, parent comment etc. were specifically about water-moderated types.

I was too grabbed by some of the later description and just connected it somewhat haphazardly to not very organized or accurate info rattling around in my head from years ago.

Thanks for pointing that out!

[paulnpace]

Additionally, they were ordered to disable all safety mechanisms and run the reactor in a known unsafe condition, causing a massive long-term disaster in... the Ukraine.

The way I attempt to explain the difference between the negative and positive coefficient of reactivity is it's like one car accelerates by pressing the gas pedal and the other car has an engine running WFO and all you ever do is press the brake pedal. It isn't a perfect analogy, but I think it gets the general concept across.

[dgroshev]

As an aside, "the Ukraine" has mildly offensive connotations. https://en.wikipedia.org/wiki/Name_of_Ukraine#English_defini...