Chernobyl happened for a lot of reasons, you’re being reductive. The reactor design, testing procedure, and crew all contributed to that disaster.

If I had to pick one reason, I would blame reactor design. The RBMK reactor had what’s known as a positive void coefficient. When voids (boiling water pockets) formed, these reduced the reactor’s ability to moderate neutrons, thus accelerating the reaction rate. This makes the reactor fundamentally unsafe, and guaranteed that something was going to go wrong.

This is a design flaw that has thankfully been fixed in other reactors, including all other RBMK reactors. The flaw was known to the Soviet’s at the time, but the RBMK was designed to produce nuclear weapons, so they swept the flaws under the rug.

> When voids (boiling water pockets) formed, these reduced the reactor’s ability to moderate neutrons, thus accelerating the reaction rate.

Actually, what the loss of water did was reduce capture of neutrons on hydrogen. The moderator, graphite, remained in place and was more effective.

Oops, thank you.

Yes, Chernobyl was a stupid disaster (though I would blame the delay of the test more than the test itself). Chernobyl was also unusually expensive since today's plants at least can't literally explode.

But human error happens and has to be in your risk calculation. Just like natural disasters happen (Fukushima was insured, just not against earthquakes, tsunamis and the like)

Edit: for context, "unusually expensive" means we hope to get the whole thing fixed within the 21st century.