TVA meltdown

Ok so TMI, is actually a cumulation of three primary factors, none of which should've happened.

The first was that they had bad/broken gauges and equipment that was hard to read, meaning they didn't know the state of the reactor exactly like they should have been able to.

Second, they had the auxiliary cooling pumps shutdown for maintenance. Now normally this wouldn't be a huge problem if you're following procedure, as procedure dictates (this was also the law btw) that when your reactor core is running, the auxiliary cooling lines need to be active.

And third, the reactor core was operated under only the primary water feed lines. I.E. power generating feed water. Which as i said is not allowed.

Once they had a crew change, the new crew immediately realized everything had melted down, and called for shutdown. Only took like 8 hours or something.

Chernobyl was a little more complicated, because chernobyl wasn't designed with a PCV (primary containment vessel), technically it had a secondary containment, which would be the building around it, but obviously that didn't help. It was theorized and believed by the engineers (and the operators, ignoring a certain condition where this wouldn't be true, which nobody knew about) that it was impossible for the plant to have a meltdown, primarily due to the fact that it was such a spread out core, making it react slower to immediate changes in the power production.

However, during the day when this happened, (i wont go into all the detail because i will be here for hours otherwise.) the plant was in a xenon well, meaning that it was able to produce virtually zero fission (xenon absorbs neutrons) and the operators didn't realize it, so they pulled out nearly all of the control rods trying to bring it back up (for a loss of power test) only to then have all the xenon start decaying at about that period of time, which meant the power level started to aggressively increase, and since there were no control rods, it sort of hit a runaway condition. Leading to the entire plant fucking exploding, due to a steam explosion specifically. This one was a lot more like a steam boiler explosion in a locomotive than a nuclear explosion or even hydrogen explosion (though that could've been the second explosion that happened)