>In fission, a radioactive isotope (such as uranium or plutonium) is constantly shedding particles and melting down. And the reactor uses graphite rods and heavy water cooling to moderate and manage how much of a chain reaction can take place. Without the rods and water, you could have a runaway chain reaction which could lead to reactor core meltdown… and if there’s more than critical mass of nuclear fuel… an A-bomb explosion to go with the meltdown.

Formerly in the industry. Most of this is a pretty OK explanation, but that last bit is very wrong and I fear you've been* exposed to some disinformation or are repeating misinformation. You may want to read up on the definition and usage of 'critical mass', but more importantly, there are no fission plants that could accidently result in a nuclear explosion. nuclear accidents can result in meltdown, as you stated correctly, and there are - in extreme and sometime unprecedented circumstances - risks of gas explosions or explosive pressure events... But not nuclear explosions or spontaneous nuclear bombs made from reactor cores. That exists purely in the realm of propaganda and cinema