An situation when the temperature inside reactor core reaches such high levels that the fuel rods (made from Uranium) begin to melt their way down and destroy the reactor.
It usually happens when fuel rods cannot be cooled by a cold substance (for example heavy water) and they start vaporising every single molecule of water nearby, thus accelerating the process of building heat. The only thing operators can do now to stop it is to drop control rods, but it can be impossible because of built up pressure (that's how Chernobyl happened). When the control rods are down, the nuclear power plant crew still needs to resume water delivery to the reactor because of nuclear decay
TL:DR Nuclear fuel gets hot and burn it's way down the reactor.
Guy from Fukushima: The tsunami shut down our water pumps and now now the heat level is over the critical point. We have to evacuate whole wing of power plant and get fireman over here. Tell them there's high risk of Nuclear meltdown.
Normally caused by A drink that is used to kill zombies in COD WAW but can also be used as an anal cleanser. Manufacturer is not responsible for nuked restrooms. therefore PHD flopper is prohibited by casual consumption
Joe: "where's jenny?"
Alex: "she took a swig of PHD flopper which contains prunes, ate some haribo sugar free gummy bears and has been in the hospital ever since"
Joe: "what happened, wait isn't she allergic to prunes?"
Alex: "I heard a fart, then an explosion. I think she might've blown her ass apart. she did say she had to take a nuclear shit"
Joe: "Oh my god!"
(Alex takes a swig of PHD)
Joe: "you didn't just drink that!"
Alex: (stomach gurgles) "Must've been the drink i had" (runs into restroom)
Joe: "MOTHER OF G0-"
(house explodes)