@saxnot I am not a nuclear engineer, but I cant go into detail on how I am related to nuclear projects. I can tell you I'm not an expert on nuclear though even if I am well read on certain aspects.

Basically the meltdown-proof reactors take usually take the form of a liquid. They work on two principles 1) the fissible material can not go critical as a fundamental property, this has two advantages: no meltdowns, and you cant easily make nuclear bombs out of it either for the same reason. 2) Since they are liquid a -plug on the bottom is designed to melt away above a certain temperature. This drains the liquid material into a protected pit far under ground. So no matter how hot it gets it cant melt through stuff or get out.

In this way they are considered melt-down proof. The most common example of this design is here: https://en.wikipedia.org/wiki/Liquid_fluoride_thorium_reactor