"So, the simplest way to explain a nuclear reactor is that it has fuel, something to make the reaction go faster, and something that it boils to move around. Just putting a big enough pile of super-expensive uranium together makes a reaction begin, but once you put it into some heavy water, it means that a much smaller amount of uranium makes a much stronger reaction. It's the thing that makes the reaction go faster, that I mentioned. The easiest thing to boil would be water; but water isn't quite perfect for this, in large part because it boils too easily. Reactors today, compared to those of old, are almost boring in how simple they are. They use a special metal alloy, that's a perfect mixture of various parts, that means that you can just stuff super-simple uranium into it, and it works both to make the reaction go faster and as replacing the water as the thing boiled. And the boiling off of the alloy works to slow down the reaction, meaning that it's impossible for a reactor to just get going faster and faster and faster."
"The almost-boringly-simple solution is the key thing to understand about engineering: it should be boringly simple and obvious. 'A clever solution is an idiot's solution', as the slogan says. Electrostatic headphones used to be patented, meaning that the far less elegant kind had to be used instead. But when they finally came off patent, doing things in the sensible and correct way finally became an option. It used to be like this dystopian novel where someone patented 'using steam pressure to convert thermal energy into mechanical', and so people were condemned to pre-industrial poverty for centuries, because the patent holder was an idiot who made it illegal for anyone to do better than him. And things went from bad to worse, when another patent became introduced: using the extract of one rare plant grown in distant corners of the world, that had proved itself a cure for a raging pandemic. Its inventor wasn't a very useful botanist, and condemned the world to go without it until he'd be able to finally get his greenhouse working, decades later."