If you recall the story of the demon core, you already know how ghastly it is. If you don't, well, you're about to find out. The demon core was a ball of plutonium that killed two scientists, one of them only 24 years old. Despite the name, it wasn't personal; that's just what plutonium does. In the 1940s, nuclear scientists knew how dangerous radioactive materials could be, and that's how the US developed those first nuclear bombs that ended World War II. But despite this knowledge, and the safety protocols around nuclear experimentation (inadequate as they were), these scientists took risks led to their drawn out and agonizing deaths.
This video from SciShow tells the story in a matter-of-fact way, but focuses specifically on the science underneath the danger. Savannah Geary explains the difference between uranium and plutonium down to the atomic level, and why plutonium is super spicy, meaning dangerous. And yet the research these scientists were doing was dedicated to making it more dangerous. There have been a lot of nuclear accidents since then, but the name demon core still haunts the world of nuclear science. There's a 45-second skippable ad at 8:00.