In Spider-Man 2 (2004) Harry Osborn provides Dr. Octopus with a rare element for his reactor. Is there any source that specifies said rare material?
He says it multiple times in the movie. It's Tritium. Or atleast a fictional version of it. Supposedly only 25 pounds of it in the whole world in the movie. In real life, it's just an uncommon on Earth isotope of hydrogen. Just like how the real life kryptonite is an inert gas instead of a radioactive crystal, the movie embellishes the nature of the material.
You can get glow in the dark Keychain with Tritium in it.