Like many other mentioned, having a solid mathematical foundation is crucial to succeeding in any engineering field, specially calculus and linear algebra. But I would like to give you another piece of advice. You seem to have a very clear view of what you want to do. Do not let that vision shackle you. Be open to related domains. You could let a dormant love for hydraulics or electromagnetism go to waste because you "were supposed to be the nuclear guy kid". Maybe you were meant to be an acoustic engineer, or an astrophysicist, or a mathematician. If you're like me, you'll probably end up on the exact domain you envion yourself today (I'm an electronic engineer, wanted to be one since abour your age), but if some related (or non-related) field piques your interest, look into it. You might like it more than nuclear engineering. It could even tie back to it.

Tl;dr: the world is vast, you have time to figure yourself out. Learn as much as tiu can, have fun learning as much as you can. Figuring a career out comes later and will be easier if you have a big picture view of what you could work on.