Really cool concept! Balancing multiple factors at once with the player and the reactor made it feel like a true balancing act.
I noticed that the graphics felt a bit lopsided cus the reactor took up a huge portion of the screen, making the player and enemy graphics seem tiny in comparison. Not a huge deal, and i liked the pixel art but it might be nice giving players a bit more room to maneuver? Also be careful with "mixels" where different assets have different pixel sizes. Having the background have the same small pixel size would help unify the scene overall.
There's a lot of potential here and I liked the feedback you would give to the players heat balance! Good work!