I enjoyed this a lot! It shows how you can get interesting systems and dynamics with very few controls and variables. I tried something similar with my game for the last global game jam, check it out here if you like: https://elegantfish.itch.io/ggj2025
If you were going to expand this, I think upgrades would be nice, e.g. faster fill rate for the water reservoir, an additional set of fuel rods (maybe toggled separately from the first), etc.
Anyway, great job on this!