First time I played I got to level 9, but the second time I played, I made it all the way up to level 156. I stopped playing seeing how endless it was and how crunchy my audio was becoming once the screen was filled with grey. xD
Around level 50 I no longer needed to shoot or move rods anymore since having a bigger neuron kept reactions going infinitely and enough cooling upgrades + reduced heat upgrades made it so reactions no longer produced heat.
If this idea gets further developed, I would suggest having a cap on upgrade %s (if there weren't any already) and giving us a way to view our obtained upgrades. A cap would allow reactions to keep generating heat and keep the player from essentially afk farming energy. At one point, I had to avoid getting the Big Beautiful Neuron upgrade since already levelling up once or twice was more enough to make reactions continue endlessly.
Had fun watching things bounce for a while! :)