If you re-calibrate the HP of your reactor, it is instantly back with nothing preventing you from using it to its fullest.
It should be similar to the current reboot mechanic. X seconds after pressing it before it actually happens, and while that is happening, you have no recharge. Can't build or remove either, and any block damage will bring it out of the reboot timer and you would need to restart it again.
Needs config values for seconds though, based on initial reactor size.