The button 'Y' is set for 'Reboot Systems'
Rebooting through this popup dialogue, doesn't actually reboot the reactor HP.
You have to use the "RECALIBRATE HP" button in the Tree reactor panel.
Note that the old reboot works slightly different, in this case, rebooting through Y should also be denied if it's still on cooldown (for new power reactors). The cooldown could be put as a countdown in the top left corner perhaps: "X secs till you can reboot".