My build of 8 reactors (120 * 4 + 160 * 4 = 1.12 GW), 12 heat exchangers plus 24 steam turbines worked fine in 1.2. However after the update to 1.3/1.5/1.6, they seems to generate very little actual heat despite the nominal 160 MW per reactor output. Reverting back to 1.2 fixes the problem.

Investigating.

Thank you for the bug report.

I've been able to reproduce with one of my own save files.

4 existing reactors produce 171 MW under full load, which is almost, but not exactly, 40 MW each, while newly placed reactors work as expected, with each producing exactly 160 MW. It seems that the update causes existing reactors to update their neighbor bonus to 0, and their nominal power to 160 MW, but not their actual power. (Tho why they seem to produce to 42.75 MW per reactor rather than exactly 40 MW is unclear.)

I might have thought a way to fix this, but I'm not sure. In the meantime, you can work around this removing and replacing the reactors. This will lose the partially burnt fuel, but given that a 1.2 build won't have had cores available yet, this should waste far less uranium than you've already saved by having the mod installed.

I've added a migration script that fixes this issue in 1.6.1.