My bug has been that no overclocking has been possible at some point in game at all. That wasn't limited to nuclear power plants only.
It might have happened when I placed the last HDD in the M'AM, but I'm not sure.
What I can say is that somehow the slug research got broken and required me to use a savegame editor to get it running again.
when the new models for Nuclear Power Plants were introduced and finally released to early access I've started to plan my base around them.
This included the ability to overclock the plants as I'm really limited in space here (the price of a really cool location), but with OC it was perfectly fitting.
Last week I've finished all productions and waste disposal for the plants, directly starting the output. Yesterday I wanted to insert the power shards in order to overclock the plants to their maximum - but I couldn't do so any more!
At the place where the shards had been inserted before there's just a locked cover now.
Assuming that this isn't just a bug:
=> PLEASE let us overclock the nukes again!
To those who think of nukes being OP:
When I'm using them @250% I'm paying 2.5 times the supply and waste for getting double the output, so in principle it's less efficient than before. We should be able to decide ourselves if we want to pay that price or not.