There seems to be a compatibility issue with the mod Nuclear Fuel. With that mod enabled, it is not possible to research the techs Californium Processing and Large Atomic Weapons. The tech tree visually shows that all prerequisites are researched, but the start research button is still greyed out. I suspect it's because those techs are still checking for Kovarex as a prereq even though that tech is removed (renamed?), though I haven't dived into files to check. Californium and Large Atomics do (presumably correctly) show Nuclear Fuel's Plutonium Breeding as a prereq.

E: It could also partly be my save being really old, but the behaviour persists even in a new save made via map editor to get all prereq researches.

Hi,
I'll have a look.

Yeah, you're entirely right, the game still knows about the technology internally, but doesn't show it.
I'm gonna have to check all the other mods, and see if they do the same - I didn't notice because going in sandbox and clicking 'research all' gives you kovarex, as if it had always been there.

The latest version should fix it (including for apm, where this was also an issue).

Just tested it, for me Californium is working properly now but Large Atomics is still not. I also noticed that the Atomic Bomb research (the first test one) is no longer a prereq of either, so that's probably related.

Yeah, I worked out what happened there. Try the latest version

Everything looks like it's working properly on my end. Thank you!

Awesome!