I suggest the following changes:

-- sensible progression of portable reactors (fission -> plutonium -> fusion)
table.insert(data.raw["technology"]["cerys-applications-of-radioactivity"].prerequisites, "fission-reactor-equipment")
table.insert(data.raw["technology"]["fusion-reactor-equipment"].prerequisites, "cerys-applications-of-radioactivity")

-- plutonium rounds actually need uranium rounds as ingredient, so the tech should have them as prereq as well
table.insert(data.raw["technology"]["cerys-applications-of-radioactivity"].prerequisites, "uranium-ammo")

Thanks, the tweaks to Applications of radioactivity have been implemented. As for Fusion reactor, installing Cerys does not tend to gate existing vanilla content, so it has been left alone.

When using the mod Nuclear Science by Atanvarno, gating applications of radioactivity, particularly the mixed-oxide reactor, behind behind portable fusion reactors and uranium ammo makes going to Nauvis essentially a hard requirement to unlock the reactor.

The issue arises with the any planet start mod. I started on Fulgora, and figured that I can leave a nauvis ultra-deathworld as a crownpiece for the end. Cerys's reactor was the obvious alternative. None of that is possible however without going to Nauvis and mining uranium to unlock "uranium processing" which blocks all the nuclear research dead in it's track.

Since the mixed-oxide reactor does not require uranium, would you consider adding it as a separate tech that only requires mixed-oxide waste reprocessing, making it a stand-alone reward for Cerys? Otherwise there is fairly little point to going to Cerys before Nauvis with the Nuclear Science mod installed, and I think the intention is for Cerys to be stand-alone yes?

Thanks for writing that. There seem to be a few inaccuracies in this comment – it is only 'Applications of radioactivity' (optional side-tech) which was newly gated; we've always had Any Planet Start compatibility so nothing crucial is locked behind Uranium processing, the Cerys discovery tech grants anything necessary; and portable fusion reactors were never part of this change – however, I agree that the 4.12.1 change is not ideal when APS is installed, so it no longer takes effect when the mod detects APS.

Oh you're absolutely right, I mixed up some words resulting in something that made little sense if taken to the letter, glad you could read around my fumbling mind's errors :) I meant to write fission reactors, of course!

I did explain myself poorly. Cerys itself or the progress on it was never gated as you say, only the (optional) reward, which to me is in this situation the major pull with a Fulgora start (personal solar panel limitation for power armors, very bad). On second thought, I don't think that the Nuclear Science mod has any impact on this anyway, as I might misremember the 2.0 vanilla uranium tech chain; I guess it always required you to mine uranium to unlock the chain. I mistakenly attributed that to the Nuclear Science mod (was still thinking in 1.0 tech unlocks).

Thank you for sorting through my muddy message and thank you very much for the change. I just updated and that indeed solves the issue -- Cerys and it's rewards can now be unlocked in any order with APS, which makes it a lot more interesting in my opinion! Unlocking plutonium ammo and plutonium fuel before the prerequisite uranium ammo and uranium fuel to craft it is a non-issue in my opinion, so all good. What really matters is that the portable mixed-oxide reactor is now available without going to Nauvis first.

Have a great day (: