Been flying around in spectator mode & I've used a scanner from the Mod scanner to search only for the block.
It's nowhere to be found. I'm guessing this is an issue so iI'll report it.

Versions:

Minecraft: 1.17.1
Forge: 37.0.53
Bigger Reactors: 0.6.0-alpha.3.1 | https://www.curseforge.com/minecraft/mc-mods/biggerreactors/files/3448811
Phosphophyllite: 0.6.0-alpha.3 | https://www.curseforge.com/minecraft/mc-mods/phosphophyllite/files/3447790

More:

I did this on a clean instance with no mods installed other than Bigger Reactors & Phosphophyllite and the same result is present there. No Uranium Ore.

- Gbergz

Registry issue, there was a small log error about it, but logging is kinda broken in 1.17 still, so i missed it
[18:48:56] [Worker-Main-8/ERROR]: Cannot register unknown block for world generation