I traveled back from the Nether and got the following in the console:

02:22:50] [Server thread/WARN] [FML]: NuclearCraft loaded a new chunk (18, -28 Dimension: -1) during chunk population, causing cascading worldgen lag. Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

The world gen for glowing mushrooms was a little lazy - I think I have it fixed in the next release :)