Fluffy, but with a bow!

So uh, some basics I usually stick to:

Their fur is either quite short or completely absent (I flipflop on this a lot.)

They are slightly shorter than other Eeveelutions at 2'4" (0.7 m)(though whether this is because their ears are not measured or not, I don't know)

I usually only give them 2 pawpads on the back legs, similar to Vaporeon.

Their floaty bits, paws, and eyes all glow, this is involuntary, they cannot willingly change it, though increasing or decreasing activity (ie their adrenaline levels) will determine the brightness they are, with white being full powered, and they are at their dullest when asleep and completely disappear when KO'd.

Their control over their radiation output depends on their ability:

For Nucleon's with the Atomizate ability, their output is usually about the same level as if you were to stand outside all day, but the increase/decreasing brightness of their floaty bits also coincides with their radioactive output.

For Nucleon's with the Geiger Sense ability, they do not produce any radiation by default, and instead their glowy bits will interact with the local exposure levels, growing brighter as they get closer to a source of radiation and function as a good way to detect such things.

Their floaty bits are quasi-solid, but enough pressure will cause your body to simply pass through them. Regardless touching them feels like TV static buildup from old televisions. They have no feeling in these 'limbs', and can be freely moved within a foot of the body. Pulling them any farther will cause them to dissipate

While the floaty bits function similarly to real limbs when it comes to movement (ear twitching, tail wagging, etc) they do not actually perform any functions for balance, hearing, etc, instead Nucleon have ears similar to that of a seal, which are built into the head rather than protruding. Their hearing is as a result fairly poor.

Nucleon are very resistant to their own radiation (or all radiation should they have the Geiger sense ability) but that does not make them immune. Generally speaking Nucleon are more at risk of developing physical disorders commonly associated with chronic radiation exposure in their older age, an unfortunate reality all Nuclear types tend to face.