Radnox is a rare element in No Man's Sky found only on planets with Extreme environments.
Radnox is generally located either on the ground as smooth green rocks or floating high above the ground on Extreme Ice planets as a Jellyfish-esque creature.
Radnox is used in some high level blueprints that upgrade your weapon capabilities.
Where To Obtain
Here is a planet where Radnox can be found as Smooth Green Rocks (see image below) that only requires the first 2 Portal Glyphs to dial:
It is an Extreme Heat planet with a somewhat hellish feel, just about everything is a shade of Red and Orange.