How does Radiation work?
Randy Pitchford confirmed further details about the new element type. The nuclear radiation effect will increase the vulnerability of your enemies that have been affected by it. On top of that, it also has a damage-over-time component.
This sounds a lot like the Slag element from Borderlands 2, however, that didn’t have the damage-over-time effect. Randy Pitchford also confirmed that Slag won’t be returning in Borderlands 3. If an enemy dies from radiation, they will explode and the radiation can spread to nearby targets.
What is Slag?
In Borderlands 2, Gearbox Software added Slag to the game. Elemental weapons could harness Slag. Damaging an enemy with it would have a chance to Slag them. Turning your enemy purple would then make them vulnerable to other damage types. So you would should them with a Slag weapon and once they are weak you would switch over to another weapon to deal double damage.
While this was a fun gameplay mechanic in your first playthrough, on the overpowered levels this became a chore. Because enemies have so much health you were forced to slag them or else you would be up against giant bullet sponges.
Other Elemental Types
Borderlands 3 has multiple elemental damage types. There is fire, corrosive, cryo, shock, and not also radiation. I have a full breakdown of all the damage types here. You want to use the correct damage type for each enemy as they all have pros and cons on how effective they are against a specific enemy type. While this probably won’t have much impact on the early game, once you reach the end game these factors matter.