I wrote a sketch up for "Nuke" as a type of energy damage for 5e in general, but if you would like to adapt it for your game, here's the draft.
Nuke damage type (Mischief, May 1, 2018)
┄┅━━ Summary ━━┅┄
The material complement to mental psychic energy, "nuke" is the magical force of physicality. Its energies are associated with vital force and the decomposition and reassembly of matter. Different than realistic radioactivity, nuke energy is the nuclear that creates superheroes and giant mutant monsters. Nuke is an unpredictable, two-faced force. Warriors can surpass their physical limits by tapping nuke energy in moderation, but the power easily surges out of control. It can detonate violently, mutate or slay those overwhelmed by its radiation, and leave a corrupted wasteland in its wake. Those wielding nuclear powers use exploding damage dice for their nuke attacks.
┄┅━━ Why add Nuke to your game? ━━┅┄
In crunch terms, it lets martials use their might to unlock magic. Traditionally, magic is the domain of the mind, and mental stats determine magical strength and resistance. Nuke increases with physical strength and physical fighters benefit the most. Nuke's lore is designed to enable plotlines and complement tropes associated with martial arts, superheroes, mutant monsters, mad science, and post-apocalyptic scenarios. Nuke is suitable for lower magic settings or magic-abstaining character concepts because it behaves more like a force, radiation, or inner strength. It also better integrates the monk's "ki", which has never really fit into DnD's grecowestern elemental scheme, as manifestation of the nuke force which flows within living things.
And who wouldn't want to play a character who can flex a gate open to the plane of pure power?
┄┅━━ Lore ━━┅┄
Nuke energy channelers become more powerful: stronger, quicker, nimbler, and tougher. Control over the nuke force is easiest for physical fighters and agile sneaks in tune with their bodies. Beginners first learn to manipulate naturally existing nuke energy in their muscles, nerves, and bloodstream. Adepts can channel bursts of power through their strikes and weapons and even compose ranged blasts. Physically powerful warriors radiate nuke energy proportional to their fighting spirit. Skilled nuke users can detect the power levels of other fighters. Nuke energy can also be drawn from the environment, but is far less stable.
Nuke energy is notorious for running rampant and spectacularly backfiring when channeled in quantity. Mixed with other energies –like the classic four elements– nuke energy becomes more versatile and more unstable in equal measure. Strong nuke energy is unpredictably mutagenic. A human exposed to more nuke radiation than their body can handle may gain unusual physical powers, waste away, explode into ludicrous gibs, or transform into a horrific monstrosity. Mad scientists sometimes attempt to harness its power for superweapons, unique monsters, and enhanced soldiers. Most famously, nuke tends to explode, destroying laboratories and unleashing radiation-filled shockwaves on the surrounding land. Concentrated nuke pollution creates tainted wastelands crawling with corrupted creatures.
Ancient civilizations used to tap the elemental plane of nuke to power artifacts. If they can be recharged with nuke energies some of these forgotten devices may be restarted ... or they may detonate.
┄┅━━ Mechanics ━━┅┄
To represent its unpredictability, nuke damage uses exploding damage dice. When you roll an exploding damage die's maximum face value, roll another die of the same size and add it to the total. These bonus dice may also explode. Keep rolling and adding dice until they stop exploding. Exploding dice are indicated by a !bang. (An exploding d6 is !d6)
☢ Any situation that would cause a !d1 roll is replaced by a !d2 roll.
☢ Rules, features, feats, etc. that allow you to choose the best or worst of two damage rolls work as normal. You choose after rolling to completion.
☢ Rules, features, feats, etc. that let you reroll dice (e.g. reroll 1s) are not applied to bonus dice caused by explosions and rerolled dice don't explode. (Example: A feat to reroll 1s is applied to a 2!d4 roll. The initial roll is 1 and 4. The 4 explodes and the bonus 1d4 is a 1. Only the first natural 1 is rerolled thanks to the feat, and it does not explode even if a 4 is rolled.)
Here's some stats.
For any N-sided die numbered 1 to N with all sides equally likely, the exploding modifier will increase the die’s expected value (the mean) by a factor of {N}/{N-1}.
The following are some approximations
d3 --> No good approx.
d4 --> 1!d2 (A bit overpowered)
d6 --> 1!d4
d8 --> 1!d6 (A bit underpowered)
d10 --> 2!d2 or 2!d3 (A bit overpowered)
d12 --> 2!d4
┄┅━━ Creature Weaknesses and Vulnerabilities ━━┅┄
The specifics depend on the creature, but here are some guidelines.
☢ Nuke the Aliens! Aberrations are vulnerable to or at least don't resist nuke damage.
☢ Mutants and Monstrosities resist, are immune to, or absorb nuke damage.
☢ Machines No-Sell. Inorganic constructs resist or are outright immune to nuke damage.