Meltdown Elemental

"'You melted that reactor, therefore I'm going to melt you.'"

Background
The Meltdown Elemental is a Blaze-derivative mob. They are very dangerous, though fortunately they don't spawn naturally. They make Geiger Counter clicking sounds instead of the typical pained breathing of Blazes.

Spawning
Meltdown Elementals spawn near players that have experienced a nuclear reactor meltdown. All players in the vicinity of 100 blocks will be "marked". These marked players will have several Elementals spawned around them.

Dangers
Meltdown Elementals engage players wtih Tesla Coil-like beams of energy that deals radiation damage. This means that they can bypass any armor and directly damage the player. This makes fighting them off extremely dangerous and difficult. These beams deal a ludicrous 16 damage per second. This forces players to fight back at a long distance.

They have a hefty 120 health and 8 armor, outmatching even the Iron Golem. Lots of accurate and heavy hitting ammunition is recommended. There is also a mini-boss type variant that has 360 health and has flaming particles around it.

Uses/Farming
Meltdown Elementals drop rods of depleted fuel, which can be cooled down and recycled. If they die in water or rain, they drop cool depleted fuel instead. They also drop Polonium-210 nuggets equal to the Looting enchantment level of the weapon the player used. They drop nothing if they weren't killed by or at least hit by a player recently.

The mini-boss variant drops a "Radiation Coin".

Farming is not ideal, since it would be more far more efficient and safe to simply use a nuclear reactor normally, but you can attempt to get back what you lost by killing them.