Meltdown

Meltdown is a perk in Fallout: New Vegas and Fallout 2d20.

Fallout: New Vegas
When a target is killed with any energy weapon, this perk will trigger an explosion centered on the target, similar to a plasma grenade explosion, but smaller. This explosion can cause critical hits and sneak attack criticals, potentially killing nearby targets and propagating further meltdowns.

The explosion projects force, often throwing movable objects, and dismembers corpses. It can also cause friendly fire, damaging the player, companions, and bystanders.

Only direct hits can trigger Meltdown from killing an enemy. If a weapon has a damage-over-time effect (such as the flamer or holorifle), it will not trigger Meltdown if it is the cause of an enemy's death. The "bonus damage" of the pulse gun will also not trigger meltdown.

The damage dealt by Meltdown is relative to the damage dealt by the weapon that killed the target. A powerful weapon such as the Gauss rifle can trigger a high damage meltdown, whereas a flamer's meltdown would be extremely weak by comparison. The meltdown explosion carries any additional effects of the weapon that triggered it, such as igniting enemies, and any critical kill effects. Enemies critical killed by a laser triggered meltdown become ash, while a plasma triggered meltdown produces piles of goo.

Fallout 2d20
When killing an enemy with an energy weapon, they explode. Roll a number of d6's equal to half the weapon’s damage rating (rounded down). For each Effect rolled, one creature within Close range of the exploding enemy (starting with the closest) suffers energy damage equal to the total rolled on the d6's.