Meltdown

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

Fallout: New Vegas
When a target has its Hit Points reduced to 0 by an energy weapon, this perk will trigger an explosion centered on the target, similar to a plasma grenade explosion, but smaller. The explosion can cause critical hits and sneak attack criticals, potentially killing nearby targets and propagating further meltdowns.

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.