Fail-safe detonation system
From Halopedia, the Halo wiki
The MJOLNIR Powered Assault Armor is equipped with a fail-safe detonation system. If a Spartan is killed or wounded fatally, or they are forced to shed the armor in the field, the self-destruct protocol will have to be enacted by either themselves or a nearby Spartan. This is done to avoid capture of the suit and/or the wearer for identification or technological study.
The tiny access panel at the back of the suit is opened to activate the protocol, it is activated by typing several digits only Spartans know. Once activated, a fifteen-second countdown timer starts. Once it hits zero, the suit's fusion reactor overloads and burns everything in its path in a ten-meter radius with a follow-up explosion to finish anything alive.
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