A suitcase nuclear device or suitcase nuke is a definition for various types of nuclear devices developed by the former Soviet Union. They, in their various incarnations, are just what they sound like: small yeild nuclear devices placed inside suitcases. They are specifically designed to be concealable and mobile.
On Day 6 9:00am-10:00am, it was revealed that 5 of these suitcase nukes were in the hands of the enemy. Just prior to 10:00am, CTU raided a location at 351 Old Mill Road in Valencia. During the raid, one of the terrorists detonated the device.