Blast Radius is a quest in Fallout: Project Brazil. The quest begins as the player exits from the Vault 18 cavern. They have five in-game minutes to reach a safe enough distance from Vault 18 before it explodes. The player's quest markers suggests either Rockwell Tunnel or Pinehaven Gas Station. When the detonation finally goes off a massive explosion will trigger - causing a heavy amount of smoke to spread around the next few miles.
After the smoke clears Ben mentions heading to the safehouse to rest, starting the next major quest: A Night In Pinhaven. Completing the quest automatically adds the Battle Fatigue perk to the player.