The hatch wasn't destroyed when the failsafe was activated. The hatch labeled "Quarantine" was the one they blew off at the begining of the season and it was sitting next to the access down. My guess is that when the magnet was discharged it just sent everything metal nearby flying. No real explosion....Locke, Ecko and Desmond are still live!!

(That's my story and I'm stickin to it!)