vapor. >> the lng expands 600 times, as it vaporizes. wind carries the vast vapor cloud towards downtown boston. the cloud erupts in a fireball 1,000 feet high. a vast thermal radiation wave pulses through the city. this isn't fire but heat from the explosion, so hot that it ignites anything in its path. >> that circle identifies the region within which there would be harmful radiation to anyone on the shore. >> the gas would only burn for 15 minutes, but would be long enough to devastate the city. >> that's an enormous fire. we've never seen anything like that. and i hope we never do. >> but as long as lng is shipped into harbors like boston, the threat remains all too real. >> trying to run this kind of