It is a myth propogated by conspiracy theorists that fuel vapor in aircraft is not explosive and that such explosions have never happened before TWA 800. Ironically, (or maybe not) one of the sister aircraft to TWA 800 (another 747-131 built for the Iranian Air Force) was destroyed while airborne due to an explosion of fuel vapor in its #1 fuel tank. The aircraft was struck by lightening while in a descent. The lightening strike triggered an electrical surge which resulted in a spark igniting vapor in its empty #1 fuel tank.
But in total there have been at least 19 incidents of aircraft exploding due to fuel vapor igniting for various reasons. Here is a link detailing each....19 Fuel Vapor Explosions
There's no evidence that TWA 800 was struck by lightning or that any kind of equipment failure occurred on the flight that night. But there's a lot of evidence that TWA 800 was struck by a missile. The NY Times initially reported that the FBI found residue from explosives on the plane wreckage. Later they followed the Clinton party line and switched to the exploding fuel tank theory. If you want to believe the official conclusions and the CIA's rediculous animation of the crash then go right ahead. You have the right to ignore any evidence you want to ignore. But I wouldn't dismiss Cashill and other highly competent people who doubt the official conclusions too quickly. You can start by reading some of the threads about TWA 800 here at FR.