A B737-300 CAPT RPTED THAT HE AND THE FO; WHILE WAITING ON AN EMPTY ACFT; RESPONDED TO AN APU FIRE WHILE PARKED AT THE GATE AT IAD.

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ARRIVED AT GATE AFTER A 4+ HR WAIT AT THE ARPT AFTER OUR SCHEDULED DEP TIME AND NOT ADVISED OF NEW TIME. DURING PREFLT DUTIES; FO WAS IN COCKPIT AND CAPT WAS BRIEFING FLT ATTENDANTS. NO PAX WERE BOARDING YET. DURING BRIEFING; HEARD FIRE BELL AND APU AUTO SHUTDOWN. AIRPLANE WENT DARK TO BATTERY PWR. I (CAPT) IMMEDIATELY WENT TO COCKPIT AND SAW APU FIRE HANDLE ILLUMINATED RED AND FO WAS SILENCING THE BELL. I CALLED FOR THE QRC AND WENT THROUGH THE CHKLIST. TOLD FLT ATTENDANTS AND CLEANERS TO LEAVE ACFT IMMEDIATELY. CALLED RAMP TWR AND TOLD THEM OF FIRE INDICATION AND REQUESTED FIRE DEPT. DURING ALL THIS; WAS TOLD LATER BY PURSER THAT SHE HAD TO PRACTICALLY PHYSICALLY PULL AND KEEP ONE OF THE CLEANERS OFF AS SHE DIDN'T SEEM TO UNDERSTAND WHAT WAS GOING ON. SHE KEPT TRYING TO RE-ENTER THE ACFT AND KEPT SAYING I HAVE TO CLEAN THE ACFT. MAINT CAME TO STAIRS AFTER CHKLIST WAS DONE AND CREW LEFT ACFT. I MET MAINT AT BOTTOM OF THE STAIRS ON THE RAMP AS THEY HEARD THE FIRE WARNING OUTSIDE THE ACFT. HE THEN INQUIRED IF WE FIRED THE BOTTLE. FIRE DEPT WAS ARRIVING BY THIS TIME AND MAINT HAD ALREADY DETERMINED THERE WAS NO FIRE; EVEN THOUGH WE HAD ALL INDICATIONS; IE; FIRE BELL; APU AUTO SHUTDOWN; LIT APU HANDLE; HORN OUTSIDE GOING OFF; ETC. FIRE DEPT WAS THEN RELEASED AND MAINT PROCEEDED TO DEFER THE APU AND WE DEPARTED. NO FURTHER PROBS ENCOUNTERED. ONE BIG PROB; A CLEANER TRYING TO GET ON A POSSIBLY BURNING ACFT. AT THIS TIME; WE HAD NO IDEA THAT APU WAS NOT ON FIRE. IT APPEARED THAT SHE DIDN'T SPEAK A LOT OF ENGLISH. SHE WAS IN MY WAY GETTING TO COCKPIT AND I HAD TO PHYSICALLY PUSH HER OUT OF THE WAY; LET ALONE THE FLT ATTENDANT HAVING TO KEEP HER AWAY.