CRJ200 APU FIRE WAS OBSERVED AFTER PARKING ACFT. PAX WERE EVACUATED AND FIRE DEPT WAS SUMMONED TO EXTINGUISH FIRE.

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UPON ARRIVING AT THE ACFT I OBSERVED THAT THERE WERE 'MAINT IN PROGRESS' TAGS ON THE ACFT. AFTER SPEAKING WITH MAINT; I WAS TOLD THAT THEY HAD WIRED THE APU DOOR OPEN. MEL ITEM XXX AND IT WAS EITHER THAT OR DEFER THE APU. WE DISCUSSED THE DOOR AND THAT I WOULD KEEP THE APU IN OPERATION. I BRIEFED MY FO ABOUT THE DEFERRAL AND REVIEWED THE MEL WITH HIM. MY BRIEFING INCLUDED; AMONG OTHER THINGS; A DISCUSSION OF SPD LIMITATIONS AND FLT OPS WITH THE APU RUNNING. TAXI AND TKOF WERE CONDUCTED WITHIN NORMAL OPERATING PARAMETERS. I WAS PM. WHILE MAKING A DSCNT I NOTICED THAT THE APU WAS NOT RUNNING. AFTER RESTARTING THE APU; WE CONTINUED IN FOR VECTORS FOR THE APCH. LNDG WAS UNEVENTFUL. TAXIING TO GATE; WE PARKED THE ACFT AND SHUT DOWN THE ENGS. WHEN THE MAIN CABIN DOOR WAS OPENED; A RAMP WORKER BOARDED THE ACFT AND INFORMED US THAT THERE WERE FLAMES IN THE APU EXHAUST. THERE WAS NOTHING IN THE FLT DECK THAT INDICATED THERE WAS A FIRE. THE FO LEFT THE COCKPIT TO INSPECT THE ENG. PAX HAD BEGUN TO DEPLANE BY THIS TIME. THE FO RETURNED QUICKLY AND RPTED THAT THERE WERE LARGE FLAMES COMING FROM THE EXHAUST. I DIRECTED THE FO TO HAVE GND PERSONNEL CALL FIRE RESCUE. HE RESPONDED THAT EMER PERSONNEL WERE ALREADY CONTACTED. SINCE THERE WERE STILL NO INDICATIONS OF A FIRE; I CONSIDERED THE POSSIBILITY THAT THE FIRE SYS INDICATIONS MAY NOT BE OPERATING PROPERLY. THE APU WAS SHUT DOWN AND I SET OFF THE FIRE BOTTLES. AT THIS POINT; I WAS CONCERNED THAT THE SYS DID NOT SEEM TO BE OPERATING PROPERLY. I THEN WENT TO THE CABIN TO HELP THE FLT ATTENDANT DEPLANE THE REMAINING PAX. THERE WERE APPROX HALF OF THE PAX STILL ON THE ACFT AT THAT TIME. I ADVISED THE FLT ATTENDANT THAT AN EMER EVAC WAS NECESSARY. WE TRIED TO KEEP THE PAX REMAINING CALM WHILE HAVING THEM QUICKLY EXIT THE ACFT. WHEN THE LAST PAX LEFT THE SHIP; THE FLT ATTENDANT DID A WALK THROUGH OF THE CABIN AND AT MY INSTRUCTION LEFT AS WELL. I QUICKLY CHKED THE FLT DECK; CABIN; AND LAVATORY. AT NO TIME DID I OBSERVE SMOKE ENTER THE PAX CABIN. WHEN I WALKED OUTSIDE; I SAW A CLOUD OF WHITE SMOKE. I COMMANDED THAT THE PAX BE MOVED FROM THE AREA AS THEY HAD CONGREGATED BESIDE THE ACFT; WAITING FOR THEIR BAGS. AS THE FLT ATTENDANT USHERED THE PAX INSIDE THE BUILDING; I INSTRUCTED THE RAMP WORKERS TO LEAVE THE CARGO COMPARTMENT SINCE THEY CONTINUED REMOVING BAGS FROM THE ACFT DURING THIS TIME. A GND WORKER BEGAN TO SPRAY THE APU EXHAUST WITH AN EXTINGUISHER. CONCURRENTLY A FIRE TRUCK PULLED UP AND STARTED TO SHOWER WATER ON THE FLAMES. FIRE PERSONNEL WOULD NOT ALLOW ME TO RETURN TO THE ACFT. THE FIRE CHIEF; ARPT SAFETY; AND STATION MGR APCHED ME. I THEN CONTACTED DISPATCH; THE CHIEF PLT ON DUTY; AND MAINT TO INFORM THEM OF THE SITUATION. I AM NOT AWARE OF ANY RPTED INJURIES TO PAX OR CREW.