BEFORE ENG START; GND MARSHALLER NOTIFIED FLC OF SMOKE COMING FROM THE APU. NO COCKPIT INDICATIONS OF FIRE; BUT APU WAS SHUT DOWN. WHEN SO WENT TO ASSESS THE SIT; GND CREW SAID FIRE DIDN'T GO OUT WITH INTERNAL APU EXTINGUISHER. SO DIRECTED GND CREW TO FIRE EXTINGUISHER WHICH STOPPED ANY FIRE SMOKE. ANTI-ICE FLUID SUSPECTED.

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PRIOR TO ENG START; GND MARSHALLER OBSERVED SMALL AMOUNT OF SMOKE IN R WHEEL WELL AREA NEAR APU EXHAUST. SHORTLY THEREAFTER; RECEIVED FIRE SIGNAL FROM GND MARSHALLER INDICATING THE APU. CAPT DIRECTED APU SHUTDOWN (ACCOMPLISHED WITH FIRE SWITCH) AND ARPT FIRE/RESCUE SUMMONED (ACCOMPLISHED BY FO). THERE WAS NO COCKPIT APU FIRE OR WHEEL WELL FIRE INDICATION. I WENT OUTSIDE TO ASSESS SIT WHILE CAPT DIRECTED DEPLANING OF PAXS VIA JET BRIDGE. UPON ARR AT WHEEL WELL; THERE WAS STILL SOME SMOKE PRESENT AND THE GND MARSHALLER (WHO WAS NOW STANDING BY WITH A RAMP FIRE BOTTLE) STATED THAT THE FLAMES DID NOT GO OUT WHEN THE APU WAS SHUT DOWN. I DIRECTED THE GND MARSHALLER TO FIRE THE EXTINGUISHER AT THE FLAME AREA. FLAMES WERE EXTINGUISHED; FIRE RETARDANT CLEANED FROM WHEEL WELL; APU MEL'D AND FLT CONTINUED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SAYS THE FIRE WAS A RESULT OF DEICING FLUID BEING TRAPPED IN THE APU SHROUD COMBINED WITH AN EXHAUST LEAK. THE SHROUD IS DOUBLE-WALLED WITH INSULATION BTWN THE WALLS. THE ACFT HADN'T BEEN DE-ICED FOR THE LAST 2 LEGS; SO THEY SUSPECT THE FLUID LEAKED INTO THE SHROUD WALL AND THEN IF THE SHROUD HAD A CRACK; ALLOWING THE HOT EXHAUST TO COME IN CONTACT WITH THE GLYCOL-SOAKED INSULATION RESULTING IN THE FIRE. RPTR SAYS IT IS NORMAL TO HAVE SOME SMOKE COMING FROM THE APU AFTER DE-ICING; BUT THIS TIME WHEN THE GND CREW CHKED THEY SAW A DETACHMENT RING IN THE APU AREA BEING RED HOT. THE APU WAS SHUT DOWN AND RPTR THINKS THAT PUT OUT THE FIRE; BUT WHEN HE LOOKED THE GND CREW SAID IT DID NOT; SO HE DIRECTED THEM TO FIRE THEIR GND EXTINGUISHER INTO THE AREA. THAT RESULTED IN A MAJOR CLEAN UP OF THE AREA BECAUSE OF THE HIGHLY CORROSIVE EXTINGUISHING AGENT. IT REQUIRED WASHING DOWN THOROUGHLY AND DRYING WITH AN AIR HOSE. AFTER THE CLEAN UP; THE AREA WAS INSPECTED; THE APU MEL'D AND THE FLT CONTINUED.