A FOKKER 100 BEING DEICED WITH ENGS OFF AND THE APU PWRING THE ACFT HAD THE APU INCUR AN UNCONTAINED FAILURE PENETRATING THE PAX CABIN.

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I WAS CAPT ON FLT ABC FROM ZZZ-ZZZ1. FLT ABC DEPARTED GATE X ON TIME; TAXIED TO SPOT X FOR DEICING. WHEN WE WERE #1 FOR DEICING; WE WERE REQUESTED TO SHUT DOWN ENGS AND LEAVE APU RUNNING AS FLUID TYPE 1 AND 4 WERE BEING USED. ABOUT 10 MINS AFTER THE DEICING STARTED; I HEARD AND FELT A THUD NOISE FROM OUTSIDE THE ACFT AT THAT EXACT MOMENT. THE APU STOPPED RUNNING. ONE OF THE ACR X GND CREW MEMBERS CAME TO MY WINDOW AND RPTED THAT HE SAW A FLAME COME FROM THE APU EXHAUST AND HE HAD ACTIVATED THE APU GND FIRE BOTTLE DISCHARGE SWITCH. AT THE SAME TIME THE #2 FLT ATTENDANT ON MY ACFT CAME FORWARD TO THE COCKPIT WITH THE AFT EMER KIT FROM UNDER HER JUMP SEAT -- IT HAD A 3 INCH HOLE IN THE BACK OF IT AND IT HAD BEEN THROWN FROM ITS STOWAGE COMPARTMENT INTO THE AISLE FROM THE IMPACT OF A PIECE OF METAL FROM THE APU HITTING IT. I CONFIRMED WITH ACR X RAMP AND ACR X GND PERSONNEL THAT THE AFT SECTION OF THE ACFT WAS OK; IE; NO FIRE; SMOKE; HAZE OR DANGEROUS SIT EXISTED THAT WOULD PREVENT US FROM STAYING ON THE ACFT. I HAD THE ACR X MAINT DEPT COME TO SPOT X AND TOW ACFT TO Y GATE. I WAS MET BY AN ACR X MAINT SUPVR WHO OPENED THE EMER KIT FROM THE AFT BULKHEAD AND FOUND METAL FRAGMENTS AND 1 3-INCH PIECE OF METAL FROM THE APU INSIDE IT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE APU UNIT FAILED WITH A SECTION OF HOT METAL ABOUT 3 INCHES IN LENGTH PASSING THROUGH THE APU TITANIUM CONTAINMENT SHROUD CONTINUING ON THROUGH THE PRESSURE BULKHEAD AND PENETRATING THE FIRST AID KIT MOUNTED UNDER THE AFT CABIN ATTENDANT SEAT. THE RPTR SAID THE FIRST AID KIT WAS PROJECTED DOWN THE AISLE FOR A GOOD DISTANCE FROM THE IMPACT. THE RPTR STATED THE AFT CABIN ATTENDANT PICKED UP THE FIRST AID KIT AND RPTED TO THE COCKPIT; THE FIRST AID KIT HAD EXPLODED. THE RPTR SAID HE LOOKED AT THE KIT AND OBSERVED THE HOLE IN THE SIDE AND THE SMOLDERING METAL INSIDE. THE RPTR STATED THE GND CREW HAD ALREADY FIRED THE APU FIRE BOTTLE. THE RPTR STATED AFTER INSPECTION THE ACFT WAS TOWED TO A GATE FOR DEPLANING THE PAX. THE RPTR SAID IT IS SUSPECTED THE APU FAILED DUE TO THERMAL SHOCK CAUSED BY DEICING FLUID ENTERING THE VENTILATION AIR INTAKE. THE RPTR STATED NO INJURIES WERE INCURRED IN THIS INCIDENT AS THE CABIN ATTENDANT NORMALLY SEATED IN THE AFT CABIN WAS IN THE FORWARD CABIN AREA.