ACFT EQUIP PROB -- APU SMOKE.

Narrative

SHORTLY AFTER ARR AT ACFT; SO STARTED APU; AND PUT IT ON LINE (ELECTRIC AND PNEUMATIC/PACKS). WHILE HE WAS WORKING ON FUEL SLIP -- ABOUT 30 SECONDS LATER -- A RAMP PERSON RAN ONTO PLANE -- CAME TO COCKPIT AND RPTED SMOKE COMING FROM APU. JUST ABOUT SAME TIME APU AUTO-SHUT DOWN AND CABIN FILLED WITH HVY;ACRID SMOKE. GND SVC PERSONNEL CALLED SEATAC FIRE/EMER CREWS AND THEY STOOD BY BRIEFLY. I WAS AT PODIUM AREA DOING FLT PLAN AND RELEASE ETC; AND RETURNING TO THE ACFT BECAME AWARE OF PROB. AFT CABIN; ESPECIALLY ZONES C AND D HAD ESPECIALLY HVY SMOKE; DEPTH 4-5 FT FROM THE FLOOR. IT WAS VERY DIFFICULT TO SEE FLOOR. NO PAX WERE ON BOARD OPENED DOORS TO VENTILATE BUT IT TOOK A LONG TIME -- NO CONDITIONED AIR TRUCK WAS AVAILABLE. SMOKE PENETRATED FABRIC AND COATED SURFACES -- EVEN WHEN CLRED OUT AND AIR-CONDITIONING SYS HAD BEEN PURGED. ACFT ATMOSPHERE REMAINED VERY IRRITATING TO EYES AND LUNGS 3 HRS LATER. SMOKE WOULD SURELY HAVE CAUSED A PANIC SIT IF PAX HAD BEEN ON ACFT. SCHEDULED FLT EVENTUALLY CANCELED. CAUSE OF APU FAILURE UNKNOWN.