WHILE IN CRUISE FLIGHT, THE 'RIGHT REVERSER ACCUMULATOR LOW' CAUTION LIGHT ILLUMINATED. EXAMINATION FOLLOWING LANDING REVEALED THAT THE AUXILIARY POWER UNIT (APU) TURBINE WHEEL HAD SEPARATED IN AN UNCONTAINED FAILURE, AND HAD DAMAGED NUMEROUS COMPONENTS IN THE COMPARTMENT BELOW THE NUMBER TWO ENGINE PYLON. THE APU HAD BEEN PLACARDED AS INOPERATIVE TWO DAYS PRIOR TO THE INCIDENT. THE INVESTIGATION DID NOT REVEAL WHEN AN INADVERTENT ATTEMPT TO START THE APU HAD OCCURRED, BUT DID REVEAL THAT NO PROCEDURES, OTHER THAN THE INOPERATIVE PLACARD, WERE IN PLACE TO PREVENT START ATTEMPTS.