WHILE IN STRAIGHT AND LEVEL FLIGHT AT 1000 FT, THE PILOT HEARD A "POP" IN THE ENGINE AREA. HE NOTED THAT THE TURBINE OUTLET TEMPERATURE (TOT) BEGAN AN IMMEDIATE RISE TO THE HIGH YELLOW AREA INITIALLY, THE POWER WAS REDUCED TO 50% TORQUE AND THE TOT RETURNED TO THE GREEN ARC. THE PILOT STARTED TO DIVERT, BUT AFTER ABOUT 20 TO 30 SEC, THE TOT BEGAN RISING AGAIN, SO THE TORQUE WAS REDUCED TO 30%. THE HELICOPTER WAS OVER AN AREA OF TREES, BUT THE PILOT SAW A CULDESAC. AS HE WAS APPROACHING TO LAND, THE TOT BECAME UNCONTROLLABLE AND THE ENGINE WAS SHUT DOWN. AN AUTOROTATION WAS COMPLETED IN A TAIL LOW ATTITUDE TO PREVENT A GROUND RUN. DURING TOUCHDOWN, THE TAILBOOM WAS SEVERED AFT OF THE HORIZONTAL STABILIZER. DISASSEMBLY OF THE ENGINE REVEALED THAT THE SECOND STAGE TURBINE NOZZLE, PN 6893952, HAD SEPARATED FROM THE VANE PORTION OF THE NOZZLE.