A medium large aircraft could not taxi off of runway because of ice on the runway. Sand was applied and a tow vehicle pulled the aircraft to the gate.

Narrative

Landed with runway contaminated with ice/rain. Aircraft stopped OK with maximum autobrakes. During rollout; Tower suggested 180 degrees on runway and back-taxi to Taxiway R. Attempted left 180 degree turn at Taxiway U on Runway 24R. 3/4 of the way through turn; aircraft began to slide forward (right; southeast of Taxiway U). I attempted to stop; but aircraft continued moving. I reversed engines which stopped forward movement. When I stowed reverse buckets; aircraft began sliding forward again. Reversed engines again. Had to keep engines at reverse idle to keep aircraft stationary. Airport OPS brought our Ground Crew to us with chocks. Problem was; no sanding of taxiways/ramp with pavement ice and rain at 33 degrees F temperature. Had area around aircraft sanded; hooked up tug and towbar. Shut down engines. Sand trucks laid path of sand on taxiway and we were towed in behind sand trucks all the way across ramp to gate. Aircraft never left pavement; no damage or injuries. 'Perfect Storm' conditions: Freezing rain on pavement; rain at 33 degrees F not warm enough to melt ice; airport operator only sanded center 50 FT only of Runway 24R; no sand used on taxiways or ramps until we ordered it.