The thermal performance efficiency of the natural draft cooling tower at Entergy Operations' 858 MWe Arkansas Nuclear One, Unit 2 was successfully upgraded to 101% of design performance capability in April 1994 as the end result of a unique "root-cause" analysis of the cooling tower's long-standing performance deficiencies. Through application of state-of-the-art diagnostic testing methods and computer modeling techniques, Entergy was able to identify and correct air/water maldistribution problems in the 447 foot tall counterflow design at minimal cost. Entergy estimates that the savings realized, as a result of the 1.2^oF reduction in cooling tower outlet water temperature, will pay for the thermal upgrade project in approximately 18 months.