The $5 million study was launched after officials learned of leaks dating back to 1996 at Exelon's Braidwood plant in Illinois. Those leaks created slightly elevated levels of tritium in drinking water wells near the plant.
The potential carcinogen is a radioactive form of hydrogen commonly found in groundwater but more concentrated in water used in nuclear reactors.
Exelon says tests also revealed no tritium beyond permitted levels in water supplies outside plant boundaries, other than at Braidwood where a state approved cleanup is underway.
Exelon says final results of the study are expected within eight weeks.