The study also found that groundwater containing tritium is migrating into the Susquehanna River, where “it is immediately diluted to non-detectable levels, posing no safety hazard,” according to the study by AmerGen Energy, operator of TMI.

“We have completed the most extensive tritium sampling effort ever conducted at TMI. The results show that the TMI Station is operating in a manner that protects public health and safety,” said Rusty West, TMI site vice president.

The study was prompted by the discovery of leaks of tritium, which at high levels is linked to cancer, at three nuclear plants owned by Exelon Nuclear in Illinois in 2005.

A fleet-wide search discovered tritium had leaked into groundwater at one monitoring well at TMI. The levels found in the groundwater did not exceed federal drinking-water standards.

The spill occurred when a pipe at a sump pump at an auxiliary boiler at the Unit 1 reactor leaked, Exelon officials said last February.

The leak was promptly corrected and posed no safety hazard, Exelon said.

A more in-depth assessment has found that 42 of 66 samples of groundwater around TMI had detectable levels of tritium — primarily from past leaks. The leaks had been detected and reported to government officials, Exelon said.

But none of the levels violated drinking-water standards.

Exelon tested water from 31 new wells, 32 existing wells and three surface-water locations at TMI. All were analyzed by an independent laboratory.

Tritium is a normal byproduct of nuclear energy. Water with small levels of tritium in it is routinely released from nuclear plants into waterways.