CTK quoted the plant's spokesman, Milan Nebesar, as saying the problem would force the plant to go off line for about three weeks. Nebesar was not immediately available to confirm the report.He did not say whether there had been any radioactive fluid associated with the leak.
One week ago Temelin boosted output in the reactor to 75 percent, its highest level ever, as it nears completion of its testing phase.
Temelin, 60 km (37 miles) from the border, has been plagued by Austrian objections. Vienna says that despite the introduction of western safety systems to the Communist-era design, the plant is unsafe and should be shut down.
The Czechs say the plant is safe.
The plant's owner CEZ has said it hopes to have the reactor in full operation by the end of the year, more than 18 months behind schedule.
Temelin was allowed last week to conduct final tests on the second block before nuclear fuel loading later this year. The second reactor is expected to be operational next year.