An investigation is under way following a radioactive leak at the Torness nuclear power station.
Health and safety officials were alerted after the problem was found during a routine inspection.
The incident was one of three reported to ministers from UK plants which are now under investigation by the Office for Nuclear Regulation (ONR).
Groundwater at the Dunbar plant was found to be contaminated with radioactive tritium, which was leaking from two pipelines.
All three incidents occurred in February and are still under investigation by ONR - the government’s nuclear safety watchdog.
They were serious enough to be reported to ministers under safety guidelines agreed after the Chernobyl accident in Ukraine 25 years ago.
According to the ONR, the response to the incidents from the companies that run the plants was “appropriate”.
The pipelines at Torness were put out of use after the leak was discovered.
A spokesperson for EDF Energy, the French company which operates Torness, said there had never been any danger to staff or the public, and that the levels of radioactivity that leaked were “extremely low.”
The spokesperson said: “On February 11 checks carried out on water in the sumps of the active drain system Torness power station found very low levels of tritium. To put this in some context the water that we sampled has less dose uptake (litre for litre) than that found in mineral water.
“Checks earlier in February had shown no tritium present.”
“We immediately contacted the Scottish Environment Protection Agency and the Office for Nuclear Regulation to report the findings. Both agencies have been kept regularly informed during the investigations.
“Subsequent checks showed that the levels of tritium found in the sump water posed no risk.
“The water in the sumps was safely removed and disposed of via the authorised discharge routes. The sumps are sealed, preventing an unauthorised discharge to the environment from this source.
“The subsequent investigation is concentrating on one of the pipelines used for effluent discharge. The pipelines are part the site’s normal authorised discharge routes, the three other pipes have been checked with no issues found.
“This particular pipe has been taken out of service until the investigations are satisfactorily concluded.”
A spokeswoman for the Scottish Environment Protection Agency (SEPA) added: “SEPA was informed by British Energy Generation Limited (BEGL) in February 2011 that routine sampling had detected a very small leak in a pipe leading to raised level of tritium in a limited area of groundwater.
“The leak has been stopped and the levels of tritium in the ground water are returning towards normal levels.
“SEPA has investigated the matter and has requested that BEGL carry out further work on the pipe system to demonstrate its integrity in the longer term. This work is due to be completed later this year.
“The matter was discussed at the local liaison group meeting on March 23 which SEPA attended.”
The two-reactor plant began generating in May 1988 and is now over half way through its expected lifespan.
Torness is expected to be decommissioned by around 2023.
With an electrical output of 1250 MW, Torness is capable of supplying power to 1.5 million homes and is one of the biggest employers in the region, with more than 500 staff.