An investigation is taking place following an emergency at Hartlepool's nuclear power station.
British Energy declared a site incident at 1215 BST on Wednesday after a leak on a pipe carrying radioactive bi-products.
Fire services with an emergency tender, and an environmental protection unit attended the site.
No-one was injured in the incident and British Energy said at no time was the public at risk.
The incident happened after water carrying radioactive bi-products was released into the reactor building.
A British Energy spokeswoman told BBC News Online: "We follow strict safety procedures in any incident.
"But no one was injured and the public were never at risk. The leak has been repaired by staff on site.
"Our technical people will now investigate what happened and take any action to prevent it happening again at Hartlepool or at another station."