Operators EDF Energy said an alert was sparked on Tuesday teatime after hydrogen was detected on site.
But following the incident, a spokesman for EDF Energy said: “Early indications show that the probable cause of the increased hydrogen in the area was linked to a routine battery recharge which was taking place in the battery room in the site’s turbine hall.
“The area was ventilated and this saw hydrogen levels quickly return to normal.”
staff at the nuclear plant were “mustered” as a precaution after the alert.
There was no evacuation and staff returned to work.
AN EDF spokesman said: “A small amount of clean hydrogen was detected at Heysham 2. The site carried out a muster of its staff, as per its usual arrangements, with everyone safely accounted for.”
Witnesses living nearby reported seeing a large number of emergency vehicles at the scene after the alert was sparked.