An "accidental steam release" at the power station resulted in three casualties. It is understood that one person suffered burns, one a broken hip and the other a broken back.
A spokesman for energy supplier EDF, who own and operate the Morecambe site, quickly moved to reassure people that the event was an "industrial incident, not a nuclear issue", and did not pose a threat
The government's nuclear safety regulator, the Office for Nuclear Regulation (ONR) has now launched an investigation into the incident, but said it "did not believe there were any radiological consequences" from the leak.
A spokesman for the ONR said: "We have been notified of an incident which occurred last night at Heysham 1 power station and our thoughts and concerns are with those involved."The Office for Nuclear Regulation, as the independent safety regulator, will investigate the incident.
"Separately, in line with standard procedures, EDF has also launched its own investigation. "In light of these ongoing enquiries, which are at an early stage, it would be inappropriate to comment any further at this time."
It is the second incident declared at Heysham in the last three years.
In March 2015, a failed pipe at the Heysham facility released approximately 40 tonnes of clean Carbon Dioxide (CO2) into the atmosphere.
No-one was injured in the incident but the ONR's subsequent investigation concluded that the incident "could have had serious implications."
The ONR said the 2015 incident highlighted a "failure to adequately maintain the Carbon Dioxide storage and distribution system", which provides the coolant gas for the Heysham 1 reactors.
The report concluded: "Although no one was injured, CO2 is a substance that can be hazardous to health and the quantities released could have had serious implications if workers had been in the vicinity, including potential fatalities."