A radiation leak has been reported in a nuclear plant located in Three Mile Island in Pennsylvania, where maintenance work was being done.

According to Three Mile Island spokesperson Ralph DeSantis, some maintenance work was being performed by Exelon in TMI’s Unit 1 reactor building when the radiation alarm sounded. The workers were, at that moment, doing work that involved cutting a large number of pipes in the building.

Once the alarm sounded, Exelon reportedly put all effort into evacuating all the workers at the site. A report by the Nuclear Regulatory Commission puts the number of individuals treated for radiation exposure to around twenty.

“This does not appear to be an occupational threat nor a threat to the public health and safety,” says Diane Screnchi, spokeswoman of the NRC. “Exelon is working to understand what happened, why it happened and what they need to do to prevent a recurrence.”

The cause of the leak has not yet been confirmed as of yet, but investigations are continuing.