The radioactive leak was due to a technical failure that occurred during the treatment of fuel in the facility's reactor hall at 1.45pm.
Those who were in the reactor hall during the incident were evacuated when an alarm sounded. Workers have since been back in the hall in an attempt to identify the cause and extent of the radioactive discharge.
The Norwegian Radiation Protection Authority (NRPA) was notified of the incident by IFE on Tuesday morning.
“Our focus now is that IFE gets the leak stopped,” NRPA director Per Strand said in a press release.
“We are in continuous contact with IFE. We will start a new supervisory review of this incident to discover how this happened and why we were not notified until the day after,” he added.
According to the information NRPA has received so far, the leak is not expected to have an impact on the health of plant workers or on the outside environment.
IFE was founded in 1948 and according to its website is “he Norwegian centre of expertise on nuclear technology”.
IFE administers the Halden Reactor Project, an international operation on reactor safety that is the largest joint project of the OECD's Nuclear Energy Agency.
The nuclear reactor at Halden was built in the late 1950s.