The leak of radioactive iodine happened on Monday at the Institute for Energy Technology in Halden, in the country's southeast, the Norwegian Radiation Protection Authority (NRPA) said in a statement on its website.
"The leak of radioactivity was due to a technical failure when handling nuclear fuel inside the reactor hall. The amount of radioactivity is regarded as small," said the statement.
The NRPA added that it will begin a new series of inspections to identify the cause of the leak as well as to determine why authorities were not alerted until the day after the incident.
Norway does not use nuclear power to meet its energy needs but the Institute for Energy Technology maintains two reactors for research purposes.
It was announced on October 10 that they would be temporarily closed for economic reasons.