Officials said Tuesday that the leak posed no risk to workers at the plant or the environment.
Plant officials said a technical failure during the handling of fuel led to the release of radioactive Iodine-131 and -132 to the reactor hall. They said staff were immediately evacuated and "no employees have received any radioactive doses of significance."
They said the situation is under control and that local authorities were informed.
The Norwegian Radiation Protection Authority said Tuesday that based on its information, the leak was small and "will not have any consequences for health or the environment outside the facility."
The Halden reactor is used for research purposes.