The nuclear reactor at the Institute for Energy Technology in southern Norway has spilled radioactive iodine isotopes, says the Norwegian Radiation Protection Authority (NPRA).
An alarm sounded and staff at the plant in Halden were immediately evacuated after the incident on Monday 24 October. However, the NPRA was not told of what happened until the next day.
"The radioactive leak was due to a technical failure during treatment of the fuel in the reactor hall. Emissions are low," the NRPA said.
"We will investigate how this could happen and why we were not warned until the following day," the NPRA said in a statement.
NPRA Head of Safety, Per Strand, added: "We need to gather more information... But we are not happy with the situation, that we were not warned immediately. We will investigate further".
The Institute said that the leak has now been contained and it is not thought that the radioactive material was a risk to those outside the facility. Those working at the reactor did not receive "any radioactive doses of significance".
Atle Valseth, Research Director at the Institute insisted: "There is no danger to health. The radioactive dosage they have received is low." He added that no employees required hospital treatment.
In fairly small amounts, radioactive iodine, also known as I-131, can be used in the treatment of thyroid cancer. However, exposure to large quantities of I-131 can increase the risk of developing leukaemia.
Mark Foreman, a nuclear expert at the Chalmers University of Technology in Sweden, said iodine was mainly a by-product of nuclear fission in the reactor.
"Almost all the iodine is trapped inside the fuel, in a ceramic material. That is inside a metal tube that is welded shut.
That is then inside the reactor - it would then have to leak out of the reactor to enter the reactor hall," he said.
The Halden Reactor is a 25MW nuclear reactor which was built in the late 1950s inside a mountain cave around 120km south of capital city Oslo.