The crew of the facility, in a mountain cave in Halden municipality, was immediately evacuated after the radioactive iodine leak. The nuclear regulator says it wasn't informed until a day after the incident.
The leak at the reactor in southeast Norway was discovered on Monday at 1.45 p.m. local time (11:45 UTC) and "no employees have received any radioactive doses of significance," plant officials said on Tuesday.
No environmental damage is expected outside the facility following a small seepage of radioactive iodine, Norway's Radiation Protection Authority (NRPA) said in a statement, adding that it wasn't alerted about the incident until Tuesday.
"We will investigate how this (leak) could happen and why we were not warned until the following day," the NRPA said.
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.
A senior official at the regulator told the Reuters news agency the incident would "maybe" be rated a 1 on an International Nuclear Event Scale ranking from 1 to 7, where 1 is an anomaly and 7 is a major accident, such as Chernobyl or Fukushima.
Anger over delay
"We need to gather more information ... But we are not happy with the situation, that we were not warned immediately. We will investigate further," Per Strand, the head of safety, preparedness and environment at the NRPA, told Reuters.
The facility, which was built in the late 1950s, is close to the border with Sweden but the Swedish Radiation Safety Authority says it had not detected any radiation as result of the incident and did not expect to do so.
Although Norway doesn't operate any nuclear power stations, the Halden reactor - around 120 kilometers (75 miles) south of Oslo - is used for research purposes.
Halden is one of two small research reactors that study nuclear safety issues. It can produce up to 25 megawatts, a fraction of what nuclear reactors in neighboring Sweden can produce.
mm/rc (AP, Reuters)