OSLO (Reuters) - A leak at a small nuclear reactor in Norway has been contained, with no injuries sustained and no expected environmental damage outside the facility, the reactor’s operator and the country’s Radiation Protection Authority (NRPA) said on Tuesday.

General view of the Halden Reactor Project in Norway, October 25, 2016. Thomas Andersen/NTB Scanpix/via Reuters

The leak at the research reactor operated by the Institute for Energy Technology, located in a mountain cave in the middle of Halden in southern Norway, began on Monday at 7.45 a.m. ET but the regulator said it was not alerted until Tuesday.

The crew of the reactor was evacuated after the leak was detected but some staff later returned to assess the cause and extent of the accident, the NRPA said. Its operator said the reactor was isolated and the leak contained.

“We will investigate how this (leak) could happen and why we were not warned until the following day,” the regulator said in a statement.

A senior official at the regulator told Reuters 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.

“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 reactor’s operator said the leak had been contained and there had been no injuries. “The reactor is shut. The leak is contained,” Atle Valseth, research director at the Institute for Energy Technology told Reuters.

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He did not know how many staff were present when the leak occurred but said up to eight persons are allowed during this type of operation.

“There is no danger to health. The radioactive dosage they have received is low,” he said, adding the crew had not received hospital treatment.

CLOSE TO SWEDEN

The reactor 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 based on the low levels of radiation in Halden.

The reactor was built in the late 1950s in a mountain cave in Halden, some 120 km south of Oslo. Norway does not have nuclear power stations but operates two small research reactors that study nuclear safety issues. The Halden reactor can produce up to 25 megawatts, a fraction of what nuclear reactors in neighboring Sweden can produce.

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.

There were also small amounts of iodine from other sources, such as in water used to cool the reactor, he said.

Local police were not immediately available for comment.