Norwegian scientists say a Russian submarine is still leaking radiation into the ocean 30 years after it sank, but it poses no danger to people or marine life.

Key points: Researchers say the levels of radiation are below the recommended level for foods

Researchers say the levels of radiation are below the recommended level for foods They noticed a "cloud" seeping from a ventilation duct, but were unsure what it was

They noticed a "cloud" seeping from a ventilation duct, but were unsure what it was Scientists will examine the samples in greater detail and continue to monitor the wreck

A team from Norway's Institute of Marine Research took samples from the Soviet-era nuclear submarine Komsomolets, which sank in the Norwegian Sea in April 1989 after a fire broke out on board.

Samples taken in and around a ventilation duct were found to have high levels of radioactive caesium, the highest being 800,000 times the normal amount for Norwegian Sea.

The levels were around 100 becquerels per litre, while the normal level for the sea was about 0.001 per litre. A becquerel is a measurement unit for radioactivity.

But expedition leader Hilde Elise Heldal said this is was not dangerous to fish or humans, pointing out the limit for radioactive caesium in food was significantly higher.

"After the Chernobyl accident in 1986, Norwegian authorities set this limit to 600 Bq/kg", Ms Heldal said in a statement from the institute.

"The levels we detected were clearly above what is normal in the oceans, but they weren't alarmingly high.

"What we have found during our survey has very little impact on Norwegian fish and seafood.

"In general, caesium levels in the Norwegian Sea are very low, and as the wreck is so deep, the pollution from Komsomolets is quickly diluted."

Could this underwater 'cloud' be radioactive?

The crew sent a remote-operated sub down to about 1.7 kilometres under the surface, about 160 kilometres south-west of Bjornoya, or Bear Island.

They noticed a "cloud" seeping out of the ventilation duct believed to be the source of the leak, as well as a nearby grille where higher levels of radioactivity were recorded.

But Norwegian Radiation and Nuclear Safety Authority researcher Justin Gwynn said it was unclear whether this was linked to the radioactivity.

"It looks very dramatic on video, and it's definitely interesting, but we don't really know what we're seeing and why this phenomenon occurs," he said.

"It's something we want to find out more about."

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Submarine disaster kills more than half its crew

Of the 69 crew aboard Komsomolets, 42 died as a result of the sinking.

According to a report from the CIA library, the fire started when "somehow a spray of oil hits a hot surface" in a high-pressure compartment.

The crew managed to bring Komsomolets up to the surface, but the fire continued to spread.

While some seamen assembled topside, others were ordered to contain the damage in the submarine, which had begun filling with carbon monoxide.

Most of the crew climbed out of the sub before it sank, but rough seas made it difficult for them to board and cling on to life rafts.

An escape pod shot five trapped seamen to the surface as the sub went down, several hours after the initial fire, but the hatch opened and only one man was able to climb out before it filled with water.

The CIA reported that 39 men were killed when the sub sank, with three more dying as a result of smoke inhalation and exposure.

The submarine was fitted with an escape capsule, but the pod sank after reaching the surface. ( CIA )

Fears of Chernobyl-like catastrophe

At the time of sinking, Komsomolets contained two nuclear reactors and at least two torpedoes with nuclear warheads containing plutonium, the CIA report says.

With the Chernobyl disaster still fresh in the global consciousness, Russia surveyed the site in 1991, 1992 and 1993 to test for radioactive leaks.

The results of Russian radioactivity testing from 1991 and 1992 were described as "benign" by the CIA's historical department, but, according to a 1993 article in the New York Times, the real danger was still to come.

The article cited a report from Igor Spassky, the director the bureau that designed Russian submarines, who said actual leakage of plutonium was not expected until 1995 or 1996.

The submarine sank in freezing waters near Norway in 1989. ( Supplied: Institute of Marine Research Norwa )

Dr Spassky warned the nuclear material could present a "toxic danger" if carried by the ocean's currents.

He said the "substantial concentrations in water" could be present for months or even years as the parts corroded, but Woods Hole Oceanographic Institution scientist Charles Hollister said this was overstated.

"What we're looking for is a potential for enormous dilutions, rather than concentrations," he told The New York Times in 1993.

He said the diluting power of the sea would make the particles essentially harmless.

Sub's special sarcophagus

However, it was discovered there was significant cracking in the vessel during a research mission by the Institute of Oceanology of the Russian Academy of Sciences.

It used deep-sea manned apparatuses similar to the one used in the making of James Cameron's Titanic.

According to a translated Bauman Moscow State Technical University article archived from 1999, the researchers found parts of the sub's hull had "burst and crumbled from the explosion, like glass".

"Torpedoes, in which thermonuclear charges were located, are broken off as a result of an explosion, which means precise localisation, that is, the closure of combat units, is almost impossible," the article said.

Russian researchers found the sub's hull had cracks that required patching up. ( Supplied: Institute of Marine Research Norway )

It was decided the torpedoes could pose a threat, with Russia determining the need to seal the sub's bow in what CIA historians called "a special sarcophagus".

Special sealing plugs were developed to patch up the wreck.

"Russia said yesterday it had sealed a sunken nuclear submarine off Norway to prevent radioactive leaks," a Reuters report from Moscow from July, 1994 said.

Getting a closer look

Norway has been monitoring the radioactive pollution from the sub since the early 1990s.

In a press release from the institute in December 2017, Ms Heldal said radioactive leaks had not been detected since 1993.

"This is probably due to the fact that the pollution is diluted very quickly, and that our sampling equipment does not hit so closely [to] the submarine wreck that we are able to catch the pollution," she said.

However, the depth of the wreck made it difficult for researchers to reach it, meaning they could not collect sediments and seawater directly from the sub.

Norwegian Radiation Protection Authority section manager Anne Liv Rudjord said the wreckage and nuclear warheads could lead to further leaks of radioactive substances in the future.

"The monitoring is important to document that any leaks from Komsomolets have not led to extensive radioactive pollution so far," she said.

A report published in February 2018, co-authored by Ms Heldal, called for the use of remote-operated mini-subs to collect better samples.

Researchers were finally able to act on this recommendation this year using the AEgir6000.

The dive allowed researchers to capture detailed images of the wreck. ( Supplied: Institute of Marine Research Norway/Stine Hommedal )

"[It] allows us to see exactly where we are taking samples around the wreck, and equally importantly we've been able to use its cameras to zoom in and study the whole nuclear submarine section by section," Ms Heldal said.

Before this year's dive, the most recent images of the sub were from 2007.

Researchers now have freezers full of samples of seawater, sediments and living organisms from the sub, which will be examined in greater detail.

"We need good documentation of pollution levels in seawater, seabed sediments and, of course, fish and seafood," Ms Heldal said.

"So we'll continue monitoring both Komsomolets in particular and Norwegian waters in general."

The institute said its previous modelling of what could happen if all the radioactive caesium leached out at once concluded the impact on fish in the Barents Sea would be "negligible".