The Komsomolets (K-278) sank in the Norwegian Sea in 1989 after a fire onboard killed 42 sailors.
A sample showed radioactive cesium leaking from a ventilation pipe. However, researchers mentioned it was “not alarming” because the Arctic water rapidly diluted it.
The Soviet-period sub can also be deep down, at 1,680m (5,512ft), and there are some fishes within the area, they added.
For the first time, a Norwegian (ROV) remotely-operated vehicle examined and filmed the Komsomolets on 7 July, revealing severe damage.
The submarine is also referred to as K-278 in Russia, and it sank carrying two nuclear torpedoes with plutonium warheads.
It is front part has 6 torpedo tubes, and the sub could also launch Granit cruise missiles.
The news comes a week after a fire swept through a Russian nuclear-powered submersible within the Barents Sea, killing 14 naval officers.
Norway’s Radiation and Nuclear Safety Authority says the pressurized water reactor powering K-278 in April 1989 shut down rapidly when the fire broke out in another compartment.
Twenty-seven sailors survived – they have been eventually picked up by two Soviet ships.
The radiation leak discovered this week came from a pipe near the reactor. It was 800Bq (becquerels) per liter, while the average level within the Norwegian Sea is about 0.001Bq.
Nonetheless, some other water samples from the wreck didn’t show elevated levels.
The 42 sailors who died within the disaster succumbed to toxic fumes or froze within the Arctic waters after the submarine had surfaced briefly.
The commander managed to send a call about an hour after the fire broke out, however, he and four others died when their emergency capsule sank. The K-278 was doomed when the fire spread, fuelled by high-pressure air from a damaged pipe, Russia’s RIA news agency reported.