Russian divers have completed the first phase of a deep sea operation to seal holes in a sunken submarine to prevent its corroding nuclear torpedoes from leaking plutonium and other radioactive material into the North Atlantic, the head of the expedition said yesterday.

Diving more than a mile under the waves in a pair of manned sub mersibles, the Russian team succeeded in attaching nine large seals to the bow of the submarine, the Komsomolets. The first phase of the expedition began in June and ended in August. The sealing job is to be completed next year.

"It was a very big and very unusual operation," Dr. Anatoly M. Sagalevitch, head of the expedition, said yesterday in a interview. "It was the first time in world practice that such an operation has been carried out."

Dr. Sagalevitch, who directs Moscow's two deep-diving Mir sub mersibles, reported on the salvage work yesterday to the Marine Technology Society, a professional group based in Washington that is holding its annual meeting there. Top Secret Sub