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For instance, this is what the US Navy did with part of the first Seawolf, SSN-575.

Here's the referenced bit for anyone curious:

On 18 April 1959, the Navy disposed of the radioactive S2G plant by sealing it in a 30-foot (9.1 m) high stainless steel containment vessel, towing it out to sea on a barge, and then sinking the barge at a point about 120 miles (190 km) due east of Maryland in 9,100 feet (2,800 m) of water. Twenty-one years later, the Navy was unable to relocate the container, but said that the radioactive materials inside should decay before the containment vessel deteriorated.