Severomorsk is a closed town and the second largest in Murmansk Oblast.
As the administrative base of the Russian Northen Fleet this port town along the Kola Bay is usually out of boundaries for tourists. However, with 75,000 inhabitants there are still a few activites going on that are not directly related to the military.
This is the tricky bit, as a closed town heavy restrictions are in place on whom may enter. Don't expect to just hop on the bus and assume things will fix themself when arriving at the checkpoints. They won't.
Nearest train station is in Murmansk, 25 km away, only freight traffic continues all the way to Severomorsk. Night trains arrive from all of northwestern Russia and Moscow. Expect a long trip; most other major cities are 18-36h away.
Driving is easy since it's situated at the absolute end of highway M-18, coming all the way from Saint Petersburg.