KITTERY – The USS Alexandria and its crew of more than 150 arrived Thursday at the Portsmouth Naval Shipyard, where it will be undergoing unspecified maintenance and system upgrades.

How long the Groton, Conn.-based Los Angeles-class nuclear attack submarine remains at the shipyard is unclear.

The shipyard no longer provides advance notice of submarine movements or any indication of the type of maintenance that might give clues as to the duration of the submarine’s stay.

The policy was adopted by the Navy after the Sept. 11 terrorist attacks to ensure the security of the nuclear submarines and the safety of their sailors, said shipyard spokeswoman Deborah White.