The Groton-based attack submarine USS Pittsburgh had to return to the Naval Submarine Base recently from being out to sea due to a "minor" leak within its nuclear propulsion plant, the Navy says.

"The USS Pittsburgh (SSN 720) recently returned from sea to conduct logistics, maintenance, and repairs," a statement from Cmdr. Sarah Self-Kyler, spokeswoman for the Navy's Atlantic submarine fleet, says. "This action is not uncommon, and ensures the ship is maintained at a high state of readiness."

The Pittsburgh, a Los Angeles-class attack submarine, was commissioned in 1985.

The "minor moisture leakage" was discovered during a routine inspection while the boat was underway. The source of the moisture was near a valve in the propulsion plant. The issue did not impact the ability of the propulsion plant to function properly.

"The amount of moisture was so small that it was only detected by sensitive humidity detection equipment inside the ship's reactor compartment, a typically unmanned space," Self-Kyler said.

The moisture was fully contained within the reactor compartment, and did not affect the safety of the reactor. There was no risk posed to the crew or the ship, she said.

Each U.S. Navy submarine has a pressurized-water reactor, which has a primary coolant system and a secondary coolant system. The primary system circulates water, which is pressurized to keep it from boiling. The water is heated as it moves through the reactor, then goes through the steam generator, and then flows back to the reactor to be reheated.

The heat from that water is transferred to the secondary system, which is isolated from the primary system, to create steam. The steam flows through the secondary system to the propulsion turbines, which turn the propellers, and the turbine generators, which supply the ship with electricity.

Sub base personnel and the ship's crew are working to fix the problem, which is best addressed in port, the Navy said.

Self-Kyler indicated there would be no impact on the submarine force's ability to carry out missions.

"USS Pittsburgh's return to port does not create a lack of presence," she said. "The Submarine Force remains fully ready to support combatant commanders worldwide."

Ninety-four Navy ships, including submarines, were deployed as of Wednesday, according to the Navy's website. The Navy's fleet currently consists of 285 ships. The Navy currently has 67 submarines, including 50 attack submarines.

j.bergman@theday.com