The U.S. Navy hasn’t fully mitigated attack submarine fleet maintenance and other challenges affecting readiness, at a cost of billions of dollars, the Government Accountability Office concludes in a new report.

Over the last decade, the U.S. Navy has lost more than 15,000 operational days due to maintenance delays and idle time on active attack submarines, resulting in an estimated $3.4 billion in costs to sustain crews and submarines that provided no operational capability, the report says.

Lost operational days include ones spent on depot maintenance or active idle time, a period when an attack submarine and its crew must remain pier-side because they aren’t certified to conduct normal operations but also can’t be inducted into a maintenance period.

“In addition to lost operational days, the Navy faces significant challenges with managing inactive idle time for attack submarines,” the report says. “Inactive idle time describes when an attack submarine designated for decommissioning cannot be inducted into a dry dock in a shipyard. As a result, the submarine and its crew must remain pier-side until a shipyard has the capacity in a dry dock to defuel their nuclear reactor.”

The Navy has reduced active idle time of late, but inactive idle time has grown and is expected to get worse.

“Without mitigation, 15 attack submarines will enter inactive idle time from fiscal year 2026 through fiscal year 2030 — during which time the Navy could incur more than 14,000 days of inactive idle time and $3.1 billion in costs to operate and support these inactive submarines,” the report says.

The report makes two recommendations: The Navy should fully evaluate alternative options to more efficiently decommission attack submarines, and develop an attack submarine inactivation plan that reduces inactive idle time, which it notes would save billions.

The report is a public version of a classified GAO report issued in July. The GAO notes the Navy “verbally concurred” with its recommendations but didn’t provide written comments.