Start practicing your duck and cover — America’s best defense against a North Korean nuclear attack isn’t all that great, an expert said Wednesday.

America’s ballyhooed $40 billion Ground-Based Midcourse Defense system only has a 50-percent success rate — and that’s under unrealistic test conditions, physicist David Wright told The Post.

“You simply wouldn’t rely on it,” said Wright, co-director of the Global Security Program at the Union of Concerned Scientists.

“It has been tested 18 times in the last 12 to 15 years, and even under the controlled circumstances of tests, it has failed half the times.”

Still, the Air Force in May did successfully pit the system against an intercontinental ballistic missile, launching an interceptor from California’s Vandenberg Air Force Base and the ICBM target from the Marshall Islands in the Pacific.

The $200-million trial was the first time it had been tested against the kind of long-range weapon North Korea successfully fired this week. But Wright says the system — which features 36 ground-based interceptors at Vandenberg and in Alaska — won’t work that well in an actual emergency due to its inability to ferret out decoy missiles Kim Jong-un might use.