WASHINGTON — The U.S. Air Force “sniffer plane” was collecting air samples off Russia’s Kamchatka Peninsula on Sept. 3, 1949, when it gathered evidence of radioactivity, confirming that the war-shattered Soviet Union had tested a nuclear device. The Soviets’ Aug. 29, 1949, test had come faster than expected. Dating from the detonation at Alamogordo, New Mexico, on July 16, 1945, the basic science of nuclear explosions is more than 72 years old — three years older than the North Korean nation.