Yeah, looks like boron coated cadmium tape. The relevant excerpt from Command and Control... > To avoid the embarrassment of relying on a Los Alamos design, Te

Yeah, looks like boron coated cadmium tape. The relevant excerpt from Command and Control... > To avoid the embarrassment of relying on a Los Alamos design, Teller used Livermore’s new core but added a mechanical safing device to it. A strip of cadmium tape coated with boron was placed in the center of the core. Cadmium and boron absorb neutrons, and the presence of the tape would stop a chain reaction, making a nuclear detonation impossible. During the warhead’s arming sequence, the tape would be pulled out by a little motor before the core imploded. It seemed like a clever solution to the one-point safety problem—until a routine examination of the warheads in 1963 found that the tape corroded inside the cores. When the tape corroded, it got stuck. And the little motor didn’t have enough torque to pull the tape out. Livermore’s mechanical safing device had made the warheads too safe. A former director of the Navy’s Strategic Systems Project Office Reentry Body Coordinating Committee explained the problem: there was “almost zero confidence that the warhead would work as intended.” A large proportion of W-47 warheads, perhaps 75 percent or more, wouldn’t detonate after being launched. The Polaris submarine, the weapon system that McNamara and Kennedy considered the cornerstone of the American arsenal, the ultimate deterrent, the guarantor of nuclear retaliation and controlled escalation and assured destruction, was full of duds. For the next four years, Livermore tried to fix the safety mechanism of the W-47, without success. The Navy was furious, and all the warheads had to be replaced. The new cores were inherently one-point safe.