The Department of Energy’s National Nuclear Security Administration (NNSA) completed the first production unit of the W76/Mk4B reentry body for the Navy’s submarine-launched ballistic missile warhead, the agency said Wednesday. The first unit was produced nearly three months ahead of schedule, according to an email from NNSA. The Mk4B reentry body, developed in coordination with the Navy, is designed to protect the W76 warhead during atmospheric reentry, or the process of an object entering the atmosphere at extreme speeds. The…