A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission (fission bomb) or from a combination of fission and fusion reactions (thermonuclear bomb).
The United States Army first developed nuclear weapons in the 1940's and the first nuclear test took place in Alaska on June 1944. President Noah Grace gave a speech confirming the development of the nuclear bomb and referred to the tests as a clear message of national pride, stating "Let it be known that American isolationism is not cowardice, but restraint." However, efforts to further advancing the nuclear bomb went sidelined as the United States possessed insufficient quantities of enriched uranium to create a more effective nuclear bomb with large energy yields in mass productions. This setback had redirected the military's and SRPA's investment to conventional weapons development and biological research instead.
It was not until after the success of Operation Allied Shield in December 1952 that SRPA gained access to recovered data from British nuclear reactor technology research (which is reverse engineered from a Chimeran reactor core) in Axbridge which provided the nuclear research team in SRPA NT the needed resources to create a powerful, six kiloton fission bomb.
The SRPA nuclear research team led by J. Robert Oppenheimer built a strong yielding fission bomb code-named "Nellie" on June 16, 1953 to be used in Operation Black Eden as a last resort to destroy the Chimeran fleet in the Yucatan peninsula, which the bomb was set to explode in the center of the fleet at the radius range of 500 feet away. Sentinels Nathan Hale, Joseph Capelli, Richard Blake and other remaining Black Ops, including X-Ray Squad, infiltrated the Chimeran flagship hovering above the Chicxulub crater in Mexico to set and detonate the bomb. The entire Chimeran fleet was completely destroyed in the ensuing explosion, leaving Hale and Capelli the only survivors. However, the bomb's explosive yield inadvertently "kick-start" the Chimeran tower network, creating an energy surge that led to the opening of the wormhole to the Chimeran homeworld.
- The fission bomb that decimated the Chimeran Fleet had the markings: "513-U20A".