A fission bomb is a standard nuclear bomb developed in the 1940's by the United States Army. President Noah Grace permitted the first detonation test of a fission bomb in Alaska on June 1944. In his speech confirming the bomb, Grace stated: "Let it be known that American Isolationism is not cowardice, but restraint." However, efforts to further creating the nuclear weapon 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 insight had redirected the military's and SRPA's investment to development and research of conventional weapons and other military fields instead. It is not until after the success of Operation Allied Shield in December 1952 that SRPA gained access to recovered data from nuclear reactor technology research in Axbridge, England which provided the nuclear research team in SRPA NT the needed resources to create a powerful, six kiloton fission bomb.
SRPA NT, under the director of J. Robert Oppenheimer, built a strong yielding fission bomb code-named "Nellie" on June 18, 1953 to be used in Operation Black Eden as a last resort to the destruction of 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 results only allowed the bomb to "kick-start" the Chimeran tower in New York City, opening up a wormhole to the Chimeran homeworld.
- The Fission Bomb that decimated the Chimeran Fleet had the markings: "513-U20A".