Mark 4 (nuclear weapon)
One of the earliest United States nuclear weapons, the Mark 4 was still first-generation but a production-quality, re-engineered version of the Fat Man bomb that had been used on Nagasaki. An implosion device, its yield of which could be varied from 1, 3.5, 8, 14, 21, 22, and 31 kt by exchanging the plutonium pits; it also contained uranium. Mark 4 was the first weapon made on an assembly line rather than by hand. 550 were produced.
Relation to later devices
It was among the design ancestors of the first deployed British nuclear weapon, the Blue Danube bomb. Mark 4 was also the basis of the first systematic engineering testing of U.S. bombs since World War II, in the 1951 Operation Ranger a series of air drops over the Nevada desert. The test program explored variations in the Mark 4 core, and then tested the Mark 6 (nuclear weapon), essentially an improved, lighter-weight Mark 4.
The Mark 4 was the first bomb involved in an operational accident. On 10 November 1950. a B-50, in mechanical distress, jettisoned its bomb over the St. Lawrence River, approximately 300 miles northeast of Montreal, Quebec. The weapon's HE [high explosive] detonated on impact, scattering nearly 100 pounds (45 kg) of uranium. Its plutonium pit remained aboard the aircraft, which later landed safely.