After the
United States developed and deployed its first atomic bombs, the
Soviet Union detonated
its first bomb in 1949, leading to a
nuclear arms race during which the number of nuclear weapons escalated from nine in 1946 to 20,000 by 1960. in 1958 The first nuclear weapons to be developed relied solely on
nuclear fission of
plutonium or
enriched uranium as their source of nuclear energy. Later came the development of
thermonuclear weapons, commonly called hydrogen bombs, which use a pure fission or
boosted fission primary stage to ignite
nuclear fusion in a secondary stage, using the hydrogen isotopes
deuterium and
tritium as fuel. Thermonuclear weapons can be made to be far more powerful than those that rely solely on fission. The
Castle Bravo test in 1954 had a yield of 15 megatons; a thousand times more powerful than
Little Boy, the bomb dropped on
Hiroshima. The first tests to utilize nuclear fusion came during
Operation Greenhouse in 1951, which included the first trial of a boosted fission weapon in the
Item test. The first test of a true thermonuclear device, codenamed
Ivy Mike, took place on November 1, 1952. The explosion had a yield of 10.4 megatons and destroyed the small island of
Elugelab where it was tested. This first device, however, was too heavy to work as a deliverable weapon. The first deployable thermonuclear weapon designs were tested during
Operation Castle in 1954. ==Proposal and development==