II. EARLY DESIGN
A. PRELIMINARY DESIGN CONSIDERATIONS
Early in the Experiment it was decided that a choice had to be made about a fissile material and a method of assembly because of the time and effort it would take to develop more than one type of explosive. This section outlines the important considerations which influenced our decision as to how to proceed, and describes those features of the explosive which were recognized at the time the decision was made.
Fissile Material Considerations
Economic and Political Considerations — U233
was eliminated because of the prohibitive cost of production (thorium breeder reactor). U235 and Pu239 cost about the same to manufacture, but we were informed by the "Nth Country Treasury Department" that only one or the other could be produced. The production of Pu239 has a long range economic advantage over because it requires the development of reactor technology.
Physics Considerations — Pu239 has the advantage of a lower critical mass and a low density phase (δ phase) with a greater compressibility, both of which lead to a shorter chain reaction time. U235 has the advantage of a low neutron background, while the Pu240 impurity produced in the Pu239 has a relatively high spontaneous fission rate.
Method of Assembly Considerations
The gun method appeared easier to accomplish because it involved familiar technology. The long assembly times and the Pu240 impurity in Pu239 rule out the use of plutonium with this method.
The implosion method appeared more difficult because of its unfamiliar technology, but it seemed to have a greater potential for future development and more efficiency because the 1945 implosion bombs gave greater yields. The development of the implosion method seemed to be a more sophisticated, challenging, and hence appealing problem.
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