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THE NTH COUNTRY FISSION WEAPON DESIGN
D. A. DOBSON, D. N, PIPKORN AND R. W. SELDEN
(December 14, 1966)
I. BASIC CONCEPTS
The basic concept of how a bomb works, preliminary design considerations, and our first complete design were significant stages in the evolution of our understanding about nuclear explosives. We present the basic concepts as we understood them early in the Experiment and not from our current knowledge. 1. A nuclear fission explosion results when a supercritical mass of fissile material is assembled and held together long enough for the chain reaction to take place. 2. Critical mass numbers are readily available from the literature (Paxton, Los Alamos Critical-Mass Data and Paxton, Critical Dimensions of Systems Containing U235, Pu239, and U233
| Fissile material | Critical mass (kg) | |
|---|---|---|
| Bare sphere | Sphere surrounded by 4-cm-thick U reflector | |
| U235 (93.5%), U238 (6.5%) | 48.0 | 26.0 |
| U233 | 14.5 | 8.5 |
| a phase Pu239 (density 19.8 g/cc) | 9.5 | 6.2 |
| b phase Pu239 (density 15.6 g/cc)a | 15.5 | 8.0 |
a containing 1 wt % gallium
4. The time for the chain reaction to take place can be estimated. The neutron multiplication time, a-1, is the mean time a neutron spends in a supercritical fissile assembly before producing a fission.
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