Thread: Nuclear bombs
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Astronuc
#6
Apr15-08, 10:18 AM
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Both U-235 and U-238 release 2 or 3 neutrons, with an average value in between. U-238 is more likely to absorb neutrons (to become U-239 which decays to Np-239, which decays to Pu-239) than fission, except for fast neutrons with E typically > 1 MeV.

U-235 has a high fission cross-section for neutrons with thermal energies, < 0.1 eV, and is morely likely to fission than simply absorb neutrons.

In nuclear explosions, the mass of fissile material is compressed into a supercritical mass and the fission process is really prompt supercritical - hence the rapid fissioning within microseconds - before the mass is dispersed.