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Between uranium-235 and plutonium-239, which nuclide has the smaller critical mass?
APlutonium-239 has the smaller mass
BUranium-235 has the smaller mass
CThey have exactly the same critical mass
DNeither nuclide has a critical mass
Answer & Solution
Correct answer: A. Plutonium-239 has the smaller mass
1. Critical mass depends on how many neutrons a fission produces and how easily the nuclide undergoes fission.
2. Plutonium-239 produces more neutrons per fission than uranium-235 and is comparatively easy to fission.
3. Both factors together make the critical mass smallest for plutonium-239.
4. The two nuclides do not share the same critical mass, and both clearly do have one.
5. So plutonium-239 is correct.
_Source: OpenStax Physics (CC BY 4.0), Ch 22 "The Atom", section 22.4 Nuclear Fission and Fusion_
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