Fissile vs fertile: which of the following is not a fissile nuclear material under thermal neutrons?
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AU-233
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BU-235
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CU-238
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DPu-239
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EPu-241
Answer
Correct Answer: U-238
Explanation
Introduction / Context:Fissile isotopes can undergo fission with low-energy (thermal) neutrons. Fertile isotopes do not fission readily with thermal neutrons but can transmute into fissile materials upon neutron capture and subsequent decay.
Given Data / Assumptions:
- We compare common reactor-relevant nuclides.
- Standard behavior under thermal neutron spectrum is implied.
Concept / Approach:U-233, U-235, Pu-239, and Pu-241 are classic fissile isotopes in thermal reactors. U-238 is fertile: it captures a neutron to form U-239, which beta-decays to Np-239 and then to Pu-239, a fissile nuclide. Therefore, U-238 itself is not fissile under thermal neutrons.
Step-by-Step Solution:
1) List fissile examples: U-233, U-235, Pu-239, Pu-241.2) Identify U-238 behavior: primarily fertile, not fissile with thermal neutrons.3) Conclude: U-238 is the non-fissile option.Verification / Alternative check:Reactor physics references consistently classify U-238 as fertile and the others as fissile for thermal spectra.
Why Other Options Are Wrong:
- U-233/U-235/Pu-239/Pu-241: all fissile with thermal neutrons.
Common Pitfalls:Confusing ”fissile” with ”fissionable” at high energies; U-238 can fission with fast neutrons but not efficiently with thermal ones.
Final Answer: U-238