Fissile vs fertile: which of the following is not a fissile nuclear material under thermal neutrons?

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:

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