Fertile vs. fissile: which of the following is NOT a fertile material in the nuclear fuel cycle?

Introduction:
Fertile nuclides can absorb a neutron and transmute into fissile nuclides. Distinguishing fertile from fissile isotopes is essential for fuel-cycle strategy and breeder-reactor concepts.

Given Data / Assumptions:

• Common fertile nuclides: Th-232, U-238 (convert to U-233 and Pu-239 respectively).
• Fissile nuclides: U-233, U-235, Pu-239, etc.

Concept / Approach:
U-233 is already fissile, produced from neutron capture by Th-232 followed by beta decays. Therefore, U-233 is not fertile. Th-232 and U-238 are classic fertile materials. Some other heavier plutonium isotopes (e.g., Pu-240) are predominantly fertile rather than fissile.

Step-by-Step Solution:
Identify the fissile candidate: U-233 is fissile.Recognize the classic fertile isotopes: Th-232 and U-238.Hence, the one that is NOT fertile is U-233.

Verification / Alternative check:
Fuel-cycle diagrams confirm Th-232 → U-233 breeding and U-238 → Pu-239 breeding, verifying the classification.

Why Other Options Are Wrong:

• Th-232 and U-238: canonical fertile nuclides.
• Pu-240: largely fertile; poor fissile performance with thermal neutrons.
• “None of these” would imply all are fertile, which is incorrect because U-233 is fissile.

Common Pitfalls:
Confusing “fissile” (readily fissions with thermal neutrons) with “fertile” (neutron capture leads to fissile product).

Final Answer:
U-233