Fertile versus fissile: identify the nuclide that is fertile (can be converted into a fissile isotope upon neutron absorption and decay) rather than directly fissile.

Introduction / Context:
Nuclear fuel-cycle discussions hinge on the distinction between fissile materials (which readily fission with thermal neutrons) and fertile materials (which convert into fissile isotopes after neutron absorption and subsequent decay). Correct identification guides fuel design and reactor strategy.

Given Data / Assumptions:

• Fissile examples: U-235, Pu-239, U-233, Pu-241.
• Fertile examples: Th-232, U-238.
• We compare the listed nuclides and select the fertile one.

Concept / Approach:
Th-232 is fertile; it does not fission easily with thermal neutrons. Instead, it absorbs a neutron to become Th-233, which beta-decays to Pa-233 and then to fissile U-233. In contrast, U-235, Pu-239, and Pu-241 are fissile with thermal neutrons. U-233 is itself the fissile endpoint of the thorium cycle.

Step-by-Step Solution:
Classify each isotope: U-233 (fissile), U-235 (fissile), Pu-239 (fissile), Pu-241 (fissile), Th-232 (fertile).Identify the single fertile nuclide in the list.Select Th-232.

Verification / Alternative check:
Thorium fuel-cycle literature consistently presents Th-232 → U-233 breeding via Pa-233, validating Th-232 as the fertile starting point.

Why Other Options Are Wrong:

• U-233: Fissile product of thorium cycle.
• U-235: Canonical fissile isotope in uranium fuels.
• Pu-239: Fissile, widely used in MOX and fast reactors.
• Pu-241: Also fissile.

Common Pitfalls:
Confusing fertile Th-232 with fissile U-233; assuming all actinides listed are fissile without checking their neutron-induced behavior.

Final Answer:
Th-232