Fission likelihood: among the following nuclides, which undergoes thermal-neutron-induced fission most readily?

Introduction / Context:
Thermal reactors depend on nuclides with large fission cross-sections for slow (thermal) neutrons. Identifying which common actinides are fissile versus merely fertile clarifies fuel cycle choices and moderation requirements.

Given Data / Assumptions:

• Thermal neutron spectrum considered.
• Standard isotopes U-235, U-238, Th-232, Pu-240 listed.
• “Undergoes fission easily” = large thermal fission cross-section.

Concept / Approach:
Fissile nuclides (e.g., U-235, Pu-239, U-233) readily fission with thermal neutrons. Fertile nuclides (U-238, Th-232) capture neutrons to become fissile isotopes but do not fission efficiently at thermal energies. Pu-240 has low thermal fission probability compared to Pu-239 and mainly undergoes capture.

Step-by-Step Solution:
Classify each nuclide: U-235 (fissile), U-238 (fertile), Th-232 (fertile), Pu-240 (poor thermal fissile).Pick the fissile candidate for thermal neutrons: U-235.Confirm others are not “easy” thermal fissioners.

Verification / Alternative check:
Cross-section tables show U-235 thermal fission cross-section orders of magnitude higher than U-238 or Th-232; Pu-240 is predominantly a capture isotope.

Why Other Options Are Wrong:

• U-238: Primarily captures to form Pu-239.
• Th-232: Captures to become U-233 via Pa-233.
• None of these: Incorrect because U-235 is clearly fissile thermally.
• Pu-240: Not favored for thermal fission.

Common Pitfalls:
Assuming all uranium isotopes behave similarly; conflating fertile and fissile definitions.

Final Answer:
U-235