Fertile-to-fissile conversion: after absorbing a neutron, thorium-232 is ultimately converted into which fissile nuclide?

Introduction / Context:
Thorium-based fuel cycles exploit the fertile nature of Th-232. Upon neutron capture and subsequent beta decays, thorium breeds a fissile isotope that can sustain a chain reaction in thermal spectra, offering alternative fuel-cycle pathways.

Given Data / Assumptions:

• Single neutron capture by Th-232 followed by decay sequence.
• Intermediate nuclide protactinium appears temporarily.
• We seek the final fissile product.

Concept / Approach:
The chain is: Th-232 (n,γ) → Th-233 → β− → Pa-233 → β− → U-233. U-233 is fissile with thermal neutrons and is the endpoint of interest. Protactinium-233 is an intermediate that decays to U-233; it is not the long-term fuel in the closed cycle.

Step-by-Step Solution:
Start with Th-232 capturing a neutron to form Th-233.Th-233 beta-decays to Pa-233.Pa-233 beta-decays to U-233 (fissile).Therefore the fissile product is U-233.

Verification / Alternative check:
Thorium fuel-cycle studies universally denote U-233 as the target fissile isotope produced from Th-232 after two beta decays.

Why Other Options Are Wrong:

• Thorium-233: Intermediate, not fissile endpoint.
• Uranium-235: Not produced directly from Th-232 capture chain.
• Plutonium-239: Breeding product of U-238, not thorium.
• Protactinium-233: Intermediate precursor, not the final fissile product.

Common Pitfalls:
Stopping at Pa-233 and assuming it is the fuel; confusing U-233 with U-235 pathways.

Final Answer:
Uranium-233