Thorium fuel cycle: Thorium-232 can be converted (bred) into fissile uranium-233 predominantly in which type of nuclear reactor?

Introduction / Context:
The thorium fuel cycle relies on neutron capture by Th-232 to form U-233, a fissile nuclide. Understanding the neutron spectrum that favors this breeding step is important in reactor design and national nuclear strategies.

Given Data / Assumptions:

• Fertile nuclide: Th-232.
• Target fissile product: U-233 after neutron capture and beta decays (Th-232 → Th-233 → Pa-233 → U-233).
• We compare reactor types by neutron spectrum and moderation.

Concept / Approach:
Th-232 has a relatively high neutron capture cross-section in the thermal (slow) neutron energy range. Consequently, thermal reactors (including heavy-water-moderated designs) are suitable for breeding U-233 from thorium. While fast systems can also produce U-233, the classic thorium cycle development emphasizes thermal spectrum breeding due to favorable capture cross-sections and well-established moderation technologies.

Step-by-Step Solution:
Identify the fertile-to-fissile route: Th-232 + n → Th-233 → Pa-233 → U-233.Match to spectra: thermal neutrons provide stronger capture probability for Th-232.Therefore, select thermal reactor as the general category that enables effective conversion.

Verification / Alternative check:
Heavy-water-moderated reactors (a subset of thermal reactors) are often cited in thorium programs because of high neutron economy, reinforcing the broader answer “thermal reactor.”

Why Other Options Are Wrong:
Fast breeder reactor: primarily optimized for breeding Pu-239 from U-238; thorium breeding is less favorable in fast spectrum. Heavy-water-moderated is indeed a thermal type, but the broader correct category is “thermal reactor.” “Enriched-uranium reactor” describes fuel rather than neutron spectrum and is not specific to thorium breeding.

Common Pitfalls:

• Confusing coolant or fuel enrichment with neutron spectrum classifications.
• Assuming only heavy-water designs can breed U-233; other thermal systems can as well, given suitable neutron economy.

Final Answer:
Thermal reactor