Thorium fuel cycle: thorium-232 can be converted (bred) into uranium-233 predominantly in which reactor spectrum/category?

Introduction / Context:
Thorium-232 is a fertile isotope that can capture a neutron and, through a decay chain, become uranium-233, a fissile material suitable for thermal reactors. Understanding the reactor spectra that favor this conversion is key to appreciating thorium-based fuel cycles explored in several countries.

Given Data / Assumptions:

• Thorium-232 + neutron → Th-233 → Pa-233 → U-233 (after beta decays).
• Good neutron economy (low parasitic absorption) enhances breeding.
• We focus on the spectrum/category most associated with Th→U-233 programs.

Concept / Approach:
While neutron capture is possible in any spectrum, thorium breeding to U-233 is classically associated with thermal-spectrum systems that feature excellent neutron economy, such as heavy-water moderated or molten-salt thermal reactors. These systems minimize neutron losses so that enough captures occur in Th-232 without requiring very high fluxes. Fast reactors typically emphasize U-238→Pu-239 breeding, not Th-232→U-233 as a primary objective.

Step-by-Step Solution:

Verification / Alternative check:
Historical programs (e.g., molten-salt thermal concepts, heavy-water thermal experiments) document effective Th→U-233 breeding with proper reprocessing or on-line fuel handling.

Why Other Options Are Wrong:

• Fast breeder and other fast-spectrum options concentrate on U-238 breeding.
• Swimming-pool reactors are a geometry/cooling style; they can irradiate thorium but are not uniquely required.

Common Pitfalls:
Assuming all breeding must be “fast”; overlooking the role of neutron economy in thermal-spectrum thorium cycles.

Final Answer:
Thermal (e.g., heavy-water or molten-salt thermal) reactor