Thermal (slow-neutron) reactors: which material is the standard primary fuel used to sustain fission in most thermal reactor designs?

Introduction / Context:
Thermal reactors rely on slow (moderated) neutrons to achieve high fission probabilities. Knowing the standard primary fuel clarifies how moderators, enrichment, and core physics interact in mainstream power reactors worldwide.

Given Data / Assumptions:

• “Thermal reactor” means a reactor where neutrons are slowed to thermal energies.
• Common fuels include uranium-based materials; enrichment level depends on design (natural, low-enriched).
• Question asks for the standard primary fuel across most thermal designs.

Concept / Approach:
Uranium—specifically the fissile isotope U-235 within natural or enriched uranium—is the dominant fuel for thermal reactors. Light-water reactors typically use low-enriched uranium (about 2–5% U-235), while heavy-water reactors may use natural uranium due to excellent neutron economy. Plutonium can be used (e.g., MOX fuel) but is not the baseline primary fuel in most introductory contexts; radium is not a reactor fuel; thorium is fertile, requiring breeding to U-233 to become fissile.

Step-by-Step Solution:
Identify the default fissile isotope for thermal systems: U-235.Recognize uranium fuel forms (UO2 pellets) as industry standard.Eliminate plutonium as a routine primary fuel (used mainly in MOX), radium as non-fuel, and thorium as fertile.Select uranium.

Verification / Alternative check:
Fuel-fabrication and reactor-operator references consistently cite uranium dioxide as the standard fuel for LWRs and PHWRs (with enrichment varying by design), supporting uranium as the canonical answer.

Why Other Options Are Wrong:

• Plutonium: Used in MOX or fast reactors; not the most common baseline fuel.
• Radium: Not used as reactor fuel.
• None of these: Incorrect because uranium is standard.
• Thorium: Fertile, requires conversion to U-233.

Common Pitfalls:
Confusing “can be used” (plutonium in MOX) with “is the standard baseline fuel.”

Final Answer:
Uranium