Fusion fuels: which of the following is a practical fuel pair for thermonuclear (fusion) reactors?

Introduction / Context:
Fusion energy seeks to combine light nuclei to release energy, as in D-T (deuterium-tritium) fusion. The choice of fuel affects reaction cross-section, ignition temperature, and radiation output (e.g., neutron energy).

Given Data / Assumptions:

• We are comparing fuels for fusion (not fission).
• Practical consideration emphasizes achievable cross-sections at attainable plasma temperatures.

Concept / Approach:
The D-T reaction has the largest cross-section at relatively ”low” fusion temperatures (order of 10^8 K compared with alternatives). It produces a 14.1 MeV neutron and a 3.5 MeV alpha, making it the leading candidate in present experimental devices (e.g., tokamaks and inertial systems). U-233, thorium, and Pu-239 are fission fuels; heavy water is a moderator, not a fuel.

Step-by-Step Solution:

Verification / Alternative check:
Fusion literature consistently treats D-T as the near-term fuel due to favorable reactivity compared with D-D or D-^3He, which require higher temperatures or have lower cross-sections.

Why Other Options Are Wrong:

• U-233, Pu-239: fissile for fission, not fusion fuel.
• Thorium: fertile material for breeding, not a fusion fuel.
• Heavy water: D2O, used as moderator and coolant in some fission reactors.

Common Pitfalls:
Conflating fusion and fission fuel cycles; assuming any ”nuclear” material is a fusion fuel.

Final Answer:
Deuterium and tritium