Thermonuclear (hydrogen) bomb: Which fusion fuel combination is predominantly used in modern designs?

Introduction / Context:
Thermonuclear weapons (hydrogen bombs) use nuclear fusion to release immense energy. The practicality of a fusion reaction depends on reaction cross-section at achievable temperatures and pressures produced by a fission primary. The common fusion fuel choice in such devices is crucial foundational knowledge in nuclear engineering and defense studies.

Given Data / Assumptions:

• We are considering the fusion stage that follows a fission-driven implosion and radiation-driven compression.
• Fuel must have a high fusion cross-section at relatively “low” thermonuclear temperatures.
• Engineering constraints favor materials that can be stored or bred on demand (e.g., lithium deuteride generating tritium in situ).

Concept / Approach:
The deuterium–tritium (D–T) reaction has the highest fusion cross-section at the lowest temperatures among light isotope combinations, making it the practical choice. In many devices, lithium deuteride serves as a solid store; under neutron bombardment, lithium produces tritium which then fuses with deuterium. Pure deuterium or pure tritium alone are not typical as standalone fuels because D–D requires higher temperature and tritium is scarce and decays, although both participate within the D–T pathway. Helium isotopes are not primary fuels in such weapons.

Step-by-Step Solution:

Verification / Alternative check:
Standard fusion cross-section plots show D–T peak at temperatures markedly lower than D–D or D–He-3, supporting its adoption.

Why Other Options Are Wrong:

• Deuterium only: Requires higher temperatures and yields lower rate.
• Tritium only: Not a practical sole fusion path; needs deuterium partner.
• Helium-3 with helium-4: Not a standard weapons fuel pair.

Common Pitfalls:
Assuming “hydrogen bomb” implies ordinary hydrogen (protium). The effective fuel is a D–T combination (often bred from lithium deuteride).

Final Answer:
Deuterium–tritium (D–T) mixture