Thermal power in nuclear reactors: by which primary nuclear process is heat actually generated in a typical thermal reactor using uranium fuel?

Introduction / Context:
In a thermal nuclear reactor (e.g., PWR or BWR), the fuel contains fissile isotopes such as U-235. Understanding the precise mechanism of heat generation is foundational to reactor physics and safety analysis.

Given Data / Assumptions:

• Low-enriched uranium fuel with significant U-235 content.
• Thermal (moderated) neutron spectrum.
• Steady-state reactor operation.

Concept / Approach:

When a U-235 nucleus absorbs a neutron, it becomes unstable and splits (fissions) into two fission fragments, releasing kinetic energy of fragments, prompt gamma radiation, and additional neutrons. The kinetic energy of fragments is deposited as heat in the fuel matrix, which is transferred to the coolant. Chemical “combustion” does not occur, and nuclear “fusion” is not part of thermal reactor operation. Simple neutron capture without fission does not yield the dominant heat output.

Step-by-Step Solution:

Verification / Alternative check:

Reactor heat balances and fission energy per event (~200 MeV) confirm fission as the heat source; capture reactions without fission are parasitic and comparatively minor for heat generation.

Why Other Options Are Wrong:

fusion: Not employed in current power reactors. absorption without fission: Yields little heat, reduces reactivity. combustion: Chemical process, not applicable. alpha decay: Too slow and weak to power a reactor.

Common Pitfalls:

Confusing neutron capture with fission; assuming “burning” means chemical combustion.

Final Answer:

fission of U-235 induced by neutrons