Nuclear weapons vs reactors — nature of chain reaction An atom bomb (fission weapon) utilises which form of nuclear chain reaction control?

Introduction / Context:
Fission weapons and power reactors both rely on neutron-induced fission, but their control philosophies are opposite. This question checks understanding of chain-reaction control in weapons versus civilian reactors.

Given Data / Assumptions:

• Fissionable material (e.g., highly enriched U-235 or Pu-239) in supercritical configuration.
• Presence or absence of engineered control systems and moderators.

Concept / Approach:
A fission weapon rapidly assembles a supercritical mass and ensures that neutron multiplication far exceeds losses, producing an exponential power rise in microseconds. There is no intent to stabilise the reaction. In contrast, nuclear power reactors maintain a controlled, near-critical state using control rods, coolant/moderator feedback, and engineered safety margins.

Step-by-Step Solution:
Define “uncontrolled”: prompt supercriticality with no regulating feedbacks engaged.Recognise weapon design goal: maximise energy release in an extremely short time.Therefore the correct description for an atom bomb is “uncontrolled nuclear fission chain reaction.”

Verification / Alternative check:
Historical accounts of fission weapons describe rapid assembly techniques (gun-type or implosion) to create prompt supercritical conditions.

Why Other Options Are Wrong:
Controlled fission (Option B) describes power reactors. Pure fusion without fission trigger (Option C) does not represent typical thermonuclear designs which use a fission primary. Option D is incorrect because chain reactions are fundamental to yield.

Common Pitfalls:
Equating “moderation” with “control”; moderation slows neutrons but control requires active reactivity management.

Final Answer:
Uncontrolled nuclear fission chain reaction