Why light-water reactors need enriched uranium When ordinary (light) water is used as both moderator and coolant, why is enriched uranium fuel required?

Introduction / Context:
Reactor criticality depends on neutron economy: enough neutrons must survive moderation and non-productive losses to sustain fission. The properties of the moderator strongly influence whether natural uranium suffices or enrichment is required.

Given Data / Assumptions:

• Light water (H2O) acts as moderator and coolant.
• Fuel is uranium with fissile U-235 present at 0.7% in natural uranium.
• Goal is to maintain a multiplication factor near unity under operating conditions.

Concept / Approach:

Hydrogen in light water possesses a relatively high absorption cross-section for thermal neutrons compared with heavy water or graphite. Consequently, many neutrons are captured by the moderator instead of inducing fission in U-235. To offset these parasitic losses and achieve criticality, the fuel must be enriched so that a higher fraction of neutrons find fissile nuclei. Heavy-water or graphite moderators, with much lower absorption, can often use natural uranium instead.

Step-by-Step Solution:

Verification / Alternative check:

Comparative designs: CANDU (heavy water) operates on natural uranium; PWR/BWR (light water) require low-enriched uranium, illustrating the absorption effect of the moderator choice.

Why Other Options Are Wrong:

Option (b) is a consequence statement but the core reason is the higher absorption; option (c) scattering helps moderation rather than causing the need for enrichment; option (d) is the opposite of the truth; option (e) confuses neutronics with thermal hydraulics.

Common Pitfalls:

Conflating moderating power (a product of scattering effectiveness and low absorption) with scattering alone; low absorption is crucial for good neutron economy.

Final Answer:

Because light water has a high neutron absorption cross-section