Moderator selection in reactor physics: ordinary (light) water is often less desirable than heavy water as a moderator because it __________.

Introduction / Context:
Moderators slow down fast neutrons to thermal energies to increase the likelihood of fission in thermal reactors. The choice of moderator impacts neutron economy, fuel enrichment requirements, and overall reactor design. Heavy water (D2O) and light water (H2O) differ importantly in neutron interaction cross-sections.

Given Data / Assumptions:

• Comparison between ordinary water (H2O) and heavy water (D2O).
• Focus on absorption and scattering cross-sections.
• Thermal reactor context.

Concept / Approach:

A good moderator should have a high scattering cross-section (to slow down neutrons efficiently) and a very low absorption cross-section (to avoid parasitic loss of neutrons). Light water moderates effectively (good scattering) but has higher neutron absorption than heavy water, which hurts neutron economy and necessitates higher fuel enrichment in light-water reactors. Heavy water, with very low absorption, enables operation with natural uranium in certain designs.

Step-by-Step Solution:

Verification / Alternative check:

Thermal reactor designs (PWR/BWR) require enriched fuel largely because of neutron absorption in light water; heavy-water reactors (e.g., CANDU) can use natural uranium, demonstrating superior neutron economy.

Why Other Options Are Wrong:

low absorption: Opposite of the drawback. does not absorb at all: Incorrect; H2O does absorb. low scattering: False; H2O scatters well. cannot slow down neutrons: Incorrect; H2O is an effective moderator in practice.

Common Pitfalls:

Confusing “less desirable than heavy water” with “not used”—light water is widely used but with enriched fuel to compensate for its absorption.

Final Answer:

absorbs neutrons significantly