In reactor terminology, “light water” used as a coolant refers to which substance?

Introduction / Context:
Light-water reactors (LWRs) include PWRs and BWRs that use ordinary water as coolant and, in many cases, as moderator. “Light” here distinguishes H2O from “heavy water,” which is D2O used in certain other reactor designs.

Given Data / Assumptions:

• Terminology: light water = H2O with protium, not deuterium.
• Coolant quality control (chemistry) is important but does not change the “light vs heavy” classification.

Concept / Approach:
Light water is ordinary water composed of hydrogen-1 (protium) and oxygen-16 predominately. While plant chemistry maintains conductivity and pH within tight windows to protect components, those adjustments do not redefine it as something other than “light water.”

Step-by-Step Solution:
Define light vs heavy: H2O (light) vs D2O (heavy).Identify coolant in LWRs: ordinary water flows through core.Eliminate pH-based distractors: acidity/alkalinity control is separate from isotopic composition.

Verification / Alternative check:
Plant specifications and textbooks universally use “light water” to mean H2O. Heavy-water reactors (HWRs) explicitly specify D2O for moderation/cooling, underlining the distinction.

Why Other Options Are Wrong:
pH 6 or pH 8 descriptors: chemistry control does not redefine isotopic identity.D2O or “demineralized heavy water”: those are heavy water, not light water.

Common Pitfalls:
Equating water-treatment details with isotope-based terminology.Assuming “light” means low mineral content rather than isotopic composition.

Final Answer:
Ordinary water (H2O)