In nuclear power systems, why is liquid water generally a more effective reactor coolant than gases such as CO2, He, or N2? Consider heat-transfer performance, equipment sizing, operating pressure, and its concurrent role in common reactor designs.

Introduction / Context:
Choosing a reactor coolant is a multidisciplinary engineering decision involving thermohydraulics, materials, neutronics, and plant economics. This question compares liquid water with common gases (CO2, He, N2) from the standpoint of heat removal and practical plant design, as encountered in light-water reactors and other thermal systems.

Given Data / Assumptions:

• Comparable core thermal power must be removed in steady state.
• Coolant properties are evaluated at typical reactor conditions.
• Focus is on generic traits: density, heat capacity, thermal conductivity, and pressurisation.

Concept / Approach:
Water’s very high density and specific heat capacity mean each unit volume carries far more energy than a gas. Its thermal conductivity is higher than most gases at comparable temperatures, giving larger heat-transfer coefficients for similar flow geometries and velocities. Because water can be readily pressurised, its saturation temperature can be raised substantially, enabling high core outlet temperatures without phase change (PWR) or controlled boiling (BWR). These traits cascade into smaller pumps and heat-exchange surfaces for a given duty. Additionally, in thermal reactors, ordinary or heavy water also moderates neutrons, providing a dual role that simplifies the overall plant layout.

Step-by-Step Solution:
1) Compare bulk properties: water has higher rho and Cp, so m_dot * Cp * ΔT per channel is large.2) Higher thermal conductivity and favorable Prandtl numbers increase h (convective coefficient) at similar Reynolds numbers.3) With higher heat capacity per volume, required volumetric flow and surface area drop → smaller pumps and exchangers.4) Pressurisation raises boiling point (PWR) or enables controlled boiling (BWR), keeping temperature margins manageable.5) In LWRs, water also moderates, reducing separate moderator needs.

Verification / Alternative check:
Helium has excellent thermal conductivity among gases, but its very low density demands high volumetric flow and large compressors. CO2 avoids activation issues and was used historically, yet its lower heat capacity per volume increases equipment size. Benchmarked plant designs consistently show more compact primary circuits with water for the same thermal rating.

Why Other Options Are Wrong:
(a) Correct: water is a strong moderator in thermal reactors.(b) Correct: higher volumetric heat capacity reduces pump and exchanger size.(c) Correct: high h and ease of pressurisation support high-temperature operation.

Common Pitfalls:
Ignoring corrosion/radiolysis control; assuming moderation is always desirable (it is not for fast reactors); overlooking activation/chemistry differences between coolants.

Final Answer:
All of the above.