Hydrogen (H2) versus carbon dioxide (CO2) as reactor coolants: which property primarily makes H2 a better heat-removal medium under similar conditions?

Introduction / Context:
Gas-cooled systems value coolants that can carry away large amounts of heat with minimal pumping power. Hydrogen has been studied and used in specialized high-temperature applications because of its superior heat-transfer characteristics compared with heavier gases like CO2. This question distinguishes the dominant property that makes H2 an efficient coolant.

Given Data / Assumptions:

• Comparable temperature and pressure ranges.
• Focus on heat removal capability at similar flow conditions.
• Materials compatibility and safety engineered via standard practices.

Concept / Approach:
Hydrogen possesses a very high specific heat and exceptional thermal conductivity for a gas, which together enable better convective heat transfer at a given mass flow. Although density influences pumping and Reynolds number, it is the strong thermophysical properties—chiefly specific heat and conductivity—that drive superior heat removal. Reactivity with structural or fuel materials and neutron absorption are separate, design-managed concerns and are not the primary reason for H2’s cooling advantage over CO2.

Step-by-Step Solution:
Compare cp: cp,H2 > cp,CO2 → more energy carried per unit mass per degree.Compare k (thermal conductivity): k,H2 >> k,CO2 → higher heat-transfer coefficients.Result: for similar flow, H2 removes heat more effectively.Therefore, the key property is higher specific heat (with high conductivity).

Verification / Alternative check:
Heat-transfer correlations (e.g., Nu–Re–Pr) highlight the influence of cp and k on convective performance; hydrogen’s properties yield larger Nu and heat flux at comparable conditions than CO2.

Why Other Options Are Wrong:
Lower density: affects pumping but does not by itself ensure better heat removal.Non-reactivity with uranium: not generally the decisive factor here.Lower neutron capture: hydrogen actually has a higher thermal absorption than C or O.Boiling point: irrelevant for gas-phase coolants at operating conditions.

Common Pitfalls:
Confusing density effects with core heat-transfer capability.Assuming neutronics dictates coolant choice in all designs; here, thermophysical properties are central.

Final Answer:
Higher specific heat (and excellent thermal conductivity)