Coolant selection: which gas has a relatively high neutron capture cross-section and is therefore unsuitable as a reactor coolant?

Introduction / Context:
Good reactor coolants should not unduly absorb neutrons, especially in thermal-spectrum systems where maintaining neutron economy is critical. Among candidate gases, differences in thermal neutron capture cross-sections inform suitability for use as coolants or cover gases.

Given Data / Assumptions:

• Comparing common gases: helium, nitrogen, carbon dioxide, hydrogen, argon.
• Qualitative comparison suffices for MCQ selection.

Concept / Approach:
Helium has an extremely small capture cross-section and is chemically inert—excellent as a coolant or cover gas. CO2 and Ar also have relatively low captures compared with nitrogen. Nitrogen-14 has a significantly higher thermal capture (~order ~1–2 barns), and captures can produce C-14 via (n,p) or (n,γ)/(p) pathways, undesirable in reactor cores. Hydrogen’s capture is moderate (~0.33 b) but it is mainly used as a moderator in water rather than as a standalone gas coolant; flammability is a separate concern, not the largest capture among the options.

Step-by-Step Solution:
Rank gases by capture qualitatively: He lowest; CO2 and Ar low; H2 moderate; N2 comparatively high.Select nitrogen (N2) as the gas with comparatively high capture making it unsuitable as a reactor gas coolant.

Verification / Alternative check:
Gas-cooled reactor designs generally choose helium (or CO2 historically) over nitrogen due to neutron economy considerations and activation products concerns.

Why Other Options Are Wrong:

• He: exceptionally low capture and inert—preferred.
• CO2: historically used in gas-cooled reactors.
• H2: capture lower than N2; safety issues aside, it is not the highest absorber here.
• Ar: relatively low absorption compared with nitrogen; can be used as cover gas.

Common Pitfalls:
Assuming hydrogen, because it moderates strongly, must also have the highest capture; in fact, nitrogen’s capture and activation issues are more problematic for gas coolant roles.

Final Answer:
N2