Nuclear reactor coolants: which gas is normally avoided as a primary coolant because it has a relatively high neutron-capture cross-section (leading to parasitic absorption and activation)?

Introduction / Context:
Gas-cooled nuclear reactors select coolants that remove heat efficiently while minimizing parasitic neutron losses. A good coolant should have a small neutron-capture cross-section so that neutrons remain available to sustain the chain reaction and to achieve high neutron economy.

Given Data / Assumptions:

• Candidate gases include nitrogen, hydrogen, helium, carbon dioxide, and others.
• We focus on thermal-neutron absorption (capture) tendencies and operational practicality.
• The question asks which gas is normally not used as a reactor coolant because of high neutron capture.

Concept / Approach:
Helium and carbon dioxide are classic gaseous coolants. Helium is chemically inert, single-phase, and has a very low thermal-neutron absorption cross-section, making it excellent for high-temperature gas-cooled reactors. Carbon dioxide has been used historically (e.g., AGR/Magnox) due to reasonable neutronic and thermal properties. Nitrogen, by contrast, has a significantly higher neutron-capture cross-section and also undergoes activation (e.g., formation of C-14 via N-14(n,p)C-14 in certain conditions), which is undesirable for both neutron economy and radiological considerations. Hydrogen gas itself is rarely used as a standalone reactor coolant despite hydrogen in light water being the primary moderator/coolant in LWRs; the molecular gas introduces other design challenges and safety concerns, but the canonical 'high capture' reason points to nitrogen.

Step-by-Step Solution:

Verification / Alternative check:
Standard reactor physics texts list helium among the best gas coolants for neutron economy; nitrogen is not standard due to capture/activation and inferior neutronic characteristics compared with helium and even CO2.

Why Other Options Are Wrong:

• Hydrogen: not used as a gaseous coolant, but the reason traditionally cited for 'not used' in this context is not mainly high capture relative to nitrogen.
• Helium: extremely low capture, ideal coolant.
• Carbon dioxide: historically used successfully.
• Neon: not typical industrial choice, but the question targets the high-capture culprit, i.e., nitrogen.

Common Pitfalls:
Conflating hydrogen in water (effective moderator/coolant as light water) with using H2 gas directly; overlooking activation issues.

Final Answer:
Nitrogen