Coolants in nuclear reactors — Which gas is commonly used as a reactor coolant specifically because of its very low neutron-capture cross-section?

Introduction:
Reactor coolants must remove heat efficiently while minimally perturbing the neutron economy. A key property is a low neutron-capture cross-section so that the coolant does not parasitically absorb neutrons needed for sustaining fission.

Given Data / Assumptions:

• We compare common gases: H2, N2, He, CO2.
• Criterion: very low neutron-capture cross-section.
• Secondary considerations (thermal capacity, chemistry) do not override the stated criterion.

Concept / Approach:
Helium is a noble gas with exceptionally low neutron absorption and chemical inertness. It does not become radioactive easily and remains single-phase over wide ranges, making it attractive in high-temperature gas-cooled reactor concepts.

Step-by-Step Solution:
Evaluate H2: Although light and a good moderator, protium absorbs neutrons more than He, and molecular hydrogen poses flammability concerns.Evaluate N2: Nitrogen has higher absorption than He and can form C-14 via activation; thus it is less desirable for neutron economy.Evaluate CO2: Widely used historically (e.g., AGR, Magnox) due to acceptable properties; however, its neutron absorption is higher than He and it can react at high temperatures with graphite.Evaluate He: Extremely low neutron-capture cross-section and chemically inert. Meets the stated criterion best.

Verification / Alternative check:
High-temperature gas-cooled reactors (HTGRs) adopt helium primarily for its neutronic and chemical advantages. Cross-section compilations consistently place He among the lowest absorbers.

Why Other Options Are Wrong:

• H2: Higher absorption than He and safety concerns.
• N2: Larger absorption and activation issues (C-14 formation).
• CO2: Acceptable coolant historically, but not the lowest absorber among the choices.

Common Pitfalls:
Confusing 'good moderator' with 'low absorber'; moderation effectiveness does not mean minimal capture losses.

Final Answer:
He