Calder Hall–type gas-cooled reactor (Magnox): the circulating carbon-dioxide coolant typically operates at approximately what pressure (expressed in kgf/cm^2) under standard reference descriptions?

Introduction / Context:
Early British gas-cooled reactors (Magnox type, exemplified by Calder Hall) used carbon dioxide as the primary coolant and graphite as moderator. Recognizing typical operating pressure ranges is valuable for quick design-type identification and safety or materials questions in general knowledge exams.

Given Data / Assumptions:

• Coolant: CO2 gas in a graphite-moderated, gas-cooled reactor.
• Question requests a representative pressure in kgf/cm^2 for exam purposes.
• We select the commonly cited textbook value used in legacy MCQs.

Concept / Approach:
Magnox reactors operated CO2 at pressures substantially above atmospheric but far below the very high system pressures of pressurised water reactors. Typical GK sources quote a single-digit to low-tens value in kgf/cm^2 for such early designs. Among the choices, 7 kgf/cm^2 aligns with widely taught figures in classic exam prep material, indicating moderately pressurised gas flow adequate for heat removal without the high structural demands of water at dozens of bar.

Step-by-Step Solution:
Identify reactor type: gas-cooled, graphite moderated (Magnox/Calder Hall lineage).Recall representative CO2 system pressure from standard GK references.Map to options: 7 kgf/cm^2 is the standard single-digit pressure often quoted.Select “7.”

Verification / Alternative check:
Comparative reasoning: water-cooled reactors operate at many tens to over a hundred bar; early CO2-cooled reactors used markedly lower pressures. Among the MCQ options, 7 is the consistent teaching value.

Why Other Options Are Wrong:
0.2 / 0.02: Near-atmospheric; inadequate for efficient CO2 heat transport.35 / 50: Much higher than typical GK values taught for early Magnox systems.

Common Pitfalls:
Assuming gas-cooled implies extremely low pressure like air-cooled systems; in practice, CO2 pressure is elevated to improve density and heat capacity while maintaining manageable mechanical loads.

Final Answer:
7