Reactor cooling comparison — Relative to water cooling, which statement about gas cooling of nuclear reactors is correct?

Introduction:
Nuclear reactors can be cooled by water, heavy water, liquid metals, or gases (e.g., CO2, He). Understanding the thermodynamic and heat-transfer implications of each coolant avoids misconceptions about attainable temperatures and efficiency.

Given Data / Assumptions:

• Gas-cooled reactors include He- and CO2-cooled designs.
• Water-cooled reactors are the most common globally.
• Statements compare temperature capability and efficiency.

Concept / Approach:
Gas coolants allow high outlet temperatures at low pressure (especially helium) because gases have low vapor pressures; this actually aids high-temperature operation. Specific heat per unit mass or volume of gases is not generally higher than water; water’s volumetric heat capacity is far superior. Steam generation mode (saturated or superheated) depends on the secondary cycle design, not strictly on coolant type.

Step-by-Step Solution:
Assess (a): False. Gas coolants often enable higher temperatures (e.g., HTGRs) than water under similar pressure constraints.Assess (b): False. Water typically has a much higher volumetric heat capacity, giving strong heat-removal capability; gas cooling is not “more efficient” on that basis.Assess (c): False. Plant design can deliver superheated steam or couple to Brayton cycles; it is not limited to saturated steam.Thus, the correct choice is 'none of these'.

Verification / Alternative check:
HTGR concepts target outlet temperatures well above typical PWR/PHWR secondary temperatures, supporting advanced power cycles.

Why Other Options Are Wrong:

• (a) Inverts the real advantage of gas cooling.
• (b) Misstates comparative heat capacities.
• (c) Restricts steam conditions without technical basis.

Common Pitfalls:
Equating coolant 'efficiency' with specific heat alone, and ignoring system design.

Final Answer:
none of these.