Pressurised Water Reactor (PWR): which statement correctly describes the role of pressure in the primary coolant loop?

Introduction / Context:
Pressurised Water Reactors (PWRs) are a dominant light-water reactor type worldwide. Understanding their fundamental thermal-hydraulic principle clarifies how heat is transferred safely from fuel to secondary systems while avoiding boiling in the core region.

Given Data / Assumptions:

• We are comparing descriptions of fuel, moderator, and coolant behavior in a PWR.
• PWRs use enriched uranium fuel, light water as both moderator and coolant, and a separate steam generator.
• The primary loop is maintained at high pressure.

Concept / Approach:
In a PWR, the primary coolant is light water kept at very high pressure so that it remains subcooled (no bulk boiling) while flowing through the core. Heat is carried to steam generators where a separate secondary loop boils to produce steam for turbines. This design contrasts with BWRs, where boiling occurs in the core, and with heavy-water reactors (e.g., PHWRs) that can use natural uranium and heavy water. Therefore, the defining correct statement is that the primary coolant in a PWR is pressurised to prevent bulk boiling in the core.

Step-by-Step Solution:
Identify PWR characteristics: light water coolant/moderator, high pressure, separate steam generator.Evaluate each option against known PWR features.Select the statement about pressurisation preventing bulk boiling.

Verification / Alternative check:
Standard nuclear engineering references describe PWR operating pressures on the order of tens of bar, ensuring subcooled operation in the core and phase change only in the secondary side.

Why Other Options Are Wrong:
Natural uranium with heavy water: describes PHWR, not PWR.Coolant boils in the core: describes BWR behavior, not PWR.No moderator required: incorrect; PWR uses light water as moderator.Thorium with CO2: not a PWR configuration.

Common Pitfalls:
Confusing PWR with BWR or PHWR; always check whether boiling occurs in-core and what moderator is used.

Final Answer:
Coolant water is highly pressurised to prevent bulk boiling in the core.