What four critical safety functions protect the RCS pressure boundary?

• A. Sub-criticality, Core Cooling, Heat Sink, and RCS Inventory
• B. Sub-criticality, Core Cooling, Heat Sink, and Containment
• C. Sub-criticality, Heat Sink, RCS Integrity, and Containment Inventory
• D. Sub-criticality, Heat Sink, RCS Integrity, and RCS Inventory

Answer: (D)

Protecting the RCS pressure boundary comes from four interrelated safeguards that keep the reactor safe by controlling the reaction, removing heat, preserving coolant, and keeping the boundary intact. First, sub-criticality ensures the reactor cannot reach a state where excessive fission occurs, preventing power and heat from driving boundary stress. Second, a reliable heat sink (coolant removal) is essential so the core can shed decay heat and any generated heat without causing pressure rise that could damage the boundary. Third, maintaining RCS inventory guarantees there is enough coolant in the system to absorb heat and maintain proper pressure and flow paths; without sufficient coolant, the boundary can overheat or dry out. Fourth, ensuring RCS integrity means the pressure boundary itself remains intact, preventing leaks that would bypass the protective barrier.

This combination—sub-criticality, heat removal, sufficient coolant inventory, and intact boundary—best matches the four protections needed for the RCS pressure boundary. Containment-related terms address environmental release, not the direct safeguarding of the RCS boundary itself.