What are the four critical safety functions that protect the fuel pellet and cladding?

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

Answer: (A)

Protecting the fuel pellet and cladding comes down to four essential protections that cover reactivity control, cooling, heat removal, and coolant availability. Sub-criticality prevents the reactor from maintaining or increasing fission, so heat generation stays within design limits and the fuel isn’t pushed toward unsafe temperatures. Core cooling ensures heat is actively removed from the fuel during normal operation and transients, keeping the cladding from overheating and potentially failing. A heat sink provides a path to carry away decay and residual heat to a safe ultimate sink when immediate cooling paths are challenged, preventing heat buildup that could stress the fuel and cladding. Maintaining adequate RCS inventory guarantees there is enough coolant in the reactor coolant system to sustain cooling, even during disturbances, so the core remains covered and heat can continue to be removed.

These four together directly address protecting the fuel and cladding by controlling reactivity, removing heat, providing a reliable heat transfer path, and ensuring sufficient coolant is available. Containment-related aspects like pressure or integrity are crucial for preventing releases, but the four listed focus on the direct protections needed to keep the fuel and cladding safe.