Pellet-clad interaction (PCI) occurs under what condition?

• A. As power increases, pellet temperature rises; if the gap is too small, the pellet contacts the cladding causing breaching stress
• B. Cladding expands faster than the pellet without contact
• C. Coolant removes all heat preventing PCI
• D. PCI only occurs at low power levels with cold pellets

Answer: (A)

Pellet-clad interaction happens when the fuel pellet swells as power rises and begins to press against the surrounding cladding. The fuel pellets are designed with a small initial gap to accommodate thermal expansion and fission-gas swelling. As power increases, the pellet temperature climbs, causing its dimensions to grow. If that gap is too small, the expanding pellet makes contact with the cladding, and the resulting mechanical interaction produces high stresses at the interface. If power continues to rise or the contact persists, those stresses can contribute to cladding breach. The essence is that PCI requires both rising pellet temperature (expansion) and a sufficiently small gap to force contact. The other scenarios don’t align with PCI because they either imply no contact, rely on cooling to prevent contact, or describe conditions (low power, cold pellets) that don’t produce the necessary pellet-cladding interaction.