Thermal conductivity of SiC composite cladding significantly decreases after irradiation, leading to cladding failure due to high tensile stress under pellet-cladding mechanical interaction (PCMI).

This study aims to address the challenges of high fuel temperature resulting from low thermal conductivity and cladding failure under PCMI in SiC cladding fuel rods.

Firstly, a design of pellet-cladding gap filled with liquid LBE (Lead-Bismuth Eutectic) duplex SiC cladding fuel rod was proposed, and the gap filling material model was developed based on FRAPCON code. Then, the model was applied to incorporating the influence of LBE filling on gap heat transfer, accounting for changes in immersion height due to variations in gap and LBE volume during operation, and evaluating the impact of LBE volume on gas space and internal pressure within the fuel rod. Subsequently, the performance of this UO₂-SiC cladding fuel rod with LBE filled gap preliminarily analyzed under normal operating conditions with different initial LBE filling heights, using a typical pressurized water reactor fuel rod power history. The effects of different initial filling heights on reducing fuel temperature during operation and their impact on internal pressure were investigated. For high burnup fuel rods, further optimization of parameters including initial internal pressure, plenum length, and gap size was carried out based on the characteristics of the LBE gap and SiC cladding, resulting in an enhanced performance of UO₂-LBE-SiC fuel rod. Finally, fuel performance of three designs of fuel rods (UO₂-SiC, UO₂-SiC with central void, UO₂-LBE-SiC) were compared.

Under the condition of high power and high burnup, the decrease of the gap size and the void volume results in a significant increase in internal pressure. Increasing plenum length can compensate for the gas volume occupied by LBE and maintain the fuel rod internal pressure lower than the original He gap. SiC cladding can withstand large compressive stress and gap heat transfer no longer depends on He, hence the initial internal pressure of fuel rods is optimized to reduce internal pressure during operation. The final optimized design parameters for UO₂-LBE-SiC include a 70% fuel stack height as the initial LBE filling height, atmospheric pressure for the initial gas, a 50% increase in plenum length, and an initial gap size raised to 99 μm. The peak fuel temperature is 1 972 K, with a peak fission gas release of approximately 20% and a peak internal pressure of about 25 MPa. The hoop stress in the Ceramic Matrix Composite (CMC) layer consistently remains below its ultimate tensile strength, and the chemical vapor deposition (CVD) layer is predominantly under compression, with a maximum tensile stress of 5 MPa. The failure probability is less than 10^-6, meeting safety criteria.

The results of this study show that the design of UO₂-SiC with pellet central void cannot avoid cladding failure. Due to the excellent thermal conductivity of the LBE gap, the pellet-cladding temperature difference is minimal, increasing the gap size can weaken PCMI and reduce the probability of fuel failure without affecting the temperature field distribution.