Pool boiling heat transfer characteristics of zircaloy and SiC claddings in deionized water at low pressure

Abstract

Since the Fukushima accident in 2011, deployment of silicon carbide (SiC) materials has been investigated as a replacement for zirconium-based cladding in light water reactor claddings. SiC cladding exhibits many advantages including an exceptionally high melting temperature, considerably slow rate of hydrogen generation from reaction with water at high temperatures, and less corrosion compared to currently used zircaloy cladding. To verify the enhanced safety margin and potential features of accident tolerance of the SiC cladding, the pool boiling heat transfer characteristics were investigated in deionized water under atmospheric pressure and compared to zircaloy-4 cladding. The SiC monolith claddings were manufactured and donated by GAMMA CTP. The zircaloy-4 claddings were heated via Joule heating using a 12kW DC power supply while measuring the inner wall temperature. However, since the electrical resistance of SiC cladding is exceptionally high, indirect heating was applied by centering a heating element consisting of a stainless steel tube with an outer diameter of 6.35mm inside the SiC cladding. The measured critical heat flux (CHF) of the SiC monolith cladding was approximately 63% higher than that of the zircaloy-4 cladding. Furthermore, after occurrence of the CHF, in spite of stable film boiling formation on the SiC surface, no physical damage was observed with the SiC cladding whereas the zircaloy-4 cladding experienced rapid physical degradation at the CHF and finally fragmented. High-speed video recorded at a frame rate of 1500 fps provided observation of the distinctive features of the characteristic boiling phenomena of the zircaloy-4 and SiC claddings. The hydrodynamic instability near the CHF, the transition from the CHF to film boiling, and the stable film boiling regime were captured successfully. For the first time, this study demonstrates that SiC cladding has the sustainable structural integrity with visual observation after the CHF occurrence and can be advantageous in securing a high safety margin for nuclear power reactor applications.