Silicon carbide performance as cladding for advanced uranium and thorium fuels for light water reactors

Abstract

There has been an ongoing interest in replacing the fuel cladding zirconium-based alloys by other materials to reduce if not eliminate the autocatalytic and exothermic chemical reaction with water and steam at above 1,200 °C. The search for an accident tolerant cladding intensified after the Fukushima events of 2011. Silicon carbide (SiC) possesses several desirable characteristics as fuel cladding in light water reactors (LWRs). Compared to zirconium, SiC has higher melting point, higher strength at elevated temperature, and better dimensional stability when exposed to radiation, as well as lower thermal expansion, creep rate, and neutron absorption cross-section. However, under irradiation, the thermal conductivity of SiC is degraded considerably. Furthermore, lack of creep down towards the fuel causes the fuel-cladding gap and gap thermal resistance to stay relatively large during in-core service. This leads to higher fuel temperature during irradiation. In order to reduce the high fuel temperature during operation, the following fuel design options were investigated in this study: using beryllium oxide (BeO) additive to enhance fuel thermal conductivity, changing the gap bond material from helium to lead-bismuth eutectic (LBE) and adding a central void in the fuel pellet. In addition, the consequences of using thorium oxide (ThO₂) as host matrix for plutonium oxide (PuO₂) were covered. The effects of cladding thickness on fuel performance were also analyzed. A steady-state fuel performance modeling code, FRAPCON 3.4, was used as a primary tool in this study. Since the official version of the code does not include the options mentioned above, modifications of the source code were necessary. All of these options have been modeled and integrated into a single version of the code called FRAPCON 3.4-MIT. Moreover, material properties including thermal conductivity, swelling rate, and helium production/release rate of BeO have been updated. Material properties of ThO₂ have been added to study performance of ThO₂-PuO₂ . This modified code was used to study the thermo-mechanical behavior of the most limiting fuel rod with SiC cladding, and explore the possibility to improve the fuel performance with various design options. The fuel rod designs and operating conditions of a 4-loop Westinghouse pressurized water reactors (PWR) and Babcock and Wilcox (B&W) mPower small modular reactors (SMR) were reactors (PWR) and Babcock and Wilcox (B&W) mPower small modular reactors (SMR) were chosen as representatives of conventional PWRs and upcoming SMRs, respectively. Sensitivity analyses on initial helium gap pressure, linear heat generation rate (LHGR) history, and peak rod assumptions have been performed. The results suggest that, because of its lower thermal conductivity, SiC is more sensitive to changes in these parameters than zirconium alloys. For a low-conducting material like SiC, an increase in cladding thickness plays a significant role in fuel performance. With a thicker cladding (from 0.57 to 0.89 mm), the temperature drop across the cladding increases, which makes the fuel temperature higher than that with the thin cladding. Reduction of fuel volume to accommodate the thicker cladding also causes negative impact on fuel performance. However, if the extra volume of the cladding replaces some coolant, the reduced coolant fraction design (RCF) has superior performance to the decreased fuel volume fraction design. In general, the most effective fuel temperature improvement option appears to be the option of mixing beryllium oxide into the fuel. This method outperforms others because it improves the overall thermal conductivity and reduces the overall temperature of the fuel. With lower fuel temperature, fission gas release and eventually plenum pressure -- one of the most life-limiting factor for SiC -- can be lowered.