Effect of gap conductance on high thermal-conductivity fuels in supercritical water-cooled reactors (SCWRS)

Chosen as one of six Generation-IV nuclear-reactor concepts, SuperCritical Water-cooled Reactors (SCWRs) will have high thermal efficiencies within the range of 45-50% owing to high reactor-outlet temperatures. A generic SCWR operates at a pressure of 25 MPa with inlet-and outlet-coolant temperatures of 350°C and 625°C. The high outlet temperature and pressure make it possible to use supercritical "steam" turbines, which have led to high thermal efficiencies at coal-fired power plants. Additionally, there is a possibility for co-generation of hydrogen using high-temperature heat from an SCWR during off-peak hours. The high operating temperatures of SCWRs lead to high fuel centerline temperatures. Previous studies have shown that the fuel centerline temperatures could exceed the industry accepted limit of 1850°C when UO2 is used at SCWR conditions. Therefore, there is a need for alternative fuels for future use in SCWRs. The objective of this paper is to investigate the possibility of using high thermal-conductivity fuels such as Uranium Nitride (UN) and Uranium Carbide (UC) in SCWRs. Previous studies did not take into account the effects of a gap, between the outer surface of the fuel and the sheath, on the fuel centerline temperature. The present study focuses on investigating the fuel-sheath gap conductance effects on the fuel centerline temperature of a generic 1200-MWel SCWR. The fuel centerline temperature was calculated for fuel channels with the maximum thermal power, i.e., +15% above average channel power. Results of this analysis showed that the fuel centerline temperature is well below the industry limit and melting points of the UC and UN fuels when it was assumed that there was a 20-36 μm gap between the fuel and the sheath. Since the fuel centerline temperatures of both UC and UN were below their associated temperature limits, other factors such as volumetric swelling, chemical stability, thermal conductivity, and melting point of these fuels were considered in order to determine the best fuel options for SCWRs. The results showed that UC is a promising fuel option for future use in SCWRs. However, its chemical compatibility with water remains ambiguous. Therefore, a study in regards to the chemical compatibility of UC with water at high temperatures should be conducted before selecting UC. (author)