An assessment of silicon carbide as a cladding material for light water reactors

Abstract

An investigation into the properties and performance of a novel silicon carbide-based fuel rod cladding under PWR conditions was conducted. The novel design is a triplex, with the inner and outermost layers consisting of monolithic SiC, while the middle layer consists of a SiC fiberwound composite. The goal of this work was evaluation of the suitability of this design for use as a fuel rod cladding material in PWRs and the identification of the effects of design alternatives on the cladding performance. An in-core loop at the MITR-II was used to irradiate prototype triplex SiC cladding specimens under typical PWR temperature, pressure, and neutron flux conditions. The irradiation involved about 70 specimens, of monolithic as well as of triplex constitution, manufactured using several different processes to form the monolith, composite, and coating layers. Post-irradiation examination found some SiC specimens had acceptably low irradiation-enhanced corrosion rates and predictable swelling behavior. However, other specimens did not fare as well and showed excessive corrosion and cracking. Therefore, the performance of the SiC cladding will depend on appropriate selection of manufacturing techniques. Hoop strength testing found wide variations in tensile strength, but patterns or performance similar to the corrosion tests. The computer code FRAPCON, which is widely used for today's fuel assessment, modified properly to account for SiC properties, was applied to simulate effects of steady-state irradiation in an LWR core. The results demonstrated that utilizing SiC cladding in a 17x17 fuel assembly for existing PWRs may allow fuel to be run to somewhat higher burnup. However, due to lack of early gap closure by creep as well as the lower conductivity of the cladding, the fuel will experience higher temperatures than with zircaloy cladding. Several options were explored to reduce the fuel temperature, and it was concluded that annular fuel pellets were a solution with industrial experience that could improve the performance sufficiently to allow reaching 40% higher burnup. Management of the fuel-cladding gap was identified as essential for control of fuel temperature and PCMI. SiC cladding performance may be limited unless cladding/fuel conductivity or gap conductance is improved.