Heat transfer origin of thermal shock fracture and its application for LWR fuel during reflood

Abstract

In water quenching, a solid experiences dynamic heat transfer rate evolutions with phase changes of the fluid over a short quenching period. Yet, such a dynamic change of heat transfer rates has been overlooked in the analysis of thermal shock fracture. In this study, we are presenting quantitative evidence against the prevailing use of a constant heat transfer coefficient for thermal shock fracture analysis in water. We conclude that no single constant heat transfer could suffice to depict the actual stress evolution subject to dynamic heat transfer coefficient changes with fluid phase changes. Use of the surface temperature dependent heat transfer coefficient will remarkably increase predictability of thermal shock fracture of brittle materials and complete the picture of stress evolution in a quenched solid. The presented results show stress prediction around -90% of the actual fracture stress with the use of the surface temperature dependent heat transfer coefficient, implying that fracture uncertainties predominantly remain in the statistical nature of brittle fracture. For thermal shock fracture analysis of fuel rods during LWR reflood, transient sub channel heat transfer coefficients obtained from a thermal-hydraulics code should be used as input to stress analysis. Such efforts will advance of thermal shock modeling, which will introduce a fundamental improvement over the currently practiced experimental empiricism for cladding fracture analysis during reflood. This study advocates that advanced boiling study on fuel rod surface should consider effects of surface oxide scale, crud, and surface pore structures which naturally arise in the course of the reactor operation.