Reflood Heat Transfer of Accident Tolerant Fuel Cladding Surfaces and Its Effects on Safety of Pressurized Water Reactors

Abstract

For the short-term deployment of the Accident Tolerant Fuel (ATF) cladding, the coating of materials having high oxidation resistance on the commercial Zr-alloy based cladding have been proposed. The ATF-coating can achieve enhanced thermal safety by controlling the morphological changes in micro/nano scale surface structures. However, the technical requirements of ATF-cladding (i.e., Ra less than 0.3 μm and having no porous medium) and the existence of continuous flow and heat generation during reflooding phase in PWR have not been considered in sufficient detail in the previous relevant studies. Thus, this study aims to investigate the individual effects of flow conditions and surface characteristics on quench performance and to identify their impacts on safety of nuclear systems. Cr considered as the most promising ATF-coating material was deposited onto the test specimen by using DC magnetron sputtering technique. The Cr-coated cladding surface showed enhanced potential of capillary wicking associated with superhydrophilic characteristics. The MELCOR, a severe accident analysis code, was utilized to evaluate flow conditions inside the core during reflooding phase depending on the pipe rupture size of postulated loss-of-coolant accident (LOCA). Reflooding heat transfer experiments with simulated decay heat were carried out. Through temperature measurement, inverse heat conduction analysis, and high-speed visualization, the quench performance including quench temperature, film boiling heat transfer coefficient, critical heat flux (CHF), and quench front velocity were quantified and compared between the bare (No nanoscale feature), fresh Cr-coated (Increased nanoscale feature), and oxidized Cr-coated (Increased micro/nanoscale feature) surfaces. First, the flow conditions were diversified to investigate the effects of system parameters. Negligible effects of reflooding velocity on the quench performance were observed in the range of postulated Large Break LOCA from 25 to 45 mm/s. The increase in coolant subcooling, however, showed remarkable enhancement in overall quench performance and it was elucidated by the reduced vapor film thickness during film boiling and instant bubble condensation at the quench front. For the Cr-coated surfaces, the surface structure only showed effective contacts when the vapor film became sufficiently thin comparable to the surface roughness scale during film boiling. In addition, due to high vapor pressure inside the film at the downstream of the quench front, the capillary wicking by the surface structure was hindered, which caused deterioration and little enhancement in the CHF for the fresh and oxidized Cr-coated surfaces, respectively. Based on the current experimental data and extensive literature reviews, a modified model for the quench temperature applicable for bottom reflooding was developed. The key idea was to evaluate the appropriate isothermal quench temperature for given quench condition. The present model showed excellent predictions for quench temperature under both flow and nonflow conditions. The modified model was incorporated into the development of multi-region quench front analysis model. The parametric investigations of initial temperature distribution, decay heat level, boiling heat transfer efficiency, and ATF-coating thickness on quench front velocity and quenching time were carried out. It was found that quench front velocity was affected substantially by the nominal parameters which do not affect the other quench performance during bottom flooding. Lastly, by implementing the modified quench temperature model into the SPACE code, a thermal-hydraulic system analysis code, the impacts of change in quench performance on safety of nuclear systems were identified. By benchmarking the FLECHT-SEASET facility, the effects of the flow conditions and surface characteristics on the peak cladding temperature and quenching time were investigated. The results suggest that higher reflooding velocity is always beneficial and coolant subcooling should be optimized when reflooding velocity is expected to be low. The changes in boiling heat transfer efficiency due to surface modification can reduce or delay the quenching time without affecting the PCT noticeably.