QUENCH OF CYLINDRICAL TUBES DURING TRANSITION FROM FILM TO NUCLEATE BOILING HEAT TRANSFER IN CANDU REACTOR CORE

Abstract

Study of quench cooling is very important in nuclear reactor safety for limiting the extent of core damage during the early stages of severe accidents after Loss of Coolant Accidents (LOCA). Quench of a hot dry surface involves the rapid decrease in surface temperature resulting from bringing the hot surface into sudden contact with a coolant at a lower temperature. The quench temperature is the onset of the rapid decrease in the surface temperature and corresponds to the onset of destabilization of a vapor film that exists between the hot surface and the coolant. Re-wetting the surface is the establishment of direct contact between the surface and the liquid at the so-called re-wetting temperature. Re-wetting is characterized by the formation of a wet patch on the surface which then spreads to cover the entire surface. Situations involving quench and re-wetting heat transfer are encountered in a number of postulated accidents in Canada Deuterium Uranium (CANDU) reactors, such as re-wetting of a hot dry calandria tube in a critical break LOCA. This accident results in high heat transfer from the calandria tube to the surrounding moderator liquid which can cause the calandria tube surface to experience dryout and a subsequent escalation in the surface temperature. If the calandria tube temperature is not reduced by initiation of quench heat transfer, then this may lead to subsequent fuel channel failure. In literature very limited knowledge is available on quench and re-wetting of hot curved surfaces like the calandria tubes. In this study, a Water Quench Facility (WQF) has been constructed and a series of experiments were conducted to investigate the quench and re-wetting of hot horizontal tubes by a vertical rectangular water multi-jet system. The tubes were heated to a temperature between 380-800°C in a controlled temperature furnace then cooled to the jet temperature. The temperature variation with time in the circumferential and the axial directions of the tubes has been measured. The twophase flow behavior and the propagation of the re-wetting front around and along the tubes were simultaneously observed by using a high-speed camera. The effects of several parameters on the cooling process have been investigated. These parameters include: initial surface temperature, water subcooling (in the range 15- 800C), jet velocity (in the range 0.15-1.60 m/s), tube solid material (brass, steel and Alumina), surface curvature, tube wall thickness, jet orientation and number of jets. The variables studied include the re-wetting delay time (time to quench after initiating the cooling process), there-wetting front propagation velocity, the quench and re-wetting temperatures, the quench cooling rates and the boiling region size. The quench and the re-wetting temperatures as well as the re-wetting delay time were found to be a strong function of water subcooling. The quench and re-wetting temperatures increase with increasing water subcooling. The rewetting delay time decreases with increasing the water subcooling, decreasing initial surface temperature, increasing liquid velocity and decreasing the surface curvature. There-wetting front velocity is mainly dependent on the initial surface temperature and water subcooling. The re-wetting velocity increases by decreasing the initial surface temperature and by increasing the water subcooling. Decreasing the surface curvature was found to also increase the re-wetting front velocity. Correlations of the phenomena studied have been developed and provided good prediction of the experimental data collected in this study and data available from literature. The. results of this study provide novel knowledge and an experimental database for mechanistic modeling of quench heat transfer on calandria tube surfaces that experience dryout and film boiling.