Experimental thermal hydraulics study of the blockage ratio effect during the cooling of a vertical tube with an internal steam-droplets flow

Abstract

During a Loss of Coolant Accident (LOCA) in a Pressurized Water Reactor, partial or complete drying of fuel assemblies may take place. In these conditions, the fuel temperature increases and may lead to substantial deformation of the fuel rod cladding and a partial blockage of the fluid sub-channels. These regions could significantly affect the cooling capacity of the nuclear core during the reflooding phase by the emergency core cooling systems. Understanding the cooling process and the thermal-hydraulic characteristics of the flow in these deformed regions is decisive to guarantee nuclear safety. Looking to provide valuable experimental data to validate existing and new models, this work is an experimental study on the thermal hydraulics during the cooling of a vertical tube with an internal steam-droplets flow, representing the LOCA conditions at sub-channel scale. The effect of the blockage ratio on the tube temperature, heat dissipation, wall rewetting, and droplets characteristics is evaluated by testing three configurations (0%, 61% and 90%). Optical techniques were used for a comprehensive characterization of the process, being them infrared thermography, phase-Doppler analyzer and three-color laser induced fluorescence thermometry. In general, wall rewetting in the test section occurs from bottom to top, although there is a discontinuity in the rewetting front with the 90% blockage ratio configuration. The droplets diameter reduces downstream of the test section because of evaporation. Droplets breakup was specifically observed with 90% blockage ratio. In all the cases, the droplets temperature was approximately the same up-and downstream of the test section, which indicates they are in nearly thermal equilibrium state and, therefore, representative of a LOCA situation.

Velocity field and flow redistribution in a ballooned 7×7 fuel bundle measured by magnetic resonance velocimetry

“…These tasks are truly difficult in LOCA conditions, where we have two-phase flow and a variety of heat and mass transfer phenomena [1]. Many studies have been carried out to improve the understanding of the nuclear reactor's core thermal-hydraulics in LOCA conditions, either with a regular fuel bundle [2,3], with blockages due to clad ballooning [4,5] and at sub-channel scale [6][7][8].…”

Experimental study of dispersed flow film boiling at sub-channel scale in LOCA conditions: Influence of the steam flow rate and residual power

“…85 Figure 3 presents the experimental set-up, which consists of a test section, a droplet supply system and a superheated steam supply system. This apparatus is the same used in previous works [1,19].…”

“…As the liquid injection continues, the water level rises and immerses the fuel assemblies, where we identify three zones with different flow conditions (Fig. 1): (i) the quenching front, corresponding predominantly to the nearly-saturated water below the water level and where the first boiling mechanisms are observed; (ii) a two-phase post-dryout zone composed of superheated steam and dispersed droplets; and, finally, (iii) a monophasic post-dryout zone of superheated steam when all the droplets disappear because of evaporation [1].…”

“…If we compare nuclear and metallurgy applications of this phenomenon, the flow characteristics and involved geometries are completely different. For this reason, there is a dense literature of disperse flow film boiling specific for nuclear engineering applications [1,5,[14][15][16], especially because this thermal-hydraulic process is very important to ensure the cooling of the fuel rods above the quenching front and, therefore, guarantee nuclear safety.…”

“…Moreover, the obtained data are valuable for the validation of simulation tools, like IRSN's code called DRACCAR [17,18], and understand with parameters play important roles on the thermalhydraulics of dispersed flow film boiling. In a previous study, we correlated the flow characteristics with the wall temperature during the cooling phase and analyzed the blockage ratio effect on the heat dissipation and the droplets characteristics after testing tubes with different geometries [1].…”