Experimental investigation of ex-vessel molten core cooling based on two-phase natural circulation

Abstract

Passive cooling based on natural circulation is utilized in core catcher system of advanced reactors to handle severe accident scenarios. Core catcher coolant channel has a unique geometry with a large scale channel including downward facing to vertical heating wall. Full-scale experimental facilities were designed via scaling analysis to assess the cooling capability of the ex-vessel core catcher system. Comprehensive thermal hydraulic database on natural circulation flow rate, flow regime, void fraction, heat transfer rate and critical heat flux (CHF) were obtained in the modeled facilities. Two sets of experimental facilities were designed, one adiabatic (air-water) and another heating (steam-water) system. The first air-water experiment was designed to understand two-phase flow behavior through flow visualization and local void fraction measurement. The second steam-water experiment applied heat to the channel wall to simulate decay heat of the corium. The removal of thermal load and two-phase flow characteristics were studied. Air-water experiment was conducted mainly to observe two-phase flow phenomena inside of the core catcher coolant channel. Natural circulation flow rate was obtained in various air injection rates simulating steam generated by decay heat. Two-phase flow regimes and the flow behavior were visualized through the transparent channel. Local two-phase flow measurement was carried out by conductivity probes. Air-water experiment provides a knowledge of two-phase flow phenomena and flow development inside of the core catcher coolant channel. Steam-water experiment is for verification of the ex-vessel core catcher system coolability. Top wall of the channel was heated to simulate the decay heat of molten core. Uniform heat flux was given to the top wall, and then buoyancy driven natural circulation was followed. Natural circulation flow rate was obtained in various flow conditions. Local void fraction distribution and re-wetting time were obtained as well to understand two-phase flow behavior inside of the channel. Surface temperature of the top wall was measured along the flow path to obtain local heat transfer rate. Heat transfer enhancement was observed at the elbow-bend due to the turbulent mixing phenomena. Steam-water experiment directly supplies the heat to the real scale channel in order to verify the performance of prototypic system. Additionally, the implementation of the microporous coated surface for enhancement of heat transfer and CHF on the reactor scale large surface area is examined by modifying steam-water experimental facility. Microporous coating technique was applied to the 120 mm × 300 mm of large surface area in the middle of the inclined channel, and parametric study was carried out in various subcooling and flow rates. Boiling heat transfer enhancement was evaluated in coated surface with large surface area, and its enhancement mechanism was identified. CHF occurs in the plain Cu surface and its phenomena on the downward facing large heater was visualized. Based on the outcome of the experimental results, the coolability of the core catcher system is discussed. Understanding of the two-phase flow development and local heat transfer coefficient in the coolant channel helps to find predictive models for the two-phase flow behavior and heat transfer characteristics. CHF data available for 10 ° inclined large scale surface was obtained. Main contribution of this work is that the results provide a knowledge of two-phase flow structure in the coolant channel and similar geometries. The experimental results ensure the safety of the ex-vessel core catcher system. And the experimental database can be used for validation of CFD/system codes and models for reactor safety applications.