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Opponent: Dr. Michael Buck, Universität Stuttgart, Tyskland
Supervisor: Jan Dufek, KTH, Stockholm
In a severe accident of light water reactors (LWRs), the molten core materials (corium) may relocate to the lower plenum of the reactor pressure vessel (RPV) and consequently impose significant thermal and mechanical loadings on the lower head of RPV. To mitigate the risk of RPV failure due to the corium attack, the external surface of the RPV lower head can be cooled by water through an in-vessel melt retention (IVMR) strategy. In the conventional IVMR design of pressurized water reactors (PWRs), the external cooling of RPV is realized by flooding the reactor cavity and forming natural circulation boiling across the lower head. The cooling capacity is bounded by the critical heat flux (CHF) of natural circulation boiling. To overcome limitation of the conventional IVMR measure, an alternative IVMR measure was proposed in this work, which implements an efficient water spray system to cool the external surface of the lower head.
The study of this doctoral thesis is oriented to examine the feasibility of spray cooling in the alternative IVMR strategy. For this purpose, an extensive investigation on spray cooling of a downward-facing heater surface was accomplished in this thesis, including both experimental and numerical studies on the cooling performance and influential factors that affect performance of multiple spray nozzles. The numerical models with high-fidelity predictive capabilities for multi-nozzle spray cooling of the downward-facing heater surface were validated against the experimental data. The key points of the research and the results are as follows.
· In experimental studies, heat transfer characteristics of water spray of an array of 2×2 spray nozzles over a relatively large heater surface were investigated. The effects of surface inclination, nozzle-to-surface distance and flowrate on spray heat transfer were investigated for the first time on the downward-facing heater surface. The heater was made of a 150 μm thick SA302B steel foil with a surface of 120 mm ´ 80 mm. Joule heating was applied by connecting the specimen to the electrodes of a DC power supply of low voltage and high current. Four full-cone pressure-swirl spray nozzles were used in the experiment. The test matrix includes the following variations of parameters: six surface inclinations (between 15 deg and 90 deg), four coolant flowrates, and three nozzle-to-surface distances. The experimental results indicated an efficient cooling potential of the multi-nozzle water spray over the downward-facing heater surface, with a maximum heat removal up to 2.50 MW/m2. The cooling performance was enhanced by increasing flowrate, but the enhancement by flowrate was diminishing after an intermediate flowrate. The surface inclination had a negligible impact on the cooling performance.
· In numerical studies, simulation models to predict the spray cooling process were developed by adopting a coupled Eulerian-Lagrangian methodology implemented in the open-source CFD code package OpenFOAM. Prior to simulation of experiments, a validation of the models was conducted against analytical solutions of relevant hydrodynamic problems, and the results confirmed the predictive accuracy in liquid film dynamics and spray impingement on inclined surfaces. The predicted liquid film morphology had a good agreement with the experimental observation under adiabatic conditions.
· The numerical models were further extended to the modeling of heat transfer, thin film boiling and conjugate heat transfer in the spray cooling problem. The simulation results of full- and partial-coverage of the heater surface by respective 6- and 4-nozzle array showed reasonable agreements with experimental data across various surface inclinations. The validation exercises demonstrated the capability of the numerical approach to model complex multiphase heat transfer phenomena encountered in the spray cooling.
This work did not only improve our understanding of multi-nozzle spray cooling mechanisms over a downward-facing heater surface, but also provided basic experimental data and numerical approach for further assessment of spray cooling application to the IVMR strategy in nuclear reactor safety.