| Summary: | In Pressurized Water Reactor (PWRs), heat generated by nuclear fissions is effectively transferred to the water coolant by subcooled flow boiling. The maximum reactor power is limited by the critical heat flux (CHF), at which the boiling crisis occurs. Understanding the mechanisms that trigger this boiling crisis and predicting the CHF limit is key to the safety and efficiency of nuclear reactors. The CHF limit depends on cladding material, thickness, and surface conditions. Importantly, the surface of a fuel rod cladding evolves during operation due to oxidation and crud deposition. Many studies have investigated the effects of surface properties (e.g., surface roughness, wettability, and porosity) on boiling heat transfer and CHF. Still, the results of these investigations are not always in agreement with each other. We believe that the reason for these discrepancies is due to the lack of control of the surface conditions.
This thesis aimed at developing experimental capabilities and protocols to conduct “separate effect” studies and investigating the real effect of surface oxidation and Accident Tolerant Fuel (ATF) coatings on subcooled flow boiling heat transfer. To that end, we prepared Zircaloy-4 heaters that mimic the commercial PWRs fuel cladding, and conducted subcooled flow boiling experiments at 1 bar, 10 K subcooling, and 1000 kg/m2 s mass flux, using high-resolution high-speed video (HSV) and infrared (IR) diagnostics. A computational model solving a 3-D inverse conduction problem was developed to post-process infrared (IR) measurements. An HSV post-processing approach including a deep-learning tool, U-net, and a global optical flow algorithm was proposed to quantify boiling parameters from HSV images. The parameters were incorporated into a heat flux partitioning model, where we introduced a term to account for the non-symmetric growth of the microlayer.
The experimental results showed that groove pattern, average roughness, and wettability do not affect subcooled flow boiling. Instead, they suggest that the process is determined by the location, size, and shape of cavities, and that micro-scale surface modifications (e.g., porous cracks) or nano-scale structures play a crucial role in the formation of active nucleation cavities and modify the bubble dynamics. A key takeaway from this study is that, to elucidate how surface modifications affect boiling heat transfer, one should carefully examine how the surface morphology changes at both the micro- and nano-scale and how the surface preparation process affect the formation of cavities.
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