Investigation on Effect of Control Rod Guide Tubes on Critical Heat Flux in Large-diameter Fuel Rod Bundles

The control rod guide thimble exerts a significant influence on the critical heat flux (CHF) characteristics of rod bundles through the combined effects of cold-wall heat transfer and subchannel flow restructuring. This effect is particularly pronounced in annular fuel assemblies with large rod diameters and narrow subchannel gaps, where geometric constraints enhance the coupling between thermal and hydraulic fields. In the present study, the subchannel analysis code ATHAS, coupled with the 2006 CHF look-up table method (LUT-2006), was applied to investigate CHF behavior in both a 5×5 grid-scale rod bundle and a 13×13 fuel assembly under representative operating conditions. For the 5×5 rod bundle, the introduction of the guide thimble induces a cold-wall effect that modifies local heat transfer conditions and generates axial and lateral pressure gradients. These gradients drive crossflow between subchannels, enhancing turbulent mixing and promoting flow redistribution across the bundle cross-section. As a result, the thermal-hydraulic non-uniformity is reduced, leading to more uniform distributions of void fraction and enthalpy rise. This redistribution alters the relative CHF margins among subchannels, causing the CHF location to shift from the central hot subchannel in the typical grid configuration to another hot subchannel in the guide-thimble configuration. Meanwhile, the predicted CHF shows a modest but consistent increase compared to the typical grid case, reflecting improved liquid replenishment associated with enhanced mixing. For the 13×13 fuel assembly without confining sidewalls and with a uniform radial power distribution, the CHF behavior is governed by a different mechanism. In this configuration, the absence of radial power non-uniformity weakens localized thermal driving forces, while the larger flow domain allows for more global redistribution of mass flux. Nevertheless, due to the cumulative effect of axial enthalpy rise and two-phase flow development, the central subchannel still experiences the most rapid vapor generation and reaches CHF first. Overall, the results demonstrate that the guide thimble affects CHF not only through local cold-wall effects but also through system-level flow restructuring and mixing enhancement. The interplay between geometric constraints, lateral mixing, and two-phase flow evolution determines the CHF performance. This study provides mechanistic insights into CHF enhancement in annular fuel assemblies and offers a useful reference for the thermal-hydraulic design optimization and safety assessment of advanced reactor cores.