Summary for Three Different Validation Cases of Coolant Flow in Supercritical Water Test Sections with the CFD Code ANSYS CFX 11.0

About the Thermal Hydraulic Analysis Part of a Coupled Study on a Thorium-Fueled SCWR Concept

Inherently poorer moderation in supercritical water-cooled reactors (SCWRs) due to average density lower than in light water reactors and the resulted spectral shift can be useful when we apply thorium fuel-cycle instead of uranium–plutonium one, according to an ongoing study in Budapest University of Technology and Economics (BME) Institute of Nuclear Techniques (NTI). Upon this conclusion, a thorium-fueled SCWR design (Th-SCWR) has been proposed by BME NTI. In the current feasibility study phase, detailed three-dimensional (3D) computational fluid dynamics (CFD) calculations with novel neutronics analysis were coupled and conducted separately. Neutronics calculations provided the distribution of heat source, while the CFD analysis gave back axial distribution of coolant density (this iteration was repeated until an acceptable convergence). This paper presents the CFD analysis on thermal hydraulics of the initial design (two CFD models without any spacer device and one model with wrapped wire spacer) of Th-SCWR fuel assembly. As results of the preliminary design of Th-SCWR cladding wall, coolant and fuel temperatures have been determined; the flow field with and without spacer device has been showed, and the application of wrapped wire spacer has been proposed.

Summary on the Results of Two Computational Fluid Dynamic Benchmarks of Tube and Different Channel Geometries

Two computational fluid dynamic (CFD) benchmarks have been performed to assess the prediction accuracy and sensitivity of CFD codes for heat transfer in different geometries. The first benchmark focused on heat transfer to water in a tube (first benchmark), while the second benchmark covered heat transfer to water in two different channel geometries (second benchmark) at supercritical pressures. In the first round with the experimental data unknown to the participants (i.e., blind calculations), CFD calculations were conducted with initial boundary conditions and simpler CFD models. After assessment against measurements, the calculations were repeated with the refined boundary conditions and material properties in the follow-up calculation phase. Overall, the CFD codes seem to be able to capture the general trend of heat transfer in the tube and the annular channel but further improvements are required in order to enhance the prediction accuracy. Finally, sensitivity analyses on the numerical mesh and the boundary conditions were performed. It was found that the prediction accuracy has not been improved with the introduction of finer meshes and the effect of mass flux on the result is the strongest among various investigated boundary conditions.

A Blind, Numerical Benchmark Study on Supercritical Water Heat Transfer Experiments in a 7-Rod Bundle

Heat transfer in supercritical water reactors (SCWRs) shows a complex behavior, especially when the temperatures of the water are near the pseudocritical value. For example, a significant deterioration of heat transfer may occur, resulting in unacceptably high cladding temperatures. The underlying physics and thermodynamics behind this behavior are not well understood yet. To assist the worldwide development in SCWRs, it is therefore of paramount importance to assess the limits and capabilities of currently available models, despite the fact that most of these models were not meant to describe supercritical heat transfer (SCHT). For this reason, the Gen-IV International Forum initiated the present blind, numerical benchmark, primarily aiming to show the predictive ability of currently available models when applied to a real-life application with flow conditions that resemble those of an SCWR. This paper describes the outcomes of ten independent numerical investigations and their comparison with wall temperatures measured at different positions in a 7-rod bundle with spacer grids in a supercritical water test facility at JAEA. The wall temperatures were not known beforehand to guarantee the blindness of the study. A number of models have been used, ranging from a one-dimensional (1-D) analytical approach with heat transfer correlations to a RANS simulation with the SST turbulence model on a mesh consisting of 62 million cells. None of the numerical simulations accurately predicted the wall temperature for the test case in which deterioration of heat transfer occurred. Furthermore, the predictive capabilities of the subchannel analysis were found to be comparable to those of more laborious approaches. It has been concluded that predictions of SCHT in rod bundles with the help of currently available numerical tools and models should be treated with caution.