Yuanzineng kexue jishu (Jul 2023)
Numerical Investigation on Effects from Rods Offset and Bowing on Critical Heat Flux of Fuel Assembly
Abstract
Reactor fuel assemblies may be biased and bowed as a whole due to factors such as manufacturing and installation deviations or external forces. This consequence may have an impact on heat transfer and criticality, and even the possibility of edge-rod criticality. In order to evaluate the impact size, a set of high-quality dynamic mesh generation technology was developed through CFD. The dynamic mesh technology was used for the base condition to make the overall displacement or bowing of the fuel rods. Using the Eulerian two-fluid model and the improved RPI wall boiling model, the CHF of the 5×5 rod bundle channel with different offset distances and bowing degrees was predicted, and the CFD prediction results were verified with the CHF look-up table value. The results show that the CHF predicted by the non-deformation and small deformation conditions are in good agreement with the values of the sub-channel parameter CHF look-up table, while the large deformation condition triggers the side rods critical and CHF look-up table is not applicable. Because the CHF table is made based on an 8 mm round tube, the non-8 mm round tube or rod bundle structure (such as the central sub-channel) can be corrected by the hydraulic diameter correction factor, and the prediction accuracy is often high. However, at the edge and corner channel position of the rod bundle assembly, the heated boundary and wet circumference are not uniform, and the offset or bowing of the fuel rods squeezes the corner channel, resulting in a more unbalanced distribution of the heated boundary and wet circumference at this location. As a result, the CHF look-up table is not applicable. The critical occurrence position and numerical value under different deformation conditions were compared, the distribution of mass fluxes in different sub-channels at the critical height was also compared, and the sensitivity of various deformation types to CHF was summarized in this paper. For the calculation results of the undeformed fuel assembly, the flow rate of the central sub-channel is the highest, the side channel is the second, and the corner channel is the smallest. For the deformed fuel assembly, the offset and bowing change the flow distribution of the sub-channels, the flow of the corner channels decreases, and the flow of the central sub-channel increases. When the gap of the corner channel is so small that the coolant flow cannot effectively take away the bubbles on the surface of the fuel rods, causing local congestion of the bubbles, the side rods will be critical. The XY-type condition has the strongest influence on the corner channel flow, but the weakest effect on the overall flow distribution, and the C-type bowing condition has the strongest influence on the overall flow distribution. In the calculation condition, the axial power is uniformly biased and bowed by 0.5 mm without changing the critical axial position, while bowing by 0.8 mm and 1 mm affects the critical position. The larger the three deformation degrees, the smaller the predicted CHF value and the increase of the critical probability of the side rods. Among them, the XY-type deformation is the least sensitive to the CHF value, while the C-type bowing with a cosine axial heat flux has the greatest impact on the CHF value. Compared with the same degree of bowing, the critical heat flux density value of axial power cosine is smaller. When bowing 0.5 mm, the critical position of the axial power uniform condition occurs at the center, while the critical position of the axial power cosine condition occurs at the edge. That is, the cosine power may also be the cause of the criticality of the side rods.
