Numerical Study on the Effects of Guide Thimble on Critical Heat Flux in a 5 × 5 Fuel Assembly

Rod bundle with guide thimble is a typical structure in the PWR fuel assembly. The thermohydraulics, including critical heat flux (CHF), in the channel adjacent to the guide thimble are affected by the non-heated tube. In the current work, the CHF of 5×5 rod bundle with guide thimble at the center was predicted using the Eulerian two-fluid framework coupled with the critical heat flux model for heat transfer deterioration at the heated wall. The local thermal-hydraulic parameters as well as CHF values were compared with these without guide thimble. It was found that the CHF in the fuel assembly with thimble was slightly larger than that in the assembly without thimble. The lateral locations of boiling crisis were altered by the existence of the guide thimble. The inter-channel cross flow was also suppressed by the guide thimble due to its effect on the flow and thermal boundaries of the subchannels