A Preliminary 3D steam flow analysis for CET behavior during LSTF SBLOCA experiment using FLUENT code

Irwanto, D.; Satou, Akira ; Takeda, Takeshi ; Nakamura, Hideo  

A 3D analysis of the CET phenomena on the LSTF during SBLOCA was performed for the OECD/NEA ROSA-2 Project Test 3 (SB-HL-18) using CFD ANSYS FLUENT code to clarify influences of the steam flow on CET response during core uncovery. A portion of core above the mixture level was modeled up to CET thermocouple position (13 mm above the UCP), taking into account high, medium and low power heater rod bundle, including internal structures such as end-box, UCP and core spacer. From the calculation, it is clear that inner structures indeed affect the CET due to heat transfer from hot steam to these cool structures. Mixing was happened at the boundary between high-middle-low power rod regions due to steam velocity different at the boundary between each fuel bundles and at the free area above the heater rod. These 3D flows mixing also may contribute to the final CET values and the delay readings of CET relative to the peak cladding temperatures in the core. To confirm this simulation, ROSA/LSTF experiment data was used as comparison, resulting that most of the calculated values of CETs were slightly higher than the measured ones: 608 against 592 K for high power bundle; 598 against 586 K for medium power bundle and 579 against 581 K for low power bundle.