SMR core thermal hydraulic experiments and code validation with the MOTEL test facility

Thermal hydraulic experiments with the modular integral test facility, MOTEL, were performed as a part of the European McSAFER research project. The facility models an integral pressure water small modular reactor (SMR) with a helical steam generator and a core with separate heater rod groups, in which power can be individually controlled. Different asymmetric and ring-shaped radial core power distributions were imposed in the experiments to provoke cross flows in the buoyancy-driven coolant flow. The purpose of the experiments was to produce new SMR-relevant data for the validation of computational fluid dynamic (CFD) and thermal-hydraulic subchannel codes. The experimental measurements revealed cross flow mixing effects, mainly in the top part of the core. Obtaining visible differences in the fluid temperature measurements between different heater regions required significant power gradients between the regions. CFD simulations were performed using ANSYS CFX with a detailed model comprising the whole primary side of the facility, and additional investigations were conducted with a stand-alone model of the heat exchanger. Good agreement with the measurements was obtained with the CFD simulations, which also revealed further details of the core flow characteristics in an asymmetric heating case. Furthermore, simulations with the subchannel codes, CTF and VIPRE-01, were performed. The simulations with CTF highlighted the code’s capability to handle flow rates typical to natural circulation driven SMRs, as the results agreed well with the experiments and were able to predict the correct axial temperature profiles in the different regions of the core. VIPRE-01 solution stability was found to be highly sensitive to the flow rate, the power level, and the axial nodalization. Simulations with VIPRE-01 ended unsuccessfully due to convergence issues, and it was concluded that the conditions of the experiments are beyond the current capabilities of the code.