The necessity of the latest codes and methods for safety demonstration is increased to satisfy new safety requirement and achieve improved margin management. In this circumstance, it becomes an important issue that high-fidelity and multi-physics simulation with coupled T/H (Thermal-Hydraulics) and neutronics code for light water reactor whole core. With the improved computing power, the subchannel scale T/H analysis could be used as a suitable tool for pin-by-pin whole core simulation considering both accuracy of simulation and reasonable calculation time.

CUPID is a multi-dimensional two-phase flow analysis code developed by KAERI for the analysis of reactor core component. It has been validated against various experimental data and applied for practical nuclear applications. Recently, its applicability was extended to the subchannel scale T/H analysis. It is highly parallelized with the domain decomposition and message passing interface and these features facilitated the extension to use the code for the whole reactor core pipby- pin analysis in the subchannel scale. Required physical models for the subchannel scale analysis, for example, turbulent mixing and void drift models, were implemented and validated against available subchannel experiments.

In the present study, the grid spacer model was implemented for the enhancement of accuracy of the simulation. Afterwards, the mixing vane model was implemented considering lateral momentum exchange between adjacent subchannels by the mixing vane. For the validation of models, PSBT 5x5 experiment was simulated using CUPID, and the calculation result was compared with the CTF calculation. These implemented models can contribute to improve the prediction capability of CUPID for more realistic whole reactor core transient analysis.