The objectives of this research are to establish the analysis system of thermal hydraulic field of KOFA(Korean Fuel Assembly) core using the multichannel analysis code TORC, and to improve the analysis system for critical heat flux (CHF) according to the multichannel analysis system. The thermal hydraulic models included in TORC code were reviewed, and the value of turbulent mixing parameter, which was found to be the most sensitive parameter to the variation of fuel assembly geometry, was determined from the thermal mixing test data of KOFA. The performance of the existing CHF correlations were evaluated through the CHF test data of KOFA using the TORC code models which were adjusted to KOFA. Furthermore, new CHF correlations were developed to improve the performance of CHF analysis system by using TORC code. In addition, the model for the evaluation of transition core DNBR penalty using TORC code was developed, and the application of that model for Ulchin 1/2 mixed core of KOFA and FRA-STD FA shows that about 2% of DNBR margin can be recovered in the transition core. (Author).