The use of computer codes is required in safety evaluation of light water reactors (LWRs) in order to simulate plant behaviour during design basis event (DBE) and beyond-DBE (BDBE). This involves complex multi-dimensional single-phase and two-phase flow conditions, which may include noncondensable gas in many cases. For this purpose, thermal-hydraulic safety analysis codes, especially for best estimate (BE) analyses codes for the evaluation of safety margin at reactor accidents and abnormal transients including loss-of-coolant accident (LOCA), have been developed with significant efforts to achieve high predictive capability especially for one-dimensional (1-D) phenomena such as flows in piping at rather high flow rate. Such BE codes include APROS, ATHLET, CATHARE, COSINE, MARS, RELAP5, SPACE, SPECTRA, TRAC and TRACE. However, there remain needs for experimental work and code development and validation for the complex flow conditions that may appear during the DBE and BDBE. Further, the increased use of the BE analysis codes in licensing, which is replacing traditional conservative evaluation model (EM) approaches, requires quantification of uncertainties in the simulation models and methods through validation efforts against the obtained detailed data.