Abstract: In order to develop the licensing LOCA evaluation program,a discharge model and CHF model modification and verification to RELAP5/MOD3.3 in accordance with Appendix K of 10CFR 50.46 is carried out.With the accomplishment of typical separated tests modeling,namely,Marviken Test-22 and ORNL THTF,input modeling,comparative analysis for the modification are presented.Verification to the conservatism of the employed Moody two-phase critical flow model and CHF correlations,as per related Appendix K requirements,is demonstrated and discussed,which lays a preliminary foundation for further development of LOCA licensing safety evaluation program.