Developed and applied to the WCOBRA/TRAC code for speed up is an implicit numerical method with an iterative solution scheme of semi-implicit format. For a sample problem of subcooled liquid, computation speed was increased by a factor of 2 to 3 or more. For problems involving phase transition and heat transfer, other techniques of iteration control, time-step control, adjustment of correlations, and numerical stabilizer were introduced. Consequently, computation speed was increased by a factor of 2.