Abstract: The RELAP5/MOD3.4 predicts well the sequence of events and the major phenomena during the steam generator tube rupture (SGTR) accident transient, such as the asymmetric loop behavior, the reactor coolant system (RCS) cooldown and heat transfer by natural circulation, and so on. These results show that a single steam generator tube rupture is effectively and automatically mitigated in the integral type reactor, and the reactor is then cooled down and depressurized by the passive residual heat removal system (PRHRS). Also, the loss of RCS fluid slows down following the activation of PRHRS and the subsequent increase in the steam generator pressures, and is finally terminated once the RCS and steam generator pressures equalize. Finally, a sensitivity study is performed for understanding the effects of the break models and choking models in the RELAP5.