A major concern in the design of RMWR is that sufficient cooling capability be provided to keep fuel cladding temperature below specified values, even for a postulated abnormal transient process. In this research, power increase and flow decrease transient tests are performed in 7-rod and 37-rod double-humped tight lattice bundles, under RMWR nominal operating condition (P_<ex> = 7.2 MPa, T_<in> =283℃) for mass velocity G = 300, 450, 600 kg/m^2s. Experiments are analyzed with TRAC code, in which new JAERI critical power correlation is implemented for BT judgment. For the postulated nominal power increase and flow decrease transients, when CPR is 1.3, no Boiling Transitions (BTs) are observed in experiments and TRAC code predicts the same trends. For severer conditions that BT occurs, wall temperature jumping points (BT points) can be predicted quite well within the accuracy of the implemented critical power correlation. The traditional quasi-steady-state prediction of BT in transient process is confirmed being applicable for axially double-humped-heated tight lattice bundles.
