A simulation of the loss of residual heat removal (RHR) system during midloop operations was performed using the RELAP5/MOD3 thermal-hydraulic code. The experiment was conducted at the Rig of Safety Assessment (ROSA)-IV/Large-Scale Test Facility. The experiment involved a 5% cold-leg break along with the loss of the RHR system. The transient was simulated for 3040 s. Core boiling and subsequent primary system pressurization occurred after the initiation of the transient. There was a good agreement between the measured and the calculated data until the loop seal clearing (LSC). It was found that the steam condensation was underpredicted in the calculations. This caused the calculated data after the LSC to differ from that of the measured data. The core rod surface temperature excursion around the occurrence of the LSC was not calculated. Overall, there was good qualitative agreement between the measured and the calculated data. The calculations, performed on the CRAY-YMP supercomputer, took over 60 h of CPU time for a transient of 51 min.