The thermal-hydraulic CATHARE V1.3U code has been used to simulate an International Standard Problem (ISP38) experiment conducted at BETHSY Integral Test Facility located in Grenoble, France. This experiment presents simulation of the loss of residual heat removal system during midloop operation. It involved opening of the pressurizer man way and steam generator outlet plenum man way simultaneously with switching on the heating rod power to simulate the decay heat. The total power level of 138 kW was kept unchanged throughout the test. Mass discharge through both manways led to core boiling and uncovery. The test was stopped when the primary cooling system was filled back to a midloop level. Overall, the code’s prediction and experimental data were found to be in reasonable qualitative agreement. However, the code underestimated the time of the core uncovery and the actuation of the gravity feed injection because of the overprediction of the discharge through the steam generator man way during the initial stage of the transient. This was caused by misestimation of the phase separation effect at the hot leg/surge line tee junction and significant water entrainment into the surge line at the beginning of the test. It was found that the upward tee junction model needs to be refined for the low-pressure transients.