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The estimation of leak flow rates through a narrow channel is important to ascertain safe designs of piping system in nuclear power plants. RELAP5 has been universally used as a conventional code for predicting concerned thermal hydraulics which includes the prediction of breakage mass flow during loss of coolant (LOCA) accidents. In the present work an experimental validation of RELAP5 prediction of high temperature subcooled leakage flow has been demonstrated. Avoiding uncertainties of crack morphology, narrow rectangular slits circumferentially created on a reactor grade stainless steel pipe was used for experimental observation. The simplest model of the whole arrangement has been simulated using RELAP5 Mod 3.2 to assess the theoretically maximum flow rate. Circumferential slits were analogous to through wall cracks while slits sizes under study ranged 200–300 μm with aspect ratio 100–200 on 100 mm NB SS-304L pipes of 8 mm thickness. Pressurized sub-cooled light water was used to simulate standard conditions of PWRs or PHWRs with stagnation pressures and temperatures in the range of 6 to 9 MPa and 230 to 250 °C respectively. The experimental results were analyzed and compared with RELAP5 predictions for various stagnation conditions and node geometries mentioned. Flow predictions showed a lot of divergence when compared for several slit opening geometries.