Source term evaluation is an important element in the assessment of efficiency of a Severe Accident Management (SAM) strategy. The identification of phenomena and parameters that present major contributions to the uncertainty in the magnitude and timing of the releases and quantify the uncertainty is vital for comprehensive risk analysis. In this work source term evaluation was performed using MELCOR for two accident scenarios, large break LOCA and station blackout, that leads to containment failure due to ex-vessel phenomena such as formation of non-coolable debris bed and steam explosion. Cases without containment failure were also analyzed for the effect of melt debris release characteristics on fission product release in both accident scenarios. It was observed that initial vessel failure mode was due to failure of the penetrations. When the debris being ejected was limited to only molten mass, instead of solid and molten mass, it also led to creep-rupture of the vessel lowerhead wall. This mode of ejection also shows a remarkable increase in the cesium and iodine source term release to the environment. When the containment does not fail, it is observed that there is lesser accumulation of fission products inside the containment and the retention inside the pressure suppression pool is enhanced in the scenarios with only molten debris ejection.