Present study is centered around the containment event tree (CET) code, called EVNTREISS, to quantify the release category of the CET. This code was originally developed at Sandia Nationa Laboratory (SNL) as part of the Severe Accident Risk Reduction program (SARRP), and the code has been used to analyze the containment response of a few nuclear power plants such as Surry and Zion for severe accidents. The EVNTREISS code, used as a basis of the present work, is highly compelx and versatile in comparison with the previous CET used in the WASH-1400 study. Since the construction of the EVNTREISS code is very complex and has not gone through a through validation and review process by an independent referee it is not surprising to find a few areas of improvement and several inherent problems of the code. The present study is thus initiated to identify all the problems and areas of improvement for the CET code and modify the existing code according to the insights gained from the experience of reproducing the Zion containment response analysis results obtained at Brookhaven National Laboratory. As a result of this study, several areas of improvement for the EVNTREISS code have been identified and a few problems of the code have been resolved in addition to the reproduction of the Zion results. Finally, the modified code can now be run by a personal computer and can be used in the analysis of a Large Dry PWER containment response for severe accidents.