Abstract: The core continued to heat up due to the loss of cooling under accident conditions. The core would damage as it is uncovered for a long time. The reactor core exit temperature (CET) and the reactor vessel level (RVL) can provide an indirect indication of inadequate core cooling. CET and the containment dose rate are the master criteria in the method of core damage assessment, which is based on Westinghouse Owners Group Core Damage Assessment Guidance (CDAG). RVL, as one of the assistant parameters, can be used to validate the assessment result. In some nuclear power plant, CET, which is unused under sever accident condition, must be replaced by some other parameters. Based on the analysis of sever accident sequence, RVL would be discussed to be used in the current method of core damage assessment as a master criterion.