A simplified thermal-hydraulic program was developed to quickly calculate the minimum Departure from a Nucleate Boiling Ratio (DNBR) in a pressurized water reactor (PWR) core which is a measure for the core thermal margin. The accuracy of the simplified thermal-hydraulic program would largely depend on the size of the axial nodal sections particularly for the non-iterative scheme for solving the transport coefficient conservation equations. The number of axial nodes in the faster-running DNBR program varies depending on its applications to a PWR core monitoring and protection systems and a reactor safety analysis. It is therefore important to examine the effects of the axial nodal sections in the simplified DNBR program for its application to an advanced PWR core protection system. This paper presents the uncertainties of the minimum DNBR (MDNBR) and the DNBR margin assessment depending on the number of axial nodes. The DNBR margin significantly decreases due to very large MDNBR uncertainty for the number of axial nodes of less than 20.

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