Abstract
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 of the core thermal margin. The accuracy of the simplified thermal-hydraulic program largely depends on the size of the axial nodal sections, particularly for the noniterative scheme that is used to solve the transport coefficient conservation equations. The axial nodal size in the faster-running DNBR program varies depending on its applications to PWR core monitoring and protection systems and a reactor safety analysis. It is therefore important to investigate 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 axial nodal size, i.e., the number of axial nodes. The MDNBR calculation error could significantly increase in the case of a large axial nodal segment (~15 inches). It is concluded that the axial nodal size in the fasterrunning DNBR program should be less than or equal to 7.5 inches for its application to an online reactor protection system.
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