Calculation and Analysis of Critical Heat Flux at LWR Passive Safety Design

Critical heat flux (MDNBR) is one parameter for safe nuclear reactor operation. The magnitude of MDNBR is determined by local heat flux. When it is sufficiently high, it will cause a change in heat equilibrium and deteriorate nuclear fuel structure. To obtain assurance for nuclear reactor safety in safe criteria, Calculation and analysis of Critical heat flux of 630 MWe passive LWR was carried out at steady state and transient condition. The study used COBRA IV-I sub channel program coupled with critical heat flux correlation of EPRI-Columbia and W-3. The program models the core into 1/8 sections, each of which is divided into 83 sub-channels in radial direction and 40 nodes in axial direction, including 8 sub-channels of the hottest assembly. Normalized coast down flow and reactor power decay at initial phase is 10 second after loss of flow accident (LOFA) caused by anticipated primary pump failure. The effects of all input parameters, such as radial nodes, axial nodes, mix coefficient, velocity of coolant mass entering the core and power variation, have been identified. Evaluation of the results of safety calculation indicates that MDNBR, cladding temperature, and hot channel temperature are 1.83, 349°C and 2033°C, respectively. As the licensing limits for those parameters are 1.3, 1850°C, and 2840°C, it is concluded that the design of 630 MWe passive LWR has met the licensing criteria of safety. Modelling for safety analysis of passive LWR employs COBRA IV-I program. The results show that this program can be used to calculate hydraulic safety parameters with sub-channel model. (paper)