Calculation of vver-1000 core baffle temperature distribution for it's swelling assessment

Abstract

This paper presents simplified CFD-model of VVER-1000 core baffle cooling to address swelling problems of the reactor internals. Swelling is the main limiting factor in the reactor core internals long term operation of VVER-1000 nuclear units. The material irradiation-induced swelling and creep models are very sensitive to temperature distribution in metal, thus a more detailed analysis of the core baf- fle metal thermohydraulic cooling characteristics is required. In this paper, an approach with computational fluid dynamics (CFD) using is proposed. It allows us to consider local hydrodynamic coolant flow characteristics, as well as azimuthal distributions of characteristic parameters. An analytical model was developed to obtain characteristic parameters of simplified CFD-model, and consequently reasonably narrow its limits. Computational model is limited by the height of the baffle and performed using 60-degree symmetry, which included: core, baffle, core barrel, simplified geometry of connecting studs and coolant domains. Core is simplified as a homogeneous body with considering of spatial volumetric energy release. Core baffle is presented as monolithic body with considering of gamma-ray heat generation. Model takes into account cooling flow of the coolant through the nuts grooves, which allows obtaining a more realistic temperature field in studs. Calculated convection coefficient and temperature are in good agreement with analytical model, and give a more convenient result comparing to RELAP5. Obtained temperature distributions were used to estimate baffle swelling process. Due to the less conservative results in temperature distribution swelling and creep deformations significantly decreased. Developed model was further improved and used to calculate baf- fle temperature field changes during the representative transitional process of normal operation conditions violations. Results of transient process simulation are used in assessing of progressive form-change calculating need.