Investigation of cross flow mixing on rod bundle safety margins using sub-channel analysis framework

An investigation on effect of cross flow mixing on rod bundle thermal hydraulic safety margins is carried out using a sub-channel analysis framework. In the present study, general purpose automated sub-channel analysis framework is developed to carry out the thermal hydraulic analysis of nuclear reactor core. The framework involves the preprocessor to generate the sub-channel layout of the fuel assemblies of different sizes and shapes such as square and triangular pitched pin bundle. The analyses are carried out for different simulated rod bundles of square pitched array of typical nuclear fuel assembly. The parameters considered in this study are the size of the bundle, p/d ratio, intensity of turbulent mixing, mass flux, uniform and non-uniform axial power distribution. The coolant local conditions are significantly affected due to the inter channel mixing between sub-channels. The sub-channel layouts for fuel bundles of different sizes varying from 2 × 2 to 10 × 10 and 17 × 17 for same p/d and w/d ratios of 1.05 to 1.3 are generated. The analyses are carried out by varying the intensity of turbulent mixing parameter (c) from 0 (no mixing) to 0.1 (high mixing) using the developed framework. It is found that hot channel temperature decreased by 26% in smaller bundle due to strong interaction between wall and corner channel. In case of large sized assembly above 5 × 5, hot channel temperature is less affected. The estimation of coolant temperature, fuel pin surface temperature and local quality are also carried out without considering the inter-channel mixing. The results are compared with a change in the local conditions for different degree of mixing among the fuel bundle sub-channels. The developed Framework is used for performing the sensitivity-studies during the design and analysis phase of nuclear reactor core for estimation of thermal hydraulic safety margins in an efficient way with minimum human intervention.