This report investigates the steps of a one-way coupling between two simulation codes developed by Wesinghouse Electric Sweden AB. The Westinghouse POLCA8 is a three dimensional steady-state diffusion theory code used for simulating the neutronic, thermal and hydraulic behavior of a reactor core. In the  thermal-hydraulic module of the code, each fuel assembly is simulated as a one-dimensional channel, accounting for axial variations of the fuel geometry. While sufficient for many applications, the one-dimensional thermal-hydraulic approach may lack spatial resolution in the case of tilted radial power, very inhomogeneous fuel lattices or for specific calculations such as CHF (Critical Heat Flux) in PWR [3]. This limitation will b avoided by performing a code coupling with the sub-channel analysis code, VIPRE-W, to obtain the radial distribution of thermal-hydraulic parameters for each fuel assembly. In this thesis the codes are one-way coupled .

To be able to do a coupling an interface is needed, and this has been created in Matlab. In the interface, the output from POLCA8 is converted into a form suitable to use as an input to VIPRE-W. 

As an important first step in the coupling process, I have first analyzed how consistent the codes are when simulating the simplest thermal conditions inside the core. To be able to do the comparison,all values extracted from the sub-channel analysis code VIPRE-W must be converted into assembly-average-values, this is also done in the interface. The thermal-hydraulic parameters that have been  analyzed and compared in the two codes are; mass flux, quality and void.