title: Developing a CFD informed approach for subchannel analysis investigations for a wire-wrapped fuel bundle experiment
Creators: name: Mallela Viswa, Venkata Ravindra
Publisher: name: University of Liverpool Repository
Year: 2022

description: Project FAITH, which is part of the UK government funded Advanced Manufacturing and Materials (AMM) phase 2 nuclear innovation programmes, provided an excellent opportunity to develop skills in nuclear thermal hydraulics area. The project aimed at exploring modular building techniques for a sodium rig that can be used to conduct Generation IV reactor thermal-hydraulics experiments within the UK. To achieve this, two rigs, one using water (Rig - 1) and the other one using sodium (Rig - 2) have been proposed using modular technology. A subchannel code called CTF has been used to model the Rig-1 facility which is being built at NNL’s Workington facility in a modular manner by Cammell Laird, a ship building company. To gain confidence and build expertise on using the CTF code, two validation activities have been studied. The first validation activity is performed against PSBT benchmark data where the boundary and operating conditions are that of pressurised water reactors involving multiphase, while the second validation activity is on PNNL experimental studies in which the measurements were made on the flow regimes similar to single-phase flow inside a Gen IV sodium cooled fast reactor. Both these validation activities offered invaluable experience and confidence in using CTF for practical problems to assure successful qualification in code usage. Rig-1 configuration along with its operating conditions makes it a non-standard case for analysis from CTF, not least due to the geometry that has fuel pins with wire wrapped around them, arranged in a triangular lattice in a hexagonal pipe. A method has been developed wherein CFD studies were performed for the operating Reynolds numbers, NRe (5000 ≤ NRe ≤ 100000) on Rig – 1 and frictional loss coefficients were obtained from the analysis. These frictional loss coefficients were linked in the subchannel code, CTF replacing its standard frictional correlations for pressure drop estimates. The method has been tested on various pin arrangements ranging from square array to triangular array to the current Rig-1 design that has a hexagonal pipe with 7 fuel pins with wire wrapped around them, replicating fuel rod structure inside a sodium cooled fast reactor. The CTF results with its standard frictional correlations have over predicted the pressure drops (as compared to CFD results) by about 25% for square or triangular subchannels and around 6% for Rig-1 geometry with wire wraps. However, when this novel method of applying frictional coefficients (obtained from CFD) in CTF code, the results of pressure drop from CTF matched excellent with CFD results by overpredicting only about 0.5%. This improved CTF model can also be used to perform a range of parametric studies which includes slight variations in the geometry (due to manufacturing tolerances etc.) and their effects on the flow at a quicker turnaround time. In addition to this, the frictional coefficients obtained from 7-pin, can also be used for 19-pin and 37-pin configurations, thus CTF modelling capabilities are further expanded.