CFD Verification and Validation of Wire-Wrapped Pin Assemblies

TerraPower participated in a cooperative project among industry, a national laboratory, and a university to perform verification and validation of computational fluid dynamics (CFD) methods for predicting the flow and heat transfer within fuel assemblies with hexagonally packed wire-wrapped fuel pins. This project consisted of both experimental and numerical components and used surrogate fluids and electrically heated fuel pins to substitute for liquid metal and nuclear fuel. TerraPower performed CFD simulations of the experiments using industrial-level Reynolds-averaged Navier-Stokes (RANS) turbulence modeling. These simulations of helically wire-wrapped fuel assemblies employed meshes of bare pins without the wire-wrap geometry explicitly modeled. Instead, the effect of the wire-wrap on the flow is accounted for by introducing a momentum source (MS) into the governing fluid equations.

Solution validation was conducted by benchmarking the CFD simulations to the heated bundle experiments. These simulations used the as-tested boundary and operating conditions but were conducted blind. Pressure drop measurements and local temperature measurements were compared.

Axial pressure drop simulation results compared well with the experiment measurements. The vast majority of the local CFD temperatures matched thermocouple measurements within the instrument uncertainty. The good agreement between simulation and experiment supports the use of RANS-based CFD simulation methods and the specific applied MS method to model wire-wrapped fuel assemblies.