Implementation of droplet breakup mode in TRACE to improve the prediction of reactor core reflood conditions (NUREG/IA-0543)

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Publication Information

Manuscript Completed: August 2023
Date Published: November 2023

Prepared by:
Omar S. Al-Yahia*, Matthew Bernard**, Ivor Clifford*, Grégory Perret*, Stephen Bajorek**, Hakim
Ferroukhi*

*Paul Scherrer Institute, Laboratory for Reactor Physics and Thermal-Hydraulics
5232 Villigen PSI
Switzerland

**U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
USA
 
K.Tien, NRC Project Manager
 

Division of Systems Analysis
Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

Prepared as part of:
The Agreement on Research Participation and Technical Exchange
Under the Thermal-Hydraulic Code Applications and Maintenance Program (CAMP)

Published by:
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

Reflood conditions exhibit complex two-phase flows in fuel assemblies featuring spacer grids. Spacer grids can shatter or split dispersed droplets thereby significantly reducing their diameters, increasing the local heat and mass interfacial transfer, and reducing peak cladding temperature (PCT). The diameter of dispersed droplets downstream of a spacer grid has been shown to be dependent on the Weber number of the incoming droplets and the spacer grid blockage ratio. In this work, a spacer grid droplet breakup model based on the droplet Weber number is implemented into the new three-field version of the U.S. Nuclear Regulatory Commission’s TRACE code. The selected droplet break-up model can capture interfacial heat transfer enhancement observed in experimental data and is applicable to both mixing vane and egg-crate style spacer grids. The effect of the droplet breakup model on the three-field version of TRACE is assessed against recent RBHT (Rod Bundle Heat Transfer) experiments, for which TRACE was shown to overpredict the PCT for different initial and boundary conditions. When comparing the predictions of the base capability of TRACE and the three-field code with and without the spacer grid model, the new breakup model reduced the PCT predicted by TRACE by more than 100 K, which resulted in better-predicted PCTs and quenching times for the condition assessed.