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Conference Agenda

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Session Overview

Session

Keynote 4

Time:

Wednesday, 03/Sept/2025:

9:00am - 10:00am

Session Chair: Bao-Wen Yang, Delta Energy Group, New York, United States of America
Session Chair: Tomio Okawa, The University of Electro-Communications, Japan

Location: Session Room 1 - #205 (2F)

Presentations

ID: 3081 / Keynote 4: 1
Invited Paper
Keywords: Reflood, Spacer Grid, Droplet Breakup, LOCA

Spacer Grid Rewet during Reflood andModeling Challenges for Thermal-Hydraulic Codes

Stephen M. Bajorek¹, Yue Jin², Fan-Bill Cheung³

¹United States Nuclear Regulatory Commission, United States of America; ²University of Missouri, United States of America; ³Pennsylvania State University, United States of America

Rod bundle spacer grids are known to have important, possibly dominant effects on thermal-hydraulic behavior during core uncovery. Spacer grids enhance mixing and convective heat transfer downstream of the grid Droplet breakup at a spacer grid increases interfacial area and the de-superheats steam benefiting heat removal from uncovered rods. Because they are unpowered the spacer grids can also rewet much easier than surrounding fuel rods. The liquid film on a wet grid acts as a source for droplet entrainment immediately downstream of the grid which further increases interfacial area and steam de-superheat.

The Rod Bundle Heat Transfer Facility (RBHT) data provide a unique opportunity to determine conditions for spacer grid rewet during reflood. Each spacer grid in RBHT has one or more wall-mounter thermocouples that indicate if and when the grid rewets. Tests ranged in RBHT from low flooding rates (0.5 cm/sec) to 15 cm/sec. Previous studies have shown that spacer grids can rewet well ahead of the quench front on the heater rods. However, no systematic study has been done to characterize when and which spacer grids rewet. The effects of wetted grids on the droplet dynamics and two-phase heat transfer downstream of the grids have not been seriously investigated.

In this paper results from several RBHT tests are presented and discussed with an emphasis on spacer grid rewet and the challenges in simulating the rewet process and effects on transient reflood behavior.