Development and Validation of Models for Predicting Leakage from Degraded Tube-to-Tubesheet Joints During Severe Accidents (NUREG/CR-7210)

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Publication Information

Manuscript Completed: July 2010
Date Published: July 2020

Prepared by:
S. Majumdar
K. Kasza
C. Bahn
W.J. Shack

Argonne National Laboratory
Argonne, IL 60439

Patrick Purtscher, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington DC 20555-0001

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Abstract

This report documents the development and validation of analytical models to predict steam generator (SG) tube leakage that can be expected from cracks within the tube-to-tubesheet junction at high temperature. It is recognized that the problem of predicting the leakage from cracked tubes within the tube sheet (TS) during severe accidents is too complex to be solved by either purely analytical or purely experimental means. In this study Argonne National Laboratory (ANL) adopted a combined analytical-experimental approach. The experiments were designed to simulate several key aspects of the tubesheet behavior during a postulated station blackout severe accident. The resultant predicted leak rates were determined through these efforts.

Pressure and leak rate tests were conducted at high temperatures on 12 tube-to-collar junction specimens with independent pressurization of the tube and the leakage path (crevice). A finite element model of the specimen was used to calculate the variation of contact pressure and tube-to-tubesheet gap over time. A leak rate model was developed based on plane Couette-Poiseuille flow along the interface between two rough contacting surfaces. The model parameters were determined from the leak rate tests.

A finite element model was developed for a Westinghouse Model 51 SG tube-to-tubesheet interface, including the divider plate, lower head and a short segment of the SG shell. The model was used to analyze first, the spatial variation of the temperature with time, and second, the variations of contact pressure and gap along the tube-to-tubesheet interface as functions of time during the postulated station blackout severe accident. The leak rate model was used to predict the leak rates during the severe accident.

Results in this report indicate that leakage could occur through the tube-to-tubesheet joints in station blackout accident conditions, and there are significant variations in the leak rates calculated for different paths. In addition, results show that the leak rate remains low for three hours, after which the rate is predicted to increase. In the absence of tests with realistic interface and boundary conditions, the present results should be considered as best estimates.