Consequential SGTR Analysis for Westinghouse and Combustion Engineering Plants with Thermally Treated Alloy 600 and 690 Steam Generator Tubes, Final Report (NUREG-2195)

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Publication Information

Manuscript Completed: July 2017
Date Published: May 2018

Prepared by:
S. Sancaktar, M. Salay, R. Iyengar
A. Azarm ¹
S. Majumdar ²

¹ Innovative Engineering and Safety Solutions, LLC
20817 Spinning Wheel Pl.
Germantown, MD 20874

² Argonne National Laboratory
9700 S. Cass Avenue
Argonne, IL 60439

Selim Sancaktar, NRC Project Manager

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

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Abstract

This report summarizes severe-accident-induced consequential steam generator (SG) tube rupture (C-SGTR) analyses recently performed by the U.S. Nuclear Regulatory Commission's (NRC's) Office of Nuclear Regulatory Research. C-SGTRs are potentially risk-significant events because thermally induced SG tube failures caused by hot gases from a damaged reactor core can result in a containment bypass event and a large release of fission products to the environment. The current analyses evaluate replacement SGs with thermally treated Alloy 600 and Alloy 690 heat exchange tubes and use the latest tube flaw data available in the 2010 time frame. The methods developed were intended to address the contribution of thermally induced SGTR during severe accidents and pressure-induced SGTR during a number of design-basis accidents. The study developed the methods and the pilot applications so as to establish the framework for performing a more comprehensive probabilistic risk assessment that can address the C-SGTR at a level of detail suitable for other NRC needs. The main conclusion from this work is that the SG geometry and the fluid flow rates in different SG designs can significantly influence the potential likelihood of C-SGTRs. For the cases studied, SG designs with a shallow inlet plenum (resulting in the tubesheet located closer to the hotleg inlet) and a shorter hotleg can result in a greater likelihood of a C-SGTR following a core damage event associated with accident conditions that challenge SG tubes. A shallow inlet plenum design reduces the mixing of hot gases entering the SG, thereby creating a higher thermal load on the tubes. For the specific replacement SG geometries analyzed in this study, the Combustion Engineering plant design had an increased likelihood of a C-SGTR and a higher potential for a large early release, than the Westinghouse plant design.