Criterions of UT Thickness Measurement on Thinned Pipe Management Program

Wall thinning of carbon steel pipe components due to Flow-Accelerated Corrosion (FAC) is one of the most serious threats to the integrity of steam cycle piping systems in Nuclear Power Plants (NPP). If the thickness of a pipe component is reduced below the critical level, it cannot sustain stress and consequently results in leakage or rupture. Since the mid-1990s, secondary side piping systems in Korean NPPs have experienced wall thinning, leakages and ruptures caused by FAC. Korea Electric power Research Institute (KEPRI) and Korea Hydro & Nuclear Power Co., LTD. (KHNP) have conducted a study to develop the methodology for systematic pipe management and as a result, established the Korean Thinned Pipe Management Program (TPMP) which is being implemented to all Korean NPPs. TPMP consists of several technical elements such as prediction of the FAC rate for each component based on model analysis, prioritization of pipe components for inspection, thickness measurement, calculation of wear and wear rate for each component. Additionally, decision making associated with replacement or continuous service for thinned pipe components and establishment of long-term strategic management plan based on diagnosis of plant condition regarding overall wall thinning also are essential part of the TPMP. To effectively monitor and manage the thinning pipe components, NDE person as well as FAC engineer should understand the criterions of ultrasonic thickness measurement and there background. This paper describes the technical items of TPMP and the basis of thickness measurement criterions. This paper also shows the initial thickness variations which influence wear and wear rate calculations to obtain the reasonable integrity assessment results. Introduction Flow-Accelerated Corrosion (FAC) of carbon steel occurs due to the dissolution of the protective film that is formed on the surface and directly contacted with water. FAC is a practical threat to carbon steel piping within Nuclear Power Plant (NPP) steam cycle systems. Without a systematic pipe management program, rupture or leakage of piping or pipe components due to wall thickness reduction due to FAC is inevitable. Even a single event of a high-energy pipe rupture can be a serious accident in terms of safety and economy. This comprehensive program includes measurement of wall thickness reduction, repair and/or replacement of damaged piping components. A damaged component is defined as an element that its thickness is reduced below the required minimum thickness to sustain design pressure or is expected to reach its critical thickness within a relatively short time frame. Because these pipe components amount to more than a few thousands which have to be managed for each plant, it is an enormous task to reduce the possibility of pipe rupture due to FAC damage below a significantly low level with the use of a comprehensive pipe management program. This paper focuses on the introduction of the Korean TPMP and technical criterion for thickness measurement. Key Engineering Materials Online: 2004-08-15 ISSN: 1662-9795, Vols. 270-273, pp 2204-2209 doi:10.4028/www.scientific.net/KEM.270-273.2204 © 2004 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications Ltd, www.scientific.net. (Semanticscholar.org-13/03/20,17:38:21) Title of Publication (to be inserted by the publisher) Korean Thinned Pipe Management Program Korean TPMP consists of several technical items such as performing FAC model analyses, prioritizing pipe components for inspection, obtaining reliable thickness data, calculating the wear and wear rate, and making decisions regarding replacement necessity or continuous service acceptability based on the remaining life of the component. To make a diagnosis of plant wear level and to set up a long-term monitoring plan are also included. NSAC-202L guideline [1] was used as reference for TPMP and EPRI CHECWORKS computer code as a tool [2]. Chexal-Horowitz FAC Model The general formula of Chexal-Horowitz FAC model used in CHECWORKS is as follows [3]: W=F1(T)xF2(AC)xF3(MT)xF4(O2)xF5(pH)xF6(G)xF7(α)xF8(N2H4) (1) Where: W = FAC rate F1 (T) = factor for temperature effect F2 (AC) = factor for alloy content effect F3 (MT) = factor for mass transfer effect F4 (O2) = factor for dissolved oxygen effect F5 (pH) = factor for pH effect F6 (G) = factor for geometry effect F7 (α) = factor for void fraction effect F8 (N2H4) = factor for hydrazine effect Usage of CHECWORKS Code in TPMP CHECWORKS code can be used to predict the rate of wall thinning and remaining service life on a component-by-component basis and to evaluate the wall thickness data taken during inspections. FAC analyses using CHECWORKS are performed in three stages. These are systematic piping system classification, database creation and wear rate analysis as illustrated in Fig. 1. Fig. 1. Usage of CHECWORKS code in TPMP For the first stage, piping lines and components in steam cycle systems are classified to facilitate database creation and site application. A heat balance diagram is also generated to perform water chemistry analysis and to serve as a gateway into the database. In the second stage, design and operating data such as temperature, pressure, enthalpy, power level, water chemistry, component geometry, etc., are inputted into the plant database. Finally, the wear rate analysis based on a generated database is performed to obtain the wear rate and remaining service life for each component. Key Engineering Materials Vols. 270-273 2205