EXAMINATION OF ET-INDICATIONS OF STEAM GENERATOR …envdeg2015.org/final-proceedings/ENVDEG/presentations/ENVDEG_PPT216.pdf · TTS . End of expansion . Free span . cracks . Dented

EXAMINATION OF ET-INDICATIONS OF STEAM GENERATOR TUBES FROM THE ALMARAZ UNIT 2 NPP REPLACEMENT STEAM GENERATOR SG3

Renate Kilian, Elmar Schweitzer, AREVA GmbH, Erlangen, Germany Luis Soriano, Marta Baladia CC.NN. Alamaraz Trillo, Madrid, Spain

All rights are reserved, see liability notice.

Introduction Almaraz Units 1 and 2 are three loop PWRs designed by Westinghouse. The original steam generators (SG) of Unit 1 were replaced in 1996 and of Unit 2 in 1997 by Siemens/KWU W-61 replacement steam generators (RSG) SG 1, 2 and 3. The tubing material – Alloy 800 mod. – with a nominal outer diameter of 19.05 mm and a nominal wall thickness of 1.09 mm is shot peened.

Alamaraz 2, SG 3, Examination of Pulled Tubes- 17th EnvDeg– Renate Kilian – August 11th, 2015 - © AREVA - AL: N - ECCN: N - p. 2


Background (1/2) During the outage 2R18 in May 2009 at NPP Almaraz, Unit 2, Eddy Current Tests (ECT) were performed. These tests detected the existence of circumferential oriented crack indications at Top of Tube sheet. These indications were confirmed by Ultrasonic Tests (UT). Altogether 78 tubes were affected. These tubes were stabilized and plugged with roll expanded plugs during the outage. During the following outage 2R19 in December 2010 altogether 34 tubes with circumferential indications have been detected. These tubes were also stabilized and plugged. All circumferential indications are localized at so called dented tubes in regions of the steam generators, where hard sludge was accumulated.



Background (2/2) Since a correlation of dents and the circumferential indications seems obvious, this phenomenon needs also to be examined for a clarification of the origin of the circumferential indications. Denting in Unit 2 of NPP Almaraz was first reported for Steam Generator 3 during the outage 2R17 in April 2006, where 276 tubes on the cold leg were affected. In order to determine the cause of the ET-indications two of the tubes with circumferential indications detected during the outage 2R19 were pulled. Additionally, as a reference sample, a dented tube without a circumferential indication was pulled.



Design Detail

Almaraz Unit 2, Westinghouse design plant S Siemens/KWU RSG, start up 05/1997 5130 tubes Alloy 800 tubing, shot peened

Diameter 19.05 mm Wall thickness 1.09 mm

Full depth hydraulic expansion Mechanical expansion at both ends



Design Detail (2)

Tube-to-Tubesheet Connection at Secondary Side Alamaraz 2, SG 3, Examination of Pulled Tubes- 17th EnvDeg– Renate Kilian – August 11th, 2015 - © AREVA - AL: N - ECCN: N - p. 6


ALMARAZ 2SG3-HL

View on secondary side

05

101520253035404550556065707580859095

100105

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130

Row X

Col

Y

5130 Tube13 FDP-Bolts11 Grid-AnchorFDPDeposits 2009circumfer. Ind.Denting TSH

90°270°

RSG 2009 Visual Inspection after Tube Sheet Lancing

ODI at tube C55-L64 – Sludge pile in front of fiberscope

ODI at tube C56-L55 - Fiberscope above deposit pile, TS not visible (often)

ODI at tube C56-L65 - Fiberscope above deposit pile, TS not visible



Development of Denting

SG 1 SG 2 SG 3

Scope Denting Scope Denting Scope Denting

10/2001

04/2003

10/2004 9.3 % No Denting 9.4 % No

Denting

04/2006 33 % 276

10/2007 50.3 % 201

04/2009 53 % 285 51.4 % 364 75 % 930

11/2010 75 % 485 75 % 366 75 % 897

05/2012



+probe (MRPC) - Inspection Scope and Results - History

ISI

SG 1 SG 2 SG 3

Scope Result Scope Result Scope Result

HL CL HL CL HL CL HL CL HL CL HL CL

04/06 102 0

10/07 124 0

04/09 275 1430 1 29 142 765 2 0 754 659* 40 6

11/10 485 1.051 0 9 447 816 1 0 1.163 946 20 4

05/12 0 7 0 0 16 5

*including the scope of 2006, 3 tubes out of the 2006 scope show indications in 2009



Summary of NDT Results as of 2012

First denting detection in 2006 Amount of tubes with denting indications is stable Circumferential indications detected in 2009, number decreasing Circumferential indications on dented tubes Video after tube sheet lancing 2009 show clearly tubes with hard deposits Chemical cleaning in 2011



Destructive Examination of Three Pulled Tubes ( pulled from SG3 after inner bundle lancing in December 2010)

TTS

Free span End of expansion cracks

Dented area

Tube C52R57 with denting and OD indication (13.12.10) Tube C53R66 with denting (12.12.10) Tube C48R67 with denting and OD indications (14.12.10) – at Madrid, CIEMAT

at Erlangen, AREVA



As Received – Visual Inspection

C52R57 (Denting + Indications)

End of rolling area

C53R66 (only Denting)



As Received – Visual Inspection

C52R57 (Denting + Indications)

C53R66 (only Denting)

Cracks in circumferential orientation



EDS Analysis of Deposits R57C52



EDS Analysis of Deposits R66C53



Correlation between NDT and Tube Specimen Denting

Position Predicted depth of dent according to ET

depth of dent according to profilometric measurement

(R57C52) 0,13 mm 0,15 mm

(R66C53) 0,24 mm 0,21 mm

Surface replica and profilometric determination of dent depth

Position of ET-indication could be reproduced Comparison of depth of denting give reasonable results



Residual Stress Measuements in Axial Direction

Residual stress measurement by XRD-method (sin2Ψ) MP1 = inside tube sheet MP2, MP3 = edge of denting area MP4, MP5 = free end of tube segment Tube cut into half shelles no evaluation of circumferential stress component



Residual Stress Measuements in Axial Direction



SEM Investigation after Cleaning

Overview of crack field in the 0°-position



SEM Investigation after Cleaning, 0°-Position

Detailed of gapping crack Intergranular fracture surface



Fractography with SEM

Crack initiation site (outer surface) Crack propagation (inner surface)

Fracture characteristics completely intergranular



R57C52 - Metallographic Cross Section Overview

►Cracks are limited to a region with an axial length of approx. 1.25 mm

► Intergranular crack propagation

►Grain disintegration (intergranular attack)



Line Scan across Deposit and Interface



C53/R66 – Dented Zone

►No indications for cracks in the dented zone of tube C52/R66

►Cold worked subsurface zone due to the shot peening process



Summary 1/2

ET indications of NDE could be verified by destructive examinations – denting as well as circumferential indications Fractography and cross section examination show intergranular cracks in the dented area Circumferential cracks are limited to a region with an axial length of 1.25 mm Tensile stresses are present in a local area of the dent Intergranular stress corrosion cracking in an aggressive environment



Summary 2/2

Chemical analysis of removed deposits showed appr. 80 ppm chlorine and 80 ppm fluorine as well as 330 ppm of sulfur (calculation: 990 ppm sulfate) EDX analysis: Analysis of the deposits of Tube C48R67 carried out by Ciemat qualitatively showed the same results except for the element Na.

C52R57 C53R66

free span O, Mg, Si, S, Cl, Ca, O, Si, P, S, Cl, K, Ca, Cu, Zn, Ba, C

position of denting and crack, top of tube

sheet

Si, S, Cl, K, Ca, Cu, Zn, O, Mg, C, Ba O, Mg, Si, S, Cl, Ca, Cu

shrinked area O, Mg, S, Cl, Ca, Cu C, O, Mg, P, S, Cl, K, Ca, Cu



Tube Sheet Denting Ferritic Particles

TS denting shows a tight relationship with the presence of hard deposits

OD tube damage can occur beneath hard deposits, at dented areas if additionally an aggressive environment is formed by boiling



Discussion ODSCC 1/3 1. The material needs to be susceptible for intergranular stress

corrosion cracking. 2. Tensile stresses are necessary for the initiation of stress

corrosion cracks. 3. A specific medium must be present at the crack initiation site.



Discussion ODSCC 2/3 1. Material Alloy 800 mod. showed stress corrosion cracking in several laboratory works. In addition, as reported in some years ago, intergranular stress corrosion cracking has been observed in the field. Intergranular cracks have been found in a few Alloy 800 SG tubing in the tubesheet. 2. Tensile Stress Although the shot peening process introduces compressive stresses at the tubes outer surface, XRD measurements showed that the plastic deformation caused by denting can lead to tensile stresses in axial direction. This means denting is a necessary prerequisite for cracking.



Discussion ODSCC 3/3 3. Environment

The presence of such local chemical conditions must be presumed in the crevices formed by the hard sludge deposits.

The analyzed deposits on the outer diameter of the investigated tubes show significant amounts of S and Cl and no Na. The sulfur contents can be an indication for sulfates which can lead to local acidic conditions.

On the other hand neutral or mildly alkaline conditions with reduced S species could also occur, however, there was no indication for Na. In contradiction CIEMAT found a significant amount of Na, compared the composition of the oxide on the degraded tube with laboratory results and came to the conclusion that the environmental conditions are slightly caustic.

However it should be keep in mind that the tube examined by CIEMAT was pulled without any draining water. Therefore soluble species still remain in the deposits, which was not the case in the tubes examined by AREVA. Alamaraz 2, SG 3, Examination of Pulled Tubes- 17th EnvDeg– Renate Kilian – August 11th, 2015 - © AREVA - AL: N - ECCN: N - p. 30


Root Cause of Denting and OD Indications

CP transport

Silicates

TS deposits

Hard deposits

Oxidizing condition

Impurities (composition,

amount)

Stress OD indications

Occluded crevice

DENTING

►Moisture seperator reheater replacement in 2007

►Resin input in 1996



Conclusion The intergranular stress corrosion cracking of the examined tubes in the area of the denting was caused by a local chemically aggressive environment in combination with the axial tensile stresses due to the plastic

deformation of the tube wall by denting. This result is supported by the fact that all tubes with circumferential ET-indications showed denting, whereas a dented tube does not necessarily show ET-indications.


All rights are reserved, see liability notice. Alamaraz 2, SG 3, Examination of Pulled Tubes- 17th EnvDeg– Renate Kilian – August 11th, 2015 - © AREVA - AL: N - ECCN: N - p. 33


Editor and Copyright [Date]: AREVA GmbH – Paul-Gossen-Straße 100 – 91052 Erlangen, Germany. It is forbidden to reproduce the present publication in its entirety or partially in whatever form without prior consent. Legal action may be taken against any infringer and/or any person breaching the aforementioned prohibitions.

Subject to change and error without notice. Illustrations could be similar. The statements and information in this brochure are for advertising purpose only and do not constitute an offer of contract. They shall neither be construed as a guarantee of quality or durability, nor as warranties of merchantability and fitness for a particular purpose. These statements are based on information that was available to us at the date of publication. Only the content of the individual contracts shall be authoritative for type, quantity and properties of goods and services.


Documents

EXAMINATION OF ET-INDICATIONS OF STEAM GENERATOR …envdeg2015.org/final-proceedings/ENVDEG/presentations/ENVDEG_PPT216.pdf · TTS . End of expansion . Free span . cracks . Dented