final report 3002000647, 'materials reliability program ... · final report, october 2013 ......

Materials Reliability Program:

Evaluation of the Reactor VesselBeltline Shell Forgings of OperatingU.S. PWRs for Quasi-LaminarIndications (MRP-367)3002000647

FINAL Report, October 2013

Non-Privileged/Confidential Version for NRC

(Privileged & Confidential Version submitted to NRC underAffidavit)

EPRI Project ManagerT. Hardin

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ACKNOWLEDGMENTS

The following organizations, under contract to the Electric Power Research Institute (EPRI),prepared this report:

EPRI3420 Hillview Ave.Palo Alto, CA 94304

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Westinghouse Electric Co., LLC1000 Westinghouse DriveCranberry Twp., PA 16066

Principal InvestigatorN. Palm

This report describes research sponsored by EPRI.

This publication is a corporate document that should be cited in the literature in the followingmanner:

Materials Reliability Program: Evaluation of the Reactor Vessel Beltline Shell Forgings ofOperating US. PWRsfor Quasi-Laminar Indications (MRP-367). EPRI, Palo Alto, CA: 2013.3002000647.

iii

PRODUCT DESCRIPTION

In 2012, quasi-laminar indications were discovered in the beltline ring forgings of two Belgianpressurized water reactors (PWRs) during ultrasonic inspection (UT). This report assesses theimplications of that discovery for U.S. reactor pressure vessels.

BackgroundThe Doel 3 PWR has been operating in Belgium since 1982, Tihange 2 PWR since 1983. In2012, UT of the ring forgings that constitute the cylindrical shells of the reactor pressure vessels(RPVs) identified -8,000 (Doel 3) and -2,000 (Tihange 2) quasi-laminar (Q-L) indications.These are attributed to hydrogen flakes that developed during fabrication before the vesselsentered service. Vessel construction code inspections either did not detect or did not adequatelydocument the existence of these indications. The discovery of the indications in Belgium raisedissues related to the potential for the presence of similar indications in pressure vessels in theUnited States and the effect these indications could have on RPV integrity.

Challenges and ObjectivesThe objectives of this project were twofold:

* To review fabrication examination records to assess whether or not the beltline shellforgings of U.S. PWRs experienced the phenomenon observed in the Belgian reactorvessel forgings

* To evaluate the structural significance (e.g., safety significance) of the Q-L flawcondition observed at Doel 3 and Tihange 2 were that condition to exist in a U.S. PWRbeltline ring forging

ApproachNuclear reactor vessels are inspected at time of fabrication according to applicable constructioncodes, and the inspection records are required to be retained for the life of the plant. Plant staffretrieved the inspection records (data sheets) for each U.S. PWR with beltline ring forgings andreviewed them to assess the potential for the presence of the phenomenon observed at theBelgian reactors. At the same time, the PWR Owners Group (PWROG) conducted ultrasonicmodel simulations to demonstrate the laminar flaw detection capability of typical US inspectionpractices required to be performed at the time of fabrication. Concurrent with these efforts, theMaterials Reliability Program (MRP) conducted a bounding probabilistic fracture mechanics(PFM) analysis to assess the structural significance of such indications were they to exist in aU.S. plant. The approach for this analysis was consistent with analyses for pressurized thermalshock (PTS) transients performed by the Nuclear Regulatory Commission (NRC) staff indevelopment of the Alternate PTS Rule, 1OCFR50.61a.

Results and FindingsA review of U.S. reactor vessel fabrication data found that twenty-one operating U.S. PWRs andno BWRs have beltline ring forgings. Owners of the 21 PWRs retrieved fabrication inspectionrecords - e.g., the data sheets from UT inspections mandated by applicable codes and standards.The PWROG assessment of the ability of the inspections to detect and report quasi-laminar flawssuch as observed in the Belgian plants concluded that the methods employed were adequate todetermine if extensive small laminations were present and that the reporting requirements in theengineering specifications of all plants would have caused such a condition to be recorded. Plantstaff reviewed fabrication UT examination data sheet records: there were no recordableindications, thereby establishing that the condition in the Belgian plants does not exist in U.S.PWR beltline ring forgings. Finally, MRP performed a bounding safety assessment for a vesselpostulated to have a large number of Q-L indications in the beltline shell forgings. The boundingassessment included a PFM analysis of the U.S. PWR with highest mean reference temperaturein a beltline ring forging at the end of a sixty-year license and determined that the incrementalrisk would be a factor of 10 below the risk criteria set by the USNRC. Therefore, it is unlikelythat conditions similar to those observed at Doel 3 exist in U.S. PWRs; and even if substantial Q-L indications are postulated to exist in beltline ring forgings in U.S. PWRs, the potential forvessel failure is acceptably low.

Applications, Value, and UseResults of this work will be used to demonstrate that U.S. PWRs with beltline ring forgings donot have an extensive number of Q-L indications and continue to meet applicable safety riskgoals through the extended license.

KeywordsPWRQuasi-laminar indicationHydrogen flakingDoel 3Reactor vessel integrityPressurized thermal shock

vi

EXECUTIVE SUMMARY

In mid-2012, the Belgian utility Electrabel performed an ultrasound examination of the entirevolume of material that surrounds the nuclear core in the reactor pressure vessel (RPV) at theDoel Nuclear Power Plant, Unit 3 (Doel 3). This examination revealed the presence of 857 and7205 buried flaw indications in the vessel upper and lower core shells, respectively [1].Electrabel reported that the indications are quasi-laminar - i.e., generally, parallel to or slightlyinclined relative to the inner surface of the RPV wall: the indications are also approximatelycircular in shape and most of them have diameters of 10 nmm or less, with a maximum diameterof 70 mm [1]. Subsequently, an inspection of the Tihange Unit 2 RPV - fabricated by the samemanufacturer and under the same contract and at the same time as the Doel 3 vessel - foundsimilar, but fewer indications.

The RPV at Doel 3 was fabricated using forged low alloy carbon steel (SA 508, Class 3) shellcourses connected with circumferential welds. Electrabel attributes the indications to thepresence of hydrogen flakes, which developed during vessel fabrication and prior to operation[1]. These hydrogen flakes result from small ruptures in the carbon steel material produced bythe release of high-pressure hydrogen gas, which was trapped in the metal during the steelmaking process. The indications detected in 2012 either were not detected or not reported duringthe 1975 manufacturing or preservice inspection of the Doel 3 RPV [1]. The hydrogen flakes arereported not to have grown during subsequent plant operation [1].

Various deterministic and probabilistic fracture mechanics analyses performed by Electrabeldemonstrated acceptable margins against RPV failure for the indications detected in the forgedcore shells at Doel 3 and Tihange 2 [1]. Both units were restarted in 2013.

Although similar indications have not been detected during construction, preservice, or inserviceexamination of RPVs in the United States, the work described in this report was initiated toassess the implications for U.S. reactor vessels which, like Doel 3 and Tihange 2, weremanufactured with beltline ring forgings. There are 21 operating pressurized water reactors(PWRs) and no boiling water reactors (BWRs) with beltline ring forgings in the United States.

To determine the relevance of the Doel 3 operating experience (OE) to the U.S. fleet, thefollowing activities were performed:

The owners of affected plants retrieved and reviewed the records of non-destructiveexaminations (NDE) that were conducted on vessel beltline ring forgings at the timeof fabrication. The objective of the reviews was to determine if the Doel 3phenomenon was observed in U.S. vessels. The PWR Owners' Group supported thisactivity by assessing the detection capabilities of the various UT inspectiontechniques used during fabrication of the beltline ring forgings in currently-operatingU.S. vessels to establish the relevance of the NDE records. The inspection techniques

vii

described by Electrabel for the Doel 3 shop and inservice inspections were alsobenchmarked in order to provide a direct comparison. The results of this effort werepublished in WCAP-17786-NP, Revision 1. All U.S. inspection techniques employedduring fabrication were shown to have the ability to detect indications such as those atDoel 3 and Tihange 2 if such flaws had existed in U.S. vessels.

The owners of all operating PWRs with beltline ring forgings retrieved and reviewedapplicable fabrication inspection records; all plant records and specifications wereconsistent with and were bounded by the assumptions in the capabilities studydocumented in WCAP- 17786-NP, Rev. 1. In all cases, quasi-laminar indications suchas observed at Doel 3 would have been detected and recorded; there were norecordable indications' in any vessels. This provides high confidence that thephenomenon observed at Doel 3 / Tihange 2 is not present in the U.S. PWR fleet ofRPV beltline ring forgings.

The Materials Reliability Program (MRP) performed a bounding probabilisticfracture mechanics (PFM) analysis using a conservative distribution of postulatedquasi-laminar indications and the methodology consistent with that used by the U.S.NRC in development of the Alternate PTS Rule, 10 CFR 50.61 a, "Alternate FractureToughness Requirements for Protection Against Thernal Shock Events" [2]. Thisanalysis indicated that the 95% through-wall cracking frequency (TWCF) for theRPV beltline forging with the limiting reference temperature for nil ductilitytransition (RTNDT) for the population of U.S. plants with beltline ring forgings isapproximately 1.OE-7 per operating year through the end of extended life. Thisfrequency of failure satisfies the risk goal used by the NRC to define the acceptancecriteria in 1OCFR50.61 (a), which is:

95 percentile through-wall cracking frequency < 10-6 per operating year

In addition, there is tolerance for approximately ten times the number of indicationsassumed in this study before the risk goal is reached.

Based on the efforts described above, it can be concluded that it is very unlikely that thecondition observed at Doel 3 exists in U.S. PWRs: further, it would not have structuralsignificance even if it did.

A recordable indication is a relevant indication that exceeds the specified recording threshold.There are significant differences between the recording criteria and the rejection criteria.

viii

CONTENTS

1 INTRODUCTION AND OBJECTIVES .................................................................................. 1-1

The Operating Experience at Doel 3 and Tihange 2 ............................................................ 1-1

Safety Concern and Im plications for Other Reactor Vessels ............................................... 1-2

Objectives of This Report .................................................................................................... 1-3

2BACKGROUND ................................................................................................................... 2-1

Licensee's Root Cause Evaluation, Structural Evaluation, and Conclusions ....................... 2-1

Evaluation Roadmap ...................................................................................................... 2-1

Manufacturing History of the Vessels .............................................................................. 2-1

Root Cause Evaluation / Origin of the flaws .................................................................... 2-2

Structural Evaluation ....................................................................................................... 2-2

Deterministic approach .............................................................................................. 2-2

Probabilistic approach ................................................................................................ 2-3

R e s o lu tio n ........................................................................................................................... 2 -4

H y d ro g e n F la k in g ................................................................................................................ 2 -4

C a u se s o f F la k in g ........................................................................................................... 2 -4

Metallurgical Characteristics of a Large-Forging Ingot .................................................... 2-5

M itig a tio n o f F la k in g ........................................................................................................ 2 -5

Production of a Ring Forged Product from an Ingot ........................................................ 2-5

Hydrogen Flaking and the ASME Code .......................................................................... 2-6

3 U.S. INDUSTRY RESPONSE TO THE GENERIC IMPLICATIONS OF THE DOEL 3

RPV EXPERIENCE ................................................................................................................. 3-1

U.S. Industry Response to the Doel 3 OE ........................................................................... 3-1

Meeting with U.S. NRC ................................................................................................... 3-1

O b je c tiv e s ........................................................................................................................... 3 -2

S c o p e .................................................................................................................................. 3 -2

A p p ro a c h ............................................................................................................................. 3 -5

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4 EVALUATION OF FABRICATION NDE RECORDS ...................................................... 4-1

Background and O bjective .................................................................................................. 4-1

PW RO G Assessm ent of NDE M ethods and Capabilities ..................................................... 4-2

Results of PW RO G Study ............................................................................................... 4-2

PW R NDE Records Review / Results .................................................................................. 4-3

5 BOUNDING STRUCTURAL EVALUATION FOR U.S. VESSELS (PROBABILISTIC

ANALYSES) ............................................................................................................................ 5-1

Description of Q uasi-Lam inar Indications at Doel 3 ............................................................. 5-2

Assessm ent of Postulated Q uasi-Lam inar Indications at U.S. Plants .................................. 5-8

Purpose of this W ork ...................................................................................................... 5-8

A p p ro a c h ........................................................................................................................ 5 -8

A n a ly s is In p u t ................................................................................................................. 5 -9

RPV Dim ensions and M aterial Properties .................................................................. 5-9

Postulated Transient Events .................................................................................... 5-10

Neutron Fluence Distribution .................................................................................... 5-11

Distribution of Q uasi-Lam inar Indications ................................................................. 5-11

Analysis Procedure ....................................................................................................... 5-13

S o ftw a re ....................................................................................................................... 5 -1 4

R e s u lts .............................................................................................................................. 5 -1 5

Flaw Tolerance M argins .................................................................................................... 5-16

Contribution to 95% TW CF from Other Flaw Types ...................................................... 5-29

PFM Analyses Sum m ary and Conclusions ........................................................................ 5-29

6 SUMMARY OF EVALUATION AND CONCLUSIONS ..................................................... 6-1

7 REFERENCES ..................................................................................................................... 7-1

A VERIFICATION AND VALIDATION OF SOFTWARE ER-1, REV. 2 ............................... A-1

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LIST OF FIGURES

Figure 1-1 Illustration of Flaw Orientation in RPV Wall ............................................................. 1-3

Figure 1-2 Cross-section of a typical flake (9,081.88 pm, -9 mm) ............................................ 1-3

Figure 2-1 Illustration of classic segregation microstructures in static cast hot toppeds te e l in g o t ......................................................................................................................... 2 -7

Figure 2-2 NRC Slide showing the Manufacturing Steps for RPV Forgings [9] ......................... 2-8

Figure 3-1 U SN R C Plans (M arch 2013) [9] .............................................................................. 3-2

Figure 5-1 Illustration of the Azimuthal and Radial Distributions of Indications Detected inthe Doel 3 Reactor Pressure Vessel [20] .......................................................................... 5-3

Figure 5-2 Number of Indications Detected in the Upper (blue bars) and Lower (red bars)Forged Core Shells at Doel 3 as a Function of Distance from the Vessel InnerS u rfa c e [1 ] ........................................................................................................................ 5 -4

Figure 5-3 Maximum Indication Size as a Function of Distance from the Vessel InnerS urface, D oel 3 Low er C ore S hell [1] ............................................................................... 5-5

Figure 5-4 Illustration of the Typical Shape and Maximum Length (X or Y) (mm) of aHydrogen Flake Determined from Destructive Examination of Archival Material [1] .......... 5-6

Figure 5-5 Illustration of the Inclined Angles Relative to the Vessel Inner Surface andDistance S from the Clad Base-Metal Interface and the Inner Edge of the Indication[1 ] ..................................................................................................................................... 5 -7

Figure 5-6 Overview of the Approach for Determining the 95% TWCF for a ReactorPressure with a Postulated Quasi-Laminar Flaw Distribution .......................................... 5-18

Figure 5-7 Maximum Ring Forging RTNDT Distribution for the Population of U.S. PWRshaving RPVs with Forged Beltline Rings ........................................................................ 5-19

Figure 5-8 Relative Fluence Map and Histogram Used in the PFM Calculations [32] ............. 5-20

Xi

LIST OF TABLES

Table 3-1 Operating U.S. Reactor Pressure Vessels Fabricated from Forging Materials ......... 3-4

Table 3-2 U.S. Operating PWR Forged Vessel Population by Fabricator and Designer ........... 3-5

Table 4-1 Results of Fabrication NDE Records Reviews at Operating U.S. PWRs .................. 4-4

Table 5-1 Vessel Dimensions and Irradiation Degradation Variables used in the PFMA n a ly s e s ........................................................................................................................ 5 -1 0

Table 5-2 Representation of the Distribution of the Number of Indications as a Functionof Indication Length and the Distance from the Vessel Inner Surface to the InnerIndication Edge - Developed from the Indications Detected in the Lower Core Shellin the Reactor Pressure Vessel at Doel 3 ....................................................................... 5-21

Table 5-3 Postulated Distribution for the Number of Detected Indications as a Functionof Indication Length, Inclined Angle and Through-wall Depth, and Distance from theVessel Inner Surface to the Inner Indication Edge .......................................................... 5-22

Table 5-4 Summary of the Distribution of Postulated Clustered Indications ........................... 5-23

Table 5-5 Postulated Distribution for the Number of Clustered Indications as a Functionof Indication Length, Inclined Angle and Through-wall Depth, and Distance from theVessel Inner Surface to the Inner Indication Edge .......................................................... 5-24

Table 5-6 Postulated Distribution for the Number of Clustered and Individual Indicationsas a Function of Indication Length, Inclined Angle and Through-wall Depth, andDistance from the Vessel Inner Surface to the Indication Flaw Edge .............................. 5-25

Table 5-7 Calculated Values of the 95% TWCF per Indication as a Function of IndicationLength, Inclined Angles and Through-wall Depth, and Distance from the VesselInner Surface to the Indication Inner Edge ..................................................................... 5-26

Table 5-8 95% TWCF Contribution from the Postulated Number of Clustered andIndividual Indications at Each Flaw Inner Edge Location and the Total 95% TWCFfo r th e D is trib u tio n .......................................................................................................... 5 -2 7

Table 5-9 95% TWCF Contribution from the Postulated Number of Clustered Indicationsat Each Inner Flaw Edge Location and the Total 95% TWCF for the Distribution ........... 5-28

xiii

1INTRODUCTION AND OBJECTIVES

The Operating Experience at Doel 3 and Tihange 2

The Doel 3 and Tihange 2 Nuclear Power Plants are standard 3-loop Westinghouse design typepressurized water reactors (PWRs) [1]. The reactor pressure vessels (RPVs) at both units werefabricated by Rotterdamsche Droogdok Maaschappij/Rotterdam Nuclear (RDM/RN) during theperiod 1974-1976 under the same contract. The cylindrical shells of both vessels were fabricatedusing forged low alloy carbon steel shell courses (ASME Material Specification SA508 Class 3)connected with circumferential welds. The units began operation in 1982 and 1983, respectively.

In mid-2012 the Belgian utility Electrabel performed ultrasonic testing (UT) examinations of theinner 30 nu-n of the Doel 3 vessel beltline shell (i.e., the layer closest to the inner diameter, orID). The examinations were intended to determine if the Doel 3 vessel had underclad cracking 2

similar to that detected at the Tricastin nuclear power plant (France) in 1999. The inspection atDoel 3 examined large areas of the base metal not normally inspected during the 10-yearinservice inspections performed in compliance with the American Society of MechanicalEngineers (ASME) Boiler and Pressure Vessel Code, Section XI, Division I, "Rules for InserviceInspection of Nuclear Power Plant Components" [3]. The ASME Code inspections focusprimarily on the vessel welds and the areas immediately adjacent to the welds.

No underclad cracking was found at Doel 3, but a large number of other unexpected indicationswere observed. Therefore Electrabel performed inspections of the full thickness of the vesselwall using the UT method employed for ASME Code inspections of the vessel welds.Approximately 8,000 quasi-laminar (Q-L) indications were detected in the base metal (remotefrom the welds) with a through-wall depth extending from several millimeters (mm) beyond theclad-base metal interface (CBMI) to about 60% of the wall thickness from the inner surface [1].The indications were circular in shape and nearly laminar in orientation: most of the indicationshave diameters of 10 mm or less, with a maximum diameter of 70 mm [1]. A conservativerepresentation of the distribution of indications found at Doel3 was developed for use in theintegrity assessment of RPVs in the United States and is provided in Section 5.

Electrabel's root cause analysis determined that the indications are likely the result of hydrogenflaking, which developed subsequent to the forging process and prior to operation [1]. Hydrogenflakes are small, compressed cracks that are formed when hydrogen trapped in the metal matrixduring the steel making process precipitates and accumulates at segregates, causing pressure tobuild until a small rupture in the carbon steel material occurs and relieves the pressure (seeSection 2 for additional information on hydrogen flaking). Large forgings can be susceptible tohydrogen flaking because it is difficult for hydrogen to diffuse out of a large forging duringsolidification. The flaw size and orientation of the indications observed at Doel 3 are consistent

2 Small cracks at the interface where the stainless steel cladding is deposited onto the inner surface

of the carbon steel vessel wall (the clad base metal interface, CBMI); see Reference 2

1-1

Introduction and Objectives

with previous observations of hydrogen flaking by AREVA (consultant to Electrabel) in arecently fabricated ring forging that had been rejected due to hydrogen flaking. Also, the spatialdistribution of the indications is consistent with the known propensity of flaking to concentrate inthe areas of segregation in a forging and is consistent with the spatial distribution of segregates inthe Doel 3 vessel, as confirmed by retrospective mapping of carbon segregates in the Doelforgings [1].

After the discovery at Doel 3, Electrabel inspected Tihange Unit 2 because the Doel 3 andTihange vessels had been ordered and manufactured together, according to the same contract andspecifications. Similar, but fewer, indications were discovered in the Tihange 2 vessel [4].

The indications detected in 2012 either were not detected or not reported during the finalconstruction inspection of the Doel 3 RPV in 1975 [1]. However, there are some records ofearlier (interim) inspections that recorded a limited number of flaws consistent with andsuggestive of the overall pattern of the 2012 flaw distribution, suggesting detection (if notrecording) during fabrication. It is unknown why the final, formal inspection did not record thoseindications.

Safety Concern and Implications for Other Reactor Vessels

Failure of a reactor pressure vessel is a beyond-design-basis event, and acceptable marginsagainst pressure vessel failure must be maintained throughout its service life. In RPV integrityregulations, propagation of a flaw through the vessel wall is conservatively assumed to causeRPV failure. Planar flaws are flaws whose surfaces are perpendicular to the direction ofmaximum stress in the vessel wall. Because they experience high stress, planar flaws have thepotential to grow through the vessel wall and cause vessel failure. Laminar flaws are flawswhose surfaces are parallel to the maximum stresses in the vessel wall. Consequently, thestresses acting on the surface of a laminar flaw are very small; and essentially there is nopotential to grow a laminar flaw through the vessel wall.

Laminar flaws are not a concern for vessel integrity. NUREG/CR-6817, Rev. 1 [5] states:

"It has also been observed that the largest flaws in plate and forging materials haveorientations parallel to the surfaces of the vessel. This orientation is related to the rollingand forming operations used to fabricate the vessel plates and forged rings. Althoughthese laminar-type flaws can be quite large, they have no significance to vessel integrity.In this discussion, the base metal flaws of concern are therefore only those flaws thathave some through-wall dimension."

Quasi-laminar (Q-L) flaws have a small through-wall (planar) dimension that must be consideredin vessel integrity analysis. Therefore it was necessary for Electrabel to assess the potential forthese flaws to grow and fail the vessel, particularly if the vessel were subjected to large thermaltransients such as pressurized thermral shock (PTS).

The discovery of Q-L indications or flaws 3 at these two Belgian reactors had implications forother vessels fabricated from beltline ring forgings. If the Q-L indications are a result of

3 Because Electrabel has attributed the indications to hydrogen flaking, this report will use theterms "indication" and "flaw" interchangeably when discussing the Doel 3 Q-L indications.

1-2


hydrogen flaking, then the potential for and structural significance of undetected flaking in otherRPVs fabricated from forged rings should be assessed. There are 21 operating pressurized waterreactors (PWRs) in the United States. whose vessel beltlines are constructed from forged rings.

Flaw Orientation

Typical Flake Orientation Planar FlawObserved at Doel-3' Perpendicular to OD/ID

Figure 1-1Illustration of Flaw Orientation in RPV Wall

I

Figure 1-2Cross-section of a typical flake (9,081.88 pm, -9 mm)

Copyright Electrabel Generation BeLux. Reprinted with permission

Objectives of This Report

The objectives of this report are to address the implications of the Q-L indications found at Doel3 and Tihange 2 for the U.S. reactor vessels with forged core shells (i.e., beltline ring forgings).The specific objectives are:

1. Review the Doel 3/Tihange 2 service experience (discovery of Q-L indications) andprovide background information on the phenomenon to which it has been attributed(hydrogen flaking).

2. Review the fabrication and inspection records of U.S. PWRs with beltline ringforgings and determine the likelihood that the vessel beltline ring forgings have Q-Lindications.

1-3


3. Assess the structural significance of the presence of a population of postulated Q-Lindications to demonstrate compliance with the NRC safety goal, which is a through-wall cracking frequency < 10-6 per reactor year through the end of extended license.

1-4

2BACKGROUND

Various deterministic and probabilistic fracture mechanics analyses performed by Electrabeldemonstrated acceptable margins against RPV failure for the indications detected in forged coreshells at Doel 3 and Tihange 2. A brief overview of the work is provided below. Detaileddescriptions of the evaluations are provided in Electrabel's thorough Safety Case Reports [1,4]and Safety Case Addenda [6,7]; the Belgian regulator's evaluations are documented in [8].

Licensee's Root Cause Evaluation, Structural Evaluation, and Conclusions

Evaluation Roadmap

Following the discovery of the numerous quasi-laminar indications during ultrasonicexaminations of the Doel 3 and Tihange 2 reactor vessel forgings in 2012, Electrabel generated aroadmap (action plan) to perform a comprehensive safety assessment of the vessels [1]. Theobjectives of the roadmap were to perform a root cause analysis, validate use of the weldinspection technique for the characterization of base metal flaws, establish materials propertiesby testing archive materials and similar materials with known hydrogen flakes, and perform aconservative structural integrity assessment to demonstrate ASME Code and other safety criteriaare met.

GDF Suez, parent company of Electrabel, assembled an international team of experts to assistand independently review the safety assessment activities. Separately, the Belgian nuclear safetyregulator, the Federal Agency for Nuclear Control (FANC), reviewed the safety case work andassembled its own team of international independent review experts, including members of theU.S. Nuclear Regulatory Commission staff [9].

Ultimately, Electrabel published Safety Case reports [1,4] to demonstrate the acceptability of theDoel 3 / Tihange 2 vessels for continued operation. These reports were made available to thepublic on the FANC website. Highlights and significant conclusions from the Safety Case reportsare summarized below.

Manufacturing History of the Vessels

The Doel 3 and Tihange 2 RPVs were manufactured according to the ASME Boiler & PressureVessel Code, Section III for Class 1 components, Edition 1974 up to and including the Summer1974 Addenda [10]. In addition, certain conditions from the Belgian law were also applied.

The fabrication steps most relevant to hydrogen flaking (e.g., main forging operations, heattreatment, and NDE) were performed by Rotterdamsche Droogdok Maatschappij/RotterdamNuclear (RDM/RN), which is no longer in business. RDM/RN forged the ingots, which wereprovided by a steelmaker in Germany, Friedrich Krupp Hiittenwerke. Other manufacturing steps(e.g., application of cladding, assembly of vessel head) were carried out by other organizations

2-1

Background

(e.g., Cockerill and Framatome). The cylindrical shells of both vessels were fabricated from typeSA-508 Class 3 steel; the top and bottom heads were fabricated from formed SA 533 Class 1plate material [1]. It has been reported that these were the only SA-508 Class 3 vesselsmanufactured by RDM/RN [ 11].

Electrabel reviewed all available fabrication records and determined that fabrication met allapplicable codes and standards. However, no explanation was given for failure of themanufacturing examinations to report the indications. An expert working group reviewed theRDM fabrication UT procedures and concluded that the flaws found in 2012 should have beendetected and reported during manufacture of the vessels [9].

Root Cause Evaluation / Origin of the flaws

Electrabel asserts a high level of confidence that the cause of the indications is hydrogen flaking.That determination is based on an extensive review of the literature, a thorough evaluation of allalternative possible causes, and a detailed evaluation supplied by AREVA. There is a consensusamong the licensee (Electrabel), the Belgian regulator (FANC), and most foreign regulators(including the U.S. NRC) that the most likely origin of the indications is hydrogen flaking fromthe manufacturing process. Significant factors that support hydrogen flaking as the causeinclude:

" The large number, size (typically 4-14 mm), and orientation of the indications areconsistent with flaking in a forged ring.

* The UT patterns observed in Doel 3 / Tihange 2 (indications flat and separated from eachother) were similar to the UT patterns observed in a rejected forging that was examinedby UT and then destructively examined and verified to have hydrogen flaking.

* A link was established between the profile (distribution) of the indications in the vesselwall and the expected positive macro-segregation profile in the forging, obtained from aretrospective carbon mapping analysis.

Electrabel deliberately considered many other possible explanations for the indications, butexcluded each alternative based on the available information and observations [1].

Structural Evaluation

Electrabel conducted a Structural Integrity Assessment (SIA) that included a deterministicanalysis of the flaws and a complementary probabilistic analysis. Both analyses concluded thatapplicable safety criteria are met and the vessels are fit to continue service. The analyses aredetailed in references [1] and [4].

Deterministic approach

Flaw analytical evaluation was conducted to demonstrate the criteria of ASME XI IWB-3600,"Analytical Evaluation of Planar Flaws," were satisfied, using the methods of ASME Section XI,Nonmandatory Appendix A, "Analysis of Flaws."

The high number and dense grouping of the flaws posed a challenge, and Electrabel determinedthat "flaws found in the Doel 3 RPV fall outside of the ASME XI inservice framework." [I]

2-2

Background

Therefore, some modified analytical rules were developed, intended to meet the intent of theCode criteria. These included: modified rules for the grouping of closely spaced flaws, based onASME XI IWB-3300 (flaw characterization), and modified rules for demonstrating the flawacceptability, based on ASME XI IWB-3600 (analytical evaluation of flaws). The modified rulesare described in the Safety Case report [1]. It is noted here that the possibility of adopting somealternative flaw grouping criteria in ASME XI is being studied, in case such criteria are neededin the future.

Once the modified grouping / proximity criteria were adapted, the structural integrity assessmentproceeded:

" Flaw acceptability analyses were conducted. The analyses accounted for the effects ofinclination, RTNDT, and position (proximity to vessel inside surface). All flaws wereshown to be acceptable per IWB-3612, "Acceptance Criteria Based on Applied StressIntensity Factor."

" ASME III Primary stress limits of the vessels were re-evaluated considering the presenceof the flakes. The collapse load acceptance criterion of ASME Section III NB3228.3 wasmet for the most penalizing flaw configuration.

* A Fatigue Crack Growth analysis was conducted and showed no significant growth of theflakes over the life of the plants (<1.72% growth in the core shells).

Probabilistic approach

Electrabel's Safety Case included probabilistic fracture mechanics (PFM) analyses for Doel 3and Tihange 2 following the methodology of the Alternate PTS Rule, 10 CFR 50.61 (a) [12].Because the Belgian regulator does not yet accept probabilistic analyses as a basis fordemonstrating vessel integrity, the PFM analysis was performed as a complementary analysisonly.

The PFM analyses used the methodology described in NUREG- 1806 "Technical Basis forRevision of the Pressurized Thermal Shock (PTS) Screening Limit in the PTS Rule (10 CFR50.61)" [ 13], with some modifications of the NRC methodology:

* The flaw distributions pre-programmed into the FAVOR code were replaced by theactual Doel 3/Tihange 2 flaw characteristics from the 2012 inspections.

* PTS event frequencies specific to Doel 3/Tihange 2 were used.

* An additional temperature shift of 50'C was applied to the irradiated RTNDT of the coreshell forgings to account for the presumed effect of macrosegregations on fracturetoughness.

" Acceptance criterion was Frequency of Crack Initiation (FCI) < 10-6 / reactor year,

instead of Through Wall Cracking Frequency (TWCF) < 10-6 / reactor year.

" No credit was taken for Crack Arrest or Warm Pre-Stress.

Electrabel performed the PFM analyses using the software program "Fracture Analysis ofVessels - Oak Ridge" (FAVOR) [14]; Oak Ridge National Laboratory (ORNL) was contracted

2-3

Background

for some support activities [11]. The results of the analysis for Doel 3 was FCI = 2.3 x 108 /reactor year, which meets the acceptance criterion: for Tihange 2, FCI = 1.4 x 108 / reactor year[1,4].

Resolution

In early December 2012, Electrabel submitted a technical dossier on Doel 3 / Tihange 2 RPVs tothe Belgian regulator, and concluded "...the vessels' structural integrity meets, within significantmargins, all safety criteria for each of the detected indications."

FANC provisionally agreed, but requested requirements for additional testing and examinationbefore the units would be permitted to restart. The additional prestart requirements [15] included:fracture toughness testing of samples with hydrogen flakes (toughness ahead of a flake andtoughness between the flakes); demonstration that flaw under-counting would not compromisethe safety case; and performance of load tests of both vessels with acoustic emission testing.

Electrabel completed the restart prerequisites in April 2013. In May 2013 the regulatorauthorized restart of both units [16], which was subsequently accomplished.

Hydrogen Flaking

Hydrogen flaking refers to the delayed formation of hairline cracks, shatter cracks, or flakesinternal to a forging: the phenomenon is delayed because it occurs after an incubation periodafter the material has cooled below 200'C [17]. Flaking is a recognized issue in the production oflarge forgings and certain measures (discussed below) are required to prevent it. Hydrogenflaking was certainly an issue known to RDM/RN during the fabrication of the Doel 3 andTihange 2 reactor vessels; several components destined for those vessels were rejected byRDM/RN due to hydrogen flaking [18].

Causes of Flaking

Whether or not hydrogen flakes form in a forging depends on a number of factors, includinghydrogen content, segregation of solute elements in the steels, alloying elements, sulfur, and heattreatment [17]. Stress is also key factor affecting the phenomenon [19]; the stress may be causedby the work imparted on the material during forging or simply by the too-rapid cooling of a largeingot.

Flaking had been observed in forgings since the 1920s, but it was not until the 1930s that the roleof hydrogen becamne understood [19]. Hydrogen is introduced into steel in the melting processand is soluble in molten steel; it becomes progressively less soluble as the steels cools andtransforms first to austenite and then to ferrite. As the steel cools and transforms, monatomichydrogen comes out of solution and accumulates at traps formed by discontinuities such asnonmetallic inclusions in the steel matrix (e.g., manganese sulfide (MnS) inclusions). Themonatomic hydrogen becomes H2, and pressure builds at these locations. Eventually, thepressure may be sufficient to break the bonds of the steel, causing a local rupture or fissure(crack). The H2 expands into the fissure which reduces the pressure, after which there is nofurther growth of the fissure. The flakes have a typical shape and dimension of 4 -14 mm (as

2-4

Background

observed at Doel 3 and Tihange 2); and when conditions are met for flaking, the flakes tend toappear in large numbers [20]. Flaking is an internal phenomenon; flakes do not intercept thesurface of a forging.

Metallurgical Characteristics of a Large-Forging Ingot

During cooling and solidification of an ingot, segregation of solute elements occurs because theirsolubility in the solid is less than their solubility in the liquid [17]. This results in regions ofnegative segregation and regions of positive segregation. Figure 2-1 [23] shows the types ofmacrosegregation in static cast hot topped steel ingots: hot top segregation, bottom negativesegregation, A-shaped segregation (positive segregation), and V-shaped segregation (negativesegregation) [21]. Hydrogen (and thus hydrogen flaking) tends to be located in zones of positivesegregation. The tendency to flake increases as a function of ingot size because it is moredifficult for the hydrogen to diffuse out as the size increases [17]. Reactor vessel shell forgingsare created from some of the largest ingots manufactured [22].

Mitigation of Flaking

Vacuum degassing is used to reduce hydrogen content in the steel when the ingot is poured andthus to reduce flaking. It was introduced in the late 1950s in response to some dramatic,catastrophic turbine rotor failures. Material specifications for SA 508 for Class 1 vessels requiredegassing be used for all RPV forging steels.

Because stress as well as hydrogen plays a key role in the formation of flakes, heat treatmentscan also be used to mitigate the tendency of a forging to flake. However, Fruehan [ 17] makes thecase that it is impractical to heat treat large forgings for the period necessary for hydrogen todiffuse out.

Production of a Ring Forged Product from an Ingot

To understand the potential effects of hydrogen flaking on RPV integrity, it is useful to reviewthe typical Thermo-Mechanical-Process (TMP) for production of a reactor vessel shell forging:

" A static cast ingot is first upset along its long axis (see Figure 2-2).

* A drawing operation may then occur to bring the ingot (now termed bloom) back to somemulti-sided cylindrical shape (to billet).

" The billet would then receive a second upset to a reduced height.

* A center hole is pierced or trepan forged out of the center (and that material is discarded).

* Finally, the trepanned billet can either be saddled forged, mandrel drawn, or ring rolled toa ring.

The piercing or trepan operations mostly remove the V-shaped segregates and move the Asegregation patterns near to the inner surface of the ring [21 ]. The final forging process used toproduce the ring will determine how parallel inclusions or flakes are to the inside surface of thering. A large, adequately sized ring roll press can produce a very uniform inclusion or flakedistribution. Mandrel drawing or saddle forging a ring can impart less directionality to inclusions

2-5

Background

or flakes due to its incremental nature: depending on the amount of overlap used betweenincremental forge steps, a more discontinuous and non-planar distribution'of inclusions or flakescan result [23]. (This explains the quasi-laminar nature of some flakes.) As with any layeringprocess, the greater the reduction in thickness, the more uniformly oriented and compressed (inthe working direction) are the hydrogen flakes. As noted by the techmical subsidiary of theBelgian regulator, BelV [20], the quasi-laminar orientation of the Doel 3 / Tihange 2 flaws isconsistent with the fact that hydrogen flakes are generated preferentially on MnS inclusions,oriented circumferentially, and flattened by the forging operation.

Hydrogen Flaking and the ASME Code

There is no specific provision in Section III of the ASME Boiler & Pressure Vessel Coderegarding hydrogen flaking in RPV forgings. As will be discussed in Section 4 of this report, theCode requires UT inspection during fabrication; and the rejection criteria may not necessarilycause a flaked component to be rejected. A retrospective evaluation suggests that the flaking inDoel 3 and Tihange 2 would have been detectable and recordable, but acceptable [18].According to the Code, indications/flaws detected but accepted under Section III areautomatically considered acceptable under Section XI [3]. If the Doel 3 / Tihange 2 indicationshad been properly reported in the fabrication records, the operational impact may have beenminor. However, the Belgian regulator (and others) have observed that "the presence of suchflaws does not comply with the quality level expected for a reactor pressure vessel." [18] It isconsidered highly unlikely that a component with such imperfections would, have been acceptedby any owner had the indications been properly recorded and reported, even if they were notrejectable under ASME III acceptance criteria.

2-6

Background

Hot top segregation

A segregation '/ '\

V segregation

WBottom negativesegregation

Figure 2-1Illustration of classic segregation microstructures in static cast hot topped steel ingot

2-7

Manufacturing of Forgingsfor Reactor Vessels

SU.S.NRCUnited States NudI.r Regulatory Commission

Ingot

(1) Upsetting

(2P PierG2ng

PAGE 2ý

Figure 2-2NRC Slide showing the Manufacturing Steps for RPV Forgings [9]

2-8

3U.S. INDUSTRY RESPONSE TO THE GENERICIMPLICATIONS OF THE DOEL 3 RPV EXPERIENCE

Electrabel informed the U.S. industry of the Doel 3 inservice inspection results in August 2012[24]. The Pressurized Water Reactor Materials Management Program (PMMP) then initiated aprogram to assess the implications of the inspection results for the operating U.S. PWRs withbeltline ring forgings.

U.S. Industry Response to the Doel 3 OE

In response to the Doel 3 RPV experience, utility executives on the PMMP Executive Committeerequested that the Materials Reliability Program (MRP) investigate the implications for U.S.PWRs with beltline ring forgings. The MRP initiated a research program that included:

" Fact-finding visits to Belgium for discussion with Electrabel and other interested parties(9/19/2012 and 4/9/2013)

" Documentation review to verify reactor pressure vessel (RPV) construction (formed plateor forging) and to gather other required information to assess extent of condition

Evaluations to determine relevance of the Doel 3 operating experience (OE) to U.S. fleetvia a bounding probabilistic fracture mechanics (PFM) analysis using methodologyconsistent with that used by the U.S. NRC in development of the Alternate PTS Rule, 10CFR 50.61a (see Section 5 of this report)

" A request that the owners of affected plants review and assess the records of non-destructive examinations (NDE) conducted on vessel beltline ring forgings at the time offabrication to determine if the Doel 3 phenomenon was observed in U.S. vessels [25]

Meeting with U.S. NRC

On March. 5, 2013, the U.S. NRC held a public meeting with Industry to discuss the Doel 3 RPVissue; the minutes of that meeting are provided in [26]. NRC staff presented backgroundtechnical information and announced plans to issue an Information Notice [33]; industryrepresentatives discussed plans to perform probabilistic fracture mechanics and complementarycontinuum damage mechanics analyses. The NRC staff presented "NRC Plans" which is shownin Figure 3-1. The center box shows a resolution path in which the first step is "Licensees reviewUT records" and which is consistent with the resolution path that had been adopted by Industry.

3-1

US. Industry Response to the Generic Implications of the Doel 3 RPV Experience

IPublishInformationnotice

I.

Figure 3-1USNRC Plans (March 2013) [9]

Objectives

The objectives of Industry's efforts to assess the relevance and significance of the Doel 3 /Tihange 2 RPV quasi-laminar flaw issue are twofold:

1. Review and assess the fabrication NDE records for RPV forgings to determine if U.S.plants experienced flaking

2. Conduct a bounding structural evaluation to demonstrate that reactor vessel safety goalsare met by all U.S. PWRs through the extended license period in the unlikely event thatflaking exists in beltline ring forgings

Scope

Content deleted - EPRI Privileged & Confidential Information

3-2

U. S Industry Response to the Generic Implications of the Doel 3 RP V Experience


3-3

U.S. Industry Response to the Generic Implications of the Doel 3 RPV Experience

Table 3-1Operating U.S. Reactor Pressure Vessels Fabricated from Forging Materials


3-4

US. Industry Response to the Generic Implications of the Doel 3 RPV Experience

The number of operating PWRs in each Vessel Designer / Fabricator category is shown in Table3-2.

Table 3-2U.S. Operating PWR Forged Vessel Population by Fabricator and Designer


Approach

The approach adopted to achieve the Objectives outlined above consisted of the followingelements:

1. Review of plant-specific records to verify no indication of flaking (see Section 4)

2. Performance of a probabilistic fracture mechanics analysis of a vessel with postulatedhydrogen flakes (see Section 5)

3-5

4EVALUATION OF FABRICATION NDE RECORDS

Background and Objective

In early 2013 the PMMP issued a request [29] to the chief nuclear officers of the U.S. PWRswith beltline ring forgings requesting that fabrication NDE records for the forgings be reviewedand that an assessment of those records relative to hydrogen flaking be reported to the MRP.

To assist the plants in responding to the PMMP request and facilitate the retrieval of fabricationNDE records, the PWR Owners Group issued a Project Authorization (PA)-MSC- 1172, "Supportof Electrical Power Research Institute (EPRI) Materials Reliability Program (MRP) 2013-012Regarding Doel Unit 3 Inspection Results." This effort consisted of two tasks:

* Task 1: Locate and summarize the engineering specifications (E-Specs) and UT records

" Task 2: Create a UT model for comparison of the fabrication inspection techniquesbetween Doel Unit 3 and U.S. plants

To allow time for completion of this PWROG effort and for plants to have sufficient opportunityto consider and incorporate its findings into the plant responses to the PMMP request, the PMMPrevised the request in MRP 2013-012 Rev I (and later, MRP 2013-012 Rev. 2, which simplyextended the reporting deadline). The scope of the revised request was as follows:

"a. PWRs with RPV core shell forgings should locate, retrieve and review pertinent plantinformation relating to the fabrication nondestructive examinations so they can confirm theirplant(s) comply with the generic information to be included in the summary report providedto the MRP by the PWROG-MSC. The content of the utility's reply to this request wouldanswer the following questions:

1. Have the plant-specific inspection records been reviewed and do they conform to thegeneric information, technical discussion and conclusions provided by the PWROG-MSC?

2. Are the available plant-specific records and NSSS NDE fabrication specificationsdeemed sufficient to determine whether or not hydrogen-induced fabrication defectswere present?

3. If the answer to Question 2 is yes, does the combination of plant-specific records andNSSS specifications demonstrate an absence of hydrogen-induced fabricationdefects?"

4-1

Evaluation of Fabrication NDE Records

PWROG Assessment of NDE Methods and Capabilities

WCAP-17786-NP, Rev. 1 [30] reports the results of the effort undertaken in Task 2 of PA-MSC-1172. Using ultrasonic model simulations, the various inspection techniques used duringfabrication of U.S. reactor vessels were assessed for capability to detect laminar/Q-L flaws.WCAP-17786-NP, Rev. I states,

"...the purpose of Task 2 of this report is to evaluate the shop UT indication reportingcriteria, which results in the reporting of much smaller indications than the rejectioncriteria."

Rejection criteria are specified in the edition of ASME Section III to which a plant is built.Because plants were ordered and fabricated over a range of years spanning various Codeeditions, the shop UT methods and reporting criteria changed through the years. Therefore, thespecific methods and criteria used for each of the operating PWRs with beltline ring forgingswere identified.

The report further evaluates whether a condition similar to Doel 3 could have gone unreported inthe U.S. plants. (It is noted that the Doel 3 condition was acceptable per the UT rejection criteriaunder which that vessel was built; the extensive analyses performed in the Safety Case reportsvalidates that determination.)

Each set of requirements for UT calibration and reporting threshold was modeled and presentedin terms of an equivalent flat bottom reflector sensitivity for each variation in fabricationspecification requirements. The results were applied to the reported flaw distribution from Doel3, and capability of each set to detect and report some portion of the Doel 3 flaws was thusestablished.

Results of PWROG Study

Significant conclusions from WCAP- 17786 include the following:

1. U.S. forging inspection practices during fabrication were somewhat less sensitivethan the 2012 ISI at Doel 3, but sufficiently adequate to determine if extensive smalllaminations were present in the forged rings. Due to the higher frequencies employed,the U.S. inspections would be more sensitive to off angle flaws as compared to theshop inspections performed during manufacture of the Doel3 / Tihange 2 vessels.

2. U.S. forging inspection specifications contained requirements and criteria that wouldhave resulted in recording of indications such as those at Doel 3. The EquipmentSpecification (E Spec) also had a requirement that a Westinghouse or manufacturerrepresentative witness and approve the forging UT.

3. ASTM A388 had an additional recording criterion that required recording of clusteredflaws (2" cube with five or more indications). This requirement increased theprobability of recording a condition such as Doel 3 if it existed in U.S. vessels.

4. Because U.S. vessels were fabricated over time and to different standards, thecapabilities of the inspections for each plant vary; however, all were capable ofdetecting and recording the condition observed at Doel 3 / Tihange 2.

4-2

Evaluation of Fabrication NDE Records

PWR NDE Records Review / Results


4-3

Table 4-1Results of Fabrication NDE Records Reviews at Operating U.S. PWRs


4-4

5BOUNDING STRUCTURAL EVALUATION FOR U.S.VESSELS (PROBABILISTIC ANALYSES)

Although similar indications have not been detected during construction, preservice, or inserviceexamination of RPVs in the United States, the work described in this chapter was initiated byMRP to demonstrate that the integrity risk goal is maintained through the end of a sixty yearextended life (EOLE) for RPVs with beltline ring forgings having postulated quasi-laminar flawindications at pressurized water reactors (PWR) in the U.S. The risk goal employed in this studyis:

95th percentile through-wall cracking frequency < 10-6 per operating year.

This goal was selected to maintain consistency with the integrity risk goal used by the NRC todefine the Alternate Fracture Toughness Requirements for Protection Against Thermal ShockEvents (Alternate PTS Rule), 10CFR50.61 (a) [12].

Probabilistic fracture mechanics (PFM) analyses were used to compute the ninety-fifth percentilethrough-wall cracking frequency (95% TWCF) and demonstrate that the integrity risk goal ismaintained through the EOLE for a RPV postulated to have quasi-laminar flaw indications.This effort involved several steps to define and obtain the data and computational tools needed tocomplete the integrity risk goal assessment. These steps included determining: the population ofU.S. PWRs with beltline ring forgings, the highest level of irradiation degradation for the beltlinering forgings, the transient load conditions and event frequencies, and distributions of postulatedquasi-laminar indications.

Generally, the NRC software FAVOR v06.1 [14] was used to compute the 95% TWCF.However, the PFM computational procedure in FAVOR uses flaws whose depths are set equal tofixed increments of vessel wall thickness and flaw locations that are uniformly distributedthroughout the vessel wall [20]. To enable evaluation of a more specific population of flawdepths and locations, EPRI replaced the PFM module in FAVOR with the software module ER-1, R2, which is designed to evaluate any user specified depth, aspect ratio, and location of asingle indication in the vessel wall.

Several flaw indication distributions were defined and analyzed in the PFM evaluation. Thesedistributions included a representation of the "as detected" indications found at Doel 3 [1], amodification of the "as detected" distribution to incorporate the angles at which the indicationsare inclined relative to the vessel inner surface, and a modification to model the presence ofclustered indications where two closely spaced small indications were combined into one largerindication.

Because a postulated, rather than an actual, flaw distribution was being evaluated, severalconservative assumptions were used to account for uncertainty in the postulated distributions.These assumptions included using conservative representations of indication size and locationand assuming both an axial and circumferential flaw are present at each indication location. In

5-1

Bounding Structural Evaluation for U.S. Vessels (Probabilistic Analyses)

addition, the integrity assessment was performed for the limiting irradiation degradationcondition for the population of beltline ring forgings in U.S. PWRs.

The results from the PFM analyses indicate that the contribution to the TWCF is dominated bythe clustered indications. The 95% TWCF associated with the postulated quasi-laminar flawdistribution and the limiting irradiation degradation condition is approximately 1.Oxl 0-7 peroperating year. These results demonstrate that compliance with the integrity risk goal:

95%TWCF, < 10-6 per operating year

will be maintained through the EOLE for the population of U.S. plants with beltline ringforgings.

Because the 95% TWCF for the postulated distribution of quasi-laminar flaws is less than 1E-6per operating year, the vessel has the capacity to tolerate many times the number of indicationsdetected at Doel 3, including the large clustered indications. In addition, review of the NDErecords indicate there is high confidence that the phenomenon observed at Doel 3 / Tihange 2 isnot present in the U.S. PWR fleet of RPVs with forged beltline rings. Consequently, thepostulated distribution of quasi laminar indications used in the PFM evaluation has many moreand larger flaws than could possibly be present in beltline ring forgings at PWRs in the U.S., andthe 95% TWCF for vessels with beltline ring forgings will be much less than the 1.0 x 10-7 peroperating year determined from the PFM analysis.

Description of Quasi-Laminar Indications at Doel 3

Figure 5-1 illustrates the distribution of indications detected in the upper and lower core shells atDoel 3 [20]. The information in Figure 5-1 shows indications generally are spread out along thevessel circumference and there are many more indications in the lower core shell compared tothe upper core shell. There is a higher concentration of indications in the 900 segment of thecircumference between 1800 and 2700 in the lower core shell. No indications were detected inthe imnediate vicinity of the pressure boundary welds [1].

Approximately 857 flaw indications were detected in the upper forged core shell and 7205 in thelower shell at Doel 3. Figure 5-2 provides a sun-imary of the number of indications as a functionof distance from the inner surface of the vessel for the upper and lower core shells at Doel 3 [1].The numbers of indications in the upper and lower core shells are represented by the blue and redbars, respectively, in Figure 5-2.

Based on the evaluation of the ultrasound indications detected in the Doel 3 RPV and destructiveexamination of similar archival material, Electrabel personnel concluded that the indications arequasi-laminar, i.e., generally, they are parallel to or slightly inclined relative to the inner surfaceof the RPV wall, are approximately circular in shape, and most have diameters of 10 mm or less,with a maximum diameter of 70 mm [1]. Figure 5-3 provides a summnary of the maximum lengthof the indications as a function of distance from the vessel inner surface [1]. The flaw indicationmaximum length in Figure 5-3 is the largest measured coordinate length in the horizontal orvertical directions as illustrated in Figure 5-4 [1].

5-2


Examination results indicated that the flaw indications can be inclined at an angle relative to aplane parallel to the vessel inner surface. This angle generally is about 100 with values up to amaximum of 20' [1]. The indications can be inclined in one or two directions as illustrated inFigure 5-5, where S is the distance from the CBMI to the inner edge of the indication [1].

Although most of the indications are small, many are in close proximity to each other [1]. Inthose instances, Electrabel considered combining two or more of the closely spaced smallindications into one larger indication for evaluating RPV integrity. Because the larger combinedindications can decrease the margins against failure for the pressure vessel, Electrabel performeddetailed two-dimensional finite element stress analyses to better determine the distance betweenadjacent indications that would require closely spaced small indications to be combined into asingle larger indication [ 11]. These analyses indicated that approximately 20% of the detectedindications were in close enough proximity to be grouped into various larger size indications [r].

Doel 3

0 45 1 1.3' 225 3W

1000

060

'000

4000

WO# 857

0 0 0 0NCZJe S11-00

L Ppet (Olt shell

$o$o

# 7205

Figure 5-1Illustration of the Azimuthal and Radial Distributions of Indications Detected in the Doel 3Reactor Pressure Vessel [20]

5-3


1600

1400

1200FA

P01000to

800'4-

600

E= 400z

200

<=10 10- 20- 30- 40- 50- 60- 70- 80- 90- 100- 110- 120- 1320 30 40 50 60 70 80 90 100 110 120 130 14

Indication depth (mm)

Figure 5-2Number of Indications Detected in the Upper (blue bars) and Lower (red bars) Forged CoreShells at Doel 3 as a Function of Distance from the Vessel Inner Surface [1]

Copyright Electrabel Generation BeLux. Reprinted with permission.

5-4

Bounding Structural Evaluation for US. Vessels (Probabilistic Analyses)

Doel 3 - Size of indications (max(x,y)) vs. depth

80.0

70.0

-60.0E

50.0

o 40.0

30.0

S20.0

10.0

0.0

4 5 LOEAR

• 1LO

* 2LO4 4 . 4A * 5..

A * . 4AA4 . .

'S S.'

a a

a

500 100Depth (mm)

150 200

Figure 5-3Maximum Indication Size as a Function of Distance from the Vessel Inner Surface, Doel 3Lower Core Shell [1].

Copyright Electrabel Generation BeLux. Reprinted with permissionThe Symbol Colors Represent the NDE Probe Types Used to Examine Different Regions of the Vessel Wall.

5-5


Ff-4 W*ý

450 448 446 444 442 440 438 436 434 432 430 428 426N24 422 420

Figure 5-4Illustration of the Typical Shape and Maximum Length (X or Y) (mm) of a Hydrogen FlakeDetermined from Destructive Examination of Archival Material [1]


5-6


L

Figure 5-5Illustration of the Inclined Angles Relative to the Vessel Inner Surface and Distance S fromthe Clad Base-Metal Interface and the Inner Edge of the Indication [1]


5-7


Assessment of Postulated Quasi-Laminar Indications at U.S. Plants

Purpose of this Work

The purpose of this work is to demonstrate that the integrity risk goal is met through the EOLEfor U.S. PWR vessels with beltline ring forgings having postulated quasi-laminar flawindications. The integrity risk goal employed in this work is similar to that used by the NRC todefine the Alternate PTS Rule, 1OCFR50.61 (a) [12] and is based on the criterion:

95% TWCF < 10-6 per operating year.

Approach


5-8


Analysis Input

RPV Dimensions and Material Properties


5-9

Bounding Structural Evaluation for U.S. Vessels (Probabilistic Analyses,)

Table 5-1Vessel Dimensions and Irradiation Degradation Variables used in the PFM Analyses

Variable jVariable Value

Postulated Transient Events


5-10


Neutron Fluence Distribution


Distribution of Quasi-Laminar Indications


5-11

Bounding Structural Evaluation Jbr US. Vessels (Probabilistic Analyses)

Content deleted - EPRI Privileged & Confidential Information.

5-12



Analysis Procedure


5-13



Software


5-14



Results

95% TWCF in Each Individual Indication Bin


5-15


95% TWCF for the Distribution of Indications


Flaw Tolerance Margins


5-16



5-17


Content deleted - EPRI Privileged & Confidential Infonnation

Figure 5-6Overview of the Approach for Determining the 95% TWCF for a Reactor Pressure with aPostulated Quasi-Laminar Flaw Distribution

5-18



Figure 5-7Maximum Ring Forging RTNDT Distribution for the Population of U.S. PWRs having RPVswith Forged Beltline Rings

5-19


Content deleted - EPRI Privileged & Confidential Infornation

Figure 5-8Relative Fluence Map and Histogram Used in the PFM Calculations [32]

5-20


Table 5-2Representation of the Distribution of the Number of Indications as a Function of IndicationLength and the Distance from the Vessel Inner Surface to the Inner Indication Edge -Developed from the Indications Detected in the Lower Core Shell in the Reactor PressureVessel at Doel 3


5-21


Table 5-3Postulated Distribution for the Number of Detected Indications as a Function of IndicationLength, Inclined Angle and Through-wall Depth, and Distance from the Vessel InnerSurface to the Inner Indication Edge


5-22


Table 5-4Summary of the Distribution of Postulated Clustered Indications

Twenty Percent Number ofMaximum Inclined Angle, Thru-wall Depth, of the Total Clustered

Length, 2L, mm degrees 2a, mm Number of IIndicationsIndications


5-23


Table 5-5Postulated Distribution for the Number of Clustered Indications as a Function of IndicationLength, Inclined Angle and Through-wall Depth, and Distance from the Vessel InnerSurface to the Inner Indication Edge


5-24

Bounding Structural Evaluation for U.S. Vessels (Probabilistic Analiyses)

Table 5-6Postulated Distribution for the Number of Clustered and Individual Indications as aFunction of Indication Length, Inclined Angle and Through-wall Depth, and Distance fromthe Vessel Inner Surface to the Indication Flaw Edge


5-25

Bounding Structural Evaluation/1br US. Vessels (Probabilistic Analyses)

Table 5-7Calculated Values of the 95% TWCF per Indication as a Function of Indication Length,Inclined Angles and Through-wall Depth, and Distance from the Vessel Inner Surface tothe Indication Inner Edge


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Table 5-895% TWCF Contribution from the Postulated Number of Clustered and IndividualIndications at Each Flaw Inner Edge Location and the Total 95% TWCF for the Distribution


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Table 5-995% TWCF Contribution from the Postulated Number of Clustered Indications at EachInner Flaw Edge Location and the Total 95% TWCF for the Distribution


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Contribution to 95% TWCF from Other Flaw Types


PFM Analyses Summary and Conclusions

1. PFM analyses indicate that the 95% TWCF for the RPV beltline forging with the limitingRTNDT and a conservative distribution of postulated quasi-laminar indications isapproximately 1.OE-7 per operating year.

2. The clustered indications are the dominant contributors to the 95% TWCF compared tothe distribution of detected individual indications

3. These results were obtained using the following input and assumptions:

a. Transient loads and event frequencies from the 61 transients defined by the NRCfor the PTS evaluation of Beaver Valley, Unit 1 and similar PWR designs

b. The maximum RTNDT of any forging in the beltline region for the population ofU.S. plants with beltline ring forgings

c. A postulated flaw distribution that is a conservative representation of availableservice experience, including the distribution of indications found in the lowercore shell at Doel 3.

d. Conservative assumptions are used to account for the uncertainty in the postulatedindication distribution, including

i. Results of the NDE record review (Section 4) provide high confidence thatthe phenomenon observed at Doel 3 / Tihange 2 is not present in the U.S.PWR fleet of RPVs with forged beltline rings. Consequently, thepostulated distribution of quasi laminar indications has many more andlarger flaws than could possibly be present in beltline ring forgings atPWRs in the United States.

ii. The indication length and through-wall flaw depth are taken as themaximum values in each of the respective bins.

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iii. The 95% TWCF values were calculated at both the left and right sides ofeach bin. The larger of the two values was used as the 95% TWCF for thatbin overall.

iv. The conservative assumptions employed to define indication size andTWCF provide assurance that the 95% TWCF calculated for each bin is anupper bound.

v. Each indication includes a contribution to the 95% TWCF from postulatedaxial and circumferential flaws. Both the axial and circumferential flawsare the same size and have dimensions corresponding to the sizes at themaximum values in each of the respective bins.

4. The results from this study and the previous study by the NRC to define the AlternatePTS Rule demonstrate that the total 95% TWCF for the combined populations ofpostulated quasi-laminar indications, under-clad cracks, and normally anticipated forgingand weld flaws in a vessel with beltline ring forgings connected with circumferentialwelds does not exceed 1.OE-7 per operating year: thus the 95% TWCF acceptancecriterion (<1x10-6 events/year) is met.

5. The results from this study indicate that there is tolerance for approximately ten times thenumber of indications shown in the distribution in Table 5-6 before the 95% TWCF <1E-6 per operating year integrity criterion is reached.

6. The RTNDT for 19 of the 21 PWRs with beltline ring forgings is substantially less than thebounding RTNDT used in the study. Consequently, the 95% TWCF for those 19 plantswill be less than the 1.OE-7 per operating year determined in this study for the plant withthe beltline ring forging having the limiting RTNDT. Similarly, the tolerance foradditional indications in the beltline ring forgings at these vessels will be greater than thebeltline forging with the limiting RTNDT.

7. The results in this report demonstrate that the integrity risk goal, 95% TWCF < 10.6 peroperating year, will be maintained through the end of extended life for the population ofU.S. plants with beltline ring forgings containing a conservative distribution of postulatedquasi-laminar flaws.

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6SUMMARY OF EVALUATION AND CONCLUSIONS

The Materials Reliability Program (MRP) and PWR Owners Group (PWROG) have performedcomplementary evaluations designed to address the implications of the Doel 3 and Tihange 2RPV operating experience for the U.S. reactor vessels with beltline ring forgings. Theevaluations and conclusions are summarized as follows:

1) The operating experience relative to the extensive quasi-laminar indications observedfrom the ultrasonic examination results of the Doel 3 and Tihange 2 RPVs in 2012 hasbeen reviewed. It is noted that:

a. The Licensee (Electrabel) and regulator (FANC) attribute the phenomenon tohydrogen flaking that occurred during the manufacturing phase of those vessels.

b. A detailed, comprehensive structural integrity assessment by the owner/operator(Electrabel) has demonstrated that the vessels continue to meet applicable safetycriteria. The units have been restarted.

2) The implications for U.S. RPVs of the discovery of Q-L indications in two Belgianreactor pressure vessels have been carefully assessed. U.S. Industry developed anapproach to assess those implications; the results are summarized below.

3) The PWROG modeled and assessed the detection capabilities of the various UTinspection requirements and techniques used during fabrication of the beltline ringforgings in currently operating U.S. vessels; results were published in WCAP-17786-NP,Rev. 1, and were provided for MRP review. Although the detection capabilities /sensitivities of specific inspections varied over time, all U.S. inspection requirements andtechniques employed during fabrication had the ability to detect indications such as thoseat Doel 3 and Tihange 2 if such flaws had existed in U.S. vessels.

4) The applicable fabrication inspection records were retrieved and reviewed for alloperating PWRs with beltline ring forgings; all plant records and specifications wereconsistent with and were bounded by the assumptions in the capabilities studydocumented in WCAP- 17786-NP, Rev. 1. In all cases quasi-laminar indications such asobserved at Doel 3 would have been detected and reported; there were no recordableindications in any vessels.

5) The results from the review of the inspection records conducted in response to the PMMPrequest (MRP 2013-012 Rev. 2) provide high confidence that the phenomenon observedat Doel 3 / Tihange 2 is not present in the U.S. PWR fleet of RPV beltline ring forgings.

6) PFM analyses indicate that the 95% TWCF for the RPV beltline forging with the limitingRTNDT and a conservative distribution of postulated quasi-laminar indications isapproximately 1.OE-7 per operating year. The results in this report demonstrate that the

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Suminay of Evaluation and Conclusions

integrity risk goal. 95% TWCF < 10-6 per operating year, will be maintained through theend of extended life for the population of U.S. plants with beltline ring forgingscontaining a conservative distribution of postulated quasi-laminar flaws.

Based on the above, it is concluded that the condition observed at Doel 3 is unlikely to exist inU.S. PWRs, and it would not have structural significance even if it did.

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7 REFERENCES

1. Safety Case Report, Doel 3 Reactor Pressure Vessel Assessment, Electrabel nv, Sim6nBolivarlaan 34, 1000 Brussel, December 5, 2012

2. Bamford, W. and Rishel, R., "A Review of Cracking Associated with Weld DepositedCladding in Operating PWR Plants," WCAP-15338, Westinghouse Electric Co. LLC,Pittsburgh, PA, March 2000.

3. "Rules for Inservice Inspection of Nuclear Power Plant Component," Section XI of theASME Boiler and Pressure Vessel Code, American Society of Mechanical Engineers, NewYork, NY..

4. Safety Case Report, Tihange 2 Reactor Pressure Vessel Assessment, Electrabel sa, av. Sim6nBolivar 34, 1000 Bruxelles, December 5, 2012.

5. F. A. Simonen, S. R. Doctor, G. J. Schuster, and P. G. Heasler, "A Generalized Procedure forGenerating Flaw-Related Inputs for the FAVOR Code," NUREG/CR-6817, Rev. 1 (PNNL-14268, Rev. 1), Pacific Northwest National Laboratory, October 2003 (ML051790410).

6. Safety Case Report - Addendum, Doel 3 Reactor Pressure Vessel Assessment, Electrabel nv,Sim6n Bolivarlaan 34, 1000 Brussel, April 26, 2013.

7. Safety Case Report - Addendum, Tihange 2 Reactor Pressure Vessel Assessment, Electrabelsa, av. Sim6n Bolivar 34, 1000 Bruxelles, April 15, 2013.

8. "Doel 3 and Tihange 2 reactor pressure vessel Final Evaluation Report," Federal Agency forNuclear Control, Belgium, May 2013.http://www.fanc.fgov.be/CWS/GED/pop View.aspx?LG= 1 &ID=3429

9. USNRC Presentation, "Implication of Potential Quasi-Laminar Indications in USA ReactorPressure Vessel Shell Ring Forgings," Public Meeting, USNRC Headquarters, Rockville,Maryland, March 5, 2013. (ADAMS Accession Number ML13059A225)

10. "Rules for Construction of Nuclear Facility Components," ASME Boiler and Pressure VesselCode, Section III, American Society of Mechanical Engineers.

11. Presentation, "Doel 3 - Tihange 2 Reactor Vessel Assessment, Safety Case Summary -Status April 2013," from WANO Technical Conference on Doel 3 and Tihange 2 ReactorVessel Issue, Antwerp, Belgium, April 23, 2013.

12. Alternate Fracture Toughness Requirements for Protection Against Pressurized ThermalShock Events, U.S. Nuclear Regulatory Commission, Federal Register/Vol. 75, No.1/Monday, January 4, 2010/Rules and Regulations, pp. 13-29.

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References

13. Technical Basis for the Revision of the Pressurized Thermal Shock (PTS) Screening Limit inthe PTS Rule (10 CFR 50.61) NUREG-1806, Vol. 1, August 2007.

14. Dickson, T.L., P. T. Williams, and S. Yin, "Fracture Analysis of Vessels-Oak RidgeFAVOR, v06.1, Computer Code: User's Guide," ORNL/TM-2007/0031, Oak Ridge NationalLaboratory, 2007.

15. Technical Information Note - 2013.02.01, Flaw Indications in the reactor pressure vessels ofDoel 3 and Tihange 2, Federal Agency for Nuclear Control, Belgium, February 1, 2013.http://www.fanc.fgov.be/CWS/GED/pop View.aspx?LG=l &ID=3377

16. Press release, "FANC experts give positive opinion on restart of Doel 3 and Tihange 2reactor units," Federal Agency for Nuclear Control, Brussels, May 17, 2013.http://www.fanc.fgov.be/GED/00000000/3400/3430.pdf

17. Fruehan, R., "A Review of Hydrogen Flaking and Its Prevention," Iron and Steel Maker,1997, 24 (8) 61-69.

18. "Doel 3 and Tihange 2 reactor pressure vessels - Provisional evaluation report," FederalAgency for Nuclear Control, Belgium, January 30, 2013.http://www.fanc.fgov.be/CWS/GED/pop View.aspx?LG= 1 &ID=3391

19. Murphy, E.L., and Steiner, J.E., "Hydrogen - Its Occurrence, Determination, and Control inSteel Forgings", Steel Forgings: Second Volume, ASTM STP 903, E.G. Nisbett and A. S.Melilli, Eds., American Society for Testing and Materials, Philadelphia, 1986, pp. 573-582.

20. Doel 3 - Tihange 2 Reactor Pressure Vessel Assessment, April 2013 (Available from U.S.Nuclear Regulatory Commission, ADAMS Public Documents, Accession NumberML13121A107).

21. Fei Peng, "Adverse Impact and Countermeasures of Macro Segregation for Ring ForgingProduct," Journal of Materials and Applications, 2012.http://scipub.co.uk/JMA/JMA. 1.1.1 7.pdf

22. M. Kusuhashi et. al., "Manufacturing of Low Neutron Irradiation Embrittlement SensitivityCore Region Shells for Nuclear Reactor Pressure Vessels," E-Journal of AdvancedMaintenance, Vol. 1 (2009) 87-98.

23. Private Communication, Martin M. Morra (GE Global Research) to Tim Hardin (EPRI),"Hydrogen Flake Orientation in Different Product Forms (Forging versus Plate)," July 2,2013.

24. Email, Electrabel Kerncentrale Doel (T. L6fgren) to INPO (R. Spinnato) et. al., Subject:reactor vessel problem Belgium Doel NPP, July 31, 2012.

25. MRP Letter 2013-012 Revision 2, Subject: Revised Due Date for Reactor Pressure Vessel(RPV) Forging Fabrication UT Inspection Records Request, June 26, 2013.

26. Memorandum, S. Stuchell to A. Mendiola, Subject: Summary of March 5, 2013, PublicMeeting Regarding the Potential for Quasi-Laminar Indications in Reactor Pressure VesselForgings, dated March 13, 2013. (ADAMS Public Documents, Accession NumberML13066A725)

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References

27. NUREG-1511, Supplement 2, "Reactor Vessel Status Report," U.S. Nuclear RegulatoryCommission, October 2000.

28. USNRC Memorandum, "Summary of February 9, 2000 Meeting on Reactor VesselSurveillance Specimen Testing (TAC No. M89606)," dated May 3, 2000, ADAMS PublicDocuments, Accession Number ML003721690.

29. MRP Letter 2013-012, Subject: Reactor Pressure Vessel (RPV) Forging Fabrication UTInspection Records, February 14, 2013.

30. Lareau, J.P., and Bamford, W.H., "A Review of the Shop Inspections Performed on USReactor Vessels, and the Potential for Indications such as those found in the Recent Doel3/Tihange 2 Inspections (PWROG PA-MSC- 1172, Task 2)," WCAP-17786-NP, Revision 1,Westinghouse Electric Company LLC, Cranberry Township, PA, August 2013.

31. WCAP-17539-NP, Revision 0, "Sequoyah Units 1 and 2 Time-Limited Aging Analysis onReactor Vessel Integrity," Enclosure 3, Westinghouse Electric Co, March 2010 (Availablefrom U.S. Nuclear Regulatory Commission, ADAMS Public Documents, Accession NumberML13032A253).

32. Technical Basis for Revision of the Pressurized Thermal Shock (PTS) Screening Limit in thePTS Rule (10 CFR 50.61). NUREG-1806, Vol. 2- Appendix A, U.S. Nuclear RegulatoryCommission, August 2007.

33. Recommended Screening Limits for Pressurized Thermal Shock (PTS), NUREG-1874, U.S.Nuclear Regulatory Commission, March 2010.

34. NRC Information Notice 2013-19: Quasi-Laminar Indications in Reactor Pressure VesselForgings, September 22, 2013. (U.S. Nuclear Regulatory Commission, ADAMS PublicDocuments, Accession Number ML13242A263)

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AVERIFICATION AND VALIDATION OF SOFTWARE ER-1, REV. 2


A-1