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PURPOSE AND SCOPE OF THE ROOT CAUSE …

PURPOSE AND SCOPE OF THE root CAUSE analysis REPORTP urposeDetermine the root and contributing causes for Reactor pressure vessel closure head (RPV head)damage experienced at nozzle 3 and minor corrosion at nozzle 2, to support the operabilitydetermination for the station s as-found condition and the future repair early in the development of the response to this condition, it became clear that the technicalcauses behind the cracking of the Control Rod Drive Mechanism (CRDM) nozzles and theensuing corrosion of the head material would draw much attention and comparison to thepreviously developed body of knowledge on related topics and conditions. In fact, the possibilityof nozzle cracks existing at Davis-Besse was well recognized prior to the condition, but theidentified significant damage to the RPV head had not been unexpected finding of the significant damage at Davis-Besse immediately became a concern,both for the possible extent of condition implications at Davis-Besse and the potential impact tothe industry.

PURPOSE AND SCOPE OF THE ROOT CAUSE ANALYSIS REPORT Purpose Determine the root and contributing causes for Reactor Pressure Vessel closure head (RPV head)

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Transcription of PURPOSE AND SCOPE OF THE ROOT CAUSE …

1 PURPOSE AND SCOPE OF THE root CAUSE analysis REPORTP urposeDetermine the root and contributing causes for Reactor pressure vessel closure head (RPV head)damage experienced at nozzle 3 and minor corrosion at nozzle 2, to support the operabilitydetermination for the station s as-found condition and the future repair early in the development of the response to this condition, it became clear that the technicalcauses behind the cracking of the Control Rod Drive Mechanism (CRDM) nozzles and theensuing corrosion of the head material would draw much attention and comparison to thepreviously developed body of knowledge on related topics and conditions. In fact, the possibilityof nozzle cracks existing at Davis-Besse was well recognized prior to the condition, but theidentified significant damage to the RPV head had not been unexpected finding of the significant damage at Davis-Besse immediately became a concern,both for the possible extent of condition implications at Davis-Besse and the potential impact tothe industry.

2 Therefore, the objective of the Initial Investigative Team was a prompt investigationinto the primary CAUSE (s) of the damage. This root CAUSE analysis Report supports this specificobjective. These initial findings are expected to invite input from industry experts and scientistsresulting in additional study of the evidence, and further research into the topics of CRDM nozzlecracking and boric acid to the urgency in developing useful insight to the plant and the industry, revision 0 of thisreport has been prepared with the full knowledge that a number of activities are still have been captured as action items in the report, and the results of these could lead to afuture revision to the report.

3 It is expected that these activities will provide additionalunderstanding, but should not affect the fundamental conclusions of this Team MembershipSteve Loehlein, FENOC (Beaver Valley), Team LeadChuck Ackerman, FENOC (Davis-Besse)Ted Lang, FENOC (Davis-Besse)Todd Pleune, FENOC (Davis-Besse)Neil Morrison, FENOC (Beaver Valley)William Mugge, FENOC (Davis-Besse)Joseph Rogers, FENOC (Davis-Besse)Technical expertise provided by:Dr. Mark Bridavsky, FirstEnergy, Beta Labs - Failure analysis ExpertStephen Hunt, Dominion Engineering Corrosion ExpertSteve Fyfitch, Framatome ANP, Metallurgical ExpertChristine King, EPRI, Material Reliability Program ManagerAssessment of management aspects/decision making:John B.

4 Martin, Corporate Nuclear Review BoardE. J. Galbreath, Senior Representative, Assistance, Institute of Nuclear Power OperationsRoot CAUSE analysis ReportTable of Contents iTable of Contents_____TitlePage Problem Reason for Consequences of Event/Condition Immediate Actions Event Sequence of Data Non-Destructive Examinations of RPV Head and Potential Evidentiary Request Locations of Cracks and Corrosion on RPV NDE Examinations of CRDM Visual Examinations of RPV Top Head and , Leaks and CRDM Nozzle Cracks and Propagation to Leakage Rate From CRDM Nozzle Source of Boric Acid Deposits on RPV Corrosion of RPV Top Head Investigation of Lead Sequence of Relevant CRDM Flange and RPV Head Inspections during Refueling Containment Air Cooler Containment Radiation Monitor RE4597 Observations & Filter Containment Recirculation Fan/Fan Programs Important to Preventing B&W Owners Group and Industry CRDM Nozzle Related Davis-Besse Boric Acid Corrosion Control Davis-Besse Inservice Inspection Evaluation of Condition Report Management Potential Risk to the RPV Head Deferral of Service Structure Modifications to Allow Improved Management

5 Involvement/Awareness of Plant Conditions during Recognizing the Need for Collective Significance RPV Head RPV Head Restart Causal Factors/Conclusions49 root CAUSE analysis ReportTable of Contents Experience Davis-Besse Nuclear Industry root CAUSE Probable/ root Contributing Extent of Degradation Mechanism Management Recommended Corrective Probable/ root causes Corrective Contributing causes Corrective Additional Actions Requiring Davis-Besse Vendor NRC INPO Industry Other Personnel Personnel Personnel Methodologies Employed71 root CAUSE analysis ReportTables iiiTables_____TitlePage Nozzle 1 NDE Examination Results722. Nozzle 2 NDE Examination Results733.

6 Nozzle 3 NDE Examination Results744. Nozzle 5 NDE Examination Results755. Nozzle 47 NDE Examination Results766. Comparison of Davis-Besse to Other B&W Design Plants777. Nuclear Industry Experience Review Results77 root CAUSE analysis ReportFigures ivFigures_____TitlePage RPV Top of Head Section RPV Top of Head Plan CRDM Nozzle General Acid and Iron Oxide on vessel Flange at 12 RFO935. Corrosion at Nozzle 2 Drawing Side 2 Corrosion Area Location, Size, and in Reactor vessel Head between Nozzle 3 and of Cracks and Corrosion on Davis-Besse RPV Head at 1 Crack Locations and 2 Crack Locations and 3 Crack Locations and 5 Crack Locations and 47 Crack Locations and 3 Degraded Areas10314.

7 Nozzle 3 Clad Thickness Measurements10415. Hoop Stresses and Operating Condition Deflections in CRDM Nozzles 2-510516. Location of Leaking Nozzles in B&W Design Plants10617. Distribution of Leaking Nozzles in B&W Design Plant10718. CRDM Nozzle Leakage Observed at Oconee 310819. Unidentified Leak Rate at Davis-Besse (Cycle 13)10920. As Found Locations of Boric Acid Deposits on Davis-Besse vessel Head110(10 RFO to 13 RFO)21. Nozzle Crack Leakage Rate Calculation Results11122. Finite Element Model Boundary Conditions to Simulate Axial Crack11223. Crack Opening Displacement with the Crack Surface Nodes Released11324. Boric Acid Deposits on Top of Head at Start of 13 RFO11425.

8 Corrosion Rate for EPRI Experiments (Proprietary)11526. Timeline of Key Events Related to Reactor vessel Head Boric Acid Wastage11627. Events & Casual Factor Chart117a-eRoot CAUSE analysis ReportFigures vTitlePage Leaking Flanges Found and Repaired During Each Outage11829. Flange Leakage with Stalactite Formation from Insulation and Stalagmite119 Formation on top of Reactor vessel Head (8 RFO)30. Flange Leakage Crusted On Side of Nozzles and Stalactites from Gaps in120 Insulation (8 RFO)31. Reddish Brown Boron Deposits Crusted on Side of Nozzle (8 RFO)12132. Boron Deposits Source Unclear (8 RFO)12233. North Side of Reactor vessel Head (10 RFO)12334. Boron Deposits Near Top of Reactor vessel Head (10 RFO)12435.

9 Typical Deposits for Periphery (10 RFO)12536. Red Rusty Boric Acid Deposits on vessel Flange (12 RFO)12637. Boron Piled Under the Insulation (11 RFO)12738. Boric Acid Deposits with Heavy Iron Concentration on Underside of Nozzle 3128(13 RFO)39. 2000 Interferences with CRDM Flange Inspection12940. RE4597 Sample Location13041. CTMT Radiation Monitors RE4597AA/BA (Combined Iodine Channels)13142. CTMT Radiation Monitors RE4597AA & BA (Both Noble Gas Channels)13243. Potential Effects of Boric Acid Deposits on vessel Top Head Surface133 root CAUSE analysis ReportAttachments viAttachments_____TitlePage Evidentiary Request List (Rev. 4) of Relevant Events137 root CAUSE analysis Problem Statement Problem Reason for InvestigationSignificant degradation of the reactor pressure vessel top head base metal was discovered atnozzle 3 (toward nozzle 11) and minor corrosion at nozzle 2 during the thirteenth refuelingoutage (13 RFO) in March, root CAUSE report addresses the CAUSE of the loss of RPV head base metal in the region ofnozzle 3 and 2.

10 This issue of the root CAUSE report addresses conditions and information availablethrough April 6, Consequences of Event/Condition InvestigatedThe RPV head is an integral part of the reactor coolant pressure boundary, and its integrity is vitalto the safe operation of the plant. Degradation of the RPV head or other portions of the reactorcoolant pressure boundary can pose a significant safety risk if permitted to progress to the pointwhere there is risk of a loss of coolant accident. analysis indicates that the as-found condition ofthe affected nozzles would not have been expected to result in failure of the pressure integrity ofthe reactor coolant system. However, the degraded condition had been progressing over a periodof time, without knowledge of the Immediate Actions Taken1.