RA-20-0329, Proposed Alternative for Inservice Inspection of the Containment Metallic Liner and Moisture Barriers

Proposed Alternative for Inservice Inspection of the Containment Metallic Liner and Moisture Barriers

ML21250A398
Person / Time
Site:	Robinson
Issue date:	09/07/2021
From:	Kapopoulos E Duke Energy Progress
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RA-20-0329
Download: ML21250A398 (14)

Text

Ernest J. Kapopoulos, Jr.

H. B. Robinson Steam Electric Plant Unit 2 Site Vice President Duke Energy 3581 West Entrance Road Hartsville, SC 29550 o: 843.951.1701 f: 843.951.1319 Emie.Kapopoulos@duke-energy.com 10 CFR 50.55a Serial: RA-20-0329 September 7, 2021 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 H. B. Robinson Steam Electric Plant, Unit No. 2 Docket Number 50-261 Renewed Facility Operating License No. DPR-23

Subject:

Proposed Alternative for Inservice Inspection of the Containment Metallic Liner and Moisture Barriers Ladies and Gentlemen, Pursuant to 10 CFR 50.55a(z)(1), Duke Energy Progress, LLC (Duke Energy) is submitting a request for relief from the requirements of American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME Code),Section XI, Subsection IWE at H. B. Robinson Steam Electric Plant, Unit No. 2 (RNP). Specifically, Duke Energy is seeking to perform an alternative to ASME Code Section XI requirements for containment metallic liners and moisture barriers within the Code. Relief is being requested on the basis that the alternative provides an acceptable level of quality and safety in accordance with 10 CFR 50.55a(z)(1).

A virtual pre-submittal meeting was held between Duke Energy and the U.S. Nuclear Regulatory Commission (NRC) project manager and technical support staff on February 25, 2021. The details of this public meeting can be found in the Agencywide Documents Access and Management System (ADAMS) using the accession number ML21068A417.

Duke Energy requests NRC approval of the proposed alternative within one year of the date this submittal is accepted for NRC review.

Relief Request RA-20-0329 is provided as an Enclosure to this letter.

This letter contains no new regulatory commitments. Should you have any questions concerning this letter, or require additional information, please contact Art Zaremba, Manager - Nuclear Fleet Licensing, at 980-373-2062.

Sincerely, Ernest J. Kapopoulos, Jr.

Site Vice President

RA-20-0329 Page 2

Enclosures:

1. Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1) for Inservice Inspection of the Containment Metallic Liner and Moisture Barriers cc:

L. Dudes, USNRC Regional Administrator, Region II M. Fannon, USNRC Sr. Resident Inspector - RNP J. Klos, USNRC NRR Project Manager - RNP

Duke Energy Progress, LLC.

H. B. Robinson Steam Electric Plant, Unit 2 Relief Request Serial # RA-20-0329 Proposed Alternative in Accordance with 10 CFR 50.55a(z)(1) for Inservice Inspection of the Containment Metallic Liner and Moisture Barriers

RA-20-0329 1.

ASME Code Component(s) Affected:

Containment shell metal liner covered by thermal insulation and stainless steel sheathing between Elevations 228-0 and Elevation 367-10 and related sealant materials and moisture barriers.

===2.

Applicable Code Edition and Addenda===

The applicable Edition and Addenda of the ASME Code,Section XI is identified in Table 1 and is referenced in the RNP third interval Containment Inservice Inspection (CISI) plan document, RNP-PM3-006.

Table 1 Plant/Unit(s)

Containment ISI Interval Applicable ASME Section XI Code Interval Start Date Interval End Date1 H. B. Robinson Steam Electric Plant (RNP), Unit 2 Third 2013 Edition 09/09/2019 09/08/2029 Note 1: The Interval End Date is subject to change in accordance with IWA-2430(c).

===3.

Applicable Code Requirements===

3.1 IWE-2500, Table IWE-2500-1, Examination Category E-C, requires that 100% of surfaces requiring augmented examination that are accessible for visual examination be examined using a VT-1 visual examination method during each inspection period.

3.2 IWE-2420 requires that examinations performed in accordance with Examination Category E-C shall continue to be performed during subsequent inspection periods until the flaws or areas of degradation remain essentially unchanged for the next inspection period.

3.3 IWE-2500, Table IWE-2500-1, Examination Category E-A, requires that 100% of accessible moisture barriers receive a general visual examination during each inspection period.

3.4 IWE-3512 requires that moisture barriers with wear, damage, erosion, tear, surface cracks, or other defects that permit intrusion of moisture against inaccessible areas of the pressure retaining surfaces of the metal containment shell or liner shall be corrected by corrective measures. Corrective measures may be deferred until the next regularly scheduled refueling outage if an engineering evaluation (IWE-3122.3) demonstrates that degradation from any moisture intrusion would not reduce the thickness of the base metal in local areas by more than 10% of the nominal plate thickness, or the degraded-reduced thickness can be shown by analysis to satisfy the requirements of the Design Specification.

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RA-20-0329

4. Reason for Request

4.1 Duke Energy considers aspects of currently published and endorsed Editions of the ASME Section XI IWE requirements more applicable to containment vessels rather than metallic liners and that the alternative approach described in this request is acceptable for the metallic liner design at H. B. Robinson Steam Electric Plant, Unit 2 (Robinson). For example, the IWE-3513 acceptance standard for visual examination, VT-3 defines erosion or corrosion that exceeds 10% of nominal wall thickness as a relevant condition. Wall loss on the order of 10% may be difficult to establish using visual examination only, and often requires supplemental ultrasonic examination (UT) to determine wall thickness. In addition, metallic liner designs typically have smaller design minimum wall thickness because the liner is not acting as a containment vessel. In such cases, establishing 10% wall loss as a relevant condition for liners when there is significant margin above the minimum design wall is excessive. Duke Energy is proposing an alternative to this 10%

requirement.

4.2 The interior surface of the containment liner plate is covered by thermal insulation panels, which extend from Elevation 228-0 to Elevation 367-10. Duke Energy is requesting an alternative to ASME Code Category E-C E4.11 requirements due to the impractical nature of removing insulation panels on 100% of the liner during each inspection period. During the Second Containment Inservice Inspection Interval, examination of the insulated containment liner plates was performed in accordance with Relief Request No. IWE/IWL-RR-01 (Reference 8.2). Results of these examinations revealed that numerous containment liner plate surfaces behind the insulation panels continue to experience coatings degradation and loss of material due to pitting and general corrosion. However, all examination results have shown that the remaining plate thicknesses are greater than that required to satisfy the minimum required by design. Reference Attachment 2 for further detail of historical examination results. Upon completion of each examination, the affected surfaces were cleaned and recoated as needed to protect the surfaces from continued degradation.

4.3 During the First and Second Containment Inservice Inspection Intervals, more than 400 insulation panels were removed to facilitate inspection of the containment liner with coatings reapplied as needed. As such, the liner plate behind more than 1800 of the insulation panels is now subject to VT-1 visual examination in accordance with ASME Code Category E-C, Item Number E4.11 and would be required to be inspected during each inspection period. This request provides for examinations as part of the proposed alternative consistent with examinations performed during the Second Containment Inservice Inspection Interval in accordance with Relief Request No. IWE/IWL-RR-01 (Reference 8.2).

4.4 Silicone rubber sealant is installed at the interface between adjacent stainless steel sheathing panels that cover the containment liner plate thermal insulation.

Reference Attachment 1, Liner Insulation Detail Drawing. Duke Energy understands that there are various interpretations throughout the industry on the definition of moisture barriers and the inspection requirements. Based on the guidance in NRC Regulatory Issue Summary 2016-07, Duke Energy considers that these materials could be subject to general visual examination during the Third Ten-Year Containment Inservice Inspection Interval in accordance with NRCs application of ASME Code Category E-A, Item Number E1.30. The total length of this sealant material is calculated to be in excess of 23,000 feet, which makes it Page 3 of 12

RA-20-0329 difficult to perform an examination of 100% of this material during each inspection period. Data collected during the First and Second Containment Inservice Inspection Intervals has shown that containment liner plate coatings degradation and corrosion has occurred on containment liner plate surfaces under insulation behind stainless steel sheathing that has been sealed with these silicone rubber sealant materials. Examination results from UT data collected has exceeded 10%

of nominal wall thickness. Where necessary, coatings have been restored which provides reasonable assurance that further degradation will be mitigated. A summary of historical examination results and a drawing depicting areas examined and obstructions is included in Attachment 2 of this relief request. Duke Energy is proposing an alternative to visually examining 100% of the sealant materials between the stainless steel sheathing covering thermal insulation attached to the containment shell metal liner plate.

4.5 Caulking and sealant materials are also installed between the containment concrete floor and the base of the stainless steel sheathing at Elevation 228-0, and at the top of the insulation panel sheathing at Elevation 367-10. Reference, "Liner Insulation Detail Drawing". Due to the difficulty and hardship involved in safely reaching the material at Elevation 367-10 for inspection, Duke Energy is proposing not performing visual examinations of the caulking and sealant materials at this elevation.

5. Proposed Alternatives and Bases for Use:

5.1 The metallic liner configuration of thermal insulation panels covering the majority of the metallic liner is unique within the industry. Robinson is one of two known operating US nuclear plants with this configuration. Consequently, the intrinsic focus of ASME Section XI IWE requirements on vessels creates difficulty in performing inspections as required in the code. In lieu of the requirement to perform a visual examination on 100% of the containment insulated liner plate in accordance with ASME Code Category E-C, Item Number E4.11 during each inspection period, the following alternative is proposed.

5.1.1 VT-1 visual examinations shall be performed on 10% of the containment liner plates behind insulation panels at the interface between the concrete and the containment base mat (228-foot Elevation) in accordance with Table IWE-2500-1, Examination Category E-C, Item Number E4.11. Previous inspections were performed during each of the first two intervals on 100% of the liner plate at the base mat of containment. Selection will consider previous examination results.

5.1.2 These examinations shall be performed during the First Inspection Period or may be distributed in approximately equal numbers during each inspection period.

5.1.3 A general visual examination shall be performed in accordance with Table IWE-2500-1, Examination Category E-A, Item Number E1.30 on moisture barriers installed between the containment liner plate and the interior concrete floor embedment zone at all locations examined in accordance with 5.1.1 and 5.1.2.

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RA-20-0329 5.1.4 VT-1 visual examinations shall be performed on additional areas of the containment liner that were not examined during the First or Second Containment Inservice Inspection Intervals in accordance with ASME Code Category E-C, Item Number E4.11. A minimum of 90 insulation panels shall be selected for removal to facilitate liner inspections from diverse locations from the population of accessible liner plates to be examined1. The liner will be divided into three separate areas to facilitate selection of 30 panels in each area at varying heights and azimuths. Panels adjacent to areas of past degradation will be prioritized. This selection process will provide a reasonable representation of the condition of the liner as data will be collected from many different areas of the liner. The drawing in Attachment 2 provides a conceptual idea of the liner scope for the interval. In lieu of the requirement to perform these examinations during each inspection period, these examinations shall be distributed in approximately equal numbers during each inspection period or refueling outage in the inspection interval.

5.1.4.1 There are numerous obstructions inside containment at Robinson that limit access to approximately 43% of the insulation panels.

Approximately 359 panels are inaccessible due to physical obstruction such as an HVAC ring. Approximately 575 panels are inaccessible due to locations associated with the polar crane at elevation 352 and above. See the drawing in Attachment 2 for further detail. For this reason, areas of the liner in these locations shall not be included in the population from which samples are selected for examination.

5.1.4.2 Panels at the higher elevations are considered inaccessible because their elevation poses a significant challenge for erection of scaffolding or other means to safely access and perform the examinations. For this reason, liner plates above Elevation 308 shall not be included in the population from which samples are selected for examination.

5.1.5 In addition to examinations proposed in 5.1.1, 5.1.3 and 5.1.4, when any insulation panel at any elevation is removed for maintenance activities, VT-1 visual examinations shall be performed on the exposed containment liner plate surfaces in accordance with Table IWE-2500-1, Examination Category E-C, Item Number E4.11. If any of these examinations are performed on liner plate surfaces at Elevation 228, a visual examination of the exposed moisture barrier at the containment liner plate and the interior concrete floor embedment zone shall also be performed in accordance with Table IWE-2500-1, Examination Category E-A, Item Number E1.30.

5.1.6 Successive examinations of insulated containment liner plate surfaces shall be performed, if applicable, in accordance with IWE-2420, but may be limited to surfaces whose coatings have not been restored to protect against potential future degradation.

1 This sample size satisfies the criteria in NRC Draft Regulatory Guide DG-1070, Table 1 for a lot size greater than 1000.

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RA-20-0329 5.1.7 Visual examinations of containment liner plate surfaces that reveal relevant conditions that violate 120% of the minimum liner plate wall thickness required by design shall be extended to include additional examinations during the current outage. The additional examinations shall include two accessible adjacent panels. If the additional examinations reveal flaws or relevant conditions that violate 120% of the minimum liner plate wall thickness required by design (or are otherwise unacceptable for continued service without performing repair/replacement activities), the examinations shall be evaluated to determine if further scope expansion is warranted. No additional examinations are required if the engineering evaluation concludes that either there are no other areas of the liner subject to the same condition, or no other degradation mechanism exists.

5.1.8 If visual examinations of containment moisture barriers installed between the containment liner plate and the interior concrete floor embedment zone at Elevation 228-0 that reveal relevant conditions that exceed the acceptance standards of IWE-3512, and the criteria to defer repair one refueling cycle cannot be met, examinations shall be extended to include additional examinations during the current outage. The additional examinations shall include the adjacent panel on each side of the panel where the relevant condition exists. If the additional examinations reveal conditions that exceed the acceptance standards of IWE-3512, and the criteria to defer repair one refueling cycle cannot be met, the examinations shall be further extended to include additional adjacent panels until no relevant conditions are present or the criteria to defer repair for one refueling cycle is met. No additional examinations are required if the engineering evaluation concludes that either there are no other areas of the moisture barrier subject to the same root cause condition, or no degradation mechanism exists.

5.2 Basis for use of the Proposed Alternative in 5.1 5.2.1 The containment liner at Robinson, Unit No. 2, is partially covered by insulation and stainless steel sheathing. The insulation and stainless steel sheathing form part of the defense-in-depth philosophy of the containment liner at Robinson, Unit No. 2. The removal and reinstallation of all insulation sheathing panels for the performance of visual examinations is considered impractical.

5.2.2 During the Second Ten-Year Containment Inservice Inspection Interval, examinations were performed on containment liner plate surfaces behind more than 300 insulation panels that were removed. This included the planned removal of the entire inventory of panels at the ground floor elevation, for which both liner and moisture barrier examinations were performed. Additional panels at higher elevations were removed for various reasons. The results of visual examinations (and supplemental ultrasonic thickness measurements, when required) performed at all locations revealed that minimum liner plate wall thicknesses satisfied the minimum thicknesses required by design. Liner plate coatings were reapplied as necessary, and as-left examinations were performed prior to panel insulation and sheathing replacement. Duke Energy considers these surfaces are no longer priority examination locations and that re-applying the coating following the examination provides reasonable assurance that degradation is being mitigated. Except for examinations Page 6 of 12

RA-20-0329 proposed in 5.1.1, examinations performed during the Third Ten-Year Containment Inservice Inspection Interval will be focused on those containment liner plate surfaces that have not been examined during the First or Second Containment Inspection Intervals.

5.2.3 The alternative proposed for use during the Third Ten-Year Containment Inservice Inspection Interval will provide reasonable assurance that corrective actions taken to recoat liner plate surfaces behind insulation panels at Elevation 228-0 (and other locations previously inspected and recoated) have been sufficient to protect the containment liner plate surfaces from further degradation. This alternative also provides additional assurance that other containment liner plates that have not been previously examined or recoated continue to meet the minimum required liner plate wall thickness.

The proposed alternative also includes requirements for performing additional examinations when relevant conditions exceeding specified acceptance standards are exceeded. These additional examinations will provide reasonable assurance that conditions exceeding the specified acceptance standards do not exist in other locations.

5.2.4 Both a Structural Integrity Test and an Integrated Leak Rate Test were performed during the fall 2020 outage at Robinson. Successful results of these tests provide reasonable assurance of the integrity of the containment liner.

5.3 In lieu of the requirement to perform a general visual examination in accordance with Table 2500-1, Examination Category E-A, Item Number E1.30 on 100% of the sealant and caulking materials installed between all of the stainless steel insulation panel covers during each inspection period, the following alternative is proposed.

5.3.1 A general visual examination shall be performed on all accessible sealant and caulking materials installed at the base of the bottom row of insulation panel stainless steel covers at Elevation 228 (approx.) except for very high radiological areas such as the area near the Regenerative Heat Exchanger.

Sealant and caulking material between vertical joints in the bottom row of insulation panel stainless steel covers shall also be examined. Exams shall be performed in accordance with Table 2500-1, Examination Category E-A, Item Number E1.30.

5.3.2 Sealant and caulking materials installed at the top row of insulation panel stainless steel covers between the insulation panel covers and the containment liner plate surface at Elevation 367-10 are considered inaccessible for examination and repair.

5.4 Basis for use of the Proposed Alternative in 5.3 5.4.1 Sealant and caulking materials at the base of the containment insulation panel stainless steel covers at the concrete floor interface at Elevation 228-0, and those installed in the vertical joints between containment insulation panel stainless steel covers at Elevation 228-0are considered a secondary method to prevent water from accessing inaccessible surfaces of the containment liner plate. These areas shall be examined once each inspection period accordance with Table 2500-1, Examination Category E-A, Item Number E1.30 to provide reasonable assurance that any potential standing water that could accumulate on the concrete floor cannot contact Page 7 of 12

RA-20-0329 the containment liner plate behind the insulation panels at this elevation.

5.4.2 Sealant and caulking materials installed between all other insulation panel covers will not be examined in accordance with Table 2500-1, Examination Category E-A, Item Number E1.30. However, the containment liner plate surfaces behind these insulation panels shall be subject to examination in accordance with the alternative proposed in 5.1. Examination of the containment liner plate surfaces in accordance with the proposed alternative is considered sufficient to manage the potential degradation of the containment liner plate and coatings from moisture (primarily atmospheric moisture).

5.5 In lieu of the Acceptance Standards required by IWE-3521(a) which states, pressure-retaining component corrosion or erosion that exceeds 10% of the nominal wall thickness, the following alternative is proposed:

5.5.1 Pressure-retaining component corrosion or erosion that exceeds 120% of the minimum wall thickness required by design shall require correction or evaluation to meet the requirements of IWE-3122 prior to continued service.

5.6 Basis for Use of the Proposed Alternative in 5.5 5.6.1 The Acceptance Standards in IWE-3521(a) are considered more appropriate for vessels. Use of this acceptance criteria for metallic liners is considered overly conservative given the Robinson liner has ample wall thickness margin and has a concrete containment to provide containment structural integrity.

5.6.1.1 The RNP Unit 2 containment is a post-tensioned concrete containment vessel with an A442 Gr 60 or A516 Gr 60 metallic liner. Although the containment metallic liner is not a pressure vessel, the liner is part of the containment structure whose function is maintaining a leak tight barrier. Significantly less than 90% of the liner thickness is required in order to maintain the liner design function of providing a leak tight barrier. Additionally, the thicker adjacent areas around the pitted areas provide sufficient reinforcement for any forces applied to the liner plate.

5.6.1.2 Site specific analysis of the liner has been performed as documented in Calculation RNP-C/STRU-1128 that evaluates the design conditions of the liner to ensure that the liner will function as a leak tight membrane under all design conditions. The containment liner was modeled and analyzed using a nonlinear static analysis. The analysis considered that the plate thickness must be sufficient to prevent elastic instability (buckling) of the plate, and must also be sufficient to limit design stress levels to 90% of yield stress. Consideration is given for yield at 3/8 studs from historical analysis of liner bulged conditions with conservative assumptions made with regard to adjacent broken/separated studs. This analysis considers the as-built LOCA temperatures coincident with other phenomena such as vertical prestress, dead load, and concrete shrinkage. Yield stress for 1/2 stud locations considers the ultimate strength derived from test data. The RNP-C/STRU-1128 evaluation concludes that a minimum thickness of 0.124 is required to meet the bounding conditions in the 3/8 and 1/2 nominal plate, with a higher value in the immediate 1 vicinity of the embedded support studs. At embedded stud locations minimum Page 8 of 12

RA-20-0329 thicknesses of 0.156 and 0.208 are required to sustain design loads for installed 3/8 and 1/2 diameter studs, respectively. These values are approximately 50% or less of the nominal plate thickness installed. Therefore, using an alternative acceptance criteria to the code required 90% nominal wall thickness is acceptable. Rather than setting evaluation criteria based on 90% of nominal wall, an alternative acceptance criteria of 120% of the minimum required thickness is proposed for the liner to fully function. This correlates to an acceptable minimum thickness of 0.149 for liner plate areas outside of embedded stud locations (>1), 0.1872 for areas within 1 of an embedded 3/8 stud, and 0.2496 for areas within 1 of an embedded 1/2 stud.

For the above reasons, Duke Energy requests authorization to use the proposed alternative pursuant to 10 CFR 50.55a(z)(1) on the basis that the alternative provides an acceptable level of quality and safety.

6. Duration of Proposed Alternative

This alternative is requested for the duration of the H. B. Robinson, Unit 2 Third Ten-Year Containment Inservice Inspection Interval, which is currently scheduled to end no later than September 8, 2029.

7. Precedents

Previous relief requests were approved for use during the First and Second Ten-Year IWE/IWL Inspection Intervals. The alternative requested for the Third Ten-Year IWE/IWL Inspection Interval is based, in part, on successful results obtained during the First and Second Ten-Year Containment Inservice Inspection Intervals. See References 8.1 and 8.2 for alternatives approved for use during prior Inspection Intervals.

8.

References:

8.1 Letter dated July 26, 1999, from Herbert Berkow, USNRC, to D.E. Young, Carolina Power & Light Company, related to the Evaluation of Relief Requests IWE/IWL-1 through IWE/IWL-9: Implementation of Subsections IWE and IWL of ASME Section XI for Containment Inspection for Carolina Power and Light Company's H.B.

Robinson Steam Electric Plant, Unit 2 (TAC No. MA4637) 8.2 Letter dated April 30, 2009, from Thomas Boyce, USNRC, to Eric McCartney, Carolina Power & Light Company, related to the Evaluation of Relief Requests IWE/IWL-RR-01 and IWE/IWL-RR-02: H. B. Robinson Steam Electric Plant, Unit No.2 -Request for Relief from ASME Code,Section XI, Subsection IWE Requirements for Containment Inspections, (TAC No. MD8509), (ADAMS Accession Number ML091170345) 8.3 NRC Draft Regulatory Guide DG-1070, September 1997, Sampling Plans Used for Dedicating Simple Metallic Commercial Grade Items for use in Nuclear Power Plants (ADAMS Accession Number ML003739230)

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RA-20-0329 Liner Insulation Detail Drawing CONTAJNMENT CONCRETE WALL LINER TRANSIHON TO DOME

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3/4 11 THICK FROM EL. 253'- 0" TO 352 1- 0" ANO,1/2" THICK ABOVE 352 1-0'1

• Page 10 of 12

RA-20-0329 CV Liner Insulation Panel Map and Historical Examination Summary To illustrate the typical liner findings during past inspections on the vertical cylinder, the following will be described: (1) the thinnest wall areas found, (2) typical patterns of corrosion in the areas observed, and (3) patterns in the bottom row of containment. The inspected portions of the containment liner are identified by the location of the insulation sheathing panel installed over the liner. Each insulation sheathing panel identification begins with the elevation of the location, followed by alphabetical identifier. These sheathing panels are approximately 4 high by 8 wide, varying in some areas due to interferences and installation considerations. Panels below elevation 253 are 1/2 nominal thickness, and panels above 253 are 3/8 nominal thickness. For discussion on historical liner inspections, the liner areas will be identified by the insulation sheathing label which was removed for the inspection.

In five separate locations, 50% wall thinning was found. These areas are at locations 240Y (inspected fall of 2013), 252C (inspected fall of 2008), 280E (inspected fall of 2013), 280L (inspected fall of 2013, and 308BBB (inspected fall of 2020). Panel 240Y was noted to have 55% surface area covered with tightly adhered rust. Three areas totaling 5% of the inspected surface area were noted with pitting, the deepest being 50% of the nominal wall. Panel 252C was noted to have 5 areas of pitting where the deepest pit was 50% wall loss. The total pitting area was less than 10% of the total inspected surface area. Panel 280E and 280L were each noted to have 25% surface area covered with tightly adhered rust. Pitted areas were noted, covering approximately 10% of the surface area on each panel. The deepest pit was 50% wall loss. The panel at 308 elevation had nearly 100% of the surface area covered with general corrosion with pitting. Seven pits were identified in the inspected area, with the deepest pit at 50% of the nominal wall.

Additionally, there is a single instance of a pit with 60% material loss from 1993. This was found prior to routine inspections and is an isolated pitted area behind panel 260B with a thickness of 0.155. The stud spacing in this area is 14. The design required uniform thickness of the plate at this elevation is 0.066. Therefore, a 2.3 factor of safety at the pit for material thickness is shown. It is worth noting that the calculated minimum wall is of uniform thickness, where actual field condition is a pit. The pit in the field is reinforced by adjacent thicker plate material.

Sufficient margin remains at this deepest historical pit.

The required minimum wall thickness varies by elevation due to the different spacing of the liner plate embedded studs. Stud spacing varies from 6 to 18 at different elevations of the vertical liner. The required minimum wall thicknesses vary from 0.029 to 0.071 in all areas above the floor slab. The highest value of 0.124 is required adjacent to the floor slab. For conservatism and simplicity, the bounding design minimum wall thickness of 0.124 for all stud configurations is used.

Generally, panels are found with areas of general corrosion and some pitting. Over 80% of the panels inspected have greater than 70% nominal wall remaining in all areas, with 36% of panels retaining 90% or greater of nominal wall. The bottom row of the insulation sheathing panels have historically been prioritized for removal to facilitate liner inspections. All panels have been removed to facilitate inspections twice during past intervals. The deepest pit was 60% of the nominal wall thickness, and 50% of the panels were found with all areas greater than 80% of nominal wall.

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= Inaccessible for reasons other than insulation (equipment obstructions, dose, elevation)

LEGEND

= Previously inspected area AREA #1*

AREA #2*

AREA #3*

• 30 Panels from each area will be selected for inspection, for a total of 90 panel inspections.

NOTES:

1. THE LOCKED HIGH RADIATION AREA OF THE REGENERATIVE HEAT EXCHANGER ROOM AND BENEATH THE FUEL TRANSFER CANAL IS LOCATED BETWEEN AZIMUTHS 345 - 40, AT ELEVATION 228' - 251.5'.