SVP-97-267, Submits Response to Request for Info on Verification of Adequate Wall Thickness for Mark I Torus Shells for Units 1 & 2.Rept Discussing Present Matl Condition of Torus Shell & Plant Plans to Maintain Torus in Acceptable Condition Encl

Submits Response to Request for Info on Verification of Adequate Wall Thickness for Mark I Torus Shells for Units 1 & 2.Rept Discussing Present Matl Condition of Torus Shell & Plant Plans to Maintain Torus in Acceptable Condition Encl

ML20199J377
Person / Time
Site:	Quad Cities
Issue date:	11/21/1997
From:	Pearce L COMMONWEALTH EDISON CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
SVP-97-267, TAC-M95296, TAC-M97463, NUDOCS 9711280132
Download: ML20199J377 (14)

Text

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• e SVP-97 267 November 21,1997 U. S. Nuclear Regulatory Commission Washington, D. C. 20555 Attention: Document Control Desk

Subject:

Quad Cities Nuclear Power Station Units I and 2 Comed Response to Request for Information on Verification of Adequate Wall Thickness for Mark i Torus Shells, Quad Cities Nuclear Power Station, Units 1 and 2 (TAC Nos. M95296 and M97463)

NRCJ)sskcLNumhcrt10314_ nod 50-261

Reference:

(a) Letter to Ms. Irene Johnson dated 9/24/97 in the referenced letter, the Nuclear Regulatory Commission (NRC) issued a request for information concerning verification of adequate wall thickness for Mark i Torus Shells.

The letter expiessed a concern that torus wall thinning may have occurred, as a result of corrosion of the steel shell, in areas that have defective or missing coating.

In response to the above concern, Quad Cities has completed a document search of the inspection reports, engineering evaluations and other applicable documents back to original cons;ruction. A report, discussing the present material condition of the torus shell and Quad Cities Station's plans to maintain the torus in acceptable condition, is attached.

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SVP-97 267 2 November 21,1997 l

Ifyou have any questions concerning this letter, please contact Mr. Charles Peterson, ,

Regulatory Affairs Manager, at (309) 654 2241, extension 3609.  !

Respectfully, e e MOL '

L. W. Pearce ,

Site Vice President Quad Cities Station Enclosure A, Material Condition of Primary Containment Suppression Chamber (Torus) f cc: A.13. Ileach, Regional Administrator, Region 111 ,

R. M. Pulsifer, Project Manager, NRR C. G. Miller, Senior Resident inspector, Quad Cities W. D. Leech, MidAmerican Energy Cornpany D C, Tubbs, MidAmerican Energy Company F. A. Spangenberg, Regulatory Affairs Manager, Dresden >

INPO Records Center Office of Nuclear Facility Safety, IDNS DCD License (both electronic and hard copies)

M. E. Wagner, Licensing, Comed i SVP Letter File 4

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< ENCLOSURE A A1 ATERI AI, CONDITION OF PRihlARY

. , CONTAIN51ENT SUPPRESSION CilANIHER (TORUS)

SVP-97-267 (Page 1 of 8)

IIACKGHOUND The NRC is interested in obtaining information from several licensees relating to the minimum code wall thicknen in critical areas of the torus structure. The areas of concerns are: Vicinity of the torus to ring girder support welds, at the air / water interface and in submerged areas The NRC concern is that toms shell wall thinning may have occurred as a result of corrosion in the areas that have had defective or missing coating.

A document search has been completed of the inspection reports, engineering evaluations and other applicable documents from the recent time frame back to the available construction documents including discussions with John Caturano (Comed)

Level 111 coating specialist, Mike Anderson (Quality Control), Bob Cherf(Construction),

Rick Scoville (Design), Gary McCar1y (Work Analyst) and James Trettin (System Engineer)

This report discusses the present material condition of the torus shell and Quad Cities Station plan to maintain the torus in acceptable condition.

DESIGN INFORMATION The primary containment for Quad Citi s Nuclear Station Units I and 2 is a General Electric Mark I containment system c.onsisting of a drywell and a pressure suppression chamber (torus) approximately 40% full of water. The drywell encloses the reactor pressure vessel, the reactor recirculation system, and branch connections to the reactor coolant system The torus provides a source of water to condense the steam and thereby limit the peak pressure in the primary containment in the case of a loss of coolant accident (LOCA). It also provides water for long ton cooling of the nuclear core following a LOCA. The vent system distributes the steam released from the reactor vessel in the event of a LOCA through the eight vent lines (6 8. 9 in. dia.) a common vent header (4 ft 10 in. dia.)inside the torus and 96 downcomers (2 0. dia.) which discharge the steam below the water ievelin the torus. Schematic drawings of the containment are attached for ready reference.

DESIGN TillCKNESS As per Chicago Bridge & Iron Company (the supplier of the Containment Vessel) fabrication drawings, the shell plate material is AS16 Grade 70 for both Unit I and Unit

2. The design thickness of the upper plates is 0.582 in. and the bottom plates is 0.649 in., except at penetrations, where it is locally thickened. Each bay is reinforced at each mitered joint by a T-shaped ring girder.

1

c ENCL,OSURE A q h! ATERI AL CONDITION OF PRlh1 ARY l CONTAINh!ENT SUPPRESSION CIIAh!HER (TORUS) l SVP-97-267 l (Page 2 of 8) l CORROSION ALLOWANCE There is no allowance for corrosion made in the structural design.

It is important to note that during normal unit operation the torus is purged with nitrogen and the rate of corrosion is considered negligible, the highest corrosion rate will occur during an outage when the torus is open to the environment (Ref. Torus coatings report from J.S. Caturano dated 10-15-92).

Ilased on the literature search (performed by Nutech in March 1980), the fo; lowing corrosion rates were gisen for guidance:

• In demineralized water environments containing air saturated water at or near room temperature, the general corrosion rate of unprotected carbon steel is expected to be less than 10 mils per year, pit depths may approach 20 mils per year.

Ilased on the actual conditions at Quad Cities station, a reasonable estimation of projected corrosion rates while maintaining a factor of 2 for uncertainty (water chemistry and environmental factors effecting the shell corrosion) associated with the future plant operating condition and data collection accuracy is as follows:

. General corrosion rate 5 mils per year, Pitting Corrosion 10 mils per year.

Torus Pitting Corrosion Acceptance Criteria (Calc # 64.30$.2029, April 1994.) for Quad Cities Nuclear Power Station Units I and 2 prepared by Vectra Engineering has been used for evaluating shell thinning due to corrosion or removal of surface defects by grinding or other mechanical means As per the conclusion of this report:

• 1/16 in. corrosion allowance is available without any limitations on thinned areas or isolation from other thinned regions.

. Any localized thinning (encompassed in a 21/2 in, max. dia.) that does not penetrate the shell is acceptable. However, for conservatism, a maximum depth of half the nominal shell thickness will be used.

., ENCLOSURE A A1 ATERI AL CONDITION OF l'R151 ARY CONTAINhlENT SUI'PRESSION CilA5111ER (TORUS)

SYl'-97-267 (Page 3 of 8)

SIARGINS AVAILAllLE The Quad Cities Torus was designed, fabricated, installed and tested per ash 1E Section lit,1965 Edition, including addenda up to and including vinter 1965. In the mid 1970's, new loads and load combinations were identified by General Electric for hiark I primary Containment systems The new loads are addressed by the hiark I Long Term Program (LTP) which required a complete re analysis of the toms for the new loads to demonstrate that the resulting stresses met the acceptance criteria provided in Appendix A to Nureg -0661. Where stress acceptance criteria was not met, plant modifications were required. The LTP Plant unique analysis was performed by Nutech Engineers.

As built Thickness Margins The shell thickness documented in the code data sheets is 0.582 in and 0.649 in. which is the same as in the Cil&l fabrication drawings. UT measurements taken during defect removal has documented thickness readings in excess of the minimum noted above.

Material Strength Margin The torus is fabricated from A516 Grade 70 plate. The material has a specification minimum yield strength cf 38,000 psi and a minimum tensde strength of 70,000 psi.

hiaterial certifications for the actual plates used in the fabrication of the torus have not been located at this time. Comparable Cenified hiaterial Test Reports (CMTR) for 5/8 in plate material used for fabrication of containment have the following yield and tensile strengths.

Yield Strength $ 1,900 psi Tensile Strength 79,200 psi The ASME Code allowable stress fbr A516 Grade 70 materialis based on the specification minimum tensile strength oithe material. The yield and tensile strength of the actual material used is significantly higher than the minimum requirem .its.

CORROSION PROTECTION The primary containment is protected against corrosion by coating all ferrous metal surfaces with coatings that must not deteriorate in a post accident environment.

. ENCI.OSURE A M ATERI AL CONDITION OF l'RISIARY CONTAINMENT SUI'I'RESSION CilAMllER (TORUS)

SVI'-97 267 (l' age 4 of 8)

ANAINSIS TilAT MINIMUM WA1.1,IS MET (including any UT examinations performed)

There are no documents indicating thickness measurements taken on a regular basis and comparisons made for the wall thickness to quantify any metal thinning taking place. UT thickness measurements of the torus shell have been taken during removal / repair of mechanical damage / are strikes. (* UT report for Q2R12 dated 3 27 94, for bay # 8, quadrant # 2, has documented thickness readings of 0 66 in. to 0.71 in. on one 11. square grid.

• UT report for Unit 2 dated 1-3-92 has documented min. shell thickness for bay #

2, quadrant # 4 as 0 61 in , bay # 15, quadrant # 1 as 0.69 in., bay # 15, quadrant # 2 as 0.76 in. and bay # 15 quadrant # 3 as 0.75 in.

• On 2 1-92, wall thickness reported on Unit 2 bay #16 quadrant #1 was 0.730 in.). During scheduled inspections of the submerged areas, all the bays were vacuumed to remove sludge accumulation, coating inspections and measurements of corrosiort/ pitted areas were performed The deficiencies were evaluated and necessary repairs performed. Therefore the minimum shcIl thickness seems to have been maintained for the existing life of the torps shell.

ORIGINAL, COATINGS DETAILS, PERIODIC INSPECTIONS PERFORMED AND REPAIRS PERFORMED ON COATINGS Unit I i ORIGINALS COATINGS Quad Cities Nuclear Station's Unit I toms immersion areas were originath coated in 1968 through 1970 with a three cat epoxy phenolic (Plasite 715511, manufactured by Wisconsin Protective Coatings) osog The coating system was completed following extensive rework and the toms wa initially filled with water in May 1970.

PERIODIC INSPECTIONS AND REPAIRS Inspections performed during outages from 1974 to 1977 and repairs of pressure boundary surfaces were performed using Plasite 715511.

Repairs performed after this period were accomplished using Carbo Zinc 11 SG Inorganic Zine Silicate, manufactured by the Carboline Company. Inspections and repairs through 1984 were performed following torus drain down.

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ENCLOSURE A A1 ATERI AL CONDITION Olt l'RihtARY CONTAINA1ENT SUI'I'RESSION CilA5111ER (TORUS)

SVI'-97-267 (l' age 5 of N)

Torus immeision areas were inspected by S.G. Penny commencing in Sept.,1989, utilizing level 11 inspectors trained and certified in accordance with SG Penny Training and Certification Procedures. Underwater repairs were performed on all areas where deficiencies were identified. It was also concluded that the original (poxy phenolic coating on the pressure boundary was performing well (considering the 19 years of service) Pitting ranging from 2 5 mils to 14 mils in depth were reported, evaluated and accepted 1

The next underwater inspection was performed by SG Penny during QlR12 ( Sept.  !

1992. A detailed qualitative coating inspection was completed in bays 2,7,15 and 16 and general inspection of all other bays was completed to asst as the condition of the coating and compare the changes since the last inspection The conclusion of the inspection was that no evider.cc of current or incipient general coating failure was identified.

Underwater coating repairs were performed as identified and agreed to by Comed Level 3 coating specialist.

During QlR13(Mech 1994) all 16 bays of the Unit I torus were drained,  !

decontaminated, shotblasted (shotblasting process removes surface corrosion / corrosion products and insignificant base metal ) and recoated with Keeler & Long 6548/7107 epoxy coating During QlRl4, all 16 bays of the toms shell immersion area were desludged, and inspected qualitatively and quantitatively by SG Penny. Coating deficiencies (

mechanical damage, burrs and projections) as reported were iepaired using Plasite C 790 undenvater epoxy. During this inspection and repair, metal loss due to corrosion was virtually eliminated and recommendations were made to reexamine the pressure boundary at regular scheduled intervals based on as found condition of the torus.

Unit 2 l

ORIGINAL, COATINGS Quad Cities Nuclear Station's Unit 2 torus immersion areas were originally coated with Phenoline 368 Primer and Finish, manufactured by Carboline Corporation of St. Louis Missouri. The application was completed and the torus was filled with water in June 1971.

ENCI.OSURE A hl ATERI AI, CONDITION OF l'Rihl ARY

. . CONTAINSIENT SUI'I'RESSION CilAh!HER (TORUS)

SVP-97-267 (l' age 6 of 8)

PERIODIC INSPECTIONS AND REPAIRS  :

Repairs to the original coating system were performed in March 1974,1982 and 1983 using Carbozine lI SG Inorganic Zine Silicate, manufactured by Carboline. These sepairs were made Ib!!owing drain down of the torus.

Welding operations on the torus exterior shell necessitated additional inorganic zine repairs on a 32 sq R. area due to heat induced blistering, the coating was inspected by diveis in January 1978 and the coating was reported as satisfactory at that time. Further inspections were performed aner draining the torus in Dec.1982 and Dec.1983 when minor repairs were required.

T he next inspection was performed in Jan 1990 by SG Penny utilizing level 11 inspectors trained and certi0cd in accordance with SG Penny Training and Certi6 cation Procedures.

Underwater repairs were performed on areas of minor corrosion on the pressure boundary throughout the toms.

As a part of the ongoing inspection and maintenance program, SG Penny performed inspections and necessary repairs of the coating in Feb 1992, and in March 1993.

During Q2R13, the toms was completely drained, inspected and coatings were repaired using inorganic zinc. The overall toms coating was tcported to be in good condition Metalloss due to corrosion at locations where the coating system has been breached on the torus pressure boundary has been virtually eliminated through underwater coating repair.

Recommendations made by SG Penny were to reexamine the pressure boundary at regular scheduled intervals based on as Ibund condition of the torus.

ANAINSIS OF DETECTED Fl.AWS AND REPAIRS PERFORMED ( Are Strikes etc.)

Wall thickness measurements using UT inspection techniques were performed for the fbilowing evaluations:

During March 1990 (Q2R10) inspections, a relevant indication (arc strike) 0.273 in. deep and approximately 1/2 in. by 1 in. was reported in bay # 8 quadrant # 2 of Unit 2. In accordance with recommendations (Calc No COE210.0201) made by Pacinc Nuclear, the material surrounding the are strike was ground out, and the resulting cavity was blended

ENCI.OSURE A NI ATERI AI, CONDITION OF l'RIS1 ARY '

. CONTAINS1ENT SUI'I'RESSION CIIAhlilER (TORUS)

SVI'-97-267 (l' age 7 of 8)

Arc strikes were discovered near the monorall support pads, while performing the inspections of the torus shell during OIR13 torus recoat project. These are strikes were mapped by bay number, location, approximate quantity and dimensions These are strikes were removed with the technical support from Vectra Engineers.

During Q2RI1, mechanical defect in bay // 9 was reponed and was repaired / accepted per evaluation il Calc No. COE 004 dated 312 92.

Two flaws were detected during Q2R13 in uay //14. These flaws were pits / gouges introduced in the shell apparently caused by mechanical damage.(c g. inadvertent glinding, denting rrom impact of heavy tools or equipment, etc.). These were accepted per Vectra letter i.- 1598 00034 003.

All the above evaluations were performed using guidance from the codes and fractme mechanics PRECAUTIONS TAKEN DURING WEl. DING AND OTilER WORK Coating inspections and repairs were performed per approved procedures by qualified inspectors Repairs to pitting, mechanical damage and arc strikes are completed per approved procedures There is no record of welded sepairs performed on the torus shell of either Unit.

COATING PROGRAM TO ENSURE INTEGRITY As per the documents reviewed, coatings have been inspected on a regular basis and repairs performed as recommended by the inspectors and approved by Comed.

SUMM ARY (Analysis proving that we have maintained the Torus in the past)

Hased on the above detr s, particularly the results ofinspections performed during QlR14 and Q2R14, the overall condition of the torus is considered to be good considering the 26 yrs. age (for Unit 2) and type of the coatings applied General corrosion is negligible to this date. Ilowever, inspections performed indicate areas of coating defects but no metailoss. Some localized pitting was also evident. The pit depths have ocen m aluated based on the acceptance criteria prepared by Vectra (CALC

// 64.305.2029) for Shell Thinning due to corrosion. This acceptance criteria is based on existing design margins and a study of the effects oflocalized thin areas based on

' ALGOR' FINITE El.EMENT MODELING SYSTEM.

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ENCLOSURE A SIATERIAL CONDITION OF PRISIARY

. CONTAINSIENT SUPPRESSION CilAhlHER (TORUS)

SVP-97-267 (Page 8 of 8)

The rate of coating depletion and pit depth growth rate are not well known because reference or baseline value inspections were never established. Therefore, torus inspections over the next several fuel cycles should focus on obtaining reliable information on the rate of coating depletion and pit depth growth rate, while maintaining the integrity of the torus pressure boundary.

FITI URE INSPECTIONS AND DESCRIPTION OF IWE PROGRAM Comed is committed to ASME Section XI lWE (ISI) program implementation which will assure that critical areas of containment are routinely inspected to detect and take cortective action for defects that could compromise containment structuralintegrity. All required IWE examinations will be completed for the fust period by Sept.,2001. These inspections will be performed by qualified inspectors per approved procedures. Details of this program and procedures are currently being developed for all Comed nuclear units.

For both units I and 2, Predefmed Work Requests have been aded to the Electronic Work Control System (EWCS) for torus coating inspections Thi se work requests will be reviewed Ihr each refuel outage and scope will be determined I ased on the IWE program requirements, results of the previous inspections, and an," other relevant commitments.

REFERENCES:

1. Inspection and Evaluation Reports.

( 2. Code Data Sheets.

3. Primary Containment System, Design Basis Document.

4. Torus Pitting Corrosion Criteria (Cal # 64.305.2029).

5. Investigation of corrosion rates in S A 516 Grade 70 Carbon Steel Materialin Aqueous Environments.

1 NITACilMl'NTS:

1. Plan View of Containment.

l 2. Suppression Chamber Section, Midbay Vent 1.ine Day.

3. Su; pression Chamber Section-Miter joint.

4. Elevation View of Containment.

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