ML052080113

Mtg Summary of 6/22/05, Spring 2005 Steam Generator Inspection Presentation

ML052080113
Person / Time
Site:	Oconee
Issue date:	06/22/2005
From:	Duke Energy Corp
To:	Office of Nuclear Reactor Regulation
Olshan L N, NRR/DLPM, 415-1419
References
TAC MC7234
Download: ML052080113 (21)

Text

Oconee Nuclear Station Unit #1 Steam Generator Discussion with NRC Washington, DC June 22, 2005 C. Thomas Alley

>4,.

Introduction a Design comparison of the ONS OTSG and ROTSG

• Spring Inspections Results for ONS #1 SGs a Wear Indications and Probable Causes

• Duke's Response to the Wear Issue a Future Impact a Summary

;.g. C Attachment 2 1

Hourglass Broached Hole I .

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Significant Differences in Design between OTSG and ROTSG W- ' C dgkWVWAe w OTSG 50U ..

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r-Operational Comparison OTSG to ROTSG, and "A" to "B"

• FDW Flow (ROTSG Umit - 6.0 X 106 U/hr) 5 A FDW Mass Flow (avg): - 5.42x106 /hr i 8 FDW Mass Fow (avg): - 5.35x106 #/hr (corompared lo previous cycle these values are slightly lower. -0.05 x 106 91br)

• MS Outlet Temperature

• MS Temperature Increased from -592F on A side & -582F B side to -595F on both S/Gs (with cleaner tube surfaces and better beat transfer performance, this was expected)

- RCS

• Flow Increased -3 % on A side compared with previous cycle

• Flow Increased -5% on B side compared with previous cycle

-- Rx Delta T decreased accordingly

• (Total RCS performance remained constant from previous cycle) a Water level

. Water level was lowered -160 to -140 (as expected. beter heat transfer)

Other Items Reviewed Did Not Indicate Any Operational Abnormalities (Final FDW Flow, RCS Press, FDW Pump Press. Aux Fdw Flow, MS Press, Vibrations, etc.)

C 3

Definition of the Issue a A significant number of tubes in the ONS ROTSG were found with wear indications after 14 months of operation.

100% eddy current inspection of Oconee Unit 41 "A" and "B" steam generators was conducted at the end of the initial operating cycle as recommended by industry guidance. (15,631 tubes per generator)

Eddy current inspection scope included 100%

bobbin inspection, aX" probe inspection of all tubes showing indications -3250 tubes Oconee Unit 1 EOC-22 7- Steam Generator Inspection Observations

• Identification of widespread tube wear - confirmed with multiple ECT techniques (bobbin and X probe) a Tube wear statistics

• 1,800 tubes with wear indications In 1A SG

• 1.450 tubes with wear irndicalions in 18 SG maxinmum wear depth observed - 42. rTv in the 'A' SG a 80% wear indications were between 5% and 15% TN

• wear indications primarily in peripheral regions at 9th. 10th. and 11th TSPs

• most tubes had single indication: some had indications at multiple locations

• wear indications were small (generally < 0.5 inch axial. c 35' circumferential)

• wear indications had a tapered depth profile

• minimum delectability is 4% TW 4

Oconee Unit I EOC 22 Steam Generator Mos in n Inspection Observations

• Most indications (-95%) appear in the superheated steam region in the periphery of the bundle a Highest population of indications is at the 10' TSP followed by the 11"mand the 9"'

- - -

• Population of indications in the 12 TSP in the B SG is low but in the A SG 12" TSP the population is 4" highest a The 15" TSP shows only a few indications (single side chamfer broached hole on the outer tubes -10 inches)

• The outermost 4 tubes at the 14F' TSP In both SGs has only 2 indications (1i four outer tubes are drilled not broached)

• 11"b 12"' and 13"' TSP have same broached hourglass holes with K2inch bypass between the plate and the shroud blocked. The pattern of indications are similar with two exceptions

- The lack of indications at the 12' TSP of Ithe SG

Higher number of large Indications at the I1' TSP In the *B SG

• The 9"' TSP is located just below the bleed port has broached single sided chamfer for -14 outer tube rows. In the 'A' SG this TSP has the most significant non-peripheral population of wear indications a The single sided chamfer pattem for the 15". 10' and 9" TSP covers -10 Inches of the periphery. Indicatons at the 9", and 10" TSP occur in both the single sided and hourglass broached holes Oconee IA SG Wear Size Distribution

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Wear Indications "A" Generator 153 Oconee l.A

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Wear Indications "B" Generator OnP9 o0 .> ..

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Wear Indications "B" Generator lxrm

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P '.S--l, Wear Distribution by Support Location

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Wear Depth Distribution by Support Location AS 40 25 to 0 . 2 3 4 5 6 7 a 9 10 *i2 13 14 15 19 Oconee IA OTSG Wear Indications After 20 Cycles of Operation (all elevations)

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Oconee IA ROTSG Wear Indications After First Cycle of Operation (all elevations)

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-- Outage

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Oconee 1B ROTSG Wear Indications After First Cycle of Operation (all elevations)

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................. AlWear Tube Wear Cause Analysis

• Root cause evaluation being performed by BWC, with Duke oversight

• Both analytical work (calculations, modeling) and mock-up testing being used to investigate possible causes (work currently in progress)

• Field testing to determine or evaluate wear damage causes

• Outside consultants being used by Duke for independent investigation (Framatome, MPR, EPRI)

-

• Probable cause list developed 24 12

Excessive Wear Probable Causes

• Tube Vibration Induced by ROSTG Motion

• Excessive Compressive Load in Tubes

• High Wear Rate Coefficient for Inconel 690 with 410 SS TSP

• Acoustically Induced Tube Vibration a Broached Hole Surface Finish Roughness

. Hourglass Broached Hole Divergent Nozzle Tube Vibration Induced by ROTSG Motion with Primary or Secondary Pipe Flow

• Wide distribution of wear

• Pedestal is less resistant to rocking motion

• Factors to Consider

• Upper restraint may have gaps - beyond assembly concerns a Flow in the Hot Leg (Candy Cane) can cause ROSTG vibration

• Main Steam flow restrictor can cause pressure pulsation

• Actions:

• Verify gap measurements during RSG outage

• Review loose parts monitoring data

• Mount accelerometers on ONS Unit #2 during Fall 2005 outage

• Contingency to mount instrumentation on ONS Unit 1 26 13

Excessive Compressive Load in Tubes

. m Wide distribution of wear

-- Wear marks on opposite sides of a tube at different TSPs

.

• Historical data suggest the OTSG tubes were sensitive to FIV when loads exceeded 300 lbs compression.

• Factors to Consider:

a Most wear marks are at the periphery, compressive loading is highest inthe center

• Actions:

• MPR to conduct 3 rdparty review of BWC calculations and assumptions on tube loading both preload and operation state

• Evaluate feasibility of installing strain gauges in/on a tube prior to pulling during Unit #2outage a Evaluate feasibility of installing strain gauges in a tube In operation - Unit #2or Unit #3 High Wear Rate Inconel 690 Tube to 410 Stainless Steel Support Plate

• Wide distribution of wear

• 410 stainless steel is harder than original carbon steel u Factors to consider:

• BWC wear rate coefficients seem conservative compared to values utilized by others

• Actions:

• BWC / McMaster University is evaluating out wear testing

• BWC has AECL performing independent wear simulations using VIBIC and H3DMAP

• BWC review of EC results and development of gap correlations

• Place accelerometers inside a tube to measure data

• Duke and EPRI are evaluating wear rate coefficients 14

- Acoustically Induced Tube Vibration

• Wide distribution of wear

• Significant amount of wear appears at the aspirator port and above where fluid medium is steam I

• Wear did not occur In top span where the acoustic response is likely suppressed a Factors to consider

• Original OTSG top span did not pass current industry guidance for acoustic suppression however no wear was observed

-

• No unusual noise was reported

• -

• Slender structures are not typically prone to acoustic excitation

• Actions:

• Place microphones or AE listening devices on or around the ROTSG

• Install accelerometers inside a tube during operation

• Review loose parts monitor data for vibration response

• Investigating two mock-ups for laboratory studies 29 Broached Holes Surface Finish The motRoughness

- *The majority of the wear is less than 0.007 inch deep

-* Factors to consider

• Only a few tubes had wear greater than 40% Thru wall -0.014 inch deep

• Units 2 and 3 have electro-polished holes which should have smoother surfaces

• Surface imperfections should rub out without wear reaching the depths reported

• Actions:

• Perform future inspections which should show decrease in wear rate n0 15

Hourglass Broached Hole

'*' u* Divergent nozzles have inherent flow instabilities due to jet switch from an imperfect configuration - tube is elastic c Factors to consider:

.-

• Flow through a divergent cone can cause tube instability -

- prior worldwide experiences

• Actions:

• BWC (M. Pettigrew) construct mock-up and perform tests a CFD model of tube to TSP interface 31 What We Theorize is Not Causing Wear

• Fluid Elastic Instability No ring wear patterns No mid-span wear Lasted 14 months

. a Near Instability Threshold

• Statistically not valid for 3000 tube to be close with not one tube entering instability

-

• Classic Vortex Induced Vibration

• No mid-span wear

- .. Wear predominately confined to one location round the circumference at the broached hole

• Wear noted In Interior tubes where vortex shedding is not possible due to very low velocities

• High Turbulence Level

• Cross flow in aspirator area too low

• Wear rates are too low unless wear rate coefficient is found in error 16

What We Theorize is Not Causing Wear

• Axial Flow m Very little lateral energy in the power spectral density from axial flow

• Cross Flow in the Axial Region

• Very little energy

• Flow Regime Instability

• Low probability issue based on cross-flow regime maps 33

. Other Possible Causes I Investigations aX Cross flow in the bundle periphery/ gaps

• 2D CFD model completed matches THEDA model fairly well

- 3D CFD model underway

a PORTHOS modeling by Areva/Framatome

• Thermal expansion of the 410 SS TSP when compared to original carbon steel TSP, differential expansion may increase lateral contact load between the tube and TSP 17

Preliminary Conclusions

- No identified design errors - inputs and outputs of models appear reasonable this time a Expert consensus that the ONS Unit #1 ROTSG are experiencing some flow induced wear - we do not understand excitation force

. Root cause work is continuing

- -Industiy Experts Engaged Currently

• BWC design engineering organization a, Duke Technical Services/ ONS Engineering

• Areva / Framatome

• Dr. M K Au-Yang, formerly B&W Areva/Framatome

.

• Dr. M Pettigrew, Ecole Polytechnique

a Dr. John Luxat, McMaster University

• Dr. N Mureithi, Ecole Polytechnique

• Dr. E. de Langre, Ecole Polytechnique

- E. Blandford, G Srikantiah, EPRI

• M. Rao, Stress Technology - EPRI

' Various EPRI SGMP personnel

• MPR Associates

• N. Fisher, AECL (Chalk River) a Y. Han, AECL (Chalk River)

• Dr. Alan Bilanin, Continuum Dynamics 18

Plugging Criteria

• Plug and stabilize all tube with wear indications > 28% thru wall - 30 tubes plugged "A" 18 tubes plugged in 7B"

• Wear rate assumed to be linear and based upon the highest rate observed in Unit #1 results 42% (average wear rate approximately 9.8%)

• Wear rate used in the evaluation was Increased based upon the NDE sizing uncertainty of 7.4% thru wall

• 3AP pressure drop of 3930 psi

• Material properties were evaluated by Monte Carlo analysis which is industry practice

• Physical dimension of the flaws were adjusted to take into account linear taper, maximum length of the contact area is 1 inch

• Allowable EOC maximum depth is 80.6% thru wall for a tapered flaw

• Plugged tubes were stabilized

• Plugging criteria was reviewed and accepted by BWC 37 Corrective Actions Formal Corrective Actions will be detailed in the final root cause report; however, several CAs are evident including:

- Plug tubes on Unit #1 "A"and "B" SG that exceeded that plugging criteria of wear > 28% thru wall 3 Refine plugging criteria for Units #2, #3 a Completed installation of the N-1 6 radiation monitors on each ONS unit

• Review operator actions for N-1 6 monitors and existing actions for tube leakage and rupture events 3B 19

Corrective Actions Unit 2 Outage (Oct. 2005) a Pull two tubes for metallurgical examination w Determine stress loadings on tubes when pulled a Install instrumentation (accelerometers, acoustic monitoring)

Prepare to plug up to 200 tubes Unit 1 Forced Outage

- Install instrumentation 39 Future Impact

• Duke needs additional EC-data to confirm results and validate models and assumptions. ONS will need to accumulate several cycles of data.

- Future inspections and wear rate analyses may result in three possible

- conclusions:

• Wear rates higher than expected leading to mid cycle inspection outages

• Wear rates follow the expected model leading to plugging tubes and more frequent inspections, every outage w Wear rates are less than expected and inspection schedule requires more than one additional 100% inspection to accomplish basis for returning to normal inspection frequency

-

• Possible impact on unit up-rate

• Possible impact on 24 month fuel cycles a Possible modification to the existing ROTSGS

• 0 20

Summary

• Duke and BWC continue to evaluate Generator wear using enhanced analysis methods a BWC will be conducting experiments utilizing mock-ups to prove analysis assumptions, output, and results

• We remain confident the mechanism is fretting wear. We are still searching for the excitation force

: Duke is supplementing in-house expertise with a variety of consultants to ensure that testing plans and analyses are meaningful and lead to

- corrective actions that will resolve the issue

• This issue is not simple. Considerable effort expended and no

'smoking gun" identified

-

• Expect root cause investigations and research to continue through summer

• Will keep NRC informed of root cause progress and any conclusions 1