ML20150G009

Insp Results of San Onofre Nuclear Generating Station - 3 MSIV - 8205

ML20150G009
Person / Time
Site:	San Onofre
Issue date:	07/07/1988
From:	Chiu C, Herschthal M, Jerrica Johnson SOUTHERN CALIFORNIA EDISON CO.
To:
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ML13304A344	List: ML20150F968 ML20150G009
References
NUDOCS 8807190174
Download: ML20150G009 (31)

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INSPECTION RESULTS OF SONGS 3 MSIV 8205 l

July 7,1988 San Onofre Nuclear Generation Station Southern California Edison CK 00 61 O PNU l e

]Q r INSPECTION RESULTS OF SONGS-3 MSIV 8205 July 7, 1988 Principal Investigators:

C. Chiu M. A. Herschthal

, Contributors:

J. Johnson (HKM)

J. Brinkley (HKM)

D. A. Niebruegge (SCE)

N. Quigley (SCE) i' i

CONTENTS Page INTRODUCTION 1

INSPECTION RESULTS 2 DISCUSSION OF INSPECTION RESULTS 5 POSSIBLE FAILURE SCENARIOS 6 HETALLURGICAL EXAMINATION 7 OVERT 0RQUING TEST PERFORMED BY SCE 8 STRESS ANALYSIS 9 ,

REPAIR FOR MSIV 8205 9 JUSTIFICATION FOR CONTINUED OPERATION AND RECOMMENDED '

INSPECTION PLAN 10 APPENDIX A STRESS ANALYSIS 12 11

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LIST OF FIGURES P_aga Figure 1 A Close-up View of the Chamfer for the North Closing Guide Rail 15 Figure 2 A close-up View of the Chamfer for the South Closing Guide Rail 16 Figure 3 Various Views of Closing Guide Rails and Valve Skirt 17 Figure 4 Various Views of Closing CJide Rails and Valve Skirt 18 Figure 5 A Close-up View of the Chamfer for the North Opening Guide Rail 19 Figure 6 A Close-up View of the Chamfer for the South Opening Guide Rail 20 Figure 7 The Location of Three Broken Capscrews 21 Figure 8 Fracture Face of the Broken Capscrew Stud 22 Figure 9 Magnetite Build-up on the Fracture Face (1400 X Magnification) 23 Figure 10 A Close-up View of the Galling Marks on the florth Arm Shoe 24 I

Figure 11 A Close-up View of the Galling Marks on the South Arm Shoc 25 Figure 12 A Distant View of the Galling Mark:: on the Arm Shoes 26 i

Figure A-1 Fort 3s Resulting from Interference 27 111

- I INTRODUCTION l 1

9 As stated in the companion report, "Root Cause Analysis of the Lev-R-Lock and Guide Rail Interaction Problem for SONGS liSIVs," SCE has concluded that it is I unlikely that SONGS MSIVs would experience the failure mechanism that can shear two gate skirt assembly guide rails (the guide rails interacting with the lev-r-lock arm during closing) that occurred in one of the Haterford-3 HSIVs. This conclusion is based on, among other supporting evidence, a dynamic impact analysis and the fiber optic inspection results on Unit 3 HSIVs 3HV-8204 and 3HV-8205. The dynamic impact analysis reveals that one of the key parameters determining the magnitude of the shearing energy is the strcke time. Since SONGS MSIVs stroke approximately two and one-half times slower, it is very unlikely that they are subject to the failure mechanism experienced by Waterford-3 HSIVs.

On June 20, 1988, SCE was requested by the NRC to disassemble and inspect Unit 3 3HV-8205 to see if it has experienced the Waterford-3 failure mechanism. The inspection results are documented here as an addendum to the root cause analysis.

Since the inspection confirms the main conclusion of the root cause analysis, the root cause analysis together with the safety evaluation included in the root cause analysis (which constitute the bases for JCO, Justification for Continued Operation) remains valid.

The inspections also revealed that three capscrews at the bottom of one of the two segment skirt assembly guide rails were broken. This guide rail is hereafter referred to as the north segment guide rail and the other is the south segment guide rail. This failure mechanism is believed to be different from the mechanism that has resulted in shearing of the two gate guide rails of a Waterford-3 HSIV. This failure mechanism is analyzed to be much less

9 damaging than the "Haterford-3 fai'ure medianism" and it is self-limiting.

The root cause analysis for this failure mechanism is also documented in this addendum.

INSPECTI0f. RF0dLTS Unit 3 H5IV.8205 was disassembled for visdal inspection. The inspection results are documented below:

(1) The two gate guide rails were inspected. The 45' chamfers, where the lev-r-lock arm and the guide rails interact during valve closing, do not show sign of galling. However, wear marks, that are a result of swinging the arm toward the center, are evident. Figures 1 and 2 show close-up views of these chamfers.

(2) The 18 capscrews on the two gate guide rails were inspected. All of them appeared to be tight and all of the stakes on the capscrew heads were intact. The guide ra'Is are tightly held against the valve skirt.

Figures 3 and 4 are close-up views from various angles of the guide rail capscrews.

(3) The two segment guide rails were inspected. The bottom 45' chamfers show signs of galling. The galling occurs on the lower edge of the chamfers.

Figures 5 and 6 are close-up views of galling marks for the north and south chamfers.

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(4) The capscrews on the opening guide rails were inspected. Three of the 18

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capscrews were broken. All the broken capscrews are located at the bottom of the north opening guide rail. Figure 7 shows a close-up view of the empty capscrew holes.

(5) The three empty capscrew holes were inspected for corrosion and misalignment. No galvanic corrosion pits were found. The misalignment between the guide rail holes and the valve skirt sockets ranges between 5 to 10 mils.

(6) All three broken capscrew heads were found at the bottom of the valve cavity. The broken capscrew heads were visually inspected by SCE and HKH engineers at the time they were retrieved from the bottom of the valve cavity. No corrosion pits were found.

(7) The studs of the broken capscrews were removed from the capscrew sockets. They were able to be removed from the skirt plate by hand and visually inspected. No corrosion pits were found.

(8) The fracture faces of the broken capscrews were visually inspected by a 32X light microscope. General deformation around the fracture face is evident for all three broken capscrews, indicating the failure mode is ductile overload.

(9) The fracture paths of all three broken capscrews are at the bottom of the capscrews hexagonal hole. Figure 8 shows a close-up view of the fracture 3

face. Significant magnetite build-up was found on the fracture face.

Figure 9 shows this build-up with a 1400 x magnification.

. (10) The total depth of the capscrew hexagonal hole was measured.

The depth 9

is about 3/8".

(11) The fourth bottom capscrew on the guide rail with broken capscrews was found loose; that is, it can be moved by hand.

The stake marks on the head of this capscrew appear to be slightly shallower than those for intact capscrews.

It was removed to see if it contained any incipient crack.

No incipient crack was found by a 32X microscope inspection.

(12) The shoes of both lev-r-lock arms were examined. Galling marks were four.d on the tops of both shoes.

The shoe bottoms were smooth. Figures 10 and 11 are close-up views of the galling. Figure 12 is a distant view of the galling.

(13) The dimensions of the gate, segment, guide rails, lev-r-lock arms,and distance between valve seats were measured.All dimensions were within the vendor's specifications.

(14) The length of the north and south arms was measured. It appears that the north arm is about 1/16" shorter at the point of contact with the guide rail.

However, due to the difficulty in measurement, there is some uncertainty associated with this data.

(15) The lev-r-lock arm ear and the segment slot were inspected and showed no signs of excessive wear.

(16) The valve seats were inspected visually. There are.several areas on the gate and opening side valve body seats that show signs of wear marks.

The maximum depth is measured to be less than 5 mils.

(17) The gate and segment back angles were inspected visually. Several wear marks were observed.

DISCUSSION OF INSPECTION RESULTS Based on the visual inspection results, it is reasonable to conclude the following:

1) The fracture of the three broken capscrews seems to be a result of interference with the lower edge of the chamfer and the top of the arm shoe. Figure A-1 illustrates this interference during valve opening.

There are galling marks in the area of interference on both the shoe and the chamfer of the guide rail.

2) The galvanic corrosion, if any, is insignificant.

3) The depth of the capscrew socket is 0.375". The minimum depth is specified by the ANSI 18.3 standard to be 0.220". Even though there is no maximum depth limit, this 0.375" depth was evaluated to determine if the depth is excessive. To evaluate whether or not these capscrews had experienced overtorquing induced cracks, a torque test was performed.

The results are reported later in this report.

POSSIBLE FAILURE SCENARIOS Several possible scenarios that result in overload fracture of the three capscrews on an opening guide rail are hypothesized. Based on the inspection results, only one of the hypothesized scenarios is considered likely. This can not be refuted by any evidence or observation collected so far. This scenario is stated below.

"At the beginning of valve opening, the friction on the back angles of the segment and gate prevents the assembly from unwedging. As a result, as the assembly moves upward, the top of the lev-r-lock arm shoe comes in contact with the bottom edge of the segment guide rail chamfer. In the first few opening strokes, the resultant tensile force is preferentially imparted to the north guide rail, either because the north arm is shorter or because the valve skirt is not squarely installed. Consequently, the first few bottom capscrews on the north guide rail fail by tensile overload. After some metal is removed from the bottom chamfer edge of the north guide rail, the interference force begins to be shared by both north and south guide rails. Because the tensile load is now shared by two guide rails and the friction coefficient decreases as the roughened surface gets smoothed out after a few instances of interaction, the damaging mechanism stops and no more capscrews fail."

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< HETALLtJRGICAL EXAMINATION The three broken capscrew heads and studs were metallurgically examined. One set consisting of a broker, capscrew head and stud was sent do HKM for metallurgical analysis.

T90 broken studs were sent to Truesdall Laboratory for metallurgical examination.

Two broken capscrew heads were examined by SCE for material composition.

The results of this examination are documented here.

1)

The material of two broken canscrew heads was determined by SCE to be within the specifications of ASTM A193 Gr. 87.

2)

The material of two broken studs was determined by Truesdail to be within the specifications of ASTM A193 Gr. 87.

3)

The material of one broken capscrew head and one stud was determined by HKH to be within the specifications of ASTM A193 Gr. 87.

4)

The hardness of the material for tFe capscrew kept by HKM was determined to be 302 Brinell. This hardness translates into a tensile strength of 146 ksi. This is greater than the minimum tensile stress of 125 ksi for ASTM A193 B7 bolt.

5)

The hardness of the material for a capscrew sent to Truesdail Laboratory was determined to be 362 Brinell.

This hardness translates into a tensile strength of 177 ksi, according to ASTM Specification A370.

6)

The fracture faces for all three broken capscrews have significant magnetite build-up, indicating that the fracture did not recently occur.

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7) The general deformation around the fracture face suggests that the failure mode is ductile overload. However, due to the magnetite build-up on the fracture faces, detailed microscopic examination for dimple marks is not possible.

OVERTOROVING TEST PERFORMED BY SCE The deep capscrew socket, even though its dimension does not violate any capscrew specification as documented in ANSI Standard 18.3, was evaluated to determine if it may have contributed to the failure of the three capscrews.

One scenario is that these deep socket capscrews were broken during initial assembly by overtorquing. To evaluate whether or not this scenario is valid, a capscrew eas removed from the north segment guide rail and was bench tested. A test bench was constructed with identical dimensions of the counter-sink in the guide rail. The capscrew was torqued with an appropriate lubricant at various torques to see if it developed any incipient cracks or permanent deformation. The results are summarized below.

1) Torqued to 150 ft-lbf -- no incipient cracks; no plastic deformation

2) Torqued to 180 ft-lbf -- the Allen wrench starts to deform, no incipient cracks: no plastic deformation

! 3) Torqued to 250 ft-lbf -- the Allen wrench completely deformed; no l incipient cracks; no plastic deformation l

4) Torqued greater l than 250 ft-lbf --

test discontinued because of tool 1

deformation.

Note that the~capscrew was originally installed by HKH with a torque of e

150 ft-lbf. According to the test results stated above, this installation torque will not cause incipient cracks. As a result, it is reasonable to preclude the failure scenario that these three capscrews (even if the capscrew socket is 0.375" deep) wt:e failed due to overtorquing.

STRESS ANALYSIS Based on a stress analysis documented in Appendix A of this report, the maximum tensile force generated by the interaction between the lev-r-lock arm and the guide rail is estimated to be about 95,300 lbf. The interaction is assumed to occur with a galling process between two interacting parts. The tensile force needed to fracture a capscrew is estimated to be 32,800 lbf, also considering the co-existence of the shear force. Since 32,800 lbf is less than 95,568 lbf, it is reasonable to conclude that the bottom two to three capscrews of the only interacting guide rail (north guide rail) will fracture.

Once the interfering metal was removed from the chamfer of the north segment guide rail after the first few times of interaction, the friction coefficient of interaction decreased and the total load started to be shared by both the north and south guide rails. As a result, the maximum tensile force is reduct) to a level of approximately 16,730 lbf. Note that this maximum tensile load is no longer capable of fracturing any more capscrews.

REPAIR FOR MSIV 82Q5 The galling marks on the arm shoes were removed. The chamfer edges of the opening guide rails were rounded off. Because the removal of metal provides alarger clearance between the arm shoe and the chamfer edges of the opening

guide rails, and the rounded edges provide a larger contact area, the galling process at the bottom chamfer edges should be eliminated or greatly reduced.

As a result, the friction coefficient will be reduced to a level that the interference force is not likely to cause any capscrew damage. Moreover, during the reassembly, the squareness of the valve skirt is ensured by use of a leveling gauge. The three broken capscrews, the fourth bottom capscrew, and the capscrew removed for tests were replaced with new capscrews of 17-4 pH stainless steel.

JUSTIFICATION FOR CONTINUED OPERATION AND RECOMMENDED INSPECTION PLAN Based on the inspection results and the discussion above, it seems reasonable '

to conclude the following:

1) SCE's MSIVs are unlikely to experience the Haterford-3 MSIV failure mechanism because their stroke time is significantly longer than Waterford-3 HSIVs.

2) The fracture of the bottom three capscrews on one of the two segment guide rails in Unit 3 MSIV 3HV-8205 discovered during inspection, is likely to be a result of excessive interference between the lev-r-lock arm shoe and the guide rail.

3) Based on the failure pattern and the deeper galling mark on the north arm shoe it seems reasonable to conclude that the preferential loading of the north guide rail is a reasonable explanation for the fact that all three broken capscrews are located on the north guide rail. The preferential loading can be caused either by a shorter lev-r-lock arm or by the out-of-squareness of the guide rail skirt plate installatior,.

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4) Based on the fact that (1) only three capscrews were fractured after more I

than one-hundred valve openings. (2) the fracture did not recent1/ occur, (3) the fourth bottom capscrew does not show any signs of cracking or deformation; it is reasonable to conclude that this failure mechanism is self-limiting. In other words, the failure stops once the interferring metal is removed from the chamfer and galled surface is smoothed out.

5) Since the failure mechanism is self-limiting, it is unlikely to fracture more than a few capscrews. As such, it is unlikely to dislodge the whole segment guide rail by this failure mechanism.

6) To ensure that this failure mechanism can be corrected before it results in significant damage, Unit 2 MSIV 2HV-8204 and MSIV 2HV-8205 should be inspected by borescope in the next Mode 5 outage with sufficient duration (greater than 7 days). SCE will preestablish inspection criteria so that a predetermined course of ection will be followed depending on the inspection results. In addition, a borescope inspection program will be developed for subsequent refueling outages.

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s APPENDIX A Stress Analysis The purpose of this simplified analysis is to determine whether or not the hypothesized scenario is feasible. Three parameters are calculated by a best-estimate method in this Appendix. They are described as follows:

1) The initial tensile force experienced by one guide rail.

2) The tensile force needed to tensile fracture one capscrew.

3) The tensile force experienced by two guide rails when galling marks were smcothed out.

The Initial Tensile Force The maximum opening force, Fo, can be determined by balancing the hydraulic pressure and the N2 pressure:

Fo - (4') psi) x ((12")* - 2') x - 176,000 lbf (A.1)

The normal force experienced by the guide rail, Fn, is calculated from Fo as follows:

Fn - Fo cos # - 176,000 x Cos 40'

- 135,000 lbf, where 40' is the angle between (A.2)

Fo and Fn The tensile force, Ft, is related to Fn by the following formula:

Ft - Fn . p x Cos 45'

- 95,300 lbf, where p - 1.0 for a galled surface (A.3)

l Etacture Tensile Force During the interaction, the bottom capscrews will experience both a tensile force and a shear force. The magnitude of the shear force for one capscrew (assuming the loads are equally shared) is:

Fs = Fn Sin 45' / 9 = 10,600 lbf Based on the energy distortion theory (Reference A), the capscrew will fail if the following criterion is met.

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( (as - 'T)* + "T' + "S ) = 2a (A.4)

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where o gs is the ultimate strength.

Equation (A.4) becomes the following formula if each term is multiplied by the square of the bolt area.

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((Fs - FT )' + FT' + Fs') = 2F US ^*

Fys = 125 ksi x 0.5" x x x 0.15" - 29,500 lbf Based on Equation (A.5), FT is determined to be 32,800 lbf.

Reference A J. A. Collins, "Failure of Materials in Mechanical Design -- Analysis, Prevention, and Prediction," John Wiley & Sons, Inc. (1981) 13 -

Tensile Force After metal was Removed from the !; orth Guide Rail Chamfer After enough metal is removed from the north guide rail chamfer, the tensile load will be shared by both guide rails. Also, the friction coefficient decreases as the galled surface gets smoothed out. For this case, the tensile load for the bottom capscrews for both opening guide rails is:

FT . Fn u . Cos e u - 0.35 (smoothed galled surface) 2

= 16,730 lbf (A.6)

Since 16,730 lbf is less than 32,800 lbf, no capscrews should fail.

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N-FRACTURE OPENING FORCE PATH s ]

Fo = HYDRAULIC PRESSURE '

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GUIDE RAIL ^

F RESULTANT FRICTION g N = FORCE vvWv T

FT = TENSILE FORCE F3= SHEAR FORCE I

GALLING TOP SURFACE OF THE LEV-O-LOCK ARM SHOE  ;

FT FO FN FN F3 PIVOT POINT OF THE SHOE FIGURE A-1 FORCES RESULTING FROM INTERFERENCE

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