ML20205S521

RHR Pump Wear Ring Surveillance Exam Results, Final Rept

ML20205S521
Person / Time
Site:	Pilgrim
Issue date:	01/23/1987
From:	Delwiche D, Gordon G BOSTON EDISON CO.
To:
Shared Package
ML20205S496	List: ML20205S491 ML20205S521
References
87-178-001, 87-178-1, NUDOCS 8704070047
Download: ML20205S521 (33)

Text

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F&PMT TRANSMITTAL No. 87-178-001 FUEL & PLANT MATERIALS TECHNOLOGY FINAL REPORT RHR PUMP WEAR RTNe SURVEILLANCE EXAMINAT' JSULTS PII7 RIM NUCLEAR Pt.,ER STATION WORK PERFORMED FOR BOSTON EDISON COMPANY January 23, 1987

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.repared By:

D.E. Delwiche, Principal Engineer Plant Materials Performance Approved By:

G.M. Gordon, Manager Fuel & Plant Materials Technology i

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F&PMT TRANSMITTAL NO. 37-178-001 DISCLAIMER OF RESPONSIBILITY This document was prepared by or for the General Electric Company.. Neither the General Electrical Company nor any of the contributors to this document:

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Makes any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information disclosed in this document may not infringe privately owned rights; or B.

Assumes any responsibility for liability or damage of any kind which may result from the use of any information disclosed in this document.

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F&PMT TRANSMITTAL.

NO. 87-178-001 EXECUTIVE

SUMMARY

Wear ring samples were removed from two Pilgrim RER pump impellers and sent to General Electric's Vallecitos Nuclear Center for metallurgical evaluation. This is a report the results of the metallurgical evaluation performed on the wear ring saaples.

On May 20, 1986, the Nuclear Regulatory Commission issued an Information Notice to alert licensees of RHR pump impeller wear ring failures. The notice was issued as a result of multiple RER pump failures at Philadelphia Electric's Peach Bottom Unit 2 and 3, and at Tennessee Valley Authority's Browns Ferry Urits 1, 2, and 3.

The failures affect the RHR pump impeller wear rings on pumps manufactured by Bingham-Willamette with motors supplied by General Electric. The initial cause of the failure sequence was identified as defective impeller wear rings.

The wear rings are 410 stainless steel, A 182 grade F6 with a specified Rockwell C hardness of 33 to 39.

The rings are press fit to the impeller, and attached with eight dowel pins. Metallurgical analysis of failed Peach Bottom wear rings revealed evidence that IGSCC was the cause. The failed rings may have had increased SCC susceptibility due to unusually high hardness levels of the ring material.

By visual examination, the wear ring samples which had been removed from the RHR pump impellers at Pilgrim Nuclear Power Station, showed no evidence of cracking. However, by optical metallographic, and scanning electron

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mic.coscopic evaluation, one ring sample did show minor cracking thought to be j

stress corrosion of the type which had caused the degradation of similar RER pumps at other domestic power plants.

F&PMT TRANSMITTAL NO. 87-178-001 1

The weir ring material had average hardness readings of 40 to 42.5 Rockwell C.

(The manufacturer, Bingham-Willamette, specifies a range of hardness of C34 to C39.) The ring sample with minor cracking had average hardness readings of 42 to 43 Rockwell C.

With hardness levels greater than C40, the material has increased susceptibility to crack initiation and growth by IGSCC.

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F&PMT TRANSMITTAL NO. 87-178-001 TABLE OF CONTENTS RHR PUMP WEAR RING SURVEIIIANCE l

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EXAMINATION RESULTS r.

PILGRIM NUCLEAR POWER STATION PAGE EXECUTIVE

1.0 BACKGROUND

1

2.0 INTRODUCTION

2 3.0 VISUAL EXAMINATION...........................................

3 A. Pump A Wear Ring..........................................

3 B. Pump B Wear Ring..........................................

3 4.0 METALL0 GRAPHIC ANALYSIS......................................

4 A. Pump A Wear Ring..........................................

4 B. Pump B Wear Ring..........................................

5 5.0 OTHER METALLURGICAL TESTS....................................

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A. Hardness Measurements.....................................

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B. Chemical Analysis.........................................

8 6.0 DISCUSSION...................................................

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F&PMT TRANSMITTAL

.NO.-87-178-001

1.0 BACKGROUND

On Ma-/ 20, 1986, the Nuclear Regulatory Commission (Office of Inspection-and Enforcement) isrxed-an-Information Notice (Attachment'#1) to alert-licensees of RHR pump impeller wear ring failures. The IE was issued as a

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result of multiple RHR pump failures at Philadelphia Electric's Peach Botton Units 2 and 3,'

and at Tenneesee Valley Authority's Browns Ferry Units 1, 2,.

and 3.

The_ failures affect.the RHR pump impeller wear rings on pumps manufactured by Bingham-Willamette with motors supplied by General Electric.

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The most severe degraded pump impeller wear ring condition, and subsequent total pump failure, occurred at PECo's Peach Botton Unit 3 during a refueling outage in a Bingham-Willamette single stage centrifugal-pump (Figure 1), model 18X24X28 CVIC, powered by a 2000 HP GE vertical induction motor.

The initial cause cf the failure sequence was identified as defective impeller wear rings. Subsequent inspections of other pumps revealed multiple similar-wear ring failures.

The wear rings are 410 Stainless steel, A 182 grade F6 with a specified Rockwell C hardness of 33 to 39. The rings are press fit to the impeller, and attached with eight dowel pins. Metallurgical analysis of failed wear rings reveeled evidence that IGSCC contributed to the failure. The unusually high hardness levels of the ring material may have contributed to an increased SCC susceptibility.

1. IE.Information Notico No. 86-39: FAILURES OR RHR PUMP MOTORS AND PUMP INTERNAIS, issued by United States Nuclear Regulatory Commission - Office of Inspection and Enforcement, Washington, DC., May 20, 1986.

r F&PMT TRANSMITTAL NO. 87-178-001 RHR pumps of the type which had impeller wear ring failures are in use at Boston Edison Company's PILGRIM Nuclear Power Station. In order to preclude a similar problem occurring at their facility, the RRR pump impeller wear rings at PILGRIM UNIT 1 were inspected for degradation, or susceptibility to damage.

2.0 INTRODUCTION

RHR pump impeller wear ring samples were removed from two Pilgrim pumps and sent to GENERAL ELECTRIC'S VALLECITOS NUCLEAR CENTER for metallurgical evaluation (Figure 2).

This report presents the results of the metallurgical evaluation performed on the Pilgrim wear ring samples.

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F&PMT TRANSMITTAL NO. 87-178-001 3.0 VISUAL EXAMINATION A.

PUMP "A" WEAR RING SEGMENTS A single segment of the "A" RHR pump impeller wear ring measuring approximately 10 inches in circumferential length was received at Vallecitos Nuclear Center (VNC) for mer111urgical analysis. A radiological survey was performed, and showed the maximum contact radiation level to-be approximately-50 mrem /hr. Further decontamination at VNC was not performed. Figure 3 is a photo of the sample removed from RHR pump impeller "A",

taken at the time of visual examination.

The pump "A" wear ring segment was penetrant examined for evidence of crack indications. All surfaces of the segment were examined and no relevant indications were found.

B.

PUMP "B" WEAR RING SEGMENTS The complete RHR pump "B" lower impeller wear ring was sent to VNC for metallurgical evaluation.- During removal from the impeller, the ring was separated into two approximately equal segments. A radiological survey showed the surface activity readings to be approximately 50.to 60 mres/hr. Further decontamination was not performed. Figure 4 is a composite photograph of the lower impeller wear ring removed from the RHR pump "B",

upon receipt at VNC.

The "B" pump wear ring segments were penetrant examined for evidence of crack indications. All surfaces were examined and no relevant indications were found.

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NO. 87-178-001 4.0 METALL0 GRAPHIC ANALYSIS A.

PUMP "A" WEAR RING Following visual examination, the segment of RHR Pump "A" wear ring was sectioned for metallographic examination. Figure 5 is a sketch of the sectioning plan. Since no relevant indications were found, a random " typical" area was selected for sampling. As indicated in Figure 5, seven samples were cut for metallurgical characterization. Five samples were used for optical microscopy, one for scanning electron microscopy, and one for chemical analysis.

The results of the optical microscopic metallurgical characterization are provided 12 Figures 6 through 10. Each sample was polished, etched and examined on a plane normal to the wear (convex) wear surface, on that plane marked with an "x" in Figure 5.

Each metallographic section was then photographed on two edges; the edge adjacent to the wear surface, and the edge adjacent to the surface which had been in shrink fit contact with the impeller. The two surfaces were studied to possibly reveal evidence of surface degradation that could be due to the pump operational environment.

On Section 3 (Figure 8), a shallow (1/6 inch) intergranular crack was noted at the wear surface.

Since the sections were randomly selected, it must be presumed that the presence of other shallow cracks is not uncommon and in time these cracks could propagate through the wear ring thickness and cause impeller failure (seizure). Due to the similarity of the metallurgical (hardness) design, and service condition of these rings with those of Peach Bottom, with confirmed stress corrosion cracking, it is assumed that cracking in the Pilgrim ring is also due to stress corrosion. It must be noted, however, that fabrication related quench cracking (intergranular) could be a possible, albeit improbable, cause as SCC and shallow quench cracks have similar appearances.

1 F&PMT TRANSMITTAL NO. 87-178-001 Other noteworthy microstructural features associated with the wear surface were not observed. The microstructure is typical of a correctly-fabricated martensitic type 410 S/S. Surface tearing or corrosion attack was not observed.

Sample 6, removed from RHR Pump "A" wear ring segment, was selected for study by Scanning Electron Microscopy (SEM). Figure 11 provides the results of the SEM evaluation. Figure 11 has low (20X) and high (250X) magnification views of the wear surface. Minor surface imperfections / degradation can be observed in the 250X view. This view is rather typical of the condition of the wear surface. Such imperfections tend to promote crack initiation (of the type observed in Section 3) in a susceptible material, if the stress and environmental conditions are favorable for SCC.

B.

PUMP "B" IMPET.T.FR WEAR RING Following visual examination, samples of RHR Pump "B" wear ring were removed for metallurgical evaluation. Figure 12 is a set of photos showing the RHR Pump "B" metallurgical sectioning plan. The arrows in the photos indicate the plane of polish. Since no relevant indications were found by penetrant examination on this wear ring, typical random areas were selected for sampling. As indicated in Figure 12, a total of 7 samples were selected for metallurgical characterization.

(A portion of sample #3 was used for SEM evaluation). Five samples were used for optical microscopy, one for SEM and one for chemical analysis.

The results of the optical microscopic metallurgical characterization are provided in Figures 13 through 17.

Each sample was polished, etched, and examined on a plane normal to the wear surface, on that plane indicated with an arrow in Figure 12. Each metallographic sample was then photographed on two edges; one edge was adjacent to the wear surface, and the other was adjacent to the surface which had been in shrink fit contact with the

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-F&PMT TRANSMITTAL NO. 87-178-001 impeller. The two surfaces were studied to reveal small microstructural.

features that could be due to pump operation.

I Noteworthy microstructural features associated with the wear surface were-not found. The microstructure is typical of a correctly fabricated 1

martensitic type 410 S/S. Surface tearing or. corrosion-attack was not i

observed.

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The SEM. sample (a portion of sample #3) was examined for wear surface degradation. Figure 18 provides the results of the evaluation. Figure 17 has j

low (20X) and high(250X) magnification views of the wear surface. Minor surface imperfections can be observed in the 250X view. While the j

imperfections found on the surface would not in any way impair operation of the pump, they would be the preferential sites for IGSCC crack initiation in a susceptible material, if the stress and environmental conditions are also favorable for SCC.

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HARDNESS MEASUREMENTS 5

j Microhardness measurements were made on optical metallographic samples 1 j

.through 4 removed from-the A pump wear ring, and samples ~1 through 5 removed from the B pump wear ring. The locations are identified in Figures 19, and -

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The numbers on the sketches represent the hardness readings at that' t

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gram load. The measurements 'can be summarized as follows for the "A" Pump j

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j average hardness................-42.4 Rc i

avera'ge near wear surface......

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j average near impeller surface.. 42.8 Rc i

1' and as follows for the "B" pump wear ring:

average hardness...............

40.0 Re average near wear surface...... 39.3 Rc l

average near impeller surface.. 39.9 Rc Note that in the case of each ring, the average hardness level exceeds-the specified maximum hardness level of 39 Rockwell C.

At this level of

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hardness Type 410 Stainless Steel is known to have a susceptibility to stress

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corrosion crack initiation and growth under suitable conditions of stress and i

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on sample 3) is 42 to 43 Rockwell C and represents the highest values found'on the samples. Since increasing hardness levels increase the propensity for IGSCC, it is consistent to have observed cracking in.the highest hardness region.

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F&PMT TRANSMITTAL NO. 87-178-001 B.

CHEMICAL ANALYSIS The RHR Pump wear rings were fabricated of a Type 410 Stainless steel,ll.

182, Grade F6, with a specified hardness range of'Rc 33 to 39. A sample of each wear ring was analyzed by an independent laboratory for a chemical check i

analysis. The purpose was to establish compliance with the material specification. The results are presented in Figure 21.

It is noted that the chemistry of the PILGRIM samples is in compliance with the requirements of

. ASTM A 182, Grade F6.

(See Attachment 2.)

6.0 DISCUSSION 1

By visual examination, the wear ring samples which had been removed from the RHR pump impe11ers at PILGRIM Nuclear Power Station, showed no evidence of cracking. However, by optical metallographic and scanning electron microscopic evaluation, one ring sample did show the stress corrosion cracking of the type which had caused the degradation of similar RHR pumps at other domestic power plants.

The wear ring material had average hardness readings of 360 to 44 Rockwell C.

(The manufacturer, Bingham-Willamette, specifies a range of I

hardness of C34 to C39.) The ring sample with minor cracking had average hardness readings of 42 to 43 Rockwell C.

The material has increased susceptibility to crack initiation and growth by IGSCC at hardness levels greater than C40. It is, therefore, consistent to have observed minor l

cracking in the highest hardness regions of wear ring sample "A".

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Typical edge view of section 1 removed from RHR pump "B" wear ring. The edge in the upper photo l

was adjacent to the wear surface and the edge in l

the lower photo was adjacent to the impeller.

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F&PMT TRANSMITTAL NO. 87-178-001 Wear Surface s

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Typical edge view of section 2 removed from RifR l

pump "B" wear ring. The edge in the upper photo i

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s F&PMT TRANSMITTAL NO. 87-178-001 CHEMICAL ANALYSIS COMPARISON A Iti "A"

Pump "B"

Pump Gr. F6 Sample Sample I

C 0.12 max 0.12 0.10 Cr 11.5 - 13.5 12.33 11.79 Ho 1.0 max 0.36 0.44 P

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Figure 21.

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SSINS No.:

6835 IN 86-39 UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT Washington, DC 20555 May 20, 1986 IE INFORMATION NOTICE NO. 86-39:

FAILURES OF RHR PUMP MOTORS AND PUMP INTERNALS Addressees:

All nuclear power reactor facilities holding an operating license (OL) or a constructionpermit(CP).

Purpose:

This notice is provided to alert licensees to serious damage which has occurred at a licensed nuclear power plant to residual heat removal (RHR) pumps manufac-tured by Bingham-Willamette with motors supplied by General Electric.

The damage identified at the Philadelphia Electric Co. (PECO), Peach Bottom facility involved failure of motor bearings and/or failure cf pump impeller wear rings.

It is expected that recipients will review this notice for applicability to their facilities and consider actions, if appropriate, to preclude a similar problem occurring at their facilities.

However, suggestions contained in this notice do not constitute requirements; therefore, no specific action or written response is required.

Description of Circumstances:

On November 2,1985, during core reload of PECO's Peach Bottom Unit 3, a fire occurred in the 3C RHR pump motor which totally engulfed the motor and rendered the pump inoperable. A lower guide bearing high temperature alarm had been received for 3 days prior to the pump failure, but this warning of potential pump failure went unnoticed by personnel because of the large number of other alarms that were received during the shutdown.

The motor, a 2000 HP GE verti-cal induction model 5K6348XC29, and pump, a Bingham-Willamette single stage centrifugal model 18x24x28 CVIC (Figure 1), were uncoupled, disassembled and inspected for damage.

Results of the inspection identified that the motor lower guide bearing was destroyed, the rotor bars were gouged and scorched, i

and the stator windings were burned and gouged.

The pump internals were found to have sustained internal damage.

Specifically, the lower impeller wear ring (Figure 1) was separated from the impeller and fused to the casing wear ring.

No other wear surfaces indicated wear or damage.

On November 16, 1985, while

{

replacing the lower pump casing gasket on the 3A RHR pump PECO personnel discovered that the lower pump impeller wear ring was sepa, rated from the

)

impeller and cracked in three places.

Subsequent inspections of the remaining two unit 3 RHR pumps and two unit 2 pumps (2A and 2C) revealed similar wear ring failures in three of the pumps inspected, i

'860615047k5

IN 86-3g May 20, Page 2 of On December 22, 1985, after several overcurreni. :les with pump 2D from Unit PECO inspected this pump and discovered the lower pump impeller wear ring separated and cracked, a 6-inch piece of wear ring missing, and the impeller vanes damaged.

Not all failures noted above were as severe as the ones identified on pumps 3C and 20.

However, the similarity was evident.

The RHR pump impeller wear rings are press fit to the impeller and attached I

with eight dowel pins.

The wear rings provide a wearing surface on the pump impeller.

The wear rings are 410 stainless steel, A 182 grade F6 with a Rockwell C hardness of 33 to 39. Metallurgical examinations of the wear ring fracture surfaces indicate the presence of intergranular stress corrosion cracking (IGSCC).

PECO has classified the wear ring failures as IGSCC.

On November 26, 1985, PECO rde an INPO Network notification regarding these RHR pump failures.

PECO has repaired all affacted pumps by replacement of damaged motors and pump internals.

Tennessee Valley Authority's Browns Ferry Units 1, 2, and 3 utilize the identical pumps for RHR service.

Similar motor and pump impeller wear ring failures have occurred at these facilities, but not to the extent identified at Peach Bottom.

Pumps of similar design, but different size, are utilized for core spray service both at Peach Bottom and Browns Ferry.

However, these pumps use the " integral" impeller wear ring design, i.e.,

extended impeller part replaces separate wear ring and forms a single unit, and therefore are not susceptible to the type of wear ring failure previously described.

Discussion:

These multiple events are of concern because of the potential for common-mode failures of all pumps in the same system.

At Peach Bottom, six of eight pumps inspected exhibited degraded pump impeller wear rings and internals.

j These flaws could lead to pump hydraulic degradation and under the worst conditions, complete pump failure.

ThemotorguidebearIngfailuresaresigni-ficant because they could cause failure of the pump motors and pump internal damage.

The full extent to which this type of pump may be used in safety-related services at other facilities is not known with complete certainty. According to information ascertained from Bingham-Williamette records and confirmed by contact with affected sites, other plants utilizing this type of pump in the RHR system include the following:

Cooper, Pilgrim 1, and Vermont Yankee.

The exact cause of the pump internal failures has not been fully determined, except that there is evidence that IGSCC has contributed to the impeller wear ring failures.

Operating pumps with inadequate flow and' lubrication, whereby high internal temperatures develop, is also a likely contributor, e.g., pump cavitation.

PECO is continuing to pursue root causes and wear ring redesigns to prevent such occurrences in the future.

l

IN 86-39 May 20, 1986 Page 3 of 3 No specific action or written response is required by this information notice.

If you have any questions regarding this matter, please contact the Regional

{

Administrator of the appropriate NRC regional office or the technical contact listed below.

I

Ihr, dward L Jordan, Director i

Division f Emergency Preparedness and En insering Response Office o Inspection and Enforcement Technical

Contact:

Ronald M. Young (301) 492-8985 1

Attachments:

i 1.

Figure 1 - Typical CVIC RHR Pump Sectional Assembly 1

2.

List of Recently Issued IE Information Notices l

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___________._,______---._._-____._.._____-...-_-_-_.____________.I

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t LABCRATORY CERTIFICATE MNM[M Anamet Laboratories, Inc.

d 3400 INVESTMENT BOULEVARD. HAYWARD, CALIFORNIA 94545 3811. (416) 887 8811

--c Laboratory Kurber:

187.17 Purchase Crder: 205-XI 14?

Date Subritted:

January 6, 19f7 Date Perorted:

January 12, 1987 General Electric Fucicar Energy Eusinese Operations Attn:

D. E. Delwiche 175 Curtner Avenue, P/C 785 San Jose, CA 95125 SUPJECT:

Two retal coupons were subritted for cherical analynia.

The carrier were identified as follows: PHP-A and PPP-2, 410 rartensitic stainleen steel.

SPECTPOCHEPICAL ANALYSIS

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Trcrorted in wt. Il Park:

PHP-A PHP-P Carbon (C) 0.12 0.10 Chromiur (Cr) 12 33 11 79 Pantanese (Pn) 0.?6 0.44 Phoarborus (P) 0.013 0.013 Silicon (SI) 0.28 0.?6 Sulfur (S) 0.015 0.007 Thie teeting was perfarred in accordance with the purchase order.

l Peapectfully Subritted, APAPET LAPCPATCPIES, INCCPPOPATEC l

Py b. C;, % % e w l

E. A. Forcran 3c/bb123186 Panager, Cuality Control l

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