Text

Preliminary Rept of Helium Circulator S/N C-2101 Damage Including Licensing Assessment
	ML20235K272
Person / Time
Site:	Fort Saint Vrain
Issue date:	09/11/1987
From:	; PUBLIC SERVICE CO. OF COLORADO
To:	;
Shared Package
ML20235K078	List: 05000267/LER-1987-019, Forwards LER 87-019-00 & 870911 Preliminary Rept of Helium Circulator S/N C-2101 Damage Including Licensing Assessment; ML20235K272;
References
	NUDOCS 8710050028
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PRELIMINARY REPORT OF HELIUM CIRCULATOR S/N C-2181 DRMAGE INCLUDING LICENSING ASSESSMENT l

SEPTEMBER 11, 1987 I

l FORT ST. VRRIN l NUCLERR GENERATING STRTION PUBLIC SERVICE COMPRNY OF COLORADO gg RES SN$I7

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PRELIMINARY REPORT OF HELIUM CIRCULATOR S/N C-2101 DAMAGE INCLUDING LICENSING ASSESSMENT l

September 11, 1987 FORT ST. VRAIN NUCLEAR GENERATING STATION Public Service Company of Colorado I i

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' ABSTRACT The ' purpose of _this' report is:to provide prelimin'iry information -

relative to the damago.: incurred in helium circulator S/N C-2101-Lduring; . July, 1987,~'and; justification for returning -to'. power operation.

On ' July > 22,-. 1987, while running at about;9200 rpm, 'D' circulator tripped on PPS cierspeed; On. July 24, 'D' circulator was brought up to a'. maximum speed:lof 6405 rpm when erratic speed was observed.

Wobble measurements inc'icated excessive shaft wobble; therefore, 'D' circulator.

' was taken of f line. .Further wobble indications rendered E D' circulator inoperable per the operation. and maintenance manual 1.i mi t s .

.Also, on July 27-28, 1987, an interspace leak from 'D' circulator secondary closura in excess of the Technical Specification LCO 4.2.9 l limit was measured; Therefore,, .'D' circulator- was declared

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' noperable'due to high wobble ir.dication and excess interspace leakage. On July 31, 1987, the decision Was made to~ remove the circulator. (S/N C-2101) from 'D' penetration for inspection and replace it with an available refurbished spare (C-2104).

During removal operations. of C-2101 from its penetration, failed

. circulator hardware pieces were discovered 'in its. steam outlet plenum.- These parts were initially identified as-coming from the insulation cover / labyrinth seal area of the steam-end of the machine.

Once the machine was removed and disassembled further in San Diego, the initial; identity of the failed parts was confirmed. The failed

. parts' included the insulation cover, labyrinth ' seal and spacer, labyrinth seal mounting bolts, backing plate, insulation, spring plunger and . steam ducting-to-bearing assembly bolts. Various other isteam-end parts were also damaged.

Although the ' root cause of the failures has not been determined, preliminary metallurgical observations have confirmed pre existing cracks' in both the labyrinth seal mounting bolts, the steam ducting-to-bearing assembly bolts and the spr.ing plunger. The cracks are likely due to stress corrosion cracking (SCC) although no causative contaminant has been positively identified from the corrosion products available.

The exact location of the secondary seal interspace leak has not been determined after preliminary leak tests were performed.

Disassembly and thorough inspection of the remainder of the C-2101 machine is planned. Further evidence of an interspace leak path and a contaminant which may have lead to the SCC will be sought.

As a result of the damage te circulator C-2101, a historical review including operational and refurbishment / repair history was performed in an effort to make a general evaluation of the reliability of the circulators. It was found that the circulator machine has been extremely rugged with its integrity proven over many hours of reactor service. No rotating parts failures have occurred and, of paramount importance, no shaft seizure problems have been experienced; in all cases of past component failures, the circulators have continued to operate and would have ensured safe shutdown of the reactor.

l PSC acknowledges the seriousness of the current event; however, after reviewing all of its implications, PSC considers that operation of l i

Fort St. Vrain in accordance with the Technical Specifications and FSAR is justified. This justification is based on the assumption that the root cause of the current parts failure is generic to all machines. PSC is developing a comprehensive program of monitoring, inspections and fastener replacements to ensure that the failures that occurred in circulator C-2101 do not occur again.

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1.0 PURPOSE. . . . . . . . . . . ............ 1 2.0 TRIP AND POST-TRIP SCENARIO. . .

....... .. 1 3.0 REMOVAL AND INITIAL INSPECTION OF CIRCULATOR S/N C-2101. . . . ....... .. 4 3.1 ONSITE INSPECTION AND FINDINGS. . . ... .4 3.2 0FFSITE INITIAL INSPECTION AND FINDINGS ... 6 3.2.1 Parts Still Assembled at the Lower End l of the Bearing Assembly . ....... .6 3.2.2 Parts Disassembled from the Lower End of the Bearing Assembly . . .... . .12 3.2.3 Interspace Seal and Bellows Leak Tests. . . 22 4.0 PRELIMINARY METALLURGICAL OBSERVATIONS OF PARTS RECOVERED FROM CIRCULATOR C-2101. . . ........23 5.0 PLANNED COMPLETE DISASSEMBLY AND INSPECTION OF CIRCULATOR C-2101. . ..........25 6.0 HISTORICAL REVIEW 0F HELIUM CIRCULATORS. . . . . . 26 6.1 ORIGINAL DESIGN / DEVELOPMENT PHILOSOPHY, CRITERIA AND ANALYSIS . ..... . . . 26 6.1.1 Total Machine . . . .... .....26 6.1.2 Insulation / Labyrinth Seal Area of Circulator Steam End. .. ... . . . 28 6.2 SPEED / WOBBLE MEASUREMENT SYSTEM . . . . . . . . 31 6.2.1 General Information . ... . . . . . . 31 6.2.2 Functional Information. .. . . 31 6.2.3 Interpretational Information. . ... . 32 6.2.4 Wobble Monitor System . . . .. . 32 6.3 CIRCULATOR OPERATIONAL-SERVICE HISTORY. ... . . 33 6.4

SUMMARY

OF PREVIOUS PROBLEMS, REPAIRS AND REFURBISHMENT. . . . .35 6.4.1 Replacement History of Steam Ducting Solts (3/4-16x3) and Labyrinth Seal Mounting Bolts (1/4-20x5/8). .. . 36

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6.4.2 Distorted Insulation Ocvar Found on Circulator C-2102 in 1979 . . . . . . . . . 36 6.4.3 Failed Labyrinth Seal Mounting Bolt Found on Circulator C-2104 in 1984. .....39 6.4.4 Speed Probe Bolt Failure on Circulator C-2104 in 1985. . ...... .. .40 6.4.5 Replacement of Helium-End Stainless Steel Bolts on All Machines in 1985 . . . 40 6.4.6 Fretting Identified in GA Inspection Report GA-C15847 for Circulator C-2102 in 1979 .... .... . . . . . . 41 6.4.7 Cracking of Turbine Water Drain Bellows . . 43 6.5 CIRCULATOR RELIABILITY BASED ON HISTORICAL REVIEW .43 7.0 LICENSING ASSESSMENT OF CIRCULATOR C-2101 OAMAGE . . . 46

7.1 INTRODUCTION

. ........ ........46 7.2 LICENSING BASIS FOR HELIUM CIRCULATOR OPERABILITY .47 7.3 OBSERVED FAILURES SIGNIFICANCE. . . . . . . . . 48 7.4 PREVENTION OF RECURRENCE. . . . . ...... .49 7.5 IMPACT OF CIRCULATOR C-2101 DAMAGE ON PLANT OPERATION. ..............49 ATTACHMENTS. . . . . . . ........ .... .52

Page 1 of 52 1.0 PURPOSE i The purpose of this report is to provide preliminary information relative to the damage incurred in helium.

circulator S/N C-2101 during July, 1987, and a licensing assessment of this camage. I 2.0 TRIP AND POST-TRIP SCENARIO Prior to the "C" Circulator trip, the plant was in a stable condition at approximately 70's power with all four helium circulators operating on steam turbine drive. "C" circulator was operating at a slightly higher speed than "D" circulator. For an hour preceding the trip event, no alarms i occurred.

Due to 'an apparent control system problem, "C" and "D" circulators started ramping up in speed, causing "C" d rculator to trip at 0012 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months on July 22, 1987. "C" circulator had tripped on low feedwater ficw relative to circulator speed (i.e., circulator speed to feedwater flow mismatch). Meanwhile, "D" circulator continued to ramp up in speed to compensate for the loss of "C" circulator as the program speed setpoint of "D" circulator automatically increased. The controller for "D" circulator was then placed in manual due to sagging main steam temperature and the main turbine load was dropped manually. The plant was stabilized at 50*; power. Subsequently, power was reduced to below 30% to recover "C" circulator. The trip of "C" circulator was reset at 0253 hours0.00293 days 0.0703 hours 4.183201e-4 weeks 9.62665e-5 months .

At about 0300 hours0.00347 days 0.0833 hours 4.960317e-4 weeks 1.1415e-4 months (same day), "C" circulator was rolled on steam. Hot reheat temperature fluctuations and erratic "C" circulator speeds were experienced. "C" circulator was maintained at approximately 800 rpm, "D" circulator at approximately 8600 rpm and the plant conditions restabilized.

Apparently due to misoperation of "C" circulator steam / water drain controls, the steam cavity of "C" circulator had become filled with bearing water. This water was not drained before attempting to roll "C" circulator on steam at 0300 hours0.00347 days 0.0833 hours 4.960317e-4 weeks 1.1415e-4 months . This is believed to be the reason for the atternperation of the reheat stw.. Temperature fluctuations) and the erratic speed experienced by "C" circulator upon initiating steam drive at 0300 hours0.00347 days 0.0833 hours 4.960317e-4 weeks 1.1415e-4 months .

"C" circulator was self-turbining and "D" circulator was running at about 9200 rpm in local set when "D" circulator tripped on PPS overspeed (high fixed speed) at 0516 hours0.00597 days 0.143 hours 8.531746e-4 weeks 1.96338e-4 months (about 2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months after the reheat attemperation caused by "C" 4

circulator). Due to the loss of primary coolant flow in l Loop 2, the main turbine was manually tripped, Loop 2 isolated and reactor power reduced to below 2's to recover.

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On July. 22 .and 23,.-troubleshooting took place-to try to determine why:"C" ~ circulator started ramping in speed . prior to its trip at 0012 hours1.388889e-4 days 0.00333 hours 1.984127e-5 weeks 4.566e-6 months ~, July 22. A functional test and calibration of the circulator speed control instruments on Loop' 2 was performed and. no deficiencies found. -With j certain data logger information available. and through a- j logical . analysis of the control: system,. the suspect l

components were narrowed to a few switches. 'These switches Lwere replaced even though no deficiencies were found in the original switches. Increased. data ' acquisition has 'been initiated to; allowfor moreithorough data analysis should the event reoccur.

Loop 2 was returned to service at 2230 hours0.0258 days 0.619 hours 0.00369 weeks 8.48515e-4 months , July 23, and reactor power was increased above 2?s. At 0600- hours,' July 24, with the reactor operating at 2.5?4 power, "D" circulator

.was brought up to'about 4400 rpm on steam turbine drive and balanced. with "A' circulator. . Reactor power was increased to about 5?s by 0900 hours0.0104 days 0.25 hours 0.00149 weeks 3.4245e-4 months . "0" . circulator speed reached i 5025 rpm, 6030 rpm and its peak 6405 rpm by.1000 hours0.0116 days 0.278 hours 0.00165 weeks 3.805e-4 months , 1100 hours0.0127 days 0.306 hours 0.00182 weeks 4.1855e-4 months and 1208 hours0.014 days 0.336 hours 0.002 weeks 4.59644e-4 months , respectively. Due to erratic speed ,

indication on "D" circulator at 1227 hours0.0142 days 0.341 hours 0.00203 weeks 4.668735e-4 months , Results was prompted to check the speed modules. Results reported an indication of excessively high wobble and therefore the circulator was reduced to less than 800 rpm at 1238 hours0.0143 days 0.344 hours 0.00205 weeks 4.71059e-4 months , July ' 24. " C." circulator was brought up to 6600 rpm to compensate for the loss of "D" circulator. Engineering (NED) subsequently became involved in an attempt to assess the continued operability of "D" circulator. Due to unacceptable ' wobble indication, Operations was advised not to operate "D circulator above self-turbining speed except-for diagnostic purposes (speed was limited to 2000 rpm or

-less for. this purpose). By 1345 . hours, July 24, "0" circulator: speed was reduced to about 400 rpm on self-turbining. Rise to power continued on the three remaining circulators.

On July 26, power operation was resumed, stabilizing at 45?s and approximately 615 psia PCRV pressure. During power operation, about 2000 hours0.0231 days 0.556 hours 0.00331 weeks 7.61e-4 months , July 27, an excessively high pressure was observed in the turbine water drain tank (TWDT). The source of this high pressure was found to be purified helium. The most likely source of this helium was from the water-turbine piping .of "D" circulator possibly indicative of a penetration interspace leak. A surveillance (SR 5.2.16a-Q) which determines PCRV closure leakage was then performed and resulted in a computed maximum leakage ~

through the secondary penetration seal of "D" circulator of approximately 915 pounds of helium per day. This was in excess of the 400 pounds / day allowed by Technical Specification LCO 4.2.9. The 24-hour grace period of LCO 4.2.9 was entered at 1055 hours0.0122 days 0.293 hours 0.00174 weeks 4.014275e-4 months on July 28. An orderly reactor shutdown was initiated at 2145 hours0.0248 days 0.596 hours 0.00355 weeks 8.161725e-4 months and one-hour notification of plant shutdown as required by LCO 4.2.9 was

.x Page 3 of 52 made at 2'302 hours,. July 28. Shutdown was completed at 1026 hours0.0119 days 0.285 hours 0.0017 weeks 3.90393e-4 months , . July 29.. Subsequently, PCP.V depressurization to-below 100 psia was' completed at 2108 hours0.0244 days 0.586 hours 0.00349 weeks 8.02094e-4 months , July 29.

During investigation .of this event, a deficiency.of the pressurization gas flow monitoring and alarm system..was

. identified. -The' purified helium header flow' instrumentation measures instantaneous flow in the purified helium header.

Flow through the purified helium header is constantly fluctuating due.to several- factors including changes in PCRV pressure and the identified leakage in the Loop 2. steam generator penetration interspace. These fluctuations cause intermittent actuations of the high flow alarm that are not indicative of unidentified penetration interspace leakage.

These intermittent alarms could potentially mask'an actual

. penetration interspace leakage problem. An action request has been: initiated to evaluate the indication circuitry for modifications that would reduce the frequency of j intermittent alarm actuations. Until this evaluation is complete and modifications made,- operators will log- the -I pressurization gas flow once per shift to ensure LCO 4.2.9

-compliance. (

Reference:

LER 87-018-00 submitted under P-87300).

Further "0" circulater wobble measurements were ecliected under NED supervision on July 27 and 28. Evaluation of these wobble values resulted in an indicated wobble at the lower circulator beari.ng of more than twice the available bearing clearance. However, the wobble versus shaft speed relationship was not representative of an unbalance condition Also, if bearing clearances anywhere within the cartridge had in fact increased, then one would expect cartridge bearing water delta pressure to decrease for normal flow of 165 gpm. The fact is that the bearing water flow / delta pressure relationship was observed by NED personnel to be normal at 165 gpm and 650 psid. This fact made the validity of the woo 61e indication at least on the lower bearing suspect. These apparent incongruities are yet to De fully explained.

"D" circulator thm% came to be considered inoperable on two counts: (1) interspace leak in excess of the LCO 4.2.9 limit and (2) indicated wobble magnitude greater than that permitted by the circulator operation and maintenance (0&M)

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manual for continued operation. On July 31, 1987, the decision was finally made to remove "D" ciredator (S/N C-2101) for inspection and replace it with an available refurbished spare (C-2104).

It has been postulated that the interspace leakage may be associated with the high wobble indication; however, at this point in the evaluation there is no direct evidence to l

indicate that the high wooble and the interspace leak on "D" circulator are interrelated.

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, It has been determined that no pr' mary seal leakage. existed.

This was verified by isolating the interspace from the purified helium header and monitoring the interspace

. pressure. The interspace pressure decayed below PCRV pressure when isolated from the header, and no activity was observed in the TWDT. Primary seal leakage would have l resulted in the interspace pressure equalizing with PCRV pressure, while a combination of primary and secondary seal leakage would have resulted in an increase in activity in the TWDT.

Since purified helium was discovered in the TWDT, the most probable interspace secondary seal leak paths were initially i narrowed down to three locations: (1) the double metal !

0-ring series (Items 101 and 102, Dwg. R1100-100, Sheet 5) ;

located in the ^ joint between the expansion joint bellows '

ring (Item 66, F.P. 91-M-19-9) and the steam outlet piping 1

( I ter.. 81, R1100-100, Sheet 5); (2) the expansion joint bellows itself (Item 64, F.P. 91-M-19-9); and (3) the metal 0-ring (Item 33, C2101-300) providing seal between the circulator bearing assemoly (Item 26, C2101-300) and the steam ducting (Item 23, C2101-300). (A fourth possible leak path was identified later during disassembly: puncture or crack through the steam ducting weldment (C2101-431)).

However, a means exists for monitoring leakage between the Item 101 and 102 (R1100-100) 0-rings while the circulator is installed. No leakage was in evidence between these 0-rings, which seems to eliminate that as the leak path.

Also, a failure of the expansion joint bellows (Item 64, T.P. 91-M-19-9) is unlikely in that no previous such failures to date have been experienced. The metal 0-ring (Item 33, C2101-300) is also considered unlikely because there have been no known leaks in this area to date.

However, the 0-ring must be considered due to possible distortion of the seal resulting from the forces involved during the exiting of the failed parts (described in Sections 3.1 and 3.2) through relatively small clearances and/or possible relaxation of the sealing force due to failure of three steam ducting mounting bolts (described in Section 3.2). (However, rough calculations assuming a rigid flange have shown that the loss of the three bolts would not be expected to allow leakage.)

3.0 REMOVAL AND INITIAL INSPECTION OF CIRCULATOR S/N C-2101 Removal of circulator S/N C-2101 from "D" penetration and shipment of the circulator to GA Technologies in San Diego for disassembly was completed by August 14, 1987.

3.1 ONSITE INSPECTION AND FINDINGS During removal operations of C-2101 from the PCRV, some failed circulator harcware pieces were discovered lying on

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Page 5 of.52 the top surface: of .the special weld neck flange (Item 9,;

-F.P. 91-M-19-1) of the steam inlet and water piping assembly.

(F.P. 91-M-19-1) (this flange surface. exists in the circulator steam outlet area). The pieces were initially and tentatively identified by configuration as being from.at least three different circulator stationary. parts located above'. the' steam turbine . rotor (Item 24,'C2101-300): (1) insulation cover (Item 11, C2101-300), -(2) labyrinth seal (Item .28? 'C2101-300' and (3) 1abyrinth spacer (Item 27, C2101-300). The pieces found ranged in size from large (~7" X 3/4" X 3/16") to small (1" X 1" X 1/16"), to particle sizes of-1/16" to 1/4" diameter. Visual indications on the

. pieces .suggest prolonged and severe rubbing, as the pieces were. drastically distorted. Most of the fracture surfaces exhibited corrosion. In addition to'the above parts, some 1/32" lockwire and a portion of a 1/4-20 bolt were found.

This area of the' steam end of the circulator was not directly accessible at. .this stage ~ of . disassembly for confirmation of the. identity of these parts.

Three representative sample pieces were chemically analyzed and all found to be standard type 430 ferritic stainless steel which agreed with the drawing specified material for each of.the above three parts.

The discovery of these broken parts and their accurate identity mostly assured led to the postulation that an erroneous everspeed signal and resulting trip of "D" circulator on July 22 may have been a result of these pieces exiting and/or becoming trapped in the clearance between the steam turbine rotor (Item 24, C2101-300) and the steam l

ducting.(Item 23,C2101-300) causing sufficient circulator '

shaft displacement to affect the measured speed signal (i.e., give a false indication of higher speed). This may also be supported by the fact that a few seconds prior to "0" -circulator' trip on July 22 the circulator speed ;

j controller signal increased (speed valve opening), but' actual "D" circulator speed did not increase (i.e., more steam was admitted but no. actual increase in speed resulted). If the parts failure is associated with the overspeed trip of "0" circulator, then it would indicate that "0" circulator was still able to operate up to at least 6400 rpm on July 24 af ter the damage was incurred (i.e. , "D" circulator may have been able to perform its safety function even'though high shaft wobble was indicated).

Additionally, metal to metal contact between stationary and rotating parts may have magnetized the circulator shaft which may also have an adverse effect on speed and wobble sensing.

After removal of the steam inlet and water piping assembly (F.P.91-M-19-1), a rough inspection of the pelton wheel J (Item 13, C2101-300), steam turbine stator (Item 25,

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C2101-300) and steam ducting weldment (C2101-431) became possible. Nothing unusual was observed with the pelton

' wheel and steam turbine stator. Lodged in the flow path of the steam ducting weldment were more pieces appearing to be from the labyrinth seal (Item 28, C2101-300) and labyrinth spacer, (Item 27, C2101-300). However, at this time these pieces could not be removed for further examination.

This concluded the relevant inspections and findings prior to shipment of the circulator to San Diego for further disassembly by GA Technologies.

3.2 0FFSITE INITIAL INSPECTION AND FINDINGS On August 14, 1987, initial disassembly of the steam-end components on circulator S/N C-2101 took place at GA Technologies. Upon disassembly PSC representatives were on hand to witness the inspection.

The following discussion with pictorial aids documents the visual observations made. Attachments 1 and 2 are included to show the location of the C2101-300 parts identified.

Attachments 3, 4, 5 and 6 show the assembly of interest in greater detail.

3.2.1 Parts Still Assembled at the Lower End of the Bearing Assembly (Item 26, C2101-300)

1. Lower Labyrinth (Item 11, C2101-500) (Material: 422 SST per ASTM A565, Gr. 616) (See photo below.)

The area around the bayonet locking tabs (for locking the I.D. of the insulation cover (Item 11, C2101-300))

and tFe locking tabs themselves had damage around the edges. Only a small portion of each locking tab remained (approximately 1/32" to 1/16"). There was no noticeable damage to the labyrinth teeth and no damage te the curvic coupling on the circulator shaft (C2101-521).

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ked eh There ai ed p imately n o the threaded portion of the bolt in the hole.

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3. Steam Ducting Bolts (Item 45, C2101-300) (Bolt I Material: A-286 per AMS 5737) (See photos below.) l l

Bolt No. 10 (Note: Hole No. 9 is for pressure tap bolt) I was broken approximately 0.6 inch from the bottom I surface of the hole counterbore. The bolt fracture surface had sca.le on it, with some smooth rub marks. ,

The heads of the remaining bolts (Nos. 1-8, 11-15) were !

ground roughly flush to the surface of the steam ducting (Item 23, C2101-300). In-the case of bolt No.

1, some bolt material was jammed between the bolt head and the hole counterbore. All of the bolt lockwiring (Item 81, C2101-300) (Material: Inconel 600) was missing, with the bolt heads ground below the lockwire holes.

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4. Steam Ducting Weldment (C2 D 1-431) (Material: Inconel 718 per AMS 5663, forging) .(See photos below.)

Large bolt hole No '9. for the pressure tap bolt (Item' <

52,C2101-300). look slightly distorted. There were two hairline fractures visible ~:approximately '180 apart around the bolt hole. The depth of the cracks could not be verified. The uppermost crack (see photo) extended radially approximately 3/4' inch up the outer

y. face of the . steam ducting weldment and down into the-bolt hole. The lower. crack extended radially approximately 1/4 inch down the outer surface adjacent.

to the bolt hole. There was no damage found around the-bolt hole (No.

10) of the- fractured solid bolt

.(Item 45, C2101-300).

The. outer surface (where labyrinth seal (Item'28, C2101-300) attached) on. the steam ducting.weldment-(0.0. surface in photo) had circumferential grooves with the deepest about 0.04 inch. These grooves were located about 0.6 inch radially from the bolt hole counterbore.

Two dowel pins originally existed 180 apart to locate the labyrinth seal on the steam ducting weldment surface. One dowel hole (small hole No. 9) was empty and c. lean. The other (hole No. 2) had part 'of the dowel pin still in it.

Originally there existed twelve 1/4"-20 capscrews (Item 46, C2101-300) (Material: 410 SST per ASTM A193, Gr.

B6) attaching the labyrinth seal to the steam ducting weldment. Small holes Nos. 12, 13, and 15 for these screws were empty with clean,' undamaged threads. The remaining holes (Nos. 1,3-8,10,14) had screws-broken off in them. Most of these breaks were located about one thread below the surface with the remainder flush with the surface with a rubbed appearance.

Originally - there existed one spring plunger (Item 53, C2101-300) (Material: carbon steel) in small hole No.

11. The spring plunger was used for locking the insulation ccver (Item 11, C2101-300) into place once it was mated and rotated into the cam locks of the labyrinth seal. This spring plunger was broken off with a portion of the plunger body left in the hole.

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/- A 3.2.2 Parts Disassembled from the Lower End of the Bearing Assembly (Exa'nined on the bench)

1. Insulation Cover (Item 11, C2101-300) (Material: 430 ferritic stainless steel per ASTM A479) (See photos below.)

Remnants of the insulation cover were pressed into the rim towards the outer circumference of the steam turbine rotor (Item 24, C2101-300). The 9/16-inch outer axial projection of the cover was either grossly 5ent 90 into the plane of the disc or completely I missing. About 30% of the cover flat surface was gone.

Remnants of the cover showed much evidence of deformation, rubbing, metal tearing and cracking.

There was a circumferential indentation in the cover surface phart ft appeared that the large bolt heads (Itein 45, C2101-300) had rubbed. The face of the turbine steam rotor disc had circumferential scoring marks with an exceptionally deep score mark (approximately 0.12 in, wide X 0.06 in. deep) located on a 6-1/4-in. diameter. A portion of a large bolt head (Item 45 or 52, C2101-300) was found lodged between the insulation cover and turbine disc. The six 3/8-inch holes used for installation of the insulation cover were mushroomed and distorted around the edges.

There was one large (approximately 1/2-inch diameter) ding at the outer circumference of the cover.

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2. Labyrinth Seal (Item 28, C2101-300) (Material: 430 ferritic stainless steel) (See photo below.)

Two large fragments of the labyrinth seal were found in the steam return bend of the steam ducting weldment (C2101-431). One was about 11-1/4 inches long and the other about 13-1/2 inches long, with both having 1/2 to 1 inch width. This accounts for all but about 18 inches of the total seal circumference. The 0.650-inch axial projection at the outer circumference was mostly gone and/or severely distorted. The pieces showed heavy score and rub marks. The inner locking tabs wera rubbed smooth. Part of a mounting bolt (Item 46, C2101-300) was stuck in one of the mounting holes.

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3. Labyrinth Spacer (Item 27, C2101-300) (Material: 430 ferritic stainless steel) (See photo below.)

Two large fragments of the labyrinth spacer were found in the steam return bend of the steam ducting weldment (C2101-431). One was about 14-1/4 inches and the other about 21-1/4 inches long accounting for all but about 5-3/4 inches of the total spacer circumference. There were heavy rub marks on the I.D. surface and the pieces were flattened and distorted somewhat. ,

Page 16 of 52

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4. Labyrinth Seal Mounting Bolts (Item 46,C2101-300)

(Material: 410 SST per ASTM A193, Gr. B6)

The only part of the bolts found were the pieces referred to earlier remaining in the steam ducting weldment and the portion found stuck in the labyrinth seal.

5. Lock Washer (Item 29, C2101-300) (Material: 430 ferritic stainless steel)

There was no evidence of the labyrinth seal mounting boit lock washer found during disassembly at GA (some of the small pieces founei on site during circulator removal may be part of the lockwasher).

6. Spring Plunger (Item 53, C2101-300) (Material: carbon steel) (See photo below.)

There was no evidence of the spring plunger used to lock the insulation cover into place other than the portion found remaining in the steam ducting weldment ;

(referred to earlier). (The spring plunger shown in !

the photograph is new.) !

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7. Insulation, Bearing Side and Turbine Side (Items 20 and 32, C2101-300) (Material: 430 ferritic stainless steel

(" felt metal")) (See photo below.)

There was no evidence of the " felt metal" insulation found (some of the small pieces found on site during circulater removal may be part of the insulation).

(The insulation shown in the photo is new.)

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8. Backing Plate (Item 31, C2101-300) (Material: 430 ferritic stainless steel)

There was no evidence found of the 24 gage backing {

i plate placed under the " felt metal" insulation (Item '

32, C2101-300) (some of the small pieces found on site during circulator removal may be part of the backing plate).

9. Steam Turbine Rotor (Item 24, C2101-300) (Material: I Disc - 422 SST per ASTM A565, Gr. 616; Blades -

Alloy Greek Ascolay per AMS 5616) (See photos below.)

The steam turbine rotor had circumferential grooves cut in the insulation cover side face. There was some !

distortion and material removed at the rim just inside the turbine blade mounting area (it was under this rim that the insulation cover was lodged). Reverse side of disc was normal. l The trailing edge of all turbine blades showed evidence of rubbing with small burrs and slightly rolled metal, as if the trailing edge had been scraped against a foreign piece of metal while rotating. The leading edge of most of the blades had small dings located at approximately the same circumference around the rotor.

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10. Steam Turbine Stator (Item 25, C2101-300) (Material:

Blade Ring - chrome-moly steel per ASTM A473, Type 501A annealed) (See photos below.)

The inlet side of the stator showed no damage. The trailing edge of each blade showed dings, indentation and some bending as if a metal fragment had been trapped between the rotor and stator blades.

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11. Pelton Wheel (Item 13, C2101-300) (See photo below.)

There was no evidence of abnormal damage to the pelton wheel. The flowstream splitter located down the middle of each bucket exhibited some erosion and cavitation damage, but nothing beyond normal expected wear after years of service.

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3.2.3 Interspace Seal and Bellows Leak Tests Two leak tests were performed at GA Technologies in an attempt to find the massive interspace leak (reported to be as large as 915 pounds of helium per day). The first was a coarse leak test (using air, then helium) of the metal 0-ring (Item 33, C2101-300) and the crack discovered in the steam ducting weldment (C2101-431). A special test fixture I was used and pressure applied to 20 psig. No leak was heard; however, the test fixture may have the effect of helping to seal a loose seal. A refined leak test of this l

area is not possible as it was necessary to remove the damaged steam ducting bolts (Item 45, C2101-300) prior to uprighting the assembly (uprighting the assembly would have been necessary for accessibility to effect a tighter seal for the refined leak test). Additionally, it is not j possible to pressurize the seal from the interspace side (top side) without the aid of the interspace penetration.

Later in the disassembly of the circulator it will be possible to examine the crack in the steam ducting weldment

.Page 23 of 52 .

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more thorough'ly to de'termine if it has penetrated the weldment completely, b '

The other leak ' test was performed on the expansion joint i bellows (Item 64, F.P. 91-M-19-9) using air. Pressure. for this test'.was limited to 5 psig duc to-the strength of.the available blind flange used to blank 'off the _ steam . pipe.

Pressure was held for at least 20 minutes with no pressure drop observed (no leakage was heard either). Later in the disassembly of the circulator it will be possible to access the bellows for a more thorough' inspection.

While the machine is removbd from its' penetration it is not possible to: test the' double metal,0-ring seal'(Items 101 and

'102, R1100-100, Sht. 5) located-in the joint between the expansion joint bellows ring and the steam outlet piping.

1 14.0 PRELIMINARY- METALLURGICAL OBSERVATIONS OF PARTS RECOVERED FROM CIRCULATOR C-2101 Following the failure of C-2101 a . metallurgical' investigation was initiated. Several components' have, been examined. The specific items examined were C2101-300-52 (pressure tap bolts), C2101-300-45 (steam ducting to bearing assembly bolts), C2101-300-46 (labyrinth seal mounting bolts),- C2101-300-53 (spring plunger), C2101-300-11

.(insulation cover), C2101-300-28 (labyrinth seal) and C2101-300-27 (labyrinth. spacer). The investigation is not complete and the following are preliminary findings:

Bolt C2101-300-52 which was in hole #9 (Ref. Section 3.2.1) and had fractured was examined. The bolt is a precipitation hardening austenitic stainless steel per AMS l

. 5737, Type A286. The bolt had fractured above the first >

thread in the shank. 'The fractured surface was covered with {

oxidation products. E0AX analysis found no indication of contaminants (e.g., Cl, Na, etc.). Secondary cracking was {,

observed on the fractured surface. Metallographic .I examination found stress corrosion cracks propagating from

.the fractured surface along the longitudinal axis.

Five bolts, C2101-300-45, were examined, also A286 material. The bolt from hole #8 (Ref. Section 3.2.1) was found to be cracked almost through the entire cross section.

The bolt was found to ha'e v a precracked area which initiated by fatigue. The cause of the fatigue crack has not been determined at this time. Transgranular cracking was found in two of the other four bolts examined. The cracks were propagating normal to the applied load and initiated at the i roots of the first three threads below the shank. The i cracks are likely due to stress corrosion cracking, i i

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Six C2101-300-46 bolts (hole locations known) and two {

sections of the same bolts (hole locations unknown) were 1 1

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Page 24 of.52 examined. The . bolts are a.martensitic stainless steel per ASTM A193 B6 (AISI Type 410). The fracture surfaces appearance indicated a brittle or progressive type of failure typical of stress . corrosion cracking (SCC).

The fractured surfaces were covered with oxidation products. No contaminants were.found from EDAX analysis. The six bolts and two bolt sections all exhibited cracking at the root of

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the threads. The cracks were propagating normal' to the applied load and some cracks initiated at pits. Several cracks exhibited branching typical of stress' corrosion i cracks. The hardness of the bolts, RC 22-26, would normally indicate a 410 SST material which is not susceptible to SCC, as published data indicates that RC 25 is generally an accepted lower threshold for SCC to occur.

The spring plunger, C2101-300-53, was examined. Only a portion of the plunger body was left. The plunger body material 'is carbon steel. The plunger body threaded area was found to be. heavily corroded and pitted. EDAX analysis found no evidence of contaminants. Transgranular cracking at the root of the threads was observed. It is likely that these cracks are due to SCC even though SCC in carbon steel is normally intergranular. Further investigation will be performed.

Examination of 7 samples from the insulation cover (C2101-300-11) was performed. Significant post-service rubbing damage was observed. The cover met the chemical requirements of 430 ferritic stainless steel, with a hardness of Rockwell-B 83.5. Fractographic examination revealed a brittle fracture surface, with significant oxidation of the surface. All fractures occurred at machined notches or holes, typical of the very notch sensitive 430 ferritic stainless steel. Metallographic analysis revealed extensive transgranular secondary cracking, with branching observed. EDAX analysis revealed no evidence of contamination within the oxides of the fracture surface and secondary cracks. While some of the cracks were typical of stress corrosion cracking (SCC), SCC has very rarely been observed in ferritic stainless steel, and the majority of test data reviewed indicates that SCC will not occur in ferritic stainless. When SCC has been referenced, it has almost always been intergranular. The forged and annealed microstructure observed consisted of coarse grain boundary chromium carbides, partially lamellar carbides, and fine, equiaxed recrystallized ferrite grains, in coarse delta ferrite. The coarse grain boundary carbides intensify the inherent brittleness of this material. The possibility of frictional heating producing the structure is considered unlikely because the initial temperature would have to be above 1500 F for at least 30 minutes.

Transformation would then occur at 1100 F (15 hours1.736111e-4 days 0.00417 hours 2.480159e-5 weeks 5.7075e-6 months )-1300 F (2 hours2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months ). If this had occurred, microstructural transformation of other circulator components (i.e.

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spheroidization'of the carbon steel plunger) would.have been l observed.

. Examination of' 3 samples from- the~ labyrinth seal' (C2101-300-28) was performed. Significant rubbing ' damage' b

was ; observed. . .The'sealfalso met:the chemical requirements

of 430 . stainless steel, with a hardness of HRB .188. 5'.

.Fractographic- examination- also revealed al . brittle, transgranular cleavage, fracture surface. 0xidation- of the-fracture surface was observed, and EDAX analysis revealed no evidence':of. contaminants. . Some secondary crackin.g was

/ observed,. however: no- bra nchi ng .. was, . observed. The microstructure observed was typical of that of the' cover.

a. Examination of . 2 samples , from . the labyrinth spacer (C2101-300-27) was. performed. .Very little . rubbing' damage was . observed. The spacer met.the chemical requirements'of' 430 ferritic stainless' steel, with a hardness. of : HRB 81.

Fractographic'- examination revealed a brittle fracture surface, with. oxidation present. No' secondary cracking was.

. observed. The microstructure of the spacer was considered more. typical of a uniform fo;rging structure,- with a. more uniform grain size distribution, slightly elongated' grains, and a finer distribution of matrix and grain boundary carbides.

Further metallurgical- investigations. to be performed include:

Analysis of other available components from C-2101 for evidence of SCC and corrosion, Identification of contaminants in corrosion products,

-Determination of source of contaminants, Research literature for environments which can cause SCC in specific components being analyzed, Identification of resistant replacement material (s).

5.0 PLANNED COMPLETE DISASSEMBLY AND INSPECTION OF CIRCULATOR C-2101 Upon receipt and thorough review of a disassembly and inspection work plan from GA Technologies, remaining disassembly and inspection of the C-2101 machine will commence. It is expected that the total machine will be inspected for any damage resulting from the current failure of the insulation cover assembly parts in the steam end.

Of particular interest will be a thorough inspection of the steam ducting weldment (C2101-431) for evidence that the crack observed on the lower side has extended to the upper

Page 26 of 52 side (possible interspace leak path). Also of interest will

. be the bearing and ' shaft surfaces ~ for signs of damage due to the indicated high wobble. Possible magnetization of the shaft will be investigated. Any further clues as to the positive identification of a contaminant causing the stress corrosion cracking of fasteners will also be sought.

6.0 HISTORICAL REVIEW OF HELIUM CIRCULATORS As,a result of the damage to circulator C-2101, a historical review including original design criteria and operational and refurbishment / repair history was performed in an effort.

to make a general evaluation of the reliability of the ci rcula tors . This historical review is presented below.

6.1 ORIGINAL DESIGN / DEVELOPMENT PHILOSOPHY, CRITERIA AND ANALYSIS 6.1.1 Total Machine 6.1.1.1 Design Criteria The design of the circulator was Dased on Section III of the ASME Boiler and Pressure Vessel Code for its pressure retaining parts, and on current turbomachinery practice for its turbomachine components. An important objective was a 30 year inspection- and maintenance-free lifetime. To achieve this objective, larger design margins than usual were adopted with respect te stresses and functional capability; in particular, the circulator is capable of indefinite operation at any speed up to 40's overspeed. In addition, a wear-free : design philosophy was pursued; no sliding or rubbing parts are included in the circulator.

Protection of the primary coolant system against any conceivable failure dictated the introduction of a disk catcher in order to contain any missiles that might be generated by compressor rotor failure.

6.1.1.2 Development Sequence The overall helium circu?ator design effort has progressed through three phases: a water bearing test rig, a prototype test machine, and the final production circulator configuration. Most of the testing has been accomplished within a specially constructed test facility located adjacent to the Public Service Company of Colorado Valmont Power Station near Boulde r., Colorado. The test site was selected early in the development program mainly because of the availability of steam required for the full scale prototype and production circulator testing.

The Phase I program, using the bearing test rig (also referred to as the rotor inertia simulator), consisted primarily of the prototype rotating assembly with special

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7 Page 27 of 52 i

disks to simulate the mass and inertia of the compressor and j turbine wheels. Power was supplied by the pelton turbine. l The Phase I test established _the initial dynamic rotor '

performance and facilitated developmental evaluation of the i bearings, seals and rotor dynamics prior- to full scale {

aerodynamic testing of the prototype machine.

The Phase II program included two parts. First, the aerodynamic performance of the compressor, compressor inlet i

and diffuser was evaluated in air using the pelton turbine as a power source. Next, the prototype assembly was tested f i

under simulated reactor conditions with the steam turbine l driving the compressor in helium. !

The Phase III program included the operation of the production circulator at simulated design conditions, a comprehensive series of transient tests, and a final chu.L" i out of each production machine prior to installation into I the Fort St. Vrain PCRV.

The series of transient tests was carried out .asing the first available Fort St. Vrain production circulator. The l objective of these transient tests was to determine the operational capabilities of the circulator when subjected to 3 normal and abnormal transients which could conceivably occur during reactor operation. The tests ircluded hot restart test, thermal shock test, circulator overspeed test, and rapid depressurization test. The rigorous tests the overall machine was subjected to encompassed a broad range of plant operating conditions to indicate that all_the components of the machine satisfied the expected operating requirements.

6.1.1.3 Fatigue Analysis '

The parts of the circulator that were subjected to a fatigue I analysis were the blading (rotors and stators) and the ASME I

Boiler and Pressure Vessel Code parts. The Code parts were evaluated per the requirements of Section III of the ASME Code. The analysis was performed on the basis of 20,000 reactor plant cycles between 25's and 100% power and f,00 ,

transients. !

l 6.1.1.4 Acoustics and Vibration An acoustics test program determined the noise level and frequency generated by tne compressor. Tests were performed I in air as well as helium atmospheres. The noise levels generated were found to be acceptable. No acoustic testing was i t, dune on the steam turbine. There is no indication that was even considered. In any case, it is not presently considered to be an issue because of the extremely small free space available for the propagation of sound waves in the area bounded by the steam seal.

Page 28 of 52 A vibration test program evaluated the natural frequencies and blade stresses of the steam turbine blades as well as the ccmpressor blades at various operating conditions. All stresses were found to be sufficiently low to meet a 40 year life design objective.

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6.1.2 Insulation / Labyrinth Seal Area of Circulator Steam End 6.1.2.1 Purpose of Insulation and Labyrinth Seal The insulation and insulation cover at the lower end of the bearing cartridge are provided to reduce the temperature of the lower end of the bearing cartridge and thus provide a more uniform temperature in the cartridge. This is desirable because:

1) Clearances at tM ! circ r labyrinth are easier to control,

2) Steam / water drain performance is less susceptible to

, changes in the steam temperature,

3) Temperatures of the bearings are more uniform and bearing performance is therefore not adversely affected by changes in bearing clearances (bearing temperatures are less dependent on steam conditions as wall as being more uniform from one bearing to another),

4) Lower temperatures afford more flexibility in selection of materials in the bearing cartridge.

The labyrinth seal is provided for two reasons:

1) The shape at the 0.D. provides a relatively smooth flow path for the steam, thus minimizing turbulence,

2) A seal clearance of 0.130" +/- 0.010" is accomplished by machining the seal ring at assembly to the machine.

This clearance minimizes steam flow into the area below the bearing cartridge, thus reducing turbulence and reducing the severity of the requirements on the steam / water drain design.

6.1.2.2 Original Design of Failed Parts The design and analysis of the components of the helium circulators was at three levels: (1)ASME Section III Code parts, (2)non-Code critical parts and (3)"non-critical" support components. Of the components that have failed or been damaged during the incioent, only the 3/4-inch steam ducting bolts (Items 45 and 52, C2101-300) were included in the analysis performed in accordance with the ASME Code (except for fatigde analysis). The analysis for all Code

y Page 29 of 52 w

parts:.12 included; in the design' report (GADR-13) for-the circulators.

The ~3/4-inch bolts were analyzed in conjunction with the

-ASME Section.III analysis performed on the turbine casing and. primary closurA . cone. Code allowables of less than 2 times stress intensity for initial boltup and less than_ 3 times stress! intensity for maximum membrane plus bending.

stress at the worst design conditions were used. Changes made subsequent to the original analysis were an increase in bolt torque for improved interspace sealing and the drilling of a hole through the axis of one of the fifteen (15) 3/4-inch bolts to allow sensing of turbine steam outlet pressure. A Code're-analysis as a result of the increased torque cannot be located at this time. A preliminary re-evaluation of the bolts .using stress output from the old :

1 finite element programs for varipin load ronditit." shows that stresses are all stim dtMt Mt 01v*ab m tMt the bolts are good 'ra a ' cyclic-fatigue . standpoint.

However, thit evaluation includes only the C2101-300-45 bolts and does not include the single C2101-300-52. bolt with the 1/4-in.. pressure-tap hole drilled through its axis; a separate evaluation is required for this bolt.

For the remaining failed parts (insulation cover, labyrinth seal, labyrinth spacer, labyrinth . seal mounting bolts, all were considered to be "non-critical" support etc.),

components. Sound engineering practices were used for the. .

' design and development of these parts. The actual design analysis ~ and calculations were'- not retained from the original design period (circa 1968). No specific developmental tests were performed regarding the "non-critical" support components now under evaluation. However, as stated earlier, the rigorous tests the overall machine was subjected to encompassed a broad range of plant operating. conditions to indicate that all the components of the machine satisfied the expected operating requirements.

No fatigue analysis was performed on the lower-end "non-critical" components now under evaluation because they were lightly loaded. The Code parts were evaluated for fatigue per the requirements of Section III of the ASME Code; ;

however, the 3/4-inch bolts were not included in this i original analysis.

l A seismic evaluation for the steam ducting struts was performed and is documented in the original design report l

' (GADR-13). Assum-ing that the 3/4-inch bolts instead of the steam ducting struts will take the seismic loading the struts were designed for, a rough hand calculation shows that a highly conservative seismic loading on the bolts is j significantly less than the bolt preload. l i

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a A hand calculation. on the 1/4-inch labyrinth seal mounting bolts (Item 46, C-2101-300) also shows that the seismic loading on these bolts is insignificant relative to the bolt j preload.

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Provided in the table below is a summary of the materials -

)' usad for the lower-end components now under evaluation:

C2101-300- Description Material 11 Insulation cover AISI 430 SST 20,32 Insulation AISI 430 SST 46 1/4-20 bolts A193 86 (410 SST) 45,52 3/4-16 bolts. A286 SST (AMS 5737) 28 Labyrinth seal AISI 430 SST 31 Backing plate AISI 430 SST 27 Labyrinth spacer AISI 430 SST 53 Spring plunger Carbon steel 29 Lochwasher AISI 430 SST i 81 Lockwire Inconel 600 j The rationale behind the selection of the materials for the above parts, for which no detailed design report exists (except for the 3/4-16 bolts), is as below.

Major criteria that influenced materials selection at the 3 time of circulator component design were: !

1) Appropriate mechanical properties, 4

2) Ability to survive tne circulator " hot soak" condition,

3) Adequate general corrosion resistance to avoid rusting and degradation during operation, handling and storage,

4) Resistance to stress corrosion under expected operating conditions (i.e. cold reheat steam from feedwater meeting the steam generator feedwater requirements).

After a brief review of the materials selection for the parts listed in the above table it is noted that, with xly one exception, the above criteria were clearly met. The 400-series stainless steels have appropriate mechanical and corrosion properties and were used in heat treatment conditions designed to minimize stress corrosion cracking (SCC) risks. The 410 SST parts, for example, were required to be given an additional temper for 2 hour2.314815e-5 days 5.555556e-4 hours 3.306878e-6 weeks 7.61e-7 months at 1150 F to maximize stress corrosion resistance. The A286 and Inconel

.600 and 718 parts were believed, at the time of design, to j be very resistant ts SCC in expected environments. In this ';

context it is n a.d that GA was very concerned to assure good SCC resistance during FSV design as a result of adverse

Page 31 of 52 1

adverse SCC experience encountered at the Peach Bottom I '

Reactor during late 1966.

In the years subsequent to the design of FSV, industrial !

experience has shown that A286 and certain of the Inconeis can suffer SCC under some conditions- particularly when high-oxygenated, high-temperature water or caustic are present.

At this point, it has not been possible to reconstruct the rationale behind the selection of carbon steel for the ;

spring plunger (Item 53, C2101-300). This part was '

apparently available in carbon steel and stainless steel. !

If the stainless steel available were 300 series SST, it !

also may have been unded reble.

6.2 SPEED / WOBBLE MEASUREMENT SYSTEM 6.2.1 General Information i

As the name implies, the helium circulator shaft {

speed / wobble measurement system' measures both shaft speed and wobble. Shaft speed is the number of shaf t revolutions per unit time. Shaft wobble is the displacement or movement of the shaft surface toward or away from fixed points defined by the installed location of speed elements.

6.2.2 Functional Information

.The system is comprised of reluctance-type detector probes referred to as speed elements (SE's) connected to an electronics system. The SE's are comprised of two identical coils of which one senses the gap between it and the shaft ,

j surface while the other senses a fixed reference gap. These coils are inputs to a bridge circuit in the electronics system. The output of this bridge circuit is proportional to the ratio of inductance between the two input coils which is in turn proportional to the ratio of the measured and reference gap. Shaft movement away from the SE causes an increase of output voltage from the electronics. Shoft movement toward the SE causes a decrease of output voltage.

Precision-machined slots cut into the shaft surface provide timing pulses for measuring shaft speed. These same slots provide short intervals of known displacement of the shaft surface for the purpose of calibrating system gain. One mil (0.001") change in the gap between the shaft surface and the SE will produce a 495 millivolt change in system output.

The system output can be displayed on an oscilloscope for quantification and interpretation.

The system output can also be input into an automatic wobble monitor system. The wobble monitor system, though not l operational at this time, will be briefly described in a L_ -- -

+

Page-32 of 52: o separat'e. paragraph.

6.2.3- Interpretational Information Thel shaft displacement measured. by the SE's is much less than the displacement at the bearings. This occurs. because the SE's are physically located ~ a significant distance

, inboard of the, bearings. The out-of phase relationship of-upper versus. lower displacements also in fl_ uences the displacement'at the bearings, with-the worst condition being

~

180 out of phase. At the worst out-of phase condition a 0.5-mil-(0.0005") displacement at a lower-end. SE (turbine end). represents a 2-mil (0.002") displacement at the~ lower ~

' journal bearing while the same 0.5 mil displacement' at an upper SE (compressor end) represents a 1.1-mil displacement at the upper journal. The available radial clearance;at the lower journal' is 2.25 mils; the available radial clearance at the- upper' journal is 3.25 . mils. Conservatively, a displacement at either an upper or lower SE of 0.5 mils above a "zero-wobble baseline" is sufficient- cause to reduce 1l shaft- speed to 'self-turbining. A. 0.3-mil' wobble is i sufficient cause to initiate diagnostic . procedures defined in 'thel circulator operations :and maintenance .(0&M) manual.

The term "zero-wobbl.e baseline" denotes an initial displacement value not- recognized as displacement _due to imbalance. It is obtained'immediately subsequent to machine refurbishment wherein .the assembly of rotating components

'does not exceed O'2 inch ounce imbalance;,

It is a value that includes shaft surface irregularities such as runout, pits and scratches which remain relatively constant until the next refurbishment. Consequently, the "zero-wobble baseline" value is subtracted from any subsequent i displacement values for wobble magnitude determination. !

Note that displacements are generally read as peak-to peak 'i values, however, only 1/2 peak-to peak values are used for wobble determination because' limiting clearances are radial and not diametral. To illustrate: '

present P-P mv minus intitial p-P mv / 495 my per mil 2 i (P-P means peak-to peak displacement indication less any !

superimposed noise component.)

i The physical fact that shaft displacement due to imbalance ,

varies as the square of speed is an aid in evaluating significant increases in wobble magnitude. l 6.2.4 Wobble Monitor System Much difficulty has been experienced in maintaining the Wobble Monitor System because of parts unavailability and system complexity. The Wobble Monitor System when operational utilizes a minicomputer and a display panel to

l- Page 33 of 52-I l I compute the. actual bearing clearance based upon the phase relationship. between upper and lower displacements and displays the' resulting values on Instrument Panel I-05 in the' Control Room. 'The system also sounds an alarm when !

wobble limits have been exceeded. A teletype records the !

same information for later review. l 6.3 CIRCULATOR OPERATIONAL-SERVICE HISTORY !

\

The tables in Attachment 7 provide, among other information !

to be discussed later, a summary of the following service- !

related historical data for each circulator by serial number !

(C-2101 through C-2105):

a) Time period and number of months in which penetration, or " s p a re'.' period (during which refurbishment / repair work was done),

b) Hours of operation on water and steam drive for each time period between " spare" periods, c) Number of thermal cycles experienced during each time period between " spare" periods.

Attachments 8 and 9 summarize in bar chart form the tabular information of Attachment 7.

.The definitions used for hours of operation and thermal cycles and.the met.1od by which this information was gathered is described below.

An intensive search effort was performed between August 28 and September 1, 1987, to determine cumulative circulator i duty for Fort St. Vrain. The parameters of interest were '

total run hours, both on water and stum, and total thermal cycles. A thermal cycle was defined as one startup on steam i plus the following shutdown. The data was accumulated separately for each machi .e serial number (C-2101 through C-2105) in each penetration in which it has been installed.

It is believed that the final results are accurate to about 10 percent. Data logger cata was used where available, with data manually extracted from old operator narrative logs used where necessary. As with any other measured quantity, the problem existed of defining exactly what is being measureo. In this tabulation, all startups to self-turbining condit'ons and time spent self-turbining were ignored. The principal reason for this choice is that meaningful data for these conditions is not available from th cu i,a logger tapes. When reading through the old operator logs, water-turbine time and steam-turbine time were simply computed from the information written there. If {

an operator missed logging a start or a stop, or did not l

give a time (as was often the case in the oldest set of l

1 l

i

_ - _ _ _ _ - - (

Page 34 of 52

] logs), the reviewer had no choice but to enter his best guess based on what information was available. j The data logger data presented different problems, Aside from the gaps in this data, which were filled by means of tha tedious, labor-intensive process of narrative log review, some arbitrary decisions had to be 'made to define what exactly was to be considered circulator operation, and also what was to be considered operation on water and !

operation on steam. This determination could be made easily '

by an experienced person observing all the relevant data, but there simply weren't enough experienced people and enough hours to apply this method to some 300,000 records of data logger history. What was finally decided was that a circulator was considered operating on steam if the associated cold reheat loop temperature was 250 F or greater. If this temperature was below 250 F, then the circulator was considered to be operating on water, if it was operating at all. In addition, a thermal cycle was defined to be a start on steam (speed increased through 700 I rpm while cold reheat temperature is 250 F or greater) provided that the interval since it was last run on steam is i' greater than four hours. The rationale for the four-hour interval is that a machine that has been running on steam may be restarted on steam without a prewarming period if it has been shutdown for no greater than four hours.

The totals provided in the tables in Attachment 7 for each circulator are summarized in the following table:

x k'

1 Page 35.of 52 l: Circulator' Total Time Number of Months In Hours of Operation Thermal Serial No. . Period Penetration. Scare Water Steam Total Cycles, 1

.C-2101 12/71-8/87. 165. 1 23' 22,802 41,091 63,893 136 C-2102 12/71-8/87" 126- 5' 11,934 26,073. 38,J07 108' .,

C-2103 12/71-8/87 155 33 13,862 '35,341- 49,203 99

.C-2104 12/71-8/87 131 57 14,116 34,921. 49,037i 124-C-2105 12/71-8/87 160 . 28 18,770 34,983 53,755 116 Total Cumulative Hours (5 machines): 81,484'172,411 253,895 L' ,

.Se r the FS AR ,' Section- 4.1.6, the full-temperature design operational life of primary coolant system components is at --

least- 210,000. hours. Conservatively, assuming that actual '

full-temperature operation. of each circulator equals . total hours on steam plus - water. drive, the actual hours of operation versus end-of-life design full-temperature hours-is summarized below:

Actual Total Maximum Circulator- Hours of Operation End-of-Life Design % of Design Serial No. (Water & Steam) Hours of Operation Life Used Up C-2101' 63,891- 210,000 30%

C-2102 38,003 210,000 18%

'C-2103 '49,202 210,000 23%

C-2104 49,037 210,000 23%

C-2105 53,753 210,000 26% .

The analysis' of.the ASME Code parts per Section III of the Code.was performed on the basis of 20,000 reactor plant cycles between 25% and 100% power (1% change per minute) and 600 transients. The definition of a transient for the original fatigue analysis is any event causing abnormal or l emergency operating conditions. From a design standpuint a transient would be an event causing worst' case

' conditions / stresses on the material / component (e.g., thermal shock / gradients,' pressure gradients). i For the historical review, the transient thermal cycles 1

versus thermal cycles based on 1%/ minute power changes were i not evaluated independently. However, the maximum number of tnermal cycles reported, 136 for C-2101, is significant!y ]

less than the 600 worst-case transients upon which the circulator was designed.

6.4

SUMMARY

OF PREVIOUS PROBLEMS, REPAIRS ANO REFURBISHMENT A review of refurbishment and design change documents

,. related to the cir ulators was performed to determine the H

extent of each refurbishment required and to make a general evaluation cf the reliability of the circulators.

The tables in Attachment 7 provide a complete summary of the problems encountered for each circulator by serial number 1

l

__.2-__-___ _ _ _ . _ -

Page 36 of 52 (C-2101 through C-2105), the cause of each problem, if 1 known, and the retair or corrective action taken.

I With regard to the current damage to circulator C-2101, several historical issues are worth reviewing in more detail.

6.4.1 Replacement History of Steam Ducting Bolts ( /4-16) and Labyrinth Seal Mounting Bolts (1/4-20)

A review of circulator refurbishment documentation was performed by PSC and GA to determine the replacement history of the steam ducting (3/4-16) bolts (Items 45 and 52, C2101- !

300) and the labyrinth seal (1/4-20) mounting bolts (Item 46, C2101-300), since these bolts were found failed and were identified with stress corrosion cracks on circulator C-2101.

This review shows that, except for circulator C-2104 just installed in.'D' penetration replacing C-2101, none of the above bolts have been replaced since at least August, 1972.

On circulator C-2104, all twelve (12) 1/4-20 bolts (Item 46) were replaced during the latest refurbishment. '

Although nct specifically reviewed, GA has informed PSC that oftentimes the carbon steel spring plunger (Item 53,. C2101-300), also identified with corrosion and pitting, does get replaced becaused it becomes necessary to break it to remove the insulation cover.

6.4.2 Distorted Insulation Cover Found on Circulator C-2102 in 1979.

During review of the 1980 inspection report (GA Report GA-C15847) for the 1979 ISI inspection required by Technical Specification SR 5.2.18 of circulator C-2102, it was l discovered that a distorted (dished) insulation cover was '

found during disassembly operations (see photo below). The dishing was upward (inward) in a direction away from the i steam turbine disk. Somo of the insulation, bearing side j (Item 20, C2102-300), was also found to be deteriorated. l This insulatien was replaced. In the report, the cause of distortion of the cover was assumed to be due to a thermal mechanism; however, no specific analysis was performed. The cover was deemed functionally acceptable and reused.

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In light of the current damage on circulator C-2101, it was postulated that if the dishing had occurred in the downward (outward) direction it could possibly have made contact with the steam turbine rotor. Therefore, a rough analysis was undertaken to theorize a cause of the inward dishing and to determine if the cover could dish outward from a similar mechanism. This analysis is provided below.

Bowinc of Insulation Cover Plate The insulation cover plate of circulator C-2102 was noted to be permanently bowed in the upward direction with a maximum displacement of 0.23 in. The bowing appears totally axisymmetric. Hand calculations with regard to certain

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pressure differential and thermal' effects were perfornied to .

ascertain the'possible cause of the bowing.

l' Pressure Differential'Meenanism

'A pressure;= differential of the. order of 200 psi across an.

unrestrained cover plate (positive-on: the lower side.sanc '

, negative,on1the uoper side) could cause the. observed bowing. . i L .To compress the three : insulation -diski 0.23 ;in; would-require 'an . additional" pressure load oftabout 200 psi; LA-r' pressure differential 'of. near' this magnitude c'ould be caused q

'bp a sudden quench of-the? cover plate cavity. However, the.

scenario requires almost total sealing of the cavity 1 behind the. cover 4 plate -to develop- the required : pressure P differential.

Sealing ,might .be.' achieved between: the. cover plate at its

outer radius and theJsteam ducting _'weldment. The contact here is -between two machined pieces, and as: a slight.

pressure' differential develops,' it Twill seal further, Sealing.~at the inner surface is questionable. Here, the skirt at the inner radius contacts' with the : fibrous insulation' pads, which are porous; to both a vcpor and 1.iquid flow. (See Attachments 3 and 4.)

It appears sufficient sealing of the cavity is not available to develop the required pressure' differential t o distort the' cover plate.

Thermal Mechanism Several thermal-mechanisms for producing the observed bowing were also evaluated and one mechanism is hypothesized. -The proposed mechanism matches the observed behavior'and hand calculations indicate it is,' marginally feasible. The hand-calculations do not prove.the mechanism as:the cause of the bowing. Examination of photographs of;the bowed cover plate and evaluation of several potential causes indicate that the .t phenomena involved are quite complex. A verification of the hypothesized mechanism would . require a computer thermo/ stress analysis incorporating plasti:ity, large deformations and post buckling benavior.

The mechanism assumes steady operation has heated the cover plate to approximately 600*F. A bearing water leak then quenches. the face of the cover plate while the outer ring i and outer skirt of the cover. plate remain hot. This 1 preferential quenching can occur because the face is thinner than the outer ring (0.09375 in wall versus 0.25 in. wall p lus skirt) and the face cools more rapidly. The preferential quenching of the face may also result from total vaporization of the bearing water as it flows radially

' outward (for small leak rates) or film boiling (with a low heat transfer coefficient) occurring along the outer radial o

Page 39 of 52 1 )

locations of the face, while liquid forced convection or nucleate boiling occurs along the inner radial locations. .

Either scenario with vapor generation ceuld result in !

cooling the face of the cover plate prior to cooling the ;

outer ring.

i' With the outer ring hot, a quench of the face of the cover plate by 100*F would cause it to yield as its outer radius is held by the stiffer outer ring. Further cooling down to 100-200 F causes permanent plastic membrane distortion of the face. The inward force on the outer ring due to the

' face contraction may rotate the outer ring slightly and initiate an upward (or ie.,. d) bow in the face. j As the outer ring now cool % it contracts and puts the face in compression or. moves its uuter radlui inward to cause further bowing. Cooling of the outer ring by approximately 400-600oF would create sufficient radial inward movement to bow the face the observed 0.23 in.

It- is further noted that the presence of the insulation disks may prevent complete bowing of the face while the cover plate is in place. Only partial bowing of the face may occur in place, with the face retaining in a compressed buckling mode. The total deformation of the face may then occur only when the plate is removed. (The reverse may have been experienced when the bowed cover plate was re-assembled- part of the bow was removed by the lateral forces of the insulation disks.)

With the above mechanism there is a definite preference for the face of the cover plate to bow upward (or inward). The initial inward force on the outer ring and any axial temperature gradient across the outer ring generated by ring cooling provide a rotation of the ring that initiates the face to bow upward.

6.4.3 Failed Labyrinth Seal Mounting Bolt Found on Circulator C- 1 2104 in 1984 Ouring disassembly of the circulator C-2104 steam end during the latest refurbishment (1984-85) one of the twelve 1/4-20x5/8 hex head labyrintt teal mounting bolts (Item 46, C2101-300) was found broken j place and trapped behind the flange on the insulation cover (Item 11, C2101-300) (see l

Attachments 2-5). The remainder of the bolt was still in the threaded hole. A nonconformance report was initiated and the disposition was to replace the bolt and turn the failed bolt over to Materials Engineering. Further review indicates that all twelve (12) (Item 46) bolts were replaced.

No report of analysis of the broken boit could be located, {

however. j The broken bolt itself could also not now be located for analysis. Circulator C-2104 (with the new Item 1

_ _ _ _ _ - - _ _ _ _ - _ _ 1

Page 40 of 52 I

)

] 46 bolts) was installed in 'D' penetration in August, 1987, g replacing C-2101.

6.4.4 Speed Probe Bolt Failure on Circulator C-2104 in 1985 During reassembly of circulator C-2104 during the latest refurbisruent (1984-85), a 1/4-20x1-1/4 speed probe mounting bolt (Item 51, C2101-500) broke during installation. This bolt is AISI 410 SST, quenched and tempered, and is torqued

]

4 to 7.5 ft-lb. New C2101-500-51 bolts were installed. No 1985 analysis of the failure could be located; however, the broken bolt itself was located. A recent metallurgical analysis revealed stress corrosion cracks exist, cause unknown. The normal operating environment for these bolts is bearing water. Circulater C-2104 (with the new Item 51 bolts) was installed in 'O' penetration in August, 1987, replacing C-2101.

6.4.5 Repl acernent of Helium-End Stainless Steel Bolts on All Machines in 1985 Helium circulator C-2102 (presently installed in 'A' slot) was sent to GA Technologies in 1985 originally for repair of a leak in the bearing water supply line. This leak was caused by a failed bolt (Item 42, C2101-300) at the water line connection to the bearing cartridge. The failure mechanism was a result of a manufacturing defect.

While installing the 24 3/4-in bolts (Item 40, C2101-300) w uring the bearing assembly of the compressor rotor to the helium circulator unit (primary closure), one of the bolts failed before reaching the installation torque value of 450 ft-lbs.

Metallographic examination performed on the failed bolt showed the cause of failure to be stress corrosion cracking.

Another bolt from the 24 was also selected randomly for metallographic analysis and it also showed stress corrosion cracks in the roots of the threads. Two similar bolts off C-2104 (spare) were taken for a comparative examination.

Results of the metallographic examination revealed no cracking. It als1 stated that the microstructure of the bolts off of C-2104 was identical to that found in the cracked bolts off C-210; The C2101-300-40 bolts were a chrome-moly-vanadium steel (H11 per AMS 6487). A chloride leach analysis was performed on the cracked bolts off C-2102 and another circulator y spare, C-2104. Chloride and sulfate levels present on the cracked bolts were at a level (13g/cm2 ) where stress corrosion cracking of that type material becomes possible.

The chloride level on the cracked bolts was four times higher than those on the spare; the sulfate levels were three times higher.

i; >

l Page 41 of 52 .

i l

l A study by.GA was.then performed on the 31 different types .

of fasteners used on the circulators for susceptibility to j chloride stress corrosion . cracking, of which four were i considered. susceptible! C2101-300-40, -380-10, -340-9, -and 4

-310-4. All four types are found in the helium environment i (besidos the C2101-300-40 bolts, the other three -sets of j bolts /capscrews were stainless steel (A-286)). l It .was recommended by'GA to replace these bolts with a more stress corrosion _ cracking resistant material such as Inconel 718. provided the lower mechanical properties were acceptable. All threaded fasteners are lubricated with !

Molykote-Z or Cerac SP-111 (molybdenum disulfide). There is '

no mention of the lubricant in the GA failure analysis report. ,

PSC Change Notice (CN) No. 1976 was initiated to allow the changeout of the four sets of bolts recommended,by GA to an ]

1 Inconel 718 material. Along with increased stress corrosion j resistance the mechanical strength of the Inconel was found j to be acceptable for the preload plus other mechanical l loading. The bolts were changed in all five circulators.

It was evaluated in the GA report that the consequences of failure of any of the four bolt types found to be susceptible to stress corrosion cracking would not create an j adverse impact on public health and safety. Failure of the 1 fasteners would not cause a breach in the primary coolant j boundary, but it may cause functional failure of the !

circulator. It can be concluded from the GA report that the l environment in which the bolts were exposed (primary J coolant) was the primary cause of the stress corrosion j cracking type failure. Since all other circulator fasteners !

are not in the helium envi ronment, it was not deemed necessary to replace any other fasteners. It is important i to note that the stress corrosion cracking problem was from i excessive chloride which is not present in significant j concentration level on the steam side. '

6.4.6 Fretting Identified in GA Inspection Report GA-C15847 for l Circulator C-2102 in 1979. 1 Circulator C-2102 was removed from 'C' penetration in March, I 1979, in order to comply with Technical Specification Surveillance SR 5.2.17 and 5.2.18. These are as follows: ,

i 5.2.17: One circulator pelton wheel removed from service !

at first turbine generator overhaul to be examined for a) cavitation damage, b) bucket and bucket to hub i integrity, c) curvic coupling integrity. I 5.2.18: At first turbine generator overhaul one circulator to be removed and thoroughly inspected for wear and s degradation. I 1

l l

I Page 42 of 52 I

.Results of this inspection were reported in GA Report GA-C15847. Upon removal of the support cone from C-2102, difficulty was encountered in separating the support cone from the penetration piping assembly. Jacking screws normally used for' separation were ineffective. T.o separate the pieces the support cone was slowly heated to 480 F and during cooldown a constant force was applied with the jack screws which finally separated the parts. Initially, rust was thought to be the cause of difficulty in separating the-parts since rust was found on.the outside surfaces of the support cone and on the,. mating flange face of the penetration assembly; but it was finally determined that fretting was the real cause. Fretting between the support cone register and its male counterpart on the piping penetration assembly resulted in localized self-welding.

I Fretting is a' wear phenomenon that occurs between two mating ;

surfaces; it is' adhesive in nature and vibration is its main '

cause factor. It occurs between two tight-fitting surfaces that are subjected to a cyclic, relative motion of extremely small amplitude. Fretting occurs at contacti.ng surfaces that are intended to be " fixed" in relation to each other.

The " slight" movement prevents the formation of, or destroys, protective oxide film. This creates true metal-to-metal contact.

There are eight 5/8"-11 socket head bolts used to attach the penetration assembly flange to the support cone. Per GA Report GA-C15847 there is sufficient clearance in the i boltholes so that relative movement during heatup and ,

cooldown could have allowed the fretting.

The GA report concluded that the difficulty in separating the interfaces did not affect the circulator function, nor the capability of removing the circulator from its penetration. Since the interfaces were intended.to be fixed in relation to each other and movements (to cause fretting) are extremely small, no corrective acticn other than cleaning of the surfaces was necessary. (Note: Normally these two parts are not separated except for major disassembly.)

Fretting is common between bolted machined parts that are subject to small relative movements. Localized wear and removal of material by fretting usually is not deleterious to the structural integrity of the mated parts. The mating surfaces are intended to be fixed % re' tion to each other, therefore the function of the joint <- maintained. Also, GA Report GA-CID 47 concluded no dt- d ental effect on the circulator function would result from the fretting.

It has been concluded that fretting played no part in the current damage to circulator C-2101.

I 1

Page 43 of 52 6.4.7 Cracking of Turbine Water Drain Bellows .

In 1983, cracks were found in the turbine water drain bellows (Item 27,. F.P. 91-M-19-1) in the steam inlet and water piping assembly (S/N 2000) which had been installed

. with circulator C-2105 in 'B' penetration until 1981. Both PSC and ~ GA performed a failure analysis on the Inconel 600 bellows. The results showed that the failure was cue to stress corrosion cracking with caustic being the most probable cause. A change notice (CN-1819) was written to replace the Inconel 600 bellows with a straight, sliding sleeve design that would be more resistant to stress corrosion cracking. The steam / water piping assembly (S/N 2000) was subsequently installed in 'C' penetration in June, 1985, with circulator C-2103, where it currently resides.

GA performed a review of the operability of the circulators !

with the cracked bellows. This review did not determine any I operability issues- (

Reference:

GP-1987). The same report j also states that a similar failure had been detected earlier j in another piping assembly (S/N 2004). In this case, the '

bellows was replaced with another bellows. This piping assembly was subsequently installed in 'A' penetration in May, 1985, with circulator C-2102, where it currently exists. t In 1985, cracks were discovered in the same bellows in the steam / water piping assembly (S/N 2003) which had been installed with circulator C-2103 in 'C' penetration until June, 1985. This bellows was replaced with a , split sleeve 1 design per CN-2179A instead of the solid sleeve used in CN-1819. This steam / water piping assembly was subsequently installed with circulator C-2104 in 'D' penetration in i August, 1987, replacing the currently damaged C-2101 circulator.

To summarize, steam / water piping assemblies S/N 2004 (currently with circulator C-2102 in 'A' penetration), S/N 2002 (with C-2105 in 'B') and S/N 2001 (out with C-2101 from

'D' penetration in August, 1987) still have bellows,

. Steam / water piping assembly S/N 2000 (with C-2103 in 'C') l j

has a solid sleeve in lieu of the bellows, and piping i assembly S/N 2003 (with C-2104 in 'D') has a split sleeve in I place of the bellows.

6.5 Circulator Reliability Based on H'storical Review The circulator assembly is a conservatively designed machine j with proven integrity. The circulators have been incorrectly identified with a significant fraction of the plant down time. Much of the time attributed to the s

)

circulators has in fact been a result of problems with the circulator auxiliaries, e.g., the bearing and seal water supply system. These problems were characterized by water

Page 44 of 52 ingress into the primary coolant system. The circulator machines have been much more reliable than perceived; moreover, the mechanical integrity of the circulator has been excellent.

While accumulating approximately 250,000 total combineo (five machines) operating hours (based on water plus steam drive) out of 1,050,000 hours0 days 0 hours 0 weeks 0 months full-temperature design operational life, approximately fourteen separate incidents have- required one or more circulators to be removed for repair, and one design modification was incorporated in-situ. These incidents are summarized in Table 6.5-1.

The failure incidents are categorized .in the last three columns of Table 6.5-1 headed " Design or Manufacturing Defect", " Random Failure" and " External or Other".

As indicated in the table, of the fourteen failure incidents !

there'have been tentatively eleven design or manufacturing defect failures of circulators. One failure is considered a random failure and the remaining three are not attributable to the circulator design (i.e., the " Externa' Problems / Inspections" category under the " Incident Problem" heading).

Of the eleven design or manufacturing defect failures tentatively attributed to the circulators, three are more I

directly. related to external auxiliary system failures for which modifications corrected the problem. (The three ;

auxiliary system modifications are the improved pressure l control for the shutdown seal bellows, the improved control !

of the brake / seal sequencing to prevent seal actuation while the rotor is turning, and the pressurization of the pelton !

wheel water cavity to prevent cavitation.)

Thus, only approximately eight incidents are clearly attributable to design or manufacturing defects in the circulator machine.

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OtherL ' problems' have subjected the machines. to severe

operating' conditions,that were-not anticipated during the

-design; phase,..but the circulators.have demonstrated their ruggedness by operating .through these ~ incidents. . During

'both the " test period at Valmont and service.in the plant,

~

debris l (e.g., nuts . bolts, fi_lter. screens, etc.) has inadvertently passed' through: the compressor.' and steam-

' turbine blading. . Damage to the : blades ~ was observed,- but none of' these- incidents prevented a circulator from being used to circulate helium to 'the core; i.e., no' loss of cooling capability resulted from~ the damage.

The curre - incident on C-2101 is especially an indication of the-rups c *> d - N machine. Despite extensive damage

.to the su 2m 'end, the circulator continued: to operate, and, !

.had it bniinacsssary, it could have been used to cool down the core.

i Manufactured :to high quality standards, the rotor assembly has not experienced any seizures that resulted in , loss. of-ability- to circulate helium. Even when damagc, has been incurred. to the bearing ' surfaces, the observed rotor ' wobble did not cause any damage outside.of the bearing cartridge. '

In. closing, the circulator machine is. extremely rugged.and its integrity has been ' proven in many hours. of reactor service. ' No : catastrophic disk, blade or shaft (i.e.,

rotating parts) failures have occurred. O f.- paramount importance is .the fact that no shaft seizure problems have been experienced, and in all_ cases of component failures the circulators have continued to operate and would have ensured safe shutdown of the reactor.

!7.0 LICENSING ASSESSMENT OF CIRCULATOR C-2101 DAMAGE l

l 1

7.1 INTRODUCTION

I i

o The trip of helium circulator S/N C-2101 on July 22, 1987, l and the failures that were observed during the ensuing 1 investigation have been described in considerable detail in the previous sections of this report. PSC acknowledges the i seriousness of this event, and after reviewing all of its implications, PSC considers that operation of Fort St. Vrain in accordance with the Technical Specifications and FSAR is justified.

This discussion evaluates the licensing basis requirements for circulator operability, reviews the significance of the j observed failures, examines the program PSC is developing to prevent a recurrence of the observed failures, and explairs why' plant operation at 'this time continues to be justified. ;

_1___________- - - - - - - - -

Page 47 of 52 7.2 LICENSING BASIS FOR HELIUM CIRCULATOR OPERABILITY The helium circulators are the driving force for assuring forced circulation of primary coolant. As such, they are required to function under various accident scenarios to ensure that fuel temperatures remain within acceptable limits. These accident scenarios and their circulator performance requirements are summarized as follows:

1. Safe Shutdown and Appendix R Cooling use firewater, assisted by an emergency water booster pump, to drive a circulator via its water turbine (Pelton wheel).

Primary coolant must be driven at 3.8 percent of rated flow, and any one circulator is capable of this. This is the safety-related, seismically qualified shutdown model which is relied upon in the Safety Analyses (FSAR 9.12.5.3 and 10.3.9). In the event forced circulation is lost from power levels up to 83.2 percent, if it is restored within 90 minutes, fuel temperatures will f remain below conservative limiting values. Single =

failure protection is required for Safe Shutdown Cooling functions, and this is achieved with two operable circulators. (FSAR 10.3.10 and 14.4.2.2).

2. Cooldown with a depressurized PCRV requires a higher ;

circulator speed to account for the reduced primary I coolant density and lower heat transfer rate. Analysis for a PCRV depressurization accident at a maximum credible rate, the Maximum Credible Accident (MCA)

(FSAR 14.8), shows that a circulator speed of 8000 rpm is required to achieve acceptable primary coolant flow at fully depressurized conditions. One circulator is sufficient for the MCA while two circulators operating simultaneously at 8000 rpm are required in the highly unlikely event of a rapid depressurization which is the Design Basis Accident No. 2 (DBA-2). This high speed is achievable with feedwater supplied to the Pelton wheels and provides acceptable cooling from initial power levels up to 105 percent. Due to the low likelihood of DBA-2 (probability of occurrence less than 1E-07 per year), single failure protection for this incident is not a feature of the FSV design and therefore two operable circulators provide sufficient assurance of cooldown capability. (FSAR Sections 14.11, 14.4.3.2).

3. Although the FSV Technical Specifications require two operable circulators during all power operation above 2 percent rated thermal power (LC0 4.2.1), analysis has shewn that a loss of forced circulation at power levels of 35 percent rated thermal power and below will not result in significant damage to any fission product barriers, since the PCRV liner cooling system is

Page 48?of 521 t

capable of removing all, decay' heat generated from that power level. (FSAR D.4. ) . -

- 7. 3. I OBSERVED' FAILURES S'GNIFICANCE.

.The root' cause for the. stress cor.rosion and . fatigue cracking identified previously . in this report has not yet been determined. PSC is pursuing all possibilities.

The .most si g n.i fi cant - feature- of the cbserved' stress -l corrosion cracking is that it is_.most likely a generic problem. PSC is. not aware of any special materials,

' lubricants, assembly methods, or operating history that

would. lead to the conclusion that the failures in circulator

'C-2101 are unique to that machinei Therefore it is highly.

'likely .that there are. bolts with similar cracking, to v'arying degrees, in any or all :of the helium circulators currently installed in the.PCRV. While this condition is recognized,.it is also important to realize that a single bolt failure in and of itself will not produce the damage.

observed in C-2101. There are twelve 1/4" bolts that hold ,

the labyrinth steam seal assembly away from the turbine wheel and there are fifteen 3/4" bolts that secure the steam ductingi to 'the- bearing assembly. There have been no significant manifestations of difficulty to self-turbine or other performance, anomalies that would cause PSC-to suspect that any of the currently installed helium circulators have experienced bolt failure to the extent- that circulator operability is compromised. l The damage to . circulator C-2101 has attached a 'new significance to the observation of difficulties in self-turbining. There are various known reasons for not self-turbining, and PSC has established a policy that if a ;

circulator will not start on simulated boosted firewater, it will be declared inoperable. Other motive forces up to and including steam may then be used to start rotation, and various diagnostic procedures may be used to clear bearing water passages, but the circulator must be shown capable of starting on simulated boosted firewater before it- can be considered operable.

The ejection of failed parts from circulator C-2101 did not and could not credibly have adversely affected the operation of any downstream equipment. The discharge piping is configured such that an effective trap is created before any valves could be reached. Also, strainers protect the steam generator reheaters so that any of the smaller, lighter ,

fragments could not cause damage even .if they did become j entrained .in the steam flow. (For a more detailed discussion of the impact of ejected parts on downstream l components, see Attachment 10).

1 L

f

Page 49 of 52 I

l

1 7.4 PREVENTION OF RECURRENCE o

j

.PSC .is developing a - comprehensive program'of monitoring, 1 inspections, and fastener replacements to ensure -that ;the I failures 'that.' occurred in circulator C-2101Ldo not occur. .{

-again. The major features.of this program are.as follows: ;1

~ l'

1. PSC_ will replace the fasteners that experienced stress.

-corrosion cracking on C-2101 with all new fasteners, on all' circulators installed in' the' PCRV,. Any.other corrective action deemed; appropriate as a. result of the-final evaluation of..C-2101 will also be taken. These-tasks willsbe' undertaken.as soon as the evaluation of C-2101: is completed,.the correct replacement material type .is. determined or verified, -suf ficient replacement parts become.available, and a refurbished spare helium circulator is~available. . This would most likely occur-in spring of 1988.

2. A wobble monitoring program will be implemented. The wobble is an indication of rotating element . balance, and any change in'tbis balance is evidence of material gain, loss or displacement, or -other conditions that warrant investigation. The circulator operations and

. maintenance (0&M) manual identifies appropriate' actions to be .taken for different wobble values, up to and ;

including. shutting thel machine--down. As soon as i equipment can be selected, procured and installed, PSC l will utilize a continuous monitoring system that will have alarm and data storage capabilities. Until that time, the short term monitoring system will provide'for wobble analysis on a. daily- basis and subsequent to

.every significant speed change'(800 rpm or more). This program provides an accurate record of changes in .{

circulator performance and balance condition. ( For a !

more detailed discussion of the wobble monitoring program, see Attachment 11).

3. PSC will review its existing inspection program of the circulators based upon the results of the current investigation. Any resulting changes to the program will be submitted to- the NRC as a Technical Specification Amendment Request to SR 5.2.18.

7.5 IMPACT OF CIRCULATOR C-2101 DAMAGE ON PLANT OPERATION PSC has investigated the trip of circulator C-2101 and the ,

failures discovered following that trip. Although the cause of those failures is as yet undetermined, and although the j extent of similar degradation or failures in the installed circulators is not known, PSC considers that Fort St. Vrain operation continues to be justified for the following l reasons: !

I l

Page 50 of 52

1. If an emergency had occurred, it is most likely C-2101, in its actual degraded condition, could have been operated on either its steam turbine drive or its water i turbine drive (albeit,~with high wobble indication), as demonstrated by its operation up to approximately 6000 rpm on July ~24,1987, subsequent to its trip on July 22, 1987.

A preliminary analysis was also' performed which showed that from 83.2 percent power only 3-1/2 hours of forced ci rcul a t i on ,- using a single circulator at 1100 rpm, is needed for Safe Shutdown or Appendix R Cooling (assuming a 90-minute delay) to assure a subsequent liner cooldown that would be bounded by previous liner e cooling analyses from an initial 35 percent equilibrium power. level. These analyses concluded that a liner cooldown would maintain the integrity of fission product barriers.

2. Simultaneous loss of all four installed helium circulators, such that Safe Shutdown and' Appendix R Cooling capabilities would be lost, is incredible. The circulator that was recently installed in "D" penetration, S/N C-2104, was recently refurbished and twelve new labyrinth seal retaining fasteners (Item 46, i C2101-300) were installed. The remainder of the i circulators have various service histories and it is inconceivable that they would all experience incapacitating failures at the same time, notwithstanding the fact that the failure mechanism may not occur over a predictable time interval. This is 1 also supported by the fact that circulator seizure has !

never been experienced by any machine. As long as one l helium circulator is operable, Safe Shutdown and Appendix R Cooling is assured for every case. Common- ,

mode failures and DBA-2 are addressed next. j

3. There is no single external event that could develop a sufficient forcing function on any helium circulator such that the suspect bolting, even in a degraded condition, would catastrophically fail, thereby damaging the steam-end assembly of the circulator.

Connecting piping assemblies are isolated by bellows assemblies, flanged joints, 90- and 180-degree bends, l and by about fifteen feet of piping in the penetration {

so that any pressure pulses, water hammer, or other -l forces would not be translated to the suspect bolting.

The vertical acceleration from the FSV operating basis 1

earthquake is not significant when the relatively light weight of the insulation / labyrinth seal assembly is considered. Therefore, neither a seismic evert, HELB, l DBA-2, fire, SLRDIS actuation, nor any other credible !

event could conceivably result in a common-mode failure l

h_._____._m___ _ _ _ ._ _ _ _ _____ _ _ _ ______. _-

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'jy Page 51 of 52 1

of the : helium.. circulators. in the manner observed in

.C-2101.

4.s The failure of any steam-end part of a circulator, in the manner observed in C-2101, would not affect- the operability of' any other circulator. Piping assemblies are sufficiently isolated to preclude any interrelated effects.

'The operability of all installed circulators will be

~

5.

demonstrated prior to exceeding 35 percent power. PSC will ensure that all circulators can develop 3.8 percent primary coolant flow using simulated boosted firewater; will have acceptable wobble at 8000 rpm; and be able to re-start on simulated boosted fi rewater after having been shut down without bearing water and with the brake and seal set for 90 minutes.

6 '. 'PSC will not operate FSy above 35 percent power unless all.four circulators are operating. This commitment applies .until our investigation is completed and as,sociated corrective actions are implemented. This provides maximum assurance of forced circulation and a

Safe Shutdown and Appendix R Ccoling capability,

7. An ' enhanced monitoring program, to include improved wobble monitoring arid which may also utilize such parameters as speed coast-down time, bearing water flow versus bearing cartridge pressure drop,. compressor performance data and steam turbine performance data and such instruments as accelerometers and acou 'al sensors, will allow PSC to trend circulator perfor..o ce in an effort to detect signs of degradation o'r failures before significant- damage would occur. PSC considers it highly unlikely that a single circulator could experience degradation without detection before the circulator is damaged to the extent that it seizes.

8. Even in the incredible event that all four circulators should fail and forced circulation capability would be lost, the PCRV liner cooling system is capable at any thermal power level of removing sufficient decay heat

, so that the integrity of the PCRV is assured. This ,

event has been analyzed in the FSAR as the Design Basis

Accident No. 1 (DBA-1), with acceptable offsite dose :

consequences well below the 10CFR100 guidelines. i n

I I

. ___ ____ _ _ _ _ ___ A

8

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(

' ATTACHMENTS n

. Number Description

('

1 Assembly.' Isometric of Owg. C2101-300 Assembly pir '2 Drawing C2101-300: Circulator Machine Assembly n

L 3' Blow-Up of Steam End of C2101-300 h Circulator Machine' Assembly L

p 4 Insulation Cover Assembly -- Half Cross-Section h '

'5 Insulation Cover Assembly -- Cross-Section Detail Showing a 1/4-20x5/8 Labyrinth Seal

[. Mounting Bolt

!* 6' Insulation Cover Assembly -- Cross-Section Detail

i. _

Showing the Spring Plunger-7 Helium Circulator Historical Review Summary

8 Helium Circulator History -- Time in Penetration, Hours of Operation, Thermal Cycles (Bar Chart plusTable) 9 Helium' Circulator History -- Hours of Operation (Bar Chart) 10 Impact of Ejected Parts on Downstream Components (Discussion) 11 Wobble Monitoring Program Plans (Discussion) l j .i , ,

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Attachment 10 l- Page 1 of 3 IMPACT OF EJECTED PARTS ON DOWNSTREAM COMPONENTS The steam flow path, and possibly that of any ejected parts, from the

'D' helium circulator steam turbine would be as follows: down the circulator steam outlet plenum /anuulus to the circulator steam outlet l pipe at the bottom end of the circulator and out through the ,

j circulator steam outlet nozzle to the cold reheat (CRH) steam piping; through CRH piping and the 16-in, circulator steam outlet isolation valve (HV-2252); up to the 90 wye connection shared by theC' circulator outlet; from the wye connection to the Loop 2 reheater header; from the header in parallel inlet to the CRH steam desuperheaters (S-2213, typical); and finally to the reheater inlet '

protective filters (M-2215, typical). This path is similar to all ci rcula tors .

Any ejected parts, or pieces thereof, from the circulator are most i likely to stop and be trapped in the circulator steam outlet pipe, i Parts or pieces ejected from the steam turbine would be thrown out against the steam outlet annulus wall and flow down with the steam to the bottom of the steam outlet pipe some 23 feet below. The steam flow direction then must change in order to exit through the 14-in.

diameter steam outlet nozzle, the centerline of which is approximately 19 in, above the bottom of the steam outlet pipe. The 14-in, steam outlet nozzle is a 90 elbow which is attached in such a manner to direct steam flow out and up at a 45 angle. This upward exiting piping configuration results in an approximate four-foot i

elevation increase from the outlet nozzle to the CRH piping in about a five-foot run. This overall steam outlet piping configuration is comparable to particulate collection equipment such as cyclone separators. The principal of operation of such equipment is based on the tendency of entrained particles, due to their inertia, to (

4 continue to move in a straight line when the direction of the gas l stream is changed. Therefore, it seems reasonable to assume then !

that most, if not all, of the ejected parts would be trapped in the '

bottom of the steam outlet piping. This assumption is supported by the fact that many various size parts / pieces were found in the bottom of the steam outlet pipe when it was removed from 'D' circulator. No pieces were found in the 14-in, steam outlet nozzle / elbow. The pieces found ranged in size from large (7"x3/4"x3/16"), to small (1"x1"x1/16"), to particle sizes of 1/16" to 1/4" diameter (including ,

1/32" diameter lock wire). Accounting of the pieces found in the I steam outlet piping and the damaged pieces found during the l circulator steam-end disassembly also provides reasonable assurance that all the ejected parts were collected in the ciraulator steam outlet pipe.

However, if an ejected part/ piece was carried into the CRH piping, the components in its flow path are, as stated previously, the ;

circulator steam outlet isolation valve HV-2252 (and possibly the C- I circulator outlet valve HV-2250, depending on steam flow from C-circulator), the CRH steam desuperheaters (S-2213, typical), and the reheater inlet protective filters (M-2215, typical) (See Figure 1, attached.)

i Attachment 10 Page 2 of 3 The circulator ' steam . outlet iso 1'ation valves (HV-2252 and HV-2250) are!16-in., 45 Y-configuration globe valves. These valves are not li_kely to trap parts, pieces or particles due.to their near straight-

.through design and the tight clearances (approximately 0.015" radial)

between the.. disk assembly and the body guide r i b s .. Any small

. particles / pieces that might become. lodged in the 0.015" clearances would probably' not -affect valve operation'due to the large static

. hydraulic actuator force applied to the valve stem (at leat 170,000 lb. ' closing . force greater than the steam pressure force tending to

.~open the valve).

It does not seem likely that any parts / pieces have been trapped in'or have damaged .these 1 solation valves in'any significant manner. This, statement is supported by the fact that subsequent isolation and operation of the 'C' and 'D' circulators after the overspeed trip of

.'D circulator on 7-22-87 at 0516 hours0.00597 days 0.143 hours 8.531746e-4 weeks 1.96338e-4 months has effectively stroked HV-2252 and HV-2250 closed and open at least a couple of times each. No reports o f. inoperability of these valves have been noted. Also,.

during testing of. the ability to seal 'D' circulator on 7-31-87 due to its interspace leakage problem, the steam outlet isolation valve HV-2252 was closed and appeared to. be capable of sealing approximately 80 psig ' helium ' pressure. This indicates that no :

'significant damage could have been done to the valve' disk or seat. !

The CRH steam.desuperheaters (S-2213, typical) are basically a 2-in, diameter pipe inserted normal to flow in the 10-in. CRH piping. . A

. water spray nozzle is installed at the end of the 2-in. pipe, with 3

. direction of spray the same as that of the CRH steam, i.e., .toward the reheater inlet. Any parts that.may be free flowing in the CRH !

. pipe would strike the back side of the 2-in. pipe or the back side of.

the nozzle, and therefore would not damage the water spray function of the nozzle. The nozzle and supply pipe are of sufficient size to not be damaged by any free flowing parts.

The reheater inlet protective filters (M-2215, typical) will collect any parts, pieces or particles that may reach the reheater inlet piping. These filter elements are rigidly constructed, and are designed to trap loose particles as small as 0.048 in.

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i Attachment 11 Page 1 of 3 WOBBLE MONITORING PROGRAM PLANS Existing Speed / Wobble Comouter The speed / wobble signal ~ that_ is generated by the speed element '

(SE)/ speed module (SM) electronic assembly combination has been made available for general diagnostic equipment (i.e., oscilloscopes) through a bulfer amplifier contained in the SM. Additionally . this same output was routed to an instrumentation rack in the Auxiliary Electric Room where a " continuous" speed / wobble monitoring system is installed. This system was designed to provide nearly continuous.

monitoring, indication and alarm functions, and consisted of these main components: a Digital Equipment Corporation (DEC) PDP-11/10 computer; a DEC LPS-11 analog-to-digital (A/D) converter module; a teletype; and a remote selection / alarm / indication subpanel in the Control Room. A brief discussion of how this system was designed to operate follows.

The input signal from the SM is routed to a patch panel where the signal can be teed so that both an oscilloscope and the speed / wobble computer system could be used to analyze the wobble. From this patch panel, cables were installed to the speed / wobble instrumentation rack and connected to the A/D converter module. Interconnections between the A/D module, computer, teletype and remote indication / control subpanel completes the hardware interconnects. Besides the hardware, a real-time operating system (ROS) and support subprograms were created to control A/D module setup and input / output (I/0) functions, data analysis, alert / alarm functions and Operator I/O functions.

This system, however, is not operational and the vintage of the hardware is making repair of this system difficult with minimal j chance-of success.

i Sh_ ort Term Solution After reviewing the existing system and considering that the probability that the existing system can be made operational is low.

it was decided that a newer and more serviceable system was (

necessary. An investigation into what equipment is available and j lead times associated with new equipment has lead to che conclusion "

that a continuously recording speed / wobble monitoring system is not viable in the short term. Although no short term continuous system is available, it does not mean that there is not a system that can be installed to monitor wobble on a periodic basis until a continuous system can be designed, aquired and installed.

With this in mind, the short term solution that is the most viable is the installation of four(4) dual-trace digital storage oscilloscopes and connecting the scopes in place of the existing DEC LPS-11 A/D converter module. In reviewing the available scope types, the ability to store waveforms, re-display the stored waveforms, and transfer the stored waveforms to a computer system were the items considered essential. By including these key features, data gathered by the scopes and transferred to the computer system could be re- 1 displayed anri compared with " base-line" values for changes in wobble values. Additionally, if the data is gathered o.: a daily basis, all

' Attachment 11 Page 2 of 3 of the wobble traces gathered and saved can be used for trending the circulator performance.

This system, however, has several drawbacks that only a continuous monitoring system can overcome. The first drawback is that no automatic alert or alarm function is available since the shaft wobble must he calculated by hand. Secondly, integration into the Control Room automatic wobble readout cannot be accomplished for the same reason. Finally, since the monitoring is not continuous, changes in wobble that last only.for a brief period would not be observed unless the wobble change happens while aquiring the " snapshot". Although this system is not continuous, the analyzing of wobble on a daily basis and subsequent to every significant speed change (800 rpm or more) provides an acceptable method for monitoring changes in circulator performance and can be implemented prior to plant re-start.

Lono Term Solution In order to provide e continuous wobble monitoring system that provides useful circulator performance degradation trend data, a basic set of requirements that this system must meet were developed.

This list was developed by reviewing the circulator O&M manual, the existing speed / wobble monitoring system manuals, and some of the past circulator operating history. At the conclusion of this review, the following parameters were found to be the most critical design parameters of tnis system:

The ability to continuously monitor shaft wobble.

The ability to display the shaft wobble cn either a local or remote display. j 1

The ability to remotely provide alarr of both wobble limits !

(i.e.,the 0.3-mil diagnostic and the 0.5-mil shutdown wobble values).

j The ability to store wobble values that occurred both prior to and after an alarm condition has occurred for further analysis.

With the basic list and the SM output signal characteristics, vendors are being contacted regarding what equpment they may have that can provide the desired functions. Since this ac<ivity cannot be completed prior to startup, a schedule for installing the new monitoring system is presented instead.

The proposed tentative schedule for designing, procurring and 1 installing the new speed / wobble system is as follows:

Contact vendors and solicit available equipment and estimated delivery dates for this equipment. (To be completed by 10-9-1987) l l

_ _ _ _ _ _ _ - - - _ _ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - A

Attachment 11 Page 3 of 3 Review proposed systems and resolve conflicts / design philosophy discrepancies (To be completed by 11-6-1987) 1

. Prepare Change Notice (CN) package and procure parts. (To

'be completed by 1-4-1988)

Prepare Controlled Work Procedure (CWP) and install system.

(To be completed by 3-1-1987)

I It must be noted that this tentative schedule may be affected by the delivery dates of the selected vendor since PSC will have minimal control over these delivery dates.

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ML20235K272

Contents

Text

SUMMARY

7.1 INTRODUCTION

Reference:

SUMMARY

Reference:

7.1 INTRODUCTION

Navigation menu

ML20235K272

Text

SUMMARY

7.1 INTRODUCTION

Reference:

SUMMARY

Reference:

7.1 INTRODUCTION

Navigation menu

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