ML20043C275
| ML20043C275 | |
| Person / Time | |
|---|---|
| Site: | Zion File:ZionSolutions icon.png |
| Issue date: | 05/14/1990 |
| From: | Chrzanowski R COMMONWEALTH EDISON CO. |
| To: | Murley T Office of Nuclear Reactor Regulation |
| References | |
| IEB-89-002, IEB-89-2, NUDOCS 9006040319 | |
| Download: ML20043C275 (20) | |
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Commonwealth Edison D
Zbn Generg&stalbn g
e MicNgan 700 /746 2084 May 14, 1990 Dr. Thomas E. Murley, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C.
20555
Subject:
Zion Units 1 & 2 Bulletin 89-02 Supplemental Information NRC Docket Nos. 50-295 & 50-304 L
Reference:
(a)- M.H. Richter letter to U.S. Nuclear Regulatory Commission L
dated Jan. 26, 1990
Dear Dr. Murley:
Commonwealth Edison provided the initial response for the inspection of l
L_
the Zion Station Units 1 and 2 Anchor Darling check valves in reference (a).
Zion-Station has completed the inspection of the 24 Unit 2 check valves and'12 of the 24 Unit I check valves.
This letter provides the results of those I
inspections and Attachment A to this letter provides the Engineering-Evaluation to support the decision to complete the inspection of the remainder of the Unit 1 check valves during the next Unit I refueling outage currently scheduled to begin in March of 1991.
Zion Station has completed the inspection of all the Unit 2 check valves.
l There were 5 failures discovered (2SI 9002B, 2SI 90020, 2SI 9002C, 2SI 9001C and 2SI 89568) during the inspection.
Each failure consisted of a single l
broken stud out of 2 that exist for each valve.
Zion Station has completed the inspection of 12 of the 24 check valves on Unit 1.
One failure was discovered approximately two years ago and was repaired-at that time.
Four check valves were inspected during the previous refueling outage that started in the fall of 1989.
During the current forced outage,.seven additional check valves were inspected.
Including the previous failure, there-have been three failures discovered (ISI 8956C, ISI 8956A, and IST 9002A) on Unit 1.
Each failure consisted of a single broken stud out of 2 that exist for each valve.
NW A S 9006040319 900514
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J Commonwealth Edison proposes to complete the inspection of the 12 remaining Unit I check valves during the next Unit I refueling outage currently scheduled to begin in March, Commonwealth Edison has made this decision based on the radiation exposure-savings of 100 man rem that would have been obtained when totally off-loading the core for Unit I and a reduction in the time spent at mid-loop operation of 3 weeks that would have been required to inspect 4 of the remaining check valves if the core was not-unloaded. An Engineering Evaluation has also been completed to show that if failures exist on any of the remainder of the uninspected Unit I check valves, the affected systems will still be able to perform their intended functions.
Therefore, based on the radiation exposure savings, the. reduction in time
. spent at mid-loop operation, and the Engineering Evaluation, Commonwealth Edison is requesting approval of the proposed schedule for the completion of the Anchor Darling. check valve inspection at Zion Station, Unit 1.
If any additional information is needed or any questions-arise, please address them to this office.
Very truly yours, N
R. A Chrzanowski Nuct ar Licensing Administrator
- RAC/ts I
cc: C. Patel Resident Inspector, Zion Region III Office Office of Nuclear Reactor Safety IDNS l
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b ENGINEERING EVALUATION OF ZION UNIT 1 ANCHOR DARLING CHECK l
VALVES HHICH HAVE NOT BEEN INSPECTED PER IEB-89-02 l#
1.0 OVERVIEW l
l During the current Unit 2 refueling outage, five Anchor Darling Model i
S350H swing check valves were found with broken retaining block studs.
l' Retaining block studs fasten the retaining blocks to the valve body which holds the valve disc Gssembly in place (see fig. 1).
Unit 1 is currently in an j
unrelated forced outage.
Seven similar check valves have been chosen to be inspected during this outage. All 7 valves have been inspected and identified two out of fourteen studs failed. A total of 12 identical valves will remain c
in Unit I with retaining block studs which have not been inspected, q
This report includes inspection basis and findings (Section 2), analysis l
of the effect of the broken retaining block studs with respect to valve i-functionality (Section 3), failure modes and effects analysis of the i
l; potentially affected valves in regards to system operability independent to the l
conclusions found in Section 3 (Section 4), and Hestinghouse's independent l
l analysis on the situation specific to Zion (Section 5), which concurs with 1
Commonwealth Edison's conclusions.
Similar evaluations have been performed for-Diablo Canyon and D.C. Cook Nuclear Power Stations.
This report concludes that L
Zion Unit I can safely operate until the next scheduled refueling outage, i
2.0 INSPECTION 2.1 Basis for Inspection Program NRC Bulletin IEB-89-02 was issued 7/19/89 requesting utilities to 1
identify, disassemble and inspect certain types of swing check valves which.may j
contain Type 410 stainless steel bolting. material.
Several occurrences raised concerns about the use of Anchor Darling swing check valves, model S350H (Hestinghouse ID numbers 8C48Z and 10C48Z), and l
valves of similar design, with internal preloaded bolting material of ASTM Specification A193 Grade B6 Type 410 SS. Specifically, Diablo Canyon 2 in October 1988 identified broken retaining block studs on these check valves.
In a similar incident at D. C. Cook Units 1 and 2, several broken or cracked retaining block studs were found in identical valves.
The root cause of the bolt failures was attributed to stress corrosion cracking.
Small variances in heat treating on this material can lead to large y"
variances in physical properties.
410 SS bolting material with high hardness 1
values is more susceptible to stress corrosion cracking.
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The NRC requested licensees to disassemble and inspect all safety related Anchor Darling model S-350H swing (, heck valves during the next refueling or scheduled outage of sufficient duration.
Licensees were also requested to review designs of other safety related check valves to determine if similar designs and material selections are utilized.
2.2 Population Zion Station has 24 Anchor Darling model S350H swing check valves installed per Unit on safety injection, accumulator discharge and residual heat removal piping.
A review was conducted using vendor drawings as reference to identify similar designs and materials.
The results of this review concluded there are no other types of check valves which have preloaded internal bolting of 410 SS installed at Zion. Attachment 1 identifies all check valves in the scope of IEB-89-02.
2.3'As Found Condition Prior to issuance of Bulletin IEB-89-02 the NRC issued Notice IEN-88-85 on the same issue.
Zion Station committed to disassembly and inspection of 4 Anchor Darling, model S350W swing check valves in order to ascertain the condition of the valves at Zion. These four valves in Unit 1; ISI-89560..ISI-9002C, 1SI-9002D, and ISI-8957B were found with acceptable retaining block studs.
The studs were visually inspected, magnetic particle non-destructively evaluated and hardness tested in the.as found condition.
Some bolts had slightly higher hardness values than the recommended 26 Hrc t.s defined by Westinghouse, however no indications of stress corrosion cracking were found. All'of the retaining block studs were replaced with material less susceptible to stress corrosion cracking.
During the 1990 Unit 2 spring refueling outage 23 of 24 Anchor Darling model S350H check valves were inspected. The remaining valve had maintenance performed on it during the previous refueling outage in 1988 (see section 2.4).
The results of this inspection found five broken retaining block studs in valves 2SI-9001C, 2SI-9002B, 2SI-9002C, 2S1-9002D, and 2SI-8956B.- Each valve was found with one broken and one intact stud.
In many of these cases the studs broke during the removal process which indicates that some of the studs were not completely broken prior to disassembly.
The studs were found separated between the middle of the retaining block and the block / body interface.
The separations appeared to be from brittle fracture and some corrosion was found inside the cracked surface.
In all cases, the internals were found properly aligned in the valve.
Hardness testing of the removed studs is still pending.
Fractured studs are in the process of being sent to Argonne National Laboratory for further root cause analysis.
The other 18 valves inspected were found with intact retaining block studs.
In each case, the retaining block dowels which hold the retaining block oriented properly to the valve body, were also inspected identifying no failures.
The retaining block dowels are also constructed of type 410 SS.
Unit 1 is currently in a forced outage. A population of seven Anchor Darling check valves were chosen reflecting a similar population to stud failures identified in Unit 2 which could be inspected without affecting the overall duration of time spent at mid loop operation.
All seven valves have been inspected hnd found two broken retaining block studs (valves ISI-8956C, ISI-9002A).
These stud failures are similar to those found in Unit 2.
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2.4 Maintenance Records A review of the maintenance records indicates excessive backseat leakage was found on ISI-8956A while performing PT-2L " Accumulator Backup Check Valve Leak Check" during the 1988 Unit 1 Refueling Outage. Upon disassembly, a broken retaining block stud was identified (Reference DVR-22-1-88-067).
Both retaining block studs were replaced.
Current Engineering analysis performed on ISI-8956A indicates that if a retaining block was to have lifted off the dowel pins due to the broken retaining block stud,.the resultant backseat leakage would be much greater than 20 GPM.
Therefore, we believe that other factors might have caused the backseat leakage through the check valve.
In 1988, valve 25I-9001B was repaired for backseat leakage upon identification of an adverse trend.
The valve internals, including the retaining block studs, were replaced.
The removed studs had no indications of cracking or fracture.
The backseat leakage was attributed to foreign material on the disc and seat.
The backseat leakage quantified was within acceptable limits as defined in Technical Specification 4.3.3.F. prior to and after repair.
3.0 VALVE FAILURE ANALYSIS 3.1 Valve Description - S350H The design of the S350H Anchor Darling swing check valve uses a vertical i
seat (see Figure 1).
The revolving disc assembly is mounted on a clapper arm which is hung on the clapper arm shaft set back of the seat face. The disc and clapper arm are supported on the clapper arm shaft on a hinge pin cantilevered from a structure which is mounted on two support retaining blocks located above the seat assembly.
Each retaining block has two 1/4" diameter dowels which are used to align the disc / clapper arm assembly so.that the stud can be properly
'nstalled.
Figure I does not show the two dowels per retaining block. The ocwels also serve as an extra restraint to the lateral movement of the retaining block and disc assembly which helps to maintain alignment of the disc on in seat.
The dowels are constructed of Type 410SS.. No damage was observed on the dowels (guide pins) for the 24 inspected valves on Unit 2.
Each retaining block stud is torqued down and the stud nut is tack welded to the retaining block.
3.2 Valve / Body Disc Orientation The Anchor Darling swing check valves addressed in this evaluation are of a very close tolerance design.
This type of design is hydraulically similar to a straight piece of pipe. A large diameter bonnet is placed over the body to house the disc when it is lifted by the flow.
The disc diameter is the same as the pipe 00, while the seat ID is equal to the pipe ID.
The valve body is enlarged only enough to allow the disc to swing down into the flow and cover the seat.
Little space is provided around the seat for such activitics as seat lapping.
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The effects of degraded hardware ws % evaluated by one utility with the use of a CAD model developed using the sanufacturer's original shop fabrication drawings.
The resulting model shows that the stud nuts are tack welded to the retaining block, therefore there is a low probability that parts could come loose. The tight tolerances will maintain the assembly.
Study of i
this model showed that should only one stud fail, the operation of the valve will be unaffected since the two guide pins in the retaining block will j
prevent any movement or misalignment.
The CAD model was primarily developed to evaluate the consequence of the worst case failure of both studs which hold the retalning blocks in place.
The results show that the most Ilkely effect would be undetectable in the performance of the valve.
This results from the close fabrication tolerances which create a gulde to direct the disc's travel in its normal design path.
lne four guide pins prevent lateral moveNnt.
The dise swing arm geometry provides for little uplift force until it is well up out of the flow path.
The disc centering design combined with the weight of the disc is sufficient to maintain the location of the assambly. Physical evidence tends to confirm this evaluation since in at least one instance, at another utility, the disc asse ely apparently functioned properly as a pressure isolation valve even though both retaining block studs failed.
While not predicted from the above analysis, detachment, if postulated, would occur probably two-thirds of full travel up out of the flow.
The model shows that the swing arm would prevent the top of the disc from rotating down while the flow could carry the disc forward.
This travel would be small before the disc would wedge in the valve body, well up out of the f1 w..
No tignificant flow restriction would result, although the valve may not r9 seat.
Com.nonweaMh Edison has independently reviewed the failure mode ef fects on the operation of the valve and also concludes that neither a single stud failure nor a two stud failure would afft:t the performance of any of the valves.
3.3 Valve functionality Based on an evaluation by Hestinghouse, it is been concluded that the dowel pins on their own are sufficient to assure proper operation of the valve disc. An analysis of the hydraulic forces acting on the valve internals confires the capability of these pins (two per block, four per valve) to withstand the loads imposed during opening of the valve and during full flow operation.
This demonstrates that the capability of the retaining block studs is not required to maintain the valve disc in the open position.
The disc assembly is not expected to lift off of the dowel pins and therefore proper seat / disc orientation is maintained.
The worst failed condition of the studs has been assumed in ths analysis, i.e., that both retaining block studs are broken at the retaining block / valve body interface.
Inspection data obtained to date indicates stud failure locations which vary from the block / body interface to points higher up inside the retaining block. A failure of the stud inside the retaining block leaves a stub portion of the stud which would assist in maintaining disc orientation and which could provik additional load carrying capacity.
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4.0 FAILURE MODES AND EFFECTS ANALYSIS l
4.1
Introduction:
i Reference Figure 2 for System Schematic The following Anchor Darling check valves have not been inspected:
1RH 8736A RHR hot leg injection (outboard valve) 1RH 8736B RHR hot leg injection (outboard valve) 1RH 8949A RHR hot leg injection (inboard valve) 1RH 89498 RHR hot leg injection (inboard valve)
ISI 8949C SI hot leg injection (inboard valve)
ISI 8949D SI hot leg injection (inboard valve)
ISI 89488 SI accumulator injection loop C (inboard valve)
ISI 8948C SI accumulator injection loop D (inboard valve)
ISI 8948D SI accurulator injection loop B (inboard valve)
ISI 9001A RHR cold leg injection loop B (inboard valve)
ISI 9001B RHR cold leg injection loop C (inboard valve)
ISI 9001C RHR cold leg injection loop D (inboard valve)
These valves affect the following four functions:
ECCS hot leg r uirculation flow SI accumulator injection flow ECCS cold leg injection flow Intersystem LOCA prevention These four functions have been evaluated with respect to the uninspected check valves falling to open (ccmpletely blocking flow), as well as falling to close.
4.2 Hot Leg Recirculation Injection Hot leg recirculation injection at Zion Station consists of four SI hot leg flow paths (two from each train of SI pumps) and two RHR flow paths.
Each flow path has two pressure isolation check valves installed in serie; and a normally closed motor operated 1 solation valve.
For the SI flow path check valves ISI-8905A, B, and ISI-9004C, D are not Anchor Darling check valves and in turn are not subjected to the retaining block stud failure.
Six of the hot leg recirculation check valves in 4 injection paths are uninspected Anchor Darling check valves, l
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The purpose of Hot Leg Recirculation is to preclude boron precipitation in the core, in addition to core cooling.
Hot leg recirculation is initiated approximately 20 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> after a LOCA.
The desired rate of hot leg recirculation flow is not dictated by the FSAR accident analysis, but by the decay heat rate of the core and boil off.
This does not necessarily require continuous flow. Westinghouse has estimated that approximately 300 gpm would be sufficient to accomplish this. The failure (complete flow blockage) of any two uninspected check valves would not cause the loss of all hot leg recirculation.
At least two recirculation flow paths would remain and it is judged that one flow path would provide at least 300 gpm.
The simult ueous failure of RH8736A and B check valves would cause the loss of hot leg recirculation from the RHR system, however, hot leg recirculation would still be available to all four hot legs from the SI system.
It would require the simultaneous failure of check valves SI8949C, 0 and RH8949A, B to completely prevent hot leg recirculation flow.
The simultaneous failure of all of these valves in the forward flow direction is judge to be I
highly improbable. A check valve failure would more probably result in a flow reduction, not total flow blockage. Core cooling would still be available from cold leg recirculation.
4.3 Accumulator Cold Leg Injection The accumulator injection system at Zion Station consists of four accumulators which inject into each of the cold legs.
Each injection line has
-two check valves installed in serks and a normally open isolation HOV which is isolated in post LOCA conditions.
All eight of the check valves for the accumulator injection paths are Anchor Darling model S350W 3.ving check valves.
Three accumulator injection check valves are uninspected Anchor Darling check valves.
The A Accumulator flow path is unaffected because its check valves have been inspected.
The failure (complete flow blockage) of one uninspected check valve would result in only two accumulators delivering their water to the reactor, since it is assumed that one accumulator would spill out the broken loop.
This is one less accumulator than assumed in the FSAR accident analysis. A Westinghouse assessment for a similarly designed plant stated that a best estimate small break LOCA analysis (< 6 line break) would show acceptable peak clad temperature results assuming only two accumulators inject to the reactor.
The limited break size is based on the NRC-approved leak-before-break principle app 1;ed to the reactor coolant piping.
It is our judgment that this assessment is applicable to Zion.
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In additten, recently completed analyses utilizing the MAAP computer code in connection with Commonwealth Edison Individual Plant Evaluation, IPE, work on Ilon has shown that core melt will not occur with one accumulator injecting into the reactor in combination with one RHR pump iniecting into 2 of 4 cold legs. This analysis is based on an intiating react coolant pipe break of <
3 square feet.
Peak ccre nodal temperature remaine; iess than 1200*F except for short eh ursions which returned to an acceptable value.
This analysis shows that the simultaneous failure of two uninspected accumulator check valves in combination with the remaining accumulator spilling to the broken loop will not result in core melt.
This specific failure combination (which assumes total flow blockage upon failure) is highly unlikely.
If multiple check valve failures were to occur, they would more probably involve partial flow blockages.
4.4 SI/RHR Cold Leg Injection SI cold leg injection consists of four injection lines coming from a l
common header shared by both SI pumps to each RCS cold leg.
Each injection path has two check valves installed in series and a common normally open isolation M0V.
One of the two pressure isolation check valves for each SI injection line (ISI-9012A, B, C, and D) is not an Anchor Darling check valve and in turn is not subjected to retaining block stod failure.
RHR cold leg injection consists of four injection paths into the RCS; two Injtttlon paths are supplied by each RHR pump train.
For each injection path, three check valves in series provide pressure isolation capabilities and a normally open isolation MOV.
The third check valve from the RCS is shared by tho two injection paths from each RHR pump train. All ten check valves associated with RHR cold leg injection are Anchor Darling model S350H swing check valves.
Three of the cold leg injection check valves are uninspected Anchor Darling check valves.
The "A" reactor coolant loop injection line is not affected because its check valves have been inspected. If one check valve were to fall and prevent safety injection and RHR cold leg injection flow following a LOCA, three RCS loops would still be provided with SI and RH cold leg injection. Assuming one of these three remaining SI/RHR cold leg injection lines is flowing to the broken loop, two intact loops would be supplied with SI/RH injection flow.
The IPE work discussed earlier has also shown that core melt will not occur with the SI and RHR pumps injecting into only 2 of 4 cold legs. Again, this analysis is based on an initiating pipe break of < 3 square feet. The simultaneous failure of tnree uninspected check valves (SI 9001A, B, C) would be required concurrent with a break in the A cold leg before a total loss of RH/SI cold leg injection would occur.
This is a highly improbable failure combination.
In this case, high head safety injection flow from the centrifugal charging pumps cold leg injection lines would still be available.
4.5 Intersystem LOCA Protection All of the uninspected Anchor Darling check valves are capable of being tested and monitored fo,- leakage in the reverse flow direction. Monitoring is accomplished by the check valve test system, installed control room pressure indication for ECCS injection lines, and control room ' Indication of the pressurizer relief tank parameters where the relief valves of the ECCS overpressure protection are discharged to.
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4.5.1 RHR Hot leg Recirculation In order to create an over pressurization in the RHR system, two uninspected check valves would have to fall in either injection path simultaneously, This can be monitored as the reactor coolant pressure is slowly increased after the outage. They will be tested again when the system is at pressure.
These valves would not be required to reseat after use in the hot leg recirculation mode (the RCS would be depressurized).
Should these valves fail during the operating cycle, which is highly unlikely once seated, the operator would be made aware of this by changes in the pressurizer relief tank temperature, level and pressure indications as well as alarms. Operating would isolate MOV-RH8703 which would preclude an intersystem LOCA.
4.5.2 S'l Hot Leg Recirculation The two uninspected Anchor Darling check valves in the SI hot leg injection paths pose no significant reduction in margin of safety for intersystem LOCA concerns because there is one check valve in series with each one and a common normally closed motor operated valve (MOV) all rated for system pressure.
4.5.3 RHR/SI Cold Leg Injection The uninspected cold leg injection check valves pose no significant reduction in margin of safety for intersystem LOCA concerns because there is an additional check valve in series with them in the SI system of a different design and two inspected Anchor Darling check valves in series with them in the RHR system.
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i 4.5.4 Accumulators Each of the four safety injection accumulators is isolated from the RCS by two check valves and a normally open MOV in series. One of the accumulator injection lines has both check valves inspected.
For three of the accumulators, only one of two check valves in series is an uninspected Anchor Darling Check Valve.
If any of these check valves failed to isolate RCS pressure, the accumulator would be prevented from overpressurization by the other check valve in series with it.
If the accumulator was to pressurize, then this would be indicated to the operator by increasing pressure and level on the affected accumulator.
The affected accumulator relief valve would open relieving N2, borated water, and possibly reactor coolant to the containment sump.
The affected accumulator could be isolated by energizing and closing the MOV in the discharge line upstream of the check valves, 5.0 WESTINGHOUSE ANALYSIS Reference G.P. Toth to T.P. Joyce letter CHE-90-170, NS-OPLS-0PL-I-90-276, dated 5/2/90.
5.1 Background
As a result of information received from a number of plants concerning cracked and failed bolts in ECCS injection line check valves, a potential safety concern has been identiflod regarding adequate injection flow in the event of a LOCA.
The valves in question are 8-inch, 10-inch, and 12-inch swing check valves constructed by Anchor-Darling.
Typically, these valves are located in the ECCS injection lines.
The safety concern involves the bolts that attached the hinge blocks to the valve body (two bolts in each valve).
The hinge blocks support and locate the valve disc. At Zion, as well as at other plants, a number of these bolts have been found cracked.
In the event that these systems are required for safety injection, the potential exists that the disc could break loose from the valve body, causing a significant blockage in the ECCS flow path.
This could result in reduced cooling flow reaching the reactor core with accompanying increases in peak clad temperature.
Zion Unit I contains (24) Anchor Darling check valves that were supplied by Westinghouse:
ten 10-inch check valves (10C48Z) and fourteen 8-inch check valves (8C48Z).
The 10-inch check valves are located in the accumulator discharge header and in the RHR discharge lines.
The 8-inch valves are located in the low head safety injection / residual heat removal (LHSI/RHR) injection headers to both the cold legs and the hot legs.
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5.2 Evaluation Based on analyses performed for many plants and accepted by the NRC, leak before break (LBB) has been demonstrated for the reactor coolant loop piping and large branch lines attached to the loop (down to and including 8-inch lines).
These analyses provide sufficient justification to stpport the use of a more realistic assumption regarding the accident conditions used for this assessment.
Therefore, the LOCA event used in our assessment assumes a 9-inch bna, the accumulators and the SI system would still be required to operate, but at a reduced flow, thus imposing lower loads on the check valves in these lines.
5.2.1 System Evaluation An evaluation has been performed to determine the flow conditions imposed on the Anchor Darling check valves during the assumed sniall break LOCA event.
5.2.1.1 Accumulator Discharge Check Valves The 10-inch check valves in the accumulator discharge flowpath are subjected to the flow of the discharging accumulators.
To determine the l
maximum expected accu'nulator discharge flowrate, the Zion Unit I small break LOCA analysis was reviewed.
The current small break LOCA analysis for Zion l
Unit I has been performed using the HFLASH Small Break ECCS Evaluation Model.
While this is an acceptable model for evaluation of the small break transient, i
it does not provide sufficient information for a detailed examination of the accumulator blowdown flow.
l In lieu of using the NFLASH data, an examination of the Byron /Braidwood small break LOCA accumulator behavior was undertaken using the NOTRUMP Evaluation Model to better evaluate the accumulator blowdown transient.
Based i
on this examination, it was concluded that the 9-inch break case provided the highest accumulator flow, and also indicated the greatest potential for accumulator flow oscillations.
For the assumed 9-inch small break transient, the flowrate in the accumulator discharge line has been conservatively determined to be a maximum of 1205 lbm/sec. (8,672 gpm). Generally, it was found that changes in flow l
from zero to about 300 lbm./sec., or vice versa, would require about five seconds (the 10-inch valve is fully open at approximately 486 lbm./sec.).
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5.2.1.2. SI/RHR Hot and Cold 1.99 Injection Check Valves t
The 8-inch check valves in the RHR hot and cold leg injection headers can experience a combination of RHR and SI flow.
The flowrate through an individual check valve in these flowpaths could theoretically be a maximum of approximately 338 lbm./sec. (2438 gpm), based on two RHR pump operation with no RCS backpressure.
Based on review of the small break transient, the reactor coolant system (RCS) pressure will not decrease below the RHR pump shutoff head.
Therefore, the RHRS will not inject flow to the RCS, resulting in a much lower actual flow through these valves.
However, for conservatism in the structural evaluation of these valves, this assessment assumed a 338 lbm./sec. flowrate through the valve flowpath.
5.2.2 Evaluation of Small Break LOCA The most limiting scenario for the failure of an Anchor Darling check valve for a small break LOCA for Zion Unit I would be failure of the check i
valve nearest to the reactor coolant loop in the lowest resistance accumulator /
j ECCS line at the tie of accumulator actuation.
Conservatively assuming full l
blockage of the flow path, this failure would eliminate flow from one accumulator.
In the event of this unanticipated failure, it is expected, based on engineering judgment, that the peak clad temperature for small break LOCA for lines up to nine inches in diameter would not exceed the PCT limit of 10CFR50.46.
5.2.3 Structural Evaluation of Valve Internals An evaluation of the valve internals has been completed to confirm the capability of the alignment pins (two per hinge block, four per valve) to withstand the loads imposed during opening and closing of the valve and during full flow operation, and to demonstrate that the capacity of the retaining bolts is not required to maintain the valve disc in the proper position.
The worst failed condition of the bolts has been assumed in the analysis, i.e.
that both retainer bolts are broken at the hinge block / valve body interface.
Inspection data from Zion and other plants obtained to date indicates failure locations which vary from the block / body interface to points higher up inside the hinge block. A failure of the stud inside the retaining block leaves a stub portion of the stud which would assist in maintaining disc orientation and which could provide additional load carrying capacity.
Based on the information received from Anchor Darling, the valve has a maximum opening angle of 90 degrees before it hits the body stop.
Evaluation of the dimensions demonstrate, however, that the valve disc will swing out of the flow stream at approximately 70 degrees.
Therefore, because of the valve geometry, and the relatively slow opening times expected during the transient, the valve disc will not hit the stop upon opening and will not produce a reaction load at the pin / retaining block assembly.
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4 Test results of swing check valves in water systems confirm that the valves have a natural closing tims of approximately 0.5 seconds or less for a i
slam motion.
Review of the small break data shows that the valves are exposed to velocities which require the disc to open in about five seconds.
Thus, no slam action is anticipated.
Even though the disc is not predicted to impact the valve body, an evaluation of the reaction loads at the pin / retaining block was performed which assumed that the disc hit the body stop while still within the flow stream.
This is a very conservative assumption since thi yalve internal i
geometry and flow data show t5at the disc will not impact the valve body, but rather will ride on the flow stream.
However, to conservatively bound the reaction loads at the retaining block, the disc was assumed to be against the valve body under conservatively assumed full flow conditions (70 fps).
With the above assumptions, the evaluation results show that the total load taken by the four pins on the 10C48Z is approximately 17 pounds in the horizontal direction (reacted by the pins in shear) and 25 pounds in the vertical direction.
For the 8C482, the total load is 12 pounds horizontally and 17 pounds vertically.
Each of the four dowel pins is 0.250 inches in diameter.
The resultant shear stress is less than 1000 psi.
Even with the maximum possible clearance between the pin and the hole, the vertical load will be offset by the weight of the hanger block / pin assembly and the frictional resistance from the coincident shear loads.
These results provide additional verification that, even if the disc was against the stop while still ir 'he flow stream, the loads on the block / pin assembly are not sufficient to tause the disc to become detached from the valve body, and indicate that proper functioning of the valve will be maintained.
5.3 Summary and Conclusions An evaluation of the Anchor Darling check valves has been performed for Zion Unit 1.
The results of this evaluation are as follows:
o Review of the small break LOCA analysis indicates significant margin to the applicable limits, even with the loss of an ECCS flow path.
o Evaluation of the valve internal structure indicates that, even with both bolts broken, the valve would function as designed.
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6.0 SUPPORTIVE TESTING PERFORMED Zion Station performs full flow testing (TSS 15.6.84, " Charging and SI Check Valve Verification Test, TSS 15.6.85, "RHR Check Valve Verification i
Test, and TSS 15.6.87, Accumulator Discharge Test") for all of the Anchor Darling valves.
Zion Station also performs seat back leakage tests (TSS 15.6.114, "SI and RH Check Valve Leak Check" and PT-2L " Accumulator Backup Check Valves Leak Check") for the check valves referenced on Attachment 1.
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Through the results of these tests combined, restriction in full disc lift or failure of the disc to properly seat are identified.
Since in normal plant operation these check valves are subjected to relatively static conditions, the quantified backleakage of the valve since the previous tect was performed should not change.
Technical Specification Section 4.3.3.F requires retesting of seat back leakage when the check valves have been disturbed:
1.
Each refueling outage 2.
Prior to entering MODE 2 operation if in CSD for more than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and testing has not been performed in the previous 9 months 3.
After maintenance or repair on the valve 4.
Following valve actuation resulting in water being injected into the RCS.
5.
Whenever the RCS has been within 100 psi'of SI or RH system design pressures.
Full flow testing is performed every refueling outage.
Partial flow testing is performed following maintenance on each check valve.
Accumulator full flow check valve testing was initiated in 1987 (reference OPEX Commitment No. 295-351-86-02600) per procedure TSS 15.6.87.
To date, all four accumulators have been full flow tested for Unit 1.
Accumulator partial flow check valve testing is performed each refueling outage per PT-20.
7.0 CONCLUSION
All 24 of the Anchor Darling mode S350H swing check valves installed in Unit 2 have been inspected and all retaining block studs have been replaced.
Upon completion of the current forced Unit 1 outage, 12 of 24 of the check valves will be inspected and the retaining block studs replaced.
The population of 12 check valves inspected in Unit I was based upon:
1.
Failure modes and effects analysis 2.
Inspection of a sample of sister valves found with broken studs during the Unit 2 inspection.
3.
Completing at least one valve in each of the cold leg injection paths.
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The inspection results conclude that the valves were found with the internals in a functional position and properly aligned with a maximum of one stud broken per valve.
Maintenance records identify one previous failure of a check valve whose presumed cause of failure was a broken retaining block stud.
Documentation pertaining to the failure indicates that the failure was attributed to the broken stud, however, current analysis performed conclude that some other attribute may have caused the backleakage.
In any case, this failure was identified through backseat leakage testing. This testing is required to be performed on all these check valves upon leaving cold shutdown prior to power operations.
Analysis performed specifically for the Anchor Darling model S350H swing check valve finds no effect on the ability of the valve to provide injection flow even with the worst case of two broken retaining block studs. The analysis also concludes that the valves should restat properly with the retaining block dowels maintaining' the alignment of the valve internals during and after injection.
Failure modes and effect analysis for the specific check valves in the system that have not been inspected was reviewed for worst case scenarios of blocked flow upon injection and the inability of the valve to reseat after termination of injection. These failures were compared to plant emergency or abnormal operations.
It was found that even with blocked flow failures, sufficient coolant flowpaths would be available for safety injection during a LOCA.
If the' check valves were not to reseat, other check valves or a normally closed MOV would prevent a potential inter-system LOCA event that could lead to an increased probability of an Event V incident.
Westinghouse has performed an independent review specific to Zion Station and concluded that the system was able to perform its intended safety function with broken retalning block studs.
Based upon this information, Commonwealth Edison Company believes it is acceptable for Zion Unit I to return to power and operate until the next scheduled refueling outage at which time all retaining block studs that have not been previously inspected, will be replaced per the requirements of IEB-P9-02.
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ZlDN STAflDN UNif 8 i AffACHM(NT # 1 Page 1 of 2 May 11,1990 ANCHOR DARLING CH(CK VALVE INSPECTION STATUS Sequence Croken or Cracked Studs flow From flow to from RCS Check Valve Tne inspected found during inspection RHR Not Leg Train A or B 1A Not Leg 2nd valve 1RM 8736A 8 C482 1st valve 1RM 89694 8 C482 10 Hot Leg 2nd valve 1RM 87368 8 C482 1st valve 1RM 89498 $*C482
$1 Hot Leg Train B it Hot Leg lot valve 1RH 8949C 8 C482 1C Het Leg ist valve 1RM 89490 8 C482 RHR Cold leg train A 1B & 1C Cold Leg 3rd valve 1SI 8957A 10 C482 5/D8/90 None IB Cold Leg 2nd valve ISI 9002A 8 C482 5/10/90 1 Broken Retelning Block Stud 1st valve 151 9001A 8 C482 1C Cold Leg 2nd valve 151 90025 8 C482 5/11/90 Wone 1st valve 181 90015 8 C482 RHR Cold Leg train 8 1A & 10 Cold Leg 3rd valva 151 89578 10 C482 10/09/90 None f
1A Cold leg 2nd valve 1$1 90020 8 C482 10/07/89 None 1st valve 1SI.90010 8 C482 5/03/90
_ww 10 Cold Leg 2nd valve 1SI 9002C 8 C482 10/07/C9 none 1st valve ISI 9001C 8 C482 1A Accumulator 1A Cold Leg 2nd valve 151 8956A 10 C482 4/30/88 1 Broken Retaining stock Sts$
1st valve 151 8948A 10 C482 4/28/90 None i
t 18 Accumulator 1$ Cold Leg 2nd valve 151 89560 10 C482 10/09/89 None 1st valve 151 89480 10 C482 iC Accumulator 1C Cold leg 2nd ' valve 151 89568 10 C482 5/07/90 None 1 #.6 valve 1SI 89488 10 C482 10 Accupulator 10 Cold Leg 2nd Valve 151 8956C 10 C482 4/24/90 1 Broken Retaining Block Stud 1st valve 151 8948C 10 C482 TOTALS ss>
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Page 2 of 2 Mey 11,1990 ANCHOR DARLING CHECK VALVE !NSPECflDN $fATUS Sequence Brckeri or Cracked $tuds flow f rcus Flow to from RCS Check valve Type Inspected found during inspection RHR Hot leg Train A or 8 2A Hot Leg 2nd valve 2RM 8I)6A $*C482 4/13/90 None 1st valve 2RN 9949A 8 C482 4/23/90 None 2D Hot Leg 2nd valve 2RM 87368 8' C482 4/D8/90 None 1st valve 2RN 89498 8 C442 4/17/90 None
$1 Hot Leg frein B 29 Mot Leg ist valve 2RM 8949C 8 C482 4/13/90 None 2C Mot Leg ist valve 2RH 89490 8 C482 4/11/90 None RHR Cold Les frein A 28 41C Cold Lee 3rd valve 2st 8957A 10 C482 4/08/90 None 28 Cold Leg 2nd valve 281 9002A 8 C482 4/13/90 None 1st valve 281 9001A 8 C4t4 4/12/90 None 2C Cold Leg 2nd valve 251 90028 8 C462 4/07/90 1 Broken Retaining Block stud 1st valve 2$1 90018 8 C482 10/31/88 None RHR Cold Leg Train 8 2A & 20 Cold Leg 3rd valve 2$1 89578 10 C482 4/16/90 None,_,,,
24 Cold Leg 2nd valve 251 90020 8 C482 4/19/90 1 Broken Retaining Block stud 1st valve 251 90010 8 C482 4/26/90 None 20 Cold Leg 2nd valve 2st 9002C 8 C482 4/15/90 1 proken Retaining Block stud c
ist valve '2$1 9001C 8 C482 4/ 09/90 i Brokon Retalning Block Stud 2A Accumulator 2A Cold leg 2nd valve 2SI 8956A 10 C482 4/17/ 90 _ _.
N one_ _ _.
1st valve 2$1 8948A 10 C482 4/24/90 None 28 Accumulator 2B Cold Leg ind valve 251 89540 10 C482 4/21/9r None 1st valve 2SI 89480 10 C482 4/21/90 None 2C Accumutttor 2C Cold teg 2nd valve 251 89568 10 C482 4/24/90 1 Broken Retaining Stock stud 1st valve 2SI 89488 10 C482 4/26/90 None 20 Accumulator 20 Cold Leg 2nd valve 281 8956C 10 C-82 4/07/90 None 1st valve 2$1 8948C 10 C482 4/16/90 None I
I TOTALS==>
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