ML20235N610

Summary of Engineering Evaluation of ANO-1 High Pressure Injection Sys Backflow Condition

ML20235N610
Person / Time
Site:	Arkansas Nuclear
Issue date:	02/14/1989
From:	ARKANSAS POWER & LIGHT CO.
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ML20235N608	List: ML20235N606 ML20235N610
References
NUDOCS 8903010304
Download: ML20235N610 (21)

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SUMMARY

0F ENGINEERING EVAltJATION OF ANO-1 HIGH PRESSURE INJECTION l

' SYSTEM BACKROW CONDITION 1

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SUMMARY

OF. ENGINEERING EVALUATION

'0F ANO HIGH PRESSURE' INJECTION.

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. SYSTEM BACKFLOW CONDITION >

TABLE OF CONTENTS >

1.0 INTRODUCTION

2.0. EVENT

SUMMARY

3. 0 PREVIOUS MODIFICATIONS ,.

4.0- DAMAGE ASSESSMENT 'BC LOOP 4.1 ANALYTICAL REVIEW 4.2 FHYSICAL INSPECTION -

PIPING 4.3: PHYSICAL. INSPECTION -- SUPPORTS 4.4 PIPING COMPONENT REPLACEMENT 5.0 REVIEW 0F REDUNDANT AD LOOP 6.0 ACTIONS TAKEN TO PREVENT RECURRENCE 6.1 MODIFICATIONS 6.2 VALVE TESTING 7.0 DESIGN BASIS REVIEWS 7.1 CONTAINMENT ISOLATION 7.2 SYSTEM REVIEW 7.3- OTHER HPI SYSTEM OPERATING MODES 7.4 SAR REV1SIONS TABLE 1: HPI LOOP P' 'IPING STRESS

SUMMARY

TABLE 2: LOOP AD HPI BACKFLOW TRANSIENTS FIGURE 1: POTENTIAL HPI BACKFLOW PATHS FIGURE 2: MU-66 VALVE ADDITIONS FIGURE 3: LOOP BC GENERAL ARRANGEMENT FIGURE 4: STOP-CHECK VALVE SKETCH i

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SUMMARY

OF ENGINEERING EVALUATION OF ANO-1 HIGH PRESSURE INJECTION l SYSTEM BACKFLOW CONDITION j

1.0 INTRODUCTION

This paper provides information regarding the condition which resulted in the RCS backflow through one of the High Pressure Injection (HPI) System discharge line check valves following the plant trip on Friday, January 20, 1989. The following information addresses issues related to AP&L's evaluation of the cause and consequences of the check valve backflow, as well as corrective actions taken and planned to prevent any future recurrence.

2.0 E

_ VENT

SUMMARY

A turbine trip occurred en ANO-1 on January 20, 1989, initiated by a generator lockout due to a broken pole piece in the turbine generator exciter. The subsequent reactor trip ard certain feedwater control system and electrical distribution system problems required the operators to manually initiate additional HPI flow to the RCS. It was later discovered that the check valve in the 'B' HPI injection line (MU-34B) had failed to reseat (due to c<cessive wear) after HPI flow was terminated, allowing reactor coolant to flow into the injection line through a crossconnect line and back into the RCS through the 'C' injection line, hereinafter referred to as the BC Loop. Only reactor coolant pumps (RCPs) 'B' and 'D' were operating due to the failure of the H1 6900V bus (feeding RCPS "A" AND "C")

to fast transfer following the trip; the differential pressure created by this RCP operating combination provided the driving force for the backflow.

The ANO-1 HPI System is described in ANO-1 SAR Section 6.1.1, with the associated detailed piping & instrumentation drawings (P& ids) of SAR Figures 9-3 and 7-20.

3. 0 PREVIOUS MODIFICATIONS The HPI System discharge piping configuration was modified in 1979 as a permanent resolution to an ECCS performance concern, as described in SAR Section 14.3.1.2. During Spring 1978, B&W discovered that the existing RCS small break LOCA analyses had not previously identified the most limiting break location. Resolution of the issue involved new SBLOCA analysis and ultimately required modifications to the HPI discharge lines to ensure that the necessary HPI flow reached the core given the new limiting break location (RCP discharge piping).

AP&L's proposed modifications to address this concern (discharge cross connect lines, flow restricting orifices and globe throttle valves) were described to the NRC in our letter dated January 3,1979 and subsequently approved for implementation by NRC letter dated March 1, 1979. The modifications were installed and flow tested by AP&L in the subsequent Spring 1979 refueling outage.

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As part of- the modifications, check valves were added immediately downstream of the motor-operated HPI block valves prior to the new cross-connects. The single failure of the existing discharge line check valves, in conjunction with certain combinations of RCP operation, which allows RCS fluid to flow q into the injection lines, was apparently not anticipated.- The HPI injection )

lines were not designed for RCS temperatures, j 4.0 DAMAGE ASSESSMENT - BC LOOP As a result of the event allowing introduction of RCS' fluid at temperatures greater than the piping design temperature into the BC Loop, AP&L has performed an assessment of the potential damage to the affected HPI' discharge piping. This assessment includes both analytica) and physical inspection of piping and supports to assure the integrity of the system.

4.1 Analytical Review RCS backflow.through the HPI system can occur'only during a special set of circumstances. First, there must be unbalanced Reactor Coolant Pump

, operation which provides the differential pressure required for flow. j Secondly, there must be a motive force to open the check valves (such as HPI '

injection), and finally, there must be the failure of a check valve to l reseat properly. Figure 1 demonstrates the limited possibilities of backflow considering various pump operating modes and a single check valve failure.

AP&L recognizes that as a result of the January 20, 1989 transient, the BC Loop potentially experienced a condition beyond its design basis.

Analytical efforts were undertaken to examine all aspects of this loop.

The primary input for the damage assessment effort is a preliminary thermal piping stress run with an assumed temperature of 545 F, the temperature of the RCS coldleg at the beginning of the postulated event. The assessment includes HPI injection piping inside containment from the penetration up to the RCS cold leg and piping outside containment to an anchor located adjacent to the outboard block valve. Selecting these boundaries includes  !

l all the piping in the BC Loop exposed to elevated temperatures including the crossconnects. The stress run and pipe support review utilizes existing stress models which are compared to and reconciled with as-built configurations to ensure their accuracy. Major portions of the HPI system had previously been walked down as a part of the Isometric Update Project.

Minor support location discrepancies had been previously identified and were being dispositioned prior to this review. All support discrepancies will be corrected as a part of the current effort.

The preliminary thermal analysis is based on conservative assumptions.

These conservatism include:

a) The mathematical model utilized to perform the evaluation considered rigid supports. Analysis utilizing flexible supports which accurately models how the piping system will respond to loads, normally reduces linear self-relieving loads such as thermal loads.

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s b). Gaps in the existing frame-type supports were not considered.

Consideration would reduce the stresses.in the piping and help re' duce loads on supports.

c ) -- The analysis assumed the check valve failed wide-open corresponding to 105 gpm backflow. This results in the pipe reaching cold leg

-temperature of 545 F throughout the BC Loop. This also introduces conservatism in the fatigue evaluation of the piping. A simulation conducted by Plant Engineering showed that the flow more likely was induced by a smaller leak, resulting in lower temperatures than assumed in the analysis.

d) .The 545 degrees F assumption is based on the temperature of the RCS at the HPI nozzle at initiation of injection. A more accurate temperature for the piping analysis would be RCS temperature after securing safety injection. The assumption of 545 degrees F temperature throughout the loop is conservative since some convective cooling will occur as water travels through the loop.

e) Use of nominal versus actual material strength properties adds conservatism.

The computer program used in the analysis is Bechtel proprietary Linear Elastic Program (LEAP) "ME101". A verification report of this computer code was submitted by Bechtel Power Corporation to NRC (see A.L. Cahn, BPC, to R.J. Boonack, NRC, letter dated July 23, 1980). Computer Code ME912 (Piping Thermal Program) used in the transient analysis of Class 1 piping has been used in several operating power plants designed by Bechtel and has been submitted to the NRC in various plant SARs.

Although no previous incidents have been identified of elevated temperature, a review of plant operating history since 1979 was performed. The review captured 16 transients where possiDie RCS flow reversal could have existed due to unbalanced RC pump operation and HPI initiation causing a check valve to open and not reseat. Assuming either MU34B or C failed to reseat, the BC Loop could have been subjected to a similar backflow 12 times.

A fatigue evaluation was performed to calculate the effects on the cumulative usage of these 12 occurrences. This cumulative usage is then combined with the total cumulative usage calculated based on future normal and upset operating conditions for the remainder of plant design life.

The damage assessment analysis considered 20 cycles of hydrostatic test pressures at 3125 psi in the calculation for cumulative usage factor (CUF).

Since the Section XI hydrostatic test pressure is 1.1 x Normal Operating Pressure, (2750 psi), the analysis uses very conservative hydrostatic pressure.

High stress points in BC Loop are identified utilizing the 545 F stress run.

High stress points are defined as those areas of pipe or components (fittings) for which the calculated stresses exceed ASME Code allowables.

Those areas of pipe or components for which the calculated stresses are within code are considered adequate without further analytical review. 1 Table 1 identifies the component and the (ME101) calculated versus allowable  !

stress developed at the high stressed comoonents.

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Piping pen'etrations through the containment wall have been evaluated for the postulated 545*F thermal _ load by both analysis and inspection. Piping stress anchor loads on the penetration have been compared to penetration anchor allowables. Additionally, a visual inspection of the penetrations (both -inside containment and outside containment) has been performed.

The allowable penetration loads envelope the postulated 545 F thermal loads calculated from both sides and added appropriately. No damage, deformations, cracking, or other degradation was found.

The nozzle for the HPI branch line attachment to the RCS loop has been preliminarily proven adequate by calculation. Branch line anchor loads from the 545 F stress ~run were utilized in the nozzle evaluation. The NSSS vendor (B&W) has reviewed the nozzles with respect to CUF based on the transient history outlined previousl.y. Based on the extensive efforts rcquired to validate these nozzles, the analytical approach was prior to heatup to demonstrate qualification only for one fuel cycle. The B&W l preliminary conservative results indicate that all four nozzles are qualified at least through the end of this fuel cycle. The final calculation package will be completed and reviewed by AP&L prior to heatup.

Further calculations required to demonstrate extended nozzle qualification will be performed.

In order to develop a better understanding of the potential consequences of this event, AP&L has performed finite element analyses of several piping components which would be calculated to be overstressed at 545 . Although these analyses were not used to justify the as-left conditions, they will be filed for future information. Finite element analysis _is not used to justify allowing any overstressed component to remain in the system.

4.2 Physical Inspection - Piping The visual inspection /NDE program consists of three parts:

1) Piping will be inspected for general degradation caused by thermal movement of pipe into interferences, walls, shield wall penetrations, etc. The pipe will be inspected for obvious deformations, damage and scratch marks which could result from excessive deflection caused by thermal expansion.

2) A sample of welds on pipe runs will be examined by liquid dye penetrants. Each of the HPI lines inside containment and both cross-connect loops are to be tested. Two to three welds on each line are to be examined

3) High stress areas or components (fittings) in the BC Loop identified by the stress analysis were examined by both dye penetrants and volumetric ]

examinations to the extent possible. In a few locations, due to weld geometry (socket weld or fillets), volum.etric examination was not achievable. ]

The criteria for acceptance is that no damage, deformations, cracks, or other piping degradation are found. The results of these examinations indicated that no damage was found.

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NOTE: During the NRC AIT interviews, it was reported that a Waste Control Operator had indicated that the piping had changed colors due to the high temperatures. A review of the literature indicates that stainless steel piping does not change its appearance until temperatures approach 700 to 800 F. This condition could not have occurred.

4.3 Physical Inspection - Supports Pipe support loads from the 545 F stress run were compared to the design pipe support loads to determine which pipe supports require inspection. The newly generated b45 F thermal loads + deadweight loads will be compared against existing design thermal loads + deadweight loads + seismic loads.

If the loads cannot be enveloped or reconciled against the design loads, the pipe support will require inspection (visual, PT or magnetic particle) as determined by the cognizant pipe support engineer.

In general, the visual inspection will check for bent or deformed steel I members; damage to support welds; obvious damage to support components such as snubbers, pipe clamps, or spring cans; and damage to grout and pull-out of concrete expansion anchors. Dye penetrant or magnetic particle testing will be used to identify cracked welds.

The criteria for acceptance is that no damage, deformations, cracks, or other pipe support degradation resulting from this event are found.

The results of the visual inspections and surface examinations on the pipe supports have shown that no degradation of pipe supports as a result of the postulated thermal event has occurred.

NOTE: During the NRC AII interviews, it was also reported that a Waste Control Operator had indicated that pipe supports had been observed with cracked, discolored and peeling paint. Subsequent walkdowns have not identified any paint defects attributed to high temperatures.

4.4 Piping Component Replacement The original replacement criteria was that, prior to heatup, AP&L will replace any BC Loop piping components (fitting, pipe, or weld) which:

1) do not meet visual inspection acceptance criteria; or

2) do not meet NDE acceptance criteria; or

3) do not meet code allowable stresses.

All the components passed visual and NDE; however, seven BC Loop fittings listed in Table 1 exceeded code allowable stressas, assuming a linear conservative analysis, and therefore are being r.eplaced.

This conservative position eliminates any doubt of the qualifications of this safety system piping. The piping system can then safely be considered to not be in a degraded condition. Figure 3 depicts the approximate locations of components that were replaced. An additional weld was replaced as a construction convenience near a valve.

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5. 0 REVIEW OF REDUNGANT AD LOOP The AD Loop was not affected by this event. Nonetheless, a review was l conducted to determine if a similar event might previously have occurred. A detailed review of the root cause of the check valve failure in the BC Loop including examination by the valve vendor was conducted. The check valve failure was found to be wear relatad. The exact cause of the wear has not been determined. An examination of the check valves in the AD Loop was also performed, again with the assistance of the valve vendor. A specific ,

question has been raised on the condition of MU-34A. In response to this  !

concern, it has been established that no loose parts or serious degradation of MU34A was found in the initial valve inspections following the event in question. The vendor determined that the degree of wear found in the failed valve (MU-34B) did not exist in MU-34A or D and these valves were not f subject to the failure mechanism observed in MU-348. A review of work '

history of the MU-34A and D valves was also conducted and indicated no 1 refurbishment or repair activity indicative of any past problems with these j valves.

In addition, a review has been performed of transients since 19'/9 (crossover i installation) to identify those transients in which a potential for backflow in the AD Loop existed. Due to the presence of a second check valve in the D HPI line (Fig. 2), backflow is considered to be credible only via failure of MU-34A (in the A HPI line). There have been 16 HPI injections since 1979. Only four of these injections have occurred with pump operating conditions which could have allowed such backflow in the AD loop. Table 2 provides additional information for each of these events.

Detailed visual and NDE examinations of AD Loop piping give strong evidence that no high temperature excursion has damaged the system. Pipe support and penetration walkdowns show no indication of support responses indicative of an extreme temperature loading.

In view of the observed condition of the check valves in the AD Loop, the small number of opportunities for potential backflow conditions, and the results of visual and NDE exams, it is concluded that the AD Loop has never I been subjected to significant temperature excursions.

Nevertheless, both analytical and physical reviews of the AD Loop were performed. A preliminary thermal analysis of Loop AD piping was j performed using the conservative criteria explained in Section 4.1. High l stress points were identified in the piping assuming a 545 F thermal l transient. These locations were then examined per Section 4.2. Containment j penetrations and pipe supports were also reviewed. I A visual inspection /NDE program similar to that described in Section 4.2 was applied to the Loop AD piping. Again, high stress points were examined using both dye penetrant and volumetric examination as geometry and interferences allowed. No unacceptable indications were found.

Criteria similar to Section 4.3 were utilized for pipe supports. Field examinations found no unacceptable indications which might have been the result of a temperature excursion.

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-In summary, based on the operating history of this loop, the absence of indications that the AD Loop has ever been subjected to RCS temperatures, the favorable results from field examination of analyzed high stress points, a review of past ISI results, and the check valve examinations.for these loops, the AD Loop has not been degraded and is acceptable for continued.

. service.

6.0 ACTIONS TAKEN TO PREVENT RECURRENCE AP&L evaluated several methods to ensure that the backflow condition does not rccur. The final measures selected include plant modifications and increased valve testing.

6.1 Modifications In order to preclude the potential for a similar event to occur in the future, AP&L has prepared design modifications, including installation of additional check valves (MU-66 A thru D) for both-the AD and BC Loops in series with the existing HPI discharge line check valves. (See Figure.2)

A stop-check valve manufactured by Anchor-Darling will be added to each of four HPI lines inside the reactor building between the containment penetration and the existing swing check valve in each line. The valves are added to provide redundant flow isolation in conjunction with the existing MU-34 valves to prevent a single failure from allowing RCS. flow to enter the HPI crossover loops. The valves will also serve as inboard (toward the containment penetration) containment isolation valves replacing'the MU-34 valves in that function.

The check valves to be installed are designed and manufactured to the requirements of ASME Code, 1974 Edition, Winter 1975 addenda. They are 2-1/2" valves rated in excess of RCS conditions of 2500 psi and 650 F. The valves are ASME Section III, Class I and are manufactured of 316 stainless steel.

The stop-check are being installed inside the reactor building'in existing 2 1/2" HPI lines. The capability to manually close the check valves is not required with their installation in the system. For this reason, the handwheels have been removed and the valve stem locked in the full open position, utilizing a locking bar tack welded to the valve yoke. Vendor approval of these actions has been obtained. Figure 4 provides a sketch of the new MU66 stop-check valves. The valves will be installed in horizontal pipe runs (as required by the valve vendor) as close to existing pipe supports as practical. Piping near each check valve will be provided with vents and drains as required to allow for periodic, independent, testing of valve leakage. Pipe support modifications are a part of the design change to accommodate the revised seismic loads due to the addition of the new valves. The piping and supports will be qualified to all applicable operating conditions. The piping will be designed to ANSI B31.7, 1969 Edition. A code reconciliation document is being prepared by Bechtel to reconcile the design code back to the Code of Record, ANSI B31.7, 1968 Draft with ERRATA.

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. l Post-modification testing of the check valves will include leakage testing to verify the backleakage isolation function of the valves. Maximum leakage limits will he established to ensure the upstream piping is not overheated.

A hydro will be performed on the modified piping per ASME Section XI 1980 Edition Winter 1981 Adderda per Article IWB-5000. All piping welds will be NDE inspected and accepted per AP&L Specification M-406, Rev. 8.

Pressure drop and flow reduction calculations were performed by both AP&L and B&W to verify that the flow effects caused by these ncw valves will not invalidate the HPI design basis. Calculated flow resistance for the new stop check valves was added to the original design flow calculations for the HPI legs. The resultant flow rates were within acceptance standards of a 30/70 split and total flow of greater than 500 gpm. These calculations show that HPI system flow rebalancing is not required to revalidate this basis.

However, a flow test plan is being developed to provide additional assurance of system adequacy.

Post modification examinations of all welded joints will be done to meet applicable Section XI requirements for ASME Class I components and systems.

Additionally, the newly installed check valves will be added to the ISI program which currently includes weld examinations of the injection piping from the RCS cold leg to the outside containment block valve.

Temperature instrumentation is being installed. Filled capillary contact thermometers, providing local indication, are being installed on each HPI injection line in the piping penetration rooms. Temperature data will be gathered on a regular basis in order to support analysis and to disenver any adverse trends.

This local indication is adequate to detect significant leakage through the double check valve arrangement. In addition, AP&L is evaluating an enhanced temperature indication system, including evaluation of control room indication which will be capable of detecting even very small leak rates.

This evaluation will be incorporated into AP&L's overall response to IE Bulletin 88-05. Prior to 1R9, AP&L will evaluate additional instrumentation both upstream and downstream of both check valves. AP&L has also performed conservative calculations which show that even at leakage rates up to approximately ten times greater than that observed during recent field tests, the pipe is qualified for local temperature rises which occur in the vicinity of the check valves.

6.2 Valve Testing As a part of the AND IST programs, the existing (MU-34A, B, C, and D) and new (MU-66A, B, C, and D) check valves in the HPI line will be individually leak rate tested during each refueling outage. Testing will be performed as prescribed in the ASME Code Section XI, 1980 Edition, Winter 1981 Addenda Section IWV 3420. The new and existing check valses will be full flow tested during cold shutdown in accordance with Section IWV 3520. Details of the testing are as outlined below.

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After installation, the new valves and all HDI piping between the HPI bicek valves (CV-1219, 1220, 1227, 1228) and the manual HPI isolation valves (MU-45, A, B, C, & D) will be hydro tested in accordance with applicable ASME ccde requirements.

The new valves will be tested for their ability to deliver the maximum flow rate required to meet the most limiting accident condition. They will also be tested for bacxleakage at a pressure corressending t6 the maximum pressure differential which occurs during unbalanced RCP operation. The maximum permissible leak rate will be less than that flow bounding the thermal design analysis for the HPI lines.

Based on 10CFR50.2, the HP] block valves are the RCS pressure boundary.

A class break could be taken at the MU66s if desired; however, the class break will remain at the block valve (see Figure 2). From a containment penetration and RCS pressure boundary definition standpoint, the MU66s are not RCS pressure boundary. However, for our specific configuration, the designation sf the MU66s as a functional pressure boundary with commensurate testing is of benefit. An RCS pressure test of both valves (MU34 and MU66) will be performed to verify the functional integrity of the piping configuration as part of the post-modification test plan.

A review of 30CFR50, Appendix J was performed to ensure the adequacy of the containment leakage rate testing for these HPI penetrations. The results of the assessment confirmed that a Type "A" leakage rate test is the only test required. This conclusion is based on the fact that HPI piping will remain flooded post-accident and the system will not provide a direct path for inside and outside atmospheres to communicate. The existing MU-34 check valves have been subjected to Type "A" testing (with the most receat CILRT being performed during the last refueling outage). The new MU-65 check valves will be periodically tested in the same manner beginning with the next schedulsd CILRT.

With respect to special testing requirements as specified in Section IV, paragraph A of Appendix J, the post-modification ASME hydrotest that will be performed will satisfy the requirement for a pre-operational Type "A" leakage rate test. The hydrotest will be performed at a pressure > 2370 psi which will more than adequately test the integrity of the piping rystem and associated welds with respect to leak tightness. A visual exam is coupled with the performance of the hydrotest and the results quantifying the leakage, if any, will be reported in a revision to the November 1988 ANO-1 CILRT Report.

NRC has raised questions concerning IST for the LPI and CFT check valves.

Inservice tasting for DH-13 A&B, DH-14, A&B, DH-17, DH-18, and CF-1 A&B consists of opening one of the CF-1, DH-13, DH-17 or 18, and DH-14 valves at each refueling outage to demonstrate freedom of motion of the disk throrgh its full range of travel by manually stroking thc valve and inspecting the valve internals. Pressure and level instrumentation is monitored to ensure the absence of significant beckleakage threngh these check valves.

Measurement of this backleakaga is performed after decay heat operation and shoc1d allow any significant backleakage to be detected.

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7.0 DESIGN BASIS REVIEWS As part.of the AP&L evaluation of the HPI backflow concern, substantial reviews and system design basis evaluations are being performed. Field walkdowns have been performed on much of the piping system, limiting design requirements were reconstituted or verified, and operational interfaces were reviewed with operations personnel to determine that all normal and emergency operating modes were addressed in the review of the system.

Several areas deserve additional detailed discussion and are provided with tnis section.

7.1 Containment Isolation The HPI penetrations have been reviewed against GDC 55 with particular attention to the requirements for:

1) Locating isolation valves outside containment as close as practical to-the containment, and

2) Providing other appropriate requirements to minimize the' probability or consequences of an accidental rupture of the lines.

The length of the crossover lines appears to have been minimized in the 1979 modification. The original location of the MOV block valves was not changed by the addition of the crossover lines.- For the crossover line to be effective, it had to be located inboard of the motor-operated block valves.

The limited number of containment penetrations and the normal makeup flow path were factors in the decision to place the crossover line outside containment. With classification of all piping from the motor-operated block valve inboard as ASME Section III Class 1 (requiring that the piping be included in the ISI program), and establishment of periodic testing requirements for the check valves, item 2 above is addressed.

It should be noted that with the addition of the second check valve, the ASME Class I to Class II break could be taken at the new check valve; however, we feel the intent of the GDC is better served by maintaining Section III, Class 1, for al. piping inboard of the HPI block valve. The enhanced inspection requirements and more detailed design analysis supports the intent of the GDC in addressing piping connected between-the isolation valves.

7.2 System Review In order to determine the extent to which the HPI backflow issue constituted a generic design concern, a review of other RCS connections and other safety significant fluid systems for ANO-1 and ANO-2 was conducted. This review identified potential piping systems that could be subjected to higher pressure and temperature fluids other than the original design considerations due to a failed check or block valve. These piping systems were identified based on a series of valve failures and/or initiating events which could set up a flow path through piping which might not be designed for those conditions.

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- The criteria that was used to determine whether further review of a pipe line would be performed is listed below.

A) For RCS connections and other safety significant fluid systems, assume a single check valve failure plus an additional single failure, cross connects, etc. , that could provide a flow path for hot RCS water into piping that might not be ane'yzed for higher temperature fluid.

B) Sanie as Item 1 except assume double (series) check valve failure with an additional single failure, cross connect, etc. The assumption of single or double check valve failure plus an additional single failure assumption was made considering the possibility of common mode failure mechanisms. ,

C) For RCS connections consider the additional case of a block valve failure with potential resulting flow path (i.e. irrespective of check valve failures). This case was reviewed to confirm proper RCS boundary isolation design.

Only valve failures that could produce an undesirable flow path were further j evaluated. Each identified potential flow path for the RCS and other safety significant systems was evaluated and the effects of the flow path on system ,

operations and safety was reviewed.

All scenarios reviewed (other than 1 (C) above) required the failure of at  !

least one check valve plus an additional check valve or block valve failure.

In this regard therefore, the assumptions are beyond basic design / safety requirements since double or triple failures are not typically postulated and common mode failure mechanisms have not been identified; nonetheless, the design applications were conservatively evaluated. Preliminary results ,

indicate that in all cases, the consequences of the failures were within the '

design basis and/or safety analysis; hence adequate safety margins exist in the current design.

Concerning item 1(C), no other single failures of block valves were identified which could lead to a similar backflow or high temperature / low  :

temperature design concern. i 1

7.3 Other HPI System Operating Modes  ;

l In addition to the normal makeup and emergency high pressure injection modes of operation, the HPI/LPI " piggyback" operating mode is also being .

re-evaluated to ensure its design and functional capabilities under various anticipated operational conditions. The reviews indicate that certain sizes of small break LOCAs have the potential for requiring the piggyback )

operation for long-term core cooling. Consequently, AP&L has conservatively l chosen to bound the conditions under which the piggyback mode might be required. Piping and components are being evaluated to assure their j capabilities to perform their required functions when subjected to these bounding conditions. Modifications will be made as necessary to ensure this required level of performance capability.

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4 7.4 SAR Revisions The addition of the check valves in the injection lines will restore the HPI system to the general design configuration intended by the oriG inal design.

The potential backflow through a failed single check valve was not assumed in the original 1979 small break LOCA modification project, and therefore was not mentioned in the SAR; consequently, the existing SAR descriptions are inaccurate or inadequate in several areas.

The text in the SAR will be revised to more clearly define the design basis of the HPI system and describe how the system configuration addresses the RCS pressure boundary and GDC requirements. The HPI single failure summary in the SAR will be revised to cover a single failure of one of the check valves in series.

The following sections of the SAR will be revised:

Table 6-3, page 6.7-5 Table 6-4, page 6.7-7 Section 9.1.2.5., paragraph 4 Section 6.1.1.2.4.1 Section 6.2.2.4.1 Table 5-1, pages 5.5-1 thru 5.5-5 Figures 5-6, 6-1, 6-2, 7-20 and 9-3 i

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l TABLE 1 HPI LOOP BC PIPING STRESS

SUMMARY

FOR CONSERVATIVE 545 TEMPERATURE CASE Item Component Equation 12 (NB-3650) Allowable No. (Data Pt.) Stresses Stresses (PSI) 1 Elbow 92062 53115 (B-19) 2 Elbow 74661 53115 (B-22) 3 Elbow 69169 53115 (B-50) 4 Elbow 76396 53115 (B-200) 5 Elbow 57492 53115 ,

(C-34) 6 Tee 78619 53115 (BC-170) 7 Elbow 58827 53115 (BC-180) 8 Valve Weld Not overstressed - replaced for (BC-203) construction convenience

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g n eao a eet 7 8 9 9 e t vg s t va 0 0 0 1 n v "A f r oeD / / / / i e " ak e e PR 4 2 6 8 r ch v 0 1 0 0 u f o eet e d o t l h r e

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FIGURE 1 POTENTIAL HPI BACKFLOW PATHS OPEg1NG A B C D FLOV PATHS *

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