ML20203A658

Safety Evaluation Supporting Amend 225 to License DPR-53

ML20203A658
Person / Time
Site:	Calvert Cliffs
Issue date:	02/10/1998
From:	NRC (Affiliation Not Assigned)
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ML20203A650	List: ML20203A646 ML20203A658
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NUDOCS 9802240089
Download: ML20203A658 (8)

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UNITED STATES

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(. j NUCLEAR REGULATORY COMMISSION s

I WASHINGTON, D.C. BeteHopi SAFETY EVALUATION BY THE OFFICE OF' NUCLEAR REACTOR REGUl.ATION RELATED TO AMENDMENT NO.2?STO FACILITY OPERATING LICENSE NO. DPR 53 BALTIMORE GAS AND ELECTRIC COMPANY CALVERT CLIFFS NUCLEAR POWER PLANT. UNIT No.1 DOCKET No. 50-317

1.0 INTRODUCTION

By letter dated May 16,1997, as supplemented November 14,1997, Baltimore Gas and Electric Company (BGE), the licensee for the Calver1 Cliffs Nuclear Power Plant, requested an i

amendment to Operating License No. DPR 53 to implement a modification which constitutes an unreviewed safety question as described in Title 10 to the Code of Federal Reoulations (CI'R) 50.59,' Changes, Tests, and Experiments." The proposed modification involves splacing the Unit 1 service water (SRW) heat exchangers with new plate and frame heat exen. angers (PHEs) in order to significant!, improve the thermal performance of the SRW System. A flow control scheme would be added to throttle saltwater flow to the SRW heat exchangers and the associated bypass lines. Automatic flusning strainers would be added to the Saltwater System (SW) upstream of each SRW heat exchanger. The saltwater and SRW piping configurations would be modified to accommodate the new coraponents. The licensee found that the flow control valves and the eutomatic flushing strainers are new active components in the SW that could introduce the potential for malfunctions of a different type than any evaluated previously in the Updated Final Safety Analysis Report (UFSAR). The proposed modification also includes removing one containment air cooler (CAC) from service to alicw operating with one of two PHEs on a subsystem. The November 14,1997, letter provided clarifying information that did not change the initial proposed no significant hazards consideration determination.

2.0 BACKGROUNR The SW is un open loop system, which utilizes the Chesapeake Bay as the supply source (ultimate heat sink), it consists of two subsystems that provide SW to cool the SRW heat exchangers, Component Cooling (CC) System heat exchangers, and the Emergency Core Cooling System pump room air coolers. During normal operation, both subsystems are in service with one pump on each subsystem, and a third pump in standby that can supply either subsystem. SW flow through the SRW and CC heat exchangers is throttled to provide sufficient cooling to the heat exchangers, and to maintain total subsystem flow to prevent pump runout.

Following a loss-of coolant accident (LOCA), the SW system has two phases, which include pre-and post Recirculation Actuation Signal (RAS). Each subsystem can satisfy the design heat removal requirements during both phases of the accident. During pre-RAS, a Safety injection Actuation Signal (SlAS) automatically reconfigures each SW subsystem to fully open the SRW heat exchanger SW outlet valves. An SIAS will permit the ECCS pump room air coolers to be cooled by SW if the room temperatures exceed the designated limits, and it will automatically isolate the SW flow to the CC heat exchangers. On an RAS, the SW outlet valves on the CC and SRW heat exchangers return to their pre accident positions, and the TDCS pump room air

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2 coolers continue to operate. During post RAS, an operator has to remotely throttle the SW outlet valves for the CC and SRW heat exchangers to maintain system design temperatures.

The SRW System is a closed loop system, which utilizes plant domineralized water that is treated with a corrosion inhibitor, it consists of two subsystems that remove heat from various turbine plant components, a blowdown recovery hes.t exchanger, CACs, apent fuel pool cooling heat exchangers, and diesel generator (DG) heat exchangers. During nomial operation, both sub9ystems are in service and fully redundant to assure the safe operation and shutdown of the plant, assuming a single failure. The SRW supply temperature is maintained at 5 95 degrees F for normal operation and 5105 degrees F during accident conditions. During a LOCA, each subsystem supplies hvo CACs to support the cooldown of containment, and the No.12 SRW Subsystem cools the No.1B DG to ensure continued reliable operation of the DG as an emergency power supply. The non safety-related portion of the SRW System supplies components in the turbine building, and the piping is automatica!'y isolated during a LOCA.

In response to Generic Letter 89-13," Service Water System Problems Affecting Safety.Related Equipment

• BGE performed baseline thermal performance 16ms on the SRW heat exchangers in 1993. During those tests, the licensee found that the heat removed by the SRW heat exchangers was less than expected. BGE imposed strict cleaning and bulleting requirements on the heat exchangers to compensate for the reduction in the heat removal capability. Also, flow controllers were installed on the SRW inlet valves for the CACs to throttle SRW flow during a LOCA to reduce the SRW System heat load. Despite the attdition of the flow controllers, operation at higher ultimate heat sink temperatures still required frequent heat exchanger cleaning to maintain the necessary heat removal capability.

During the spring 1994 Unit i refueling outage, the licensee also found that the existing SRW heat exchangers were susceptible to erosion and/or corrosion under normal operating conditions.

There were 140 tubes replaced in the No.11 SRW heat exchanger during the outage, which were p!ugged previously due to leakage. The licensee discovered severe tube wall thinning in the first 3 to 4 inches of the inlet end of the tubes, which was apparently caused by erosion and/or corrosion on the tube side. Similar darnage was also identified on the No. 21 SRW heat exchanger. The licensee temporarily installed sleeves on the inlet end of the tubes. As a result of the erosion and/or corrosion discovery and the reduction in heat removal capability for the SRW heat exchangers, BGE proposes the modification to replace the SRW heat exchangers with the new plate and frame heat exchangers to alleviate these problems.

3.0 EVALUATlqu 3.1 Prorosed Modification in BGE's proposed modification, the two existing shell and tube SRW heat exchangers would be replaced with four new PKEs that have an increased thermal performance capability. The PHEs are constructed with titanium plates and Ethylene Propylene Diene Monomer (EPDM) gaskets, which would deter the erosion and/or corrosion problem that has damaged the existing heat exchangers. The remaining components consist of carbon steel, which would not be directly exposed to the SW. Additional titanium plates could be added to further increase the thermal performance of the PHEs. Two PHEs would operate in parallel on each of the two SRW subsystems. Each sub,ystem would be redundant and capable of remuving the accident heat load from two CACs and a DG at SW supply temperaturer of 5 90 degree: F while maintaining SRW within its design limit, A flow control scheme would be added to throttle saltwater flow to

3-the SRW heat exchangers and the associated bypass lines. The saltwater and SRW piping configurations would be modified to accommodate the new components.

The proposed modification includes SW strainers that would be installed upstream of each PHE (two strainers per subsystem) to remove debris and minimlas macrofouling in the host exchangers. Each strainer has an automatic flushing function that consists of a flushing valve and a flow diverter, which are regulated by a control asserr.bly. A full port ball valve would be used for the flushing valve to provide for minimal obstructicn of debris removal. During normal strainer operatior,, SW would enter a strainer basket with the flow diverter open and the flushing valve closed. SW would pass through an inlet section and be forced through a screen basket before it passes through an outist. The strainers would be flushed every 60 minutes initially, then the internal could be adjusted based on operating experience with the strainers. Flushing would o.: cur automatically on high strainer differential pressure (dP). Strainer flushing or regeneration would occur in two stages. First, the flushing valve would open to commence the regeneration cycle, and the total flow through the strainer filter would increase. The increased flow would loosen the debris inside the strainer basket, which would be washed through the flushing valve to the SW discharge header downstream of the heat exchangers. For the second stage, the flow diverter would close while the flushing valve remains open, so the flow would be forced through the basket at the strainer inlet. Most of the flow would exit through the main outlet, but some would be diverted to a debris collection section. Any distodged remnants would be discharged through the flushing valve. The total flushing cycle would last for approxima'ely 80 seconds.

Any large iterns could be removed through an 8 inch handhole on the cover. The strainers on each subsystem would be interlocked to limit the regeneration cycle to perform on one strainer at a time. A manual SW isolation valve would be provided upstream of each strainer to allow for isolation of any selected strainer and PHE combination.

The proposed modification would replace the two existing SRW heat exchanger SW outlet control valves with six control valves. One control valve would be at the outlet of each of the four PHEs and one control valve would be on each of the two bypass lines. The SW outlet control valves would utilize a flow element, a flow indicating controller (FIC), a valve positioner, soleno'd valves, and instrument air valves to modulate the flow at a predetermined flow setting. The bypass line control valves would have similar valve position controls, except the position of these valves would be automatically controlled by a pressure indicating controller (PIC) to maintain SW header pressure in a pre established band. The positions of these valves would be determined by the associated FIC flow setting or Plc pressure setting. In the existing system, the SRW heat exchanger SW outlet valves go to the fully opu position on an SIAS, and retum to their pre-accident throttled position on an RAS. Under the proposed modification, these Engineered Safety Features Actuation Signals (ESFASs) would be eliminated since the flow between the minimum and maxir.um values would be automatically controlled. The staff agrees with the licensee's conclusion, and finds it acceptable to eliminate the ESFAS signals.

Control room annunciation and indication to alert the operators to high PHE outlet temperatures was also included in the proposed modification. A PHE trouble alarm would be generated for high PHE SW dP, high strainer dP. abnormal strainer flushing cycle, low SW flow, or strainer mode controlin manual. The high dP alarms would be installed to alert operators to a debris build-up or a failure of the regeneration cycle for the strainers. A local control station would be provided for indication, annunciation, and to override and take local operation of the regeneration controller.

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4 3.2 Potential Component Malfunctions Not ConsMered in UFSAR The licensee's proposed modification locludes new components that introduce the potentia *. for malfunctions that were not previously considered in the UFSAR These components include sutomatic flushing streiners and control valves. Operater actions would not t.: required to manipulate these components.

3.2.1 Automatic Flushino Strair-3 The licensee identified six potential failure modes for the strainers, which are the failure of the pressure boundary, the clogging of the strainer, the flushing valve remaining open, the flushina valve remainirig closed, the divtriar failing open, and the diverter failing closed. The "r &

justification foi these failures are as follows:

a.

Pressure Boundary Failure Since the strainer is designed and manufactured to the same codes and standards as the other system pressure boundary components, the licensee conduded that the probability of a failure of the pressure boundary we uld be no different from tt.

.,rtions of the system already evaluated in the UFSAR.

b.

Clogging of the Strainer The strainer is designed to flush automatically on a preset timing cycle and on high strainer dF. If the e.itomatic flushing cycie failed, the af'ected strainer would eventually reach its dP limit and generate a control roon) alarm. The associated PHE SW outlet control valve would cpen further to compensate for the c!cg in the strainer, and the low SW flow alarm setpoint would eventually be reached. This scenario would allow sufficient time for the licensee to investigate a id implement corrective actions. A handhole on the strainer cover plate would allow the licensee to inspect the stralacr intemcis and perform a manual cleaning. If necessary. If the clogged strainer adversely affected the subsystem operation, the licensee would deenegize the controls for the failed skainer a 51 initiate manual flushes c' the unaffected strainer, or allow the unaff*cted strainer to resume its automatic flushing soquence.

c.

Flushing Valve Rcmains Open The flushing valve is designed to fail clos # on a loss of power or air, if the flushing valve fa'!s to close, the affected strainer would continue to flush and remain relatively clean. As a result, an abnormal strainer flushing cycle alarm w culd be generated to prompt the licensee to investigate. Assuming no operator actior', an irnerlock between the twc strainers within the cubsystem would prevent flushing of the unafDeted strainer, which could eventually cause the unaffected strainer to reavi its dP limd and alarm setpoint. The associated FHE SW outlet control valve would open further to compensatt for the clog in the strainer, and the low SW flow alarm semoint would eventually be reach 6d. Both PHEs would continue to remove their design basis heat load until the heet exchanger low flow setpcint was reached due to the gradual clogging of the strainer. The PHE may remain functional with reduced few rate depending upon bay temperature and/or accidant conditions. This scenario would be a slow developing process thrt would provide suff.m.t time for the licensee to investigate and implernent corrective actions as discussed in Section 3.2.1.b.

i 5-d.

Flushing Valve Remains Closed The flushing valve is designed to fait closed on a loss of power or air. However, if the flushing va!ve fails to open during the flushing cycle, then the abnormal strainer flushing cycle would generate a control room alarm. Also, it may evertually cause the affected strainer to reach its dP limit and alarm setpoint. The PHE SW outlet control valve would open further to compensate for the clogging in the strainer, and eventually the low SW flow alarm setpoint and/or strainer dP alarm setpoint would be reached. The flushing circuit on the unaffected strainer would continue to function. Bcth PHEs would continue to remove

- their design basis heat load until the heat exchanger low flow setpoint was reached on the affected side as discussed in Section 3.2,1,c.

e.

Flow Diverter Fails Open The diverter valve is designed to fail open on a loss of power or air. If the diven.,. ails to shut during the flushing cycle, then the abnormal strainer flushing cycle would gs.ierate e control room alarm to alert operators to the condition. This failure would lead to less effective flushes that could lead to more automatically-initiated flushes due to a high strainer I

dP. Eventually this would have the same'effect as :1 flus hing valve failing Josed, but would be slower in reaching the PHE low flow alarm setpoint, f.

Flow Diverter Fails Closed The diverter valve is designed to fail or-en on e loss of power or air, if the diverter fails to.

fully reopen during the fleshing cycle, the abnormal strainer flushing cycle would generate a control room alarm. The number of automatic strainer flushes would increase due to a high dP, Eventusily this failure would have the same effect as a flushing valve falling closed.

Although, the PHE low flow setpoint may be reached soor.er tMn in other failures due to the reduction in the effective strainer area.

3.2.2 Control ygly,g1

. The licensee identified several potential failure modes for the control valves, which include a PHE SW outlet control valve failure and a bypass line control valve failure. If a control valve fa!!ure occurred, the valve would go to its fail safe position. The SW outlet control valves would faiI open on a loss of power or instrument air to ensure continued flow to the heat exchanger.

- The b; pass line control valves would fail closed on a loss of power or instrument air to prevent pump runout, and to allow the PHE SW outlet control valves to maintain flow through the PHEs

. with the FCs.

. If a bypass lin control va'- e failed in the closed position prior to an RAS, then the total SW flow would drop celow its minimum 'ow of 10,000 gpm. The impact on the pump would not be immediate, but its reliability would eventually be affecteo. An operator could increase flow by remotely opening the PHE SW outlet controls valves until an RAS occurs. If a bypass line evntrol valve failure occurred during post-RAS, pump operation would continue with the PHE FICs operating at their normal setpoints and SW header pressure would be within the prescribed limits. The failure to restore thinimum pump flow could eventually lead to a failure of the SW

- pump and subsequent loss of the associated subsystem However, the other SW subsystem would be unaffected and capable of removing the full design accident heat load.

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. If either cae of the PHE SW outlet control valves failed open, the SW flow to the associated PHE would increase, which would improve the components' heat removal capability. The other PHE on the subsystem would continue to operate, and the SW header pressure could still be automatically adjusted by the bypass line control valves. During post RAS, an operator would need to reduce flow through the bypass line in order to achieve the minimum required flow to the CC heat exchanger. The remaining SW subsystem would be unaffected by this failure and remain capable of removing the full design accident heat load.

The licensee concluded that the failure of any control valve into a position other than its fail safe position is highly unlikely, and would require operator error or equipment malfunction, along with instrument air forcing the valvo operator to hold the control valve out of position. This type of failure would not affect the other SW subsystem, which would remain capable of removing the full design accident heat load. The staff finds that no single active failure at any time, and no single pr.sive failure after recirculation from the containment sump would prevent the cafety feature systems 40m fulfilling their design function. Therefore, the staff finds the failure modes analysis perfewed by the licensee to be acceptable.

3.3 System Operation After Modification in a letter to support the proposed modification dated Novembe 14,1997, the licensee stated that the PHEs are designed for a lower minimum required SW wbsystem flow of approximate;y 8,000 gpm for each PHE rather than the minimum shell and tube heat exchanger flow of approximately 16,830 gpm. The reduced flow was based on vendor recommendations to improve the performance of the heat exchangers and to reduce the pressu s drop across the PHEs to provide a closer match to the existing SW pump operaung characteristics. Even with the reduced minimum flow, the total SW flow would be maintained near its existing value by use of the SW bypass line. In the existing SW System, total SW flow ranges between 15,000 and 20,000 gpm during normal operations. The total system flow for tM oroposed modification is between 14,000 and 18,000 gpm. Th a licensee evaluated th3 change in total system flow, and concluded that all components would receive their minimum raquirca flow and pump performance would not be adversely affected. The minimum Tow requirement would maintain sufficient turbulence in the PHF flow passages to minimize rnicrofouling on the PHE plates. The staff agrees with the licensee's conclusion, and finds the change in total system flow to be within the design limits, and is accep'able.

The operation of the SW pumps, tha ECCS pump room air coolers, and the CC heat exchangers v<ould be the same r.s the existir.g system. Either SRW subsystem could be secured for maintenance by cross-connecting the SRW system to the remaining pair of PHEs. In the event of a break in the common SW piping downstream of the v' RW heat exchangers, the e ame altemate flow path that currently exists would be available to prevent a common mode failure.

On a LOCA (pre-RAS), the proposed modification would eliminate the ESFAS signals associated with any SW equipment in the SRW pump room, which includes the SIAS and RAS to the PHE SW outlet control valvas. SW flow to the CC heat exchangers would continue to be isolated upon receipt of an Sl* J. The FICs would continue to maintain the preset flow to the PHds if the PHE SW outlet valves wer: in automatic. The strainers would continue to automatically flush.

The PIC would adjust the bypass valve positions to maintain the SW header pressure withir le established limits. No immediate actions vauld be required during this phase of the accider.

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On an RAS, the CC i eat exchanger outlet valves would retum to their pre RAS positions, and the CC heat exchanger outlet va!ves would be manually throttled to maintain CC outlet temperature. The automatic flow controller in the proposed modification would replace the actionc required by an operator to manually throttle the SW outlet valves in the existing system.

However, if the PHE SW outlet control valves were placed in their fully open position, an operator would be required to retum them to automatic after an RAS.

The licensee concluded that the design, procurement, instal,ation, and testing of the equipment associated with the pioposed modification are consistent with the applicable codes and standards goveming the original systems, structures, and components, and that the SRW System would remain redundant, and separated without any new common-mode failures.

Therefore, the staff finds the proposed moc.fication to be acceptable.

However, the licensee also proposed to operate with one of the two PHEs isolated on a subsystem, while continuing to operate with the CC heat exchanger, and the ECCS pump room j

air coolers on the affe::ted SW Subsystem. A single PHE cannot remove the entire LOCA heat load while maintaining SRW tr mperature within its design limit. As a result, the licensee proposed to remove one CAC from service, so the single PHE could handle the remaining accident heat Icad on the subsystem, which would allow the DG, the remaining Ct.C, the CC heat exchanger, and the ECCS pump room air coolers on the affected subsystem to remain operable when the one PHE is isolated. The staff reviewed the signliicance of removing a CAC from service in order to facilitate operating with one of the PHEs isolated on a subsystem, and found that it was unacceptable.

The staff reviewed the licensee's submittal, and agrees that the implementation of the proposed SRW Mat exchanger modification would provide a significant improvement in the thermal performance capability of the SRW System. Based on the considerations stated above, the staff concludes that the potential for malfunctions of a different type than any evaluated previously in the UFSAR do not preclude the SRW or SW Systems from being within their design limits.

Therefore, the proposed modification is acceptable with the exception of operating with one PHE secured, and removing one CAC from service to enable the affected subsystem to remain operable while the one PHE is secured.

4.0 STATE CONSULTATION

in accordance with the Commission's regulations, the Maryland State official was notified of the proposed issuance of the amendment. The State official had no comments.

5.0 ENVIRONMENTAL CONSIDERATION

The amendment changes a requirement with respect to installation or use J a facility component located within the restricted area as defined !n 10 CFR Part 20. The NRC staff has determined thrt the amendment ir yolves no significant increase in the amounts, and no significant change in the types, of any dhenis that may be released offsite, and that there is no significant increase in individual or cumuk n ;ccupational radiation exposure. The Commission has previously issued a preposed finding that the amendment involves no significant hazards cor,tideration, and there has been no public comment on such finding (62 FR 33118). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmentalimpact statement or enWronmental assessment need be prepared in connection with the issuance of the amendment.

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6.0 CONCLUSION

The Commission has concluded, based on the considerations discussed above, tha;: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conduc;ed in compliance with the Commissior.'s regulations, and (3) the issuance rf the amendment will not be inimical to the common defense and security or to the health anJ Safety of the public.

P.incipalContributors: V Oroaz J. Rajan Date: February 10, 1998