LICENSEE EVENT REPORT (LER)

fACIUTYNNAE (I)

Diablo Can on Unit 1

DOCKET MPASER0500027517Failure to Implement Residual Heat Removal System Overpressure Protection Requirements Dueto Insufficient Desi n Basis Information

DAY YRLER MPASER

SEQVENllALMAISER REVIQONMPA8ER

REPORT DATE

YR fACIUTYNAMESOTHER PACIUTIES ENOLVED 4

DOCKET MPASER (S)

29 94 94 0 1 5 - 0 1 2 95Diablo Can onUnit2 0 5 0 0 0 3 2 3

0 5 0 0 0TIES REPORT IS SV(PETTED PVRSVENT TO T)% REOIPREMENIS Of 10 CfR: (11)

POWERLEVEL

00)

100X 16CFR 50.738 2 v B

OTHER-

(Spoclfy In Abstract below and In text, NRC Form 366A)

UCENEEE CONTACT fOR TIES LER II

David P. Sisk- Senior Re ulato Com liance En ineerCOMPLETE ONE UNE FOR EACH COMPONENT FAILUREDESCRI8ED IN THIS REPORT 13

TELEPHONE MPESER

805 545-4420REPORTASLE

TO NEEDS

SVPPLEAAENTALREPORT EXPECTED (IA)

I I YES (Ifyes, complete EXPECTED SUBMISSION DATE) I X I NoASSTRACT (I4)

EXPECTEDSUBMISSION

DATE (15)

On June 29, 1994, with Units 1 and 2 in Mode 1 (Power Operation), PG8 E determined thatoperation of the residual heat removal (RHR) system with flow less than 3000 gpm inMode 4 (Hot Shutdown) and 5 (Cold Shutdown - with loops filled) may not meet ASMECode design basis overpressure protection requirements for the RHR heat exchangersduring postulated pressure transients. At 1236 PDT, a four-hour, non-emergency reportwas made to the NRC in accordance with 10 CFR 50.72(b)(2)(iii)(B).

On October 14, 1994, PG&E personnel determined that the design pressure of the RHRpumps could also be exceeded.

The root cause of this condition was insufficient information provided in design basisdocumentation, combined with inattention to detail regarding the design pressure of theRHR system heat exchangers and pumps.

An engineering review of the design and operation of the RHR system has been performedto ensure ASME Code overpressure protection requirements are met. A design change willbe issued to increase the design pressure rating of the RHR system components to ensurethat they comply with requirements for maximum anticipated pressure in the RHR systemfor normal and overpressure transient events. No physical modiTications of the RHRsystem will be required. PG8 E has also requested the NSSS vendor to clarify theoverpressure design basis requirements.

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Diablo Can on Unit 1TEXT(ll)

0500027594-015-0127Plant Conditions

Units 1 and 2 have operated in Modes 4 (Hot Shutdown) and 5 (Cold Shutdown - with loopsfilled) with the potential for the condition described below.

Descri tion of Problem

A. Summary

On June 29, 1994, PG&E determined that operation of the residual heat removalsystem (RHR) (BP) with a flow less than 3000 gpm in Modes 4 (Hot Shutdown) and 5

(Cold Shutdown - with loops filled) may not meet the American Society of MechanicalEngineers (ASME) Code design basis overpressure protection requirements duringpostulated pressure transients. At 1236 PDT, a four-hour, non-emergency report wasmade to the NRC in accordance with 10 CFR 50.72(b)(2)(iii)(B).

On October 14, 1994, PG&E personnel determined that the design pressure of theRHR pumps could also be exceeded during a postulated overpressure transientevent. A preliminary operability assessment was approved on October 15, 1994.

On October 21, 1994, a final operability assessment was approved for continuedoperation of the RHR sy's tem without the need for compensatory measures. Theoperability assessment approved RHR system operation with flow as low as the TSminimum flow of 1300 gpm. The operability assessment will remain in effect until theaffected RHR system component design pressures are increased by issuance of aformal plant design change.

Background

The RHR heat exchanger tube side is constructed to ASME Section III, Class C, 1968,as described in the Final Safety Analysis Report (FSAR) Update, Section 5.5.6,"Residual Heat Removal System." The relief valve located at the RHR pump suctionline is provided to comply with ASME Code requirements as described in FSARUpdate, Section 5.5.6.3.3, "Overpressure Protection," and is consistent with therequirements of Standard Review Plan Section 5.4.7, "Residual Heat Removal (RHR)System," criteria, detailed in "Branch Technical Position RSB 5-1, DesignRequirements of the Residual Heat Removal System."

RSB 5-1, Branch Position C., "Pressure Relief Requirements," indicates the RHRsystem shall satisfy the following: to protect the RHR system against accidentaloverpressurization when it is in operation (not isolated from the RCS), pressure reliefof the RHR system shall be provided in accordance with the ASME Boiler andPressure Vessel Code. The most limiting pressure transient during the plantoperating condition when the RHR system is not isolated from the RCS shall beconsidered when selecting the pressure relieving capacity of the RHR system. Forexample, the design basis should consider shutdown cooling in a PWR with no steam

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bubble in the pressurizer (water solid), coincident with inadvertent operation of anadditional charging pump.

The RHR pressure relief requirements are specified in ASME Section III, 1968,Article 9, "Protection Against Overpressure," Section N-910.3, that requires thepressure relief devices shall be sufficient to prevent a rise in pressure within thevessel they protect of to no more than 10% above the vessel's design pressure.

C. Event Description

On May 1, 1994, with Unit 1 in Mode 5, reactor coolant system (RCS) at 350 psig,190 degrees Fahrenheit and the pressurizer water solid, the pressure rapidlyincreased due to a charging flow increase to the RCS during the performance of aroutine charging system vent. The event resulted in the rapid increase in RCS andRHR pressure. Plant operators responded to alarms in the control room and isolatedthe source of the additional charging, and the pressure transient was terminatedbelow the actuation setpoint (RCS pressure of 435 psig) of the low temperatureoverpressure protection (LTOP) system. However, an RHR system pressure highalarm with a setpoint of 600 psig was received during the transient (the estimatedtransientmaximum pressurewas605psiginthe RHRsystem). Althoughtheactualpressure transient was terminated below the LTOP actuation pressure, an evaluationwas initiated to further investigate implications of RHR pressure transients.

On June 29, 1994, additional engineering calculations performed for the RHRsystem during pressure transient conditions determined that at flows less than3000 gpm, design basis overpressure protection limits at the RHR heat exchangermay not be met.

On June 29, 1994, at 1236 PDT, a four-hour, non-emergency report was madein accordance with 10 CFR 50.72(b)(2)(iii)(B). An Operations Shift Order wasissued providing RHR system flow restrictions for heat exchangeroverpressure protection requirements.

A preliminary engineering evaluation and communications with the equipmentvendor identified sufficient margin in the RHR heat exchangers such that theexisting equipment could be reanalyzed to a higher design pressure toalleviate the need for the flow restrictions provided in the shift order. A designchange was initiated to accomplish this work.

During preparation of the design change documentation on October 14, 1994, it wasdetermined that operation of the RHR system may also result in pressures exceedingthe design pressure of the RHR pumps. An initial operability assessment wasperformed that concluded the RHR system remained operable based on preliminaryevaluations performed by the Nuclear Steam Supply System (NSSS) vendor(Westinghouse) identifying sufficient margin in the pump design.

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On October 21, 1994, a final operability assessment was issued addressing theoverpressure concern. This operability assessment will remain in effect until designbasis documentation is revised to raise the maximum design pressure of the affectedRHR system components such that no new flow restrictions on RHR system operationare required. No physical modifications of the RHR system will be required.

D. Inoperable Structures, Components, or Systems that Contributed to the EventJ

None.

E. Dates and Approximate Times for Major Occurrences

1. May 1, 1994:

2. June 29, 1994:

A momentary RCS pressure transient occurred.

Event/Discovery Date: Engineering calculationsconclude that ASME Code relief protectionrequirements may not be met during RHR lowflow operation.

3. June 29, 1994, at 1236 PDT: A four-hour, non-emergency report was made to'he

NRC in accordance with 10 CFR50.72(b)(2)(iii)(B).

4. October 14, 1994 An engineering review concluded that operationof the RHR system in Modes 4 and 5 mayexceed the design pressure of the RHR pumps.

F. Other Systems or Secondary Functions Affected

None

G. Method of Discovery

As a result of investigations performed regarding the May 1, 1994, RCS pressuretransient, PG&E personnel performed detailed pressure calculations for the RHRsystem which determined that design basis pressures for the RHR heat exchangercould be exceeded for postulated pressure transients in Modes 4 and 5. Furtherreview of the RHR system identified a similar concern for the RHR pumps.

H. Operator Actions

None.

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FACNl(YNAME(1)

Diablo Can on Unit 1YEXY(1))

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I. Safety System Responses

None.

III. Cause of the Problem

A. Immediate Cause.

The immediate cause of this problem was that the ASME overpressure protectionrequirements were met for all operating conditions.

B. Root Cause

The root cause of this condition was insufficient design basis information for the RHRsystem and overpressure protection relief requirements during normal and postulatedtransient conditions in Modes 4 and 5 (with loops filled). The level of detail providedin the NSSS vendor design basis documents was inadequate regarding ASME Codeoverpressure requirements. As a result, adequate design pressure bases were notestablished to ensure RHR overpressure protection requirements were met. I

Anal sis of the Event

The most limiting overpressure transient is postulated to occur during shutdown conditionswith no steam bubble in the pressurizer (water solid conditions) when RHR flow is1300 gpm per pump (minimum flow per TS 4.9.8.1.2), and inadvertent operation ofadditional charging injection could cause a rapid rise in RCS pressure for which Codepressure relief requirements are applicable.

PG&E conservatively determined the maximum pressures attainable during both a limitingpressure transient and during normal operation. The maximum pressure that the RHR heatexchangers and pumps can experience during an overpressure transient was calculated tobe approximately 687 psig and 698 psig, respectively. These pressures are based on (1)the RHR pump suction relief valve 8707 liftsetpoint plus accumulation, (2) elevationdifferences between the relief valve and the RHR system components, (3) the developedhead of the RHR pumps at the TS minimum flow of 1300 gpm, and (4) the frictional pipinglosses between the RHR pump and heat exchanger.

The maximum pressure that the heat exchangers and pumps could experience duringnormal operation is approximately 612 psig and 623 psig, respectively.

These pressures exceed the current design pressure of 600 psig and do not meet ASMECode overpressurization requirements. However, these maximum postulated pressures arewithin the inservice test pressure (750 psig), the hydro test pressure (900 psig), and wellwithin the strength of materials such that postulated damage would not occur. I

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PG8 E and Westinghouse have performed analyses showing that the existing RHR systemcomponents, without physical modifications are adequate to with'stand the maximumachievable operating pressures during both normal and transient overpressure conditions.

Therefore, the health and safety of the public were not adversely affected by this event.

V. Corrective Actions

A. Immediate Corrective Actions

1. An operation incident summary was issued to all plant operators regarding theMay 1, 1994, event to inform operating personnel of the pressure transient andthe need to prevent operational challenges to systems.

An operations shift order was issued to all plant operators regarding theengineering evaluation of the RHR system and need to assure adequate flowto protect the heat exchanger from exceeding its maximum design pressure inModes 4 and 5 (with RCS loops filled).

B. Corrective Actions to Prevent Recurrence

1. PG&E engineering will revise the RHR design criteria memorandum (DCM) toinclude requirements regarding overpressure protection of the system duringModes 4 and 5 (with RCS loops filled).

2. PG&E operations will review and revise plant operating procedures asnecessary to assure RHR design basis overpressure protection requirementsare satisfied.

3. An FSAR Update revision willbe made to provide the design and operationalbasis regarding ASME Code overpressure protection requirements. I

4. PG&E engineering has reviewed the design bases and operatingconfigurations to ensure RHR overpressure protection requirements are met.

5. PG&E has formally requested Westinghouse to provide information to utilitiesregarding the effect of RHR flow on overpressure protection requirements.

6. PG&E will finalize the evaluations for the RHR system components and issuea design change to increase the design pressure of the RHR systemcomponents such that all applicable operational and Code requirements aremet.

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Vl. Additional Information

A. Failed Components

None.

B. Previous LERs on Similar Problems

LER 1-93-008 (Voluntary), "BlockValves Installed on Inlet/Discharge Side ofOverpressure Protection Devices Due to Vendor Design Deficiency," identified acondition where manual block valves had been provided in the pressure protectionflow stream contrary to Code requirements. The corrective actions for this LERincluded transmission of information to the original supplier and submission of a reliefrequest for the specific equipment.

As a result of LER 1-93-008 and other considerations, PG&E management initiatedadditional Code training for system engineers and reviewers involved with Codeissues as a process enhancement. This ongoing training willadd to the heightenedawareness of PG8 E personnel to a broad range of Code requirements.

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