United States Nuclear Regulatory Commission - Protecting People and the Environment

ACCESSION #: 9408110043
                       LICENSEE EVENT REPORT (LER)

FACILITY NAME: Diablo Canyon, Unit 1                      PAGE: 1 OF 6

DOCKET NUMBER:  05000275

TITLE:  Failure to Implement Residual Heat Removal System
        Overpressure Protection Requirements Due to Insufficient
        Design Basis Information

EVENT DATE:  6/29/94    LER #: 94-015-00    REPORT DATE:  7/29/94

OTHER FACILITIES INVOLVED:  Diablo Canyon, Unit 2   DOCKET NO:  05000323

OPERATING MODE:  1   POWER LEVEL:  100

THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR
SECTION:
50.73(a)(2)(v)(B)

LICENSEE CONTACT FOR THIS LER:
NAME:  David P. Sisk -  Senior Regulatory   TELEPHONE:  (805) 545-4420
       Compliance Engineer

COMPONENT FAILURE DESCRIPTION:
CAUSE:      SYSTEM:       COMPONENT:       MANUFACTURER:
REPORTABLE NPRDS:

SUPPLEMENTAL REPORT EXPECTED:  NO

ABSTRACT:

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

During an RHR system evaluation, PG&E engineering personnel determined
that a postulated pressure transient could exceed the design pressure
limit required by ASME Section VIII, 1968, UG-125, "Pressure-Relief
Devices," paragraph (c).

The root cause of this condition was insufficient design basis
information provided in design basis documentation.

An engineering review of the design and operation of the RHR system will
be performed to ensure overpressure protection requirements are met.  RHR
system design criteria memorandum will be revised to include
overpressurization design requirements.  Procedures will be reviewed and
revised as necessary to ensure the RHR overpressurization design
requirements are satisfied.  PG&E will request the NSSS vendor to clarify
the overpressure design basis requirements and provide a TS Bases
revision for the RHR system.

END OF ABSTRACT

TEXT                                                          PAGE 2 OF 6

I.   Plant Conditions

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

II.  Description of Problem

     A.   Summary

          On June 29, 1994, PG&E determined that operation of the
          residual heat removal system (RHR) (BP) with a flow less than
          3000 gpm in Mode 4 (Hot Shutdown) and 5 (Cold Shutdown - with
          loops filled) may not meet ASME Code design basis overpressure
          protection limits during postulated pressure transients.  At
          1236 PDT, a four-hour non-emergency report was made to the NRC
          in accordance with 10 CFR 50.72(b)(2)(iii)(B).

     B.   Background

          The RHR heat exchanger is constructed to ASME Section VIII,
          1968, and 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 suction line is provided
          to comply with ASME Code requirements as described in FSAR
          Update Section 5.5.6.3.3, "Overpressure Protection," and is
          consistent with the requirements of Standard Review Plan
          Section 5.4.7, "Residual Heat Removal (RHR) System," criteria,
          detailed in "Branch Technical Position RSB 5-1, Design
          Requirements of the Residual Heat Removal System."

          RSB 5-1, Branch Position C., "Pressure Relief Requirements,"
          indicates the RHR system shall satisfy the following: to
          protect the RHR system against accidental overpressurization
          when it is in operation (not isolated from the RCS), pressure
          relief in the RHR system shall be provided with relieving
          capacity in accordance with the ASME Boiler and Pressure Vessel
          Code.  The most limiting pressure transient during the plant
          operating condition when the RHR system is not isolated from
          the RCS shall be considered when selecting the pressure
          relieving capacity of the RHR system.  For example, during
          shutdown cooling in a PWR with no steam bubble in the
          pressurizer, inadvertent operation of an additional charging
          pump or inadvertent opening of an ECCS accumulator valve should
          be considered in the selection of the design bases.

          The RHR pressure relief requirements are specified in ASME
          Section VIII, 1968, UG-125, "Pressure-Relief Devices,"
          paragraph (c), which indicates all pressure vessels shall be
          protected by pressure relieving devices that will prevent the
          pressure from rising more than 10 percent above the maximum
          allowable working pressure.

          The working pressure of the RHR system is 600 psig and the
          suction relief valve setpoint is established at approximately
          450 psig.  In accordance with Code requirements, the relief
          valve setpoint plus 10 percent accumulation pressure must be

1216S

TEXT                                                          PAGE 3 OF 6

          considered for protection of downstream components, i.e.,
          piping and heat exchanger.  Therefore, Code pressure relief
          requirements must consider the sum of 495 psig at the suction
          relief valve, plus the operating RHR pump pressure increase,
          minus piping line losses, and not exceed a total of 660 psig.

     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 rapidly increased due to a charging
          flow increase to the RCS during the performance of a routine
          charging system vent.  The event resulted in the rapid increase
          in RCS and RHR pressure.  Plant operators responded to alarms
          in the control room and isolated the source of the additional
          charging and the pressure transient was terminated below the
          actuation setpoint (RCS pressure of 450 psig) of the low
          temperature overpressure protection (LTOP) system.  However, an
          RHR system pressure high alarm with a setpoint of 600 psig was
          received during the transient (a maximum of 605 psig in the RHR
          system).  Although the actual pressure transient was terminated
          below the LTOP actuation pressure, an evaluation was initiated
          to further investigate implications of RHR pressure transients.

          On June 29, 1994, additional engineering calculations performed
          for the RHR system during pressure transient conditions
          determined that at flows less than 3000 gpm, design basis
          overpressure protection limits at the RHR heat exchanger may
          not be met.

          On June 29, 1994, at 1236 PDT, a four-hour non-emergency report
          was made in accordance with 10 CFR 50.73(b)(2)(iii)(B).

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

          None.

     E.   Dates and Approximate Times for Major Occurrences

          1.   May 1, 1994:        A momentary RCS pressure transient
                                   occurred.

          2.  June 29, 1994:       Event/Discovery Date: Engineering
                                   calculations conclude that ASME Code
                                   relief protection requirements may not
                                   be met during RHR low flow operation.

          3.  June 29, 1994, at    A four-hour non-emergency report
              was 1236 PDT:        made to the NRC in accordance with
                                   10 CFR 50.72(b)(2)(iii)(B).
1216S

TEXT                                                          PAGE 4 OF 6

     F.   Other Systems or Secondary Functions Affected

          None

     G.   Method of Discovery

          As a result of investigations performed regarding a May 1,
          1994, RCS pressure transient, PG&E personnel performed detailed
          pressure calculations for the RHR system which determined that
          design basis pressures could be exceeded for postulated
          pressure transients in Modes 4 and 5.

     H.   Operator Actions

          None.

     I.   Safety System Responses

          None.

III. Cause of the Problem

     A.   Immediate Cause

          The immediate cause of this problem was inadequate control of
          RHR flow to assure the ASME Code overpressure protection for
          all operating conditions.

     B.   Root Cause

          The root cause of this condition was insufficient design basis
          information for the RHR system overpressure protection relief
          requirements during postulated transient conditions in Modes 4
          and 5 (with loops filled).  The level of detail provided in the
          NSSS vendor design basis documents was inadequate regarding
          ASME Code overpressure requirements.  As a result, procedural
          guidance was not established to ensure RHR overpressure
          protection requirements were met.

IV.  Analysis of the Event

     The most limiting overpressure transient is postulated to occur
     during shutdown conditions with no steam bubble in the pressurizer
     (water solid conditions) when inadvertent operation of additional
     charging injection could cause a rapid rise in RCS pressure for
     which Code pressure relief requirements are applicable and RHR flow
     was maintained at 1300 gpm (minimum flow per TS 4.9.8.1.2).

     PG&E conservatively evaluated a limiting pressure transient with
     1200 gpm RHR flow and determined that the maximum pressure at the
     RHR heat exchanger would be approximately 685 psig.  This pressure
     is above of the 660 psig maximum design limit and does not meet Code
     requirements.  The maximum postulated pressure is within the 125
     percent of the

1216S

TEXT                                                          PAGE 5 OF 6

     inservice test pressure, the 150 percent hydro test pressure, and
     well within the strength of materials such that postulated damage
     would not occur.

     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 the May 1, 1994, event to inform
               operating personnel of the pressure transient and the need
               to prevent operational challenges to systems.

          2.   An operations shift order was issued to all plant
               operators regarding the engineering evaluation of the RHR
               system and need to assure adequate flow to protect the
               heat exchanger from exceeding its maximum design pressure
               in Modes 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) to include requirements regarding
               overpressure protection of the system during Modes 4 and 5
               (with RCS loops filled).

          2.   PG&E operations will review and revise plant operating
               procedures as necessary to assure RHR design basis
               overpressure protection requirements are satisfied.

          3.   A TS 3/4.4.1 bases revision will be made to provide the
               design and operational basis regarding ASME Code
               overpressure protection requirements.

          4.   PG&E engineering will review design bases and operating
               configurations to ensure RHR overpressure protection
               requirements are met.

          5.   PG&E will formally request Westinghouse to provide
               information to utilities regarding the effect of RHR flow
               on overpressure protection requirements.

VI.  Additional Information

     A.   Failed Components

          None.

     B.   Previous LERs on Similar Problems

1216S

TEXT                                                        PAGE 6 OF 6


          LER 1-93-008 (Voluntary), "Block Valves Installed on
          Inlet/Discharge Side of Overpressure Protection Devices Due to
          Vendor Design Deficiency," identified a condition where manual
          block valves had been provided in the pressure protection flow
          stream contrary to Code requirements.  The corrective actions
          for this LER included transmission of information to the
          original supplier and submission of a relief request for the
          specific equipment.

          As a result of LER 1-93-008 and other considerations, PG&E
          management initiated additional Code training for system
          engineers and reviewers involved with Code issues as a process
          enhancement.  This ongoing training will add to the heightened
          awareness of PG&E personnel to a broad range of Code
          requirements.

1216S

ATTACHMENT TO 9408110043                                      PAGE 1 OF 1

Pacific Gas and Electric Company

               77 Beale Street, Room 1451    Gregory M. Rueger
               P.O. Box 770000               Senior Vice President and
               San Francisco, CA 94177       General Manager
               415/973-4684                  Nuclear Power Generation
               Fax 415/973-2313

July 29, 1994

PG&E Letter DCL-94-163

U.S. Nuclear Regulatory Commission
ATTN: Document Control Desk
Washington, D.C. 20555

Docket No. 50-275, OL-DPR-80
Docket No. 50-323, OL-DPR-82
Diablo Canyon Units 1 and 2
Licensee Event Report 1-94-015-00
Failure to Implement Residual Heat Removal System Overpressure Protection
Requirements Due to Insufficient Design Basis Information

Gentlemen:

Pursuant to 10 CFR 50.73(a)(2)(v)(B), PG&E is submitting the enclosed
Licensee Event Report regarding failure to implement residual heat
removal system overpressure protection requirements due to insufficient
design basis information.

This event did not adversely affect the health and safety of the public.

Sincerely,

Gregory M. Rueger

cc:  L. J. Callan
     Mary H. Miller
     Kenneth E. Perkins
     Sheri R. Peterson
     Diablo Distribution
     INPO

Enclosure

DC0-94-EN-N016

1216S/DDM/2246

*** END OF DOCUMENT ***


Page Last Reviewed/Updated Thursday, March 29, 2012