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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 ***
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