Part 21 Report - 1998-090

ACCESSION #: 9801130101 Duquesne Light Company Beaver Valley Power Station P.O. Box 4 Shippingport, PA 15077-0004 RONALD L. LeGRAND (412) 393-7622 Division Vice President - Fax (412) 393-4905 Nuclear Operations and Plant Manager January 7, 1998 L-98-004 Beaver Valley Power Station, Unit No. 1 Docket No. 50-334 License No. DPR-66 LER 97-039-00 United States Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 In accordance with Appendix A, Beaver Valley Technical Specifications, the following Licensee Event Report is submitted: LER 97-039-00, 10 CFR 50.73(a)(2)(ii) and 10 CFR 50.73(a)(2)(v), "Gas Accumulation in Charging/High Head Safety Injection Pump Piping." R. L. LeGrand KAM/ds Attachment January 7, 1998 L-98-004 Page 2 cc: Mr. H. J. Miller, Regional Administrator United. States Nuclear Regulatory Commission Region 1 475 Allendale Road King of Prussia PA 19406 Mr. D. S. Brinkman BVPS Senior Project Manager United States Nuclear Regulatory Commission Washington, DC 20555 Mr. David M. Kern BVPS Senior, Resident Inspector United States Nuclear Regulatory Commission Mr. J. A. Hultz Ohio Edison Company 76 S. Main Street Akron, OH 44308 Mr. Steven Dumek Centerior Energy Corporation 6670 Beta Drive Mayfield Valley, OH 44143 INPO Records Center 700 Galleria Parkway Atlanta, GA 30339-5957 Mr. Michael P. Murphy Bureau of Radiation Protection Department of Environmental Protection RCSOB-13th Floor P.O. Box 8469 Harrisburg, PA 17105-8469 Manager, Nuclear Licensing and Operations Support Virginia Electric & Power Company 5000 Dominion Blvd. Innsbrook Tech. Center Glen Allen, VA 23060 *** END OF DOCUMENT *** ACCESSION #: 9801130109 LICENSEE EVENT REPORT (LER) FACILITY NAME: Beaver Valley Power Station Unit 1 PAGE: 1 OF 5 DOCKET NUMBER: 05000334 TITLE: Gas Accumulation in Charging/High Head Safety Injection Pump Piping EVENT DATE: 12/08/97 LER #: 97-39-00 REPORT DATE: 01/97/98 OTHER FACILITIES INVOLVED: Beaver Valley Power DOCKET NO: 05000412 Station Unit 2 OPERATING MODE: 5 POWER LEVEL: NA THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR SECTION: 50.73(a)(2)(ii), 50.73(a)(2)(v) & OTHER Part 21 LICENSEE CONTACT FOR THIS LER: NAME: R.D. Hart, Senior Licensing TELEPHONE: (412) 393-5284 Supervisor, Safety and Licensing Department COMPONENT FAILURE DESCRIPTION: CAUSE: SYSTEM: COMPONENT: MANUFACTURER: REPORTABLE NPRDS: SUPPLEMENTAL REPORT EXPECTED: NO EXPECTED SUBMISSION DATE 02/28/98 ABSTRACT: On December 8, 1997 it was determined that Unit 1 and Unit 2 may have operated in a condition that is outside the design basis of the plant. A minimum of one charging/High Head Safety Injection (HHSI) pump may not have been available to provide emergency core cooling as described in the UFSAR due to intrusion of gas into the suction of the pumps, and subsequent gas binding. The determination is based on engineering evaluation of the results of scoping experiments conducted to assess fluid now patterns entering the Unit 1 and Unit 2 charging pumps. The cause of the condition was attributed in part to the design of charging/HHSI pump minimum flow recirculation line orifices. These orifices were found to strip non-condensable gas from the recirculation flow. In the event of a loss of coolant accident concurrent with a loss of offsite power, this design defect could have resulted in a loss of safety function necessary to mitigate accident consequences. Flow orifices are being replaced at Unit 1, and at Unit 2, to reduce the generation of gas bubbles. In addition, increased monitoring and vent system improvements are planned. On December 8, 1997 at 0412 hours, a one hour non-emergency notification of this condition at Unit 1 and Unit 2 was made pursuant to the requirements of 10 CFR 50.72 (b)(1)(ii). The determination that a minimum of one HHSI/charging pump may not have been available to provide emergency core cooling as described in the UFSAR, is considered a condition outside the design bases of the plant and a condition that alone could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. Therefore, this condition is reportable in accordance with 10 CFR 50.72(b)(1)(ii), 10 CFR 50.73(a)(2)(ii), and 10 CFR 50.73(a)(2)(v). The design defect associated with the charging/HHSI pump minimum flow recirculation line orifices could create a substantial safety hazard, and is reportable pursuant to 10 CFR 21 requirements. This report constitutes a 10 CFR 21 notification. The ability to provide emergency core cooling via the charging/HHSI pumps could have been adversely affected and the safety system may not have performed its' safety function. However, there were no actual concurrent failures of redundant charging/HHSI pumps at either unit. Therefore, there were no safety consequences to the health and safety of the public. END OF ABSTRACT TEXT PAGE 2 OF 5 PLANT AND SYSTEM IDENTIFICATION Westinghouse - Pressurized Water Reactor High Pressure Safety Injection System / Pump {BQ/P}* High Pressure Safety Infection System / Orifice {BQ/OR}* * Energy Industry Identification System (EIIS) codes and component function identifier codes appear in the text as {SS/CCC}. IDENTIFICATION OF OCCURRENCE Discovery Date: December 8, 1997 CONDITION PRIOR TO OCCURRENCE Unit 1: Mode 5, N/A Reactor Power (Plant Shutdown for Maintenance and Refueling) Unit 2: Mode 1, 100% Reactor Power DESCRIPTION OF EVENT On December 8, 1997 it was determined that Unit 1 and Unit 2 may have operated in a condition that is outside the design basis of the plant. A minimum of one charging/High Head Safety Injection (HHSI) pump {BQ/P} may not have been available to provide emergency core cooling as described in the UFSAR due to intrusion of gas into the suction of the pumps, and subsequent gas binding. The determination is based on engineering evaluation of the results of scoping experiments conducted to assess fluid flow patterns entering the Unit 1 and Unit 2 charging/HHSI pumps. There were no structures, components or systems that were inoperable at the start of the event and that contributed to the event. There were no automatic or manually initiated safety system responses associated with the event. No operator errors, or procedural deficiencies contributed to the event. Westinghouse Electric Corporation provided the design specifications for the HHSI system, supplied the Unit 1 and Unit 2 multistage, horizontal, centrifugal charging/HHSI pumps, and flow restricting orifices {BQ/OR)s in associated minimum flow recirculation lines (five of six total orifices are affected). Pacific Pumps Division, Dresser Industries, Inc. manufactured the pumps and orifices. The model number for the pumps is 2.5RL-IJ. The orifice part number is B-19049. CAUSE OF CONDITION Two root causes have been identified. The root causes involved design inadequacies and inadequate corrective actions including inadequate questioning attitudes toward past events. The design inadequacies included: 1) a system that generates gas, 2) a system configuration that supports accumulation of gas, and 3) a system that cannot ensure positive venting of existing gas voids. Nuclear Regulatory Commission (NRC) Information Notice 88-23, "Potential for Gas Binding of High-Pressure Safety Injection Pumps During A Loss of Coolant Accident," dated May 12, 1988, alerted licensees to potential problems resulting from hydrogen transport from the volume control tank and accumulation in emergency core cooling system piping. In June of 1988, numerous ultrasonic inspections were performed to locate and quantify hydrogen voids in system piping at both Unit 1 and Unit 2. Based on the data taken and subsequent engineering evaluation, a vent frequency was established to limit void size. Piping to support hydrogen gas venting operations was installed at Unit 1 and Unit 2, in 1989 and 1990 respectively. The inadequate corrective actions included weaknesses in the engineering evaluations performed in response to NRC Information Notice 88-23. In appropriate void fractions were established based upon superficial reviews of previous safety injection actuation signals. Additionally, the vents installed were deficient in that both units lacked the ability to positively verify adequate venting and the Unit 2 configuration presented minimal vent path differential pressure. Subsequent inadequate corrective actions and questioning attitudes were evidenced in response to actual gas voiding events in which causal analysis did not question or address gas void impact on high head safety injection system operation. TEXT PAGE 3 OF 5 ANALYSIS On September 12, 1997 an operational surveillance test showed that Unit 2 charging/HHSI pump 2CHS*P21C could not meet the required performance criteria. The pump was declared inoperable and a spare pump was installed in its place. Upon inspection the pump shaft was found to be cracked. The crack is thought to result from abnormal impact loading on the pump. The abnormal loading is attributed in part to ingestion of gas voids during pump starts. Following this event, a Multi-Disciplined Analysis Team (MDAT) was formed to comprehensively investigate the gas generation phenomenon. Past operating experience at Unit 1 and Unit 2 was reviewed and scoping experiments were conducted to assess fluid flow patterns entering the Unit 1 and Unit 2 charging/HHSI pumps. The MDAT 1) established a design basis void fraction for both units based on scaled model analysis and engineering evaluation, 2) established void generation rates for both units based on an eleven stage orifice, and during steady-state and pump swap operations, 3) established venting frequencies based on both fixed time intervals and operational pump evolutions, and 4) implemented ultrasonic tests to verify void size and to provide positive indication of venting adequacy. On December 8, 1997, it was determined, based on engineering evaluation of the results of the scoping experiments, that Unit 1 and Unit 2 may have operated in a condition that is outside the design basis of the plant. For certain design basis accidents and pump operating alignments, the potential existed for accumulated gas to create a condition in which the charging/HHSI pump flow may have been interrupted or lost due to gas-binding. Under these conditions, it cannot be conclusively stated that flow would be re-established, or that damage to the charging/HHSI pump would not occur such that long term operation could be assured. The results of the evaluation for Unit 1 and Unit 2 are discussed below. The results of the evaluation for Unit 2 indicate that under certain operating alignments, enough gas could accumulate in the suction piping of an individual charging/HHSI pump to interrupt pump suction under all flow conditions. For Unit 1, evaluations of the conditions which could have led to gas binding of the charging/HHSI pumps indicates that this would only be expected to occur under large break loss of coolant accident conditions with a single pump operating. For more than one pump operation and for other accidents which result in lower high head safety injection flow (e.g., small break loss of coolant accident), charging/HHSI pump flows are low enough at Unit 1 to preclude void fractions at the pump suction which would cause gas binding. Introduction of entrained gas into the suction of a centrifugal charging pump can result in a reduction of pump developed head, and effectively increase the required Net Positive Suction Head (NPSH) of the pump. The gas can also result in increased vibration levels and bearing loads. If very large voids are present in the suction piping, the gas voids can result in loss of pump prime. Each charging/HHSI pump was designed with a minimum flow recirculation line to protect the pump. The minimum flow recirculation lines include a flow restricting orifice. The MDAT investigation determined that the orifice strips non-condensable gas from the recirculation flow (except for the restricting orifice associated with the Unit 2 "B" pump which has a different design). Stripped gas is transported by the fluid and accumulates at local high points in stagnant or low flow sections of the charging/HHSI pump suction piping. The flow restricting orifice associated with the Unit 2 "B" charging/HHSI pump has fourteen stages. Flow restricting orifices associated with the other charging/HHSI pumps accomplish pressure reduction in eleven stages. Experience and testing demonstrated that the eleven stage orifice is subject to cavitation and gas generation. The MDAT identified a twenty-two stage orifice that significantly reduces cavitation and virtually eliminates the evolution of gas at all anticipated operating conditions. Replacement of the flow restricting orifice provided in the minimum flow recirculation line for each charging/HHSI pump is expected to reduce the stripling of non-condensable gas from solution. The replacement orifices have more stages. This decreases the pressure drop at each stage. Ultrasonic tests performed to quantify hydrogen voids in charging/HHSI pump suction piping have shown no gas accumulation following orifice replacement. This manual method of monitoring gas accumulation has beep effective thus far in ensuring recently instituted vent limits will not be exceeded. TEXT PAGE 4 OF 5 CORRECTIVE ACTIONS COMPLETED: 1. Twenty-two (22) stage flow restricting orifices have been installed in all three Unit 1 charging/HHSI pump minimum flow recirculation lines. Replacement orifices were installed and operationally accepted by December 12, 1997. 2. Twenty-two (22) stage flow restricting orifices have been installed in the Unit 2 charging/HHSI pump minimum flow recirculation lines associated with the "A" and pumps. Replacement orifices were installed and operationally accepted by January 3, 1998. 3. An acceptable gas void fraction limit has been established for Unit 1 and Unit 2 charging/HHSI pump suction piping. This limit was established on December 19, 1997 for Unit 1, and on December 23, 1997 for Unit 2. 4. Gas accumulation rates and venting frequency have been evaluated to ensure the void fraction at the Unit 1 and Unit 2 charging/HHSI pump suction is maintained below levels established by corrective action 3 above. This action was completed on December 18, 1997 for Unit 1, and on December 22, 1997 for Unit 2. 5. A manual method of providing positive monitoring for gas accumulation was developed to ensure the venting process is maintaining the void fraction at the Unit 1 and Unit 2 charging/HHSI pump suction at levels below that established by corrective action 3 above. This action was completed on December 19, 1997 for Unit 1, and on December 24, 1997 for Unit 2. FUTURE: 6. Based on the charging/HHSI pump suction void fraction limit and gas accumulation rate established by corrective action 3 above, if needed develop a vent system that will maintain gas accumulation below the limit with minimal operator action by June 30, 1998. 7. Develop an effective strategic planning process that assures safety significant issues will be resolved in a timely manner by June 30, 1998. 8. Formalize the MDAT process that was initiated and piloted in response to this event, by June 30, 1998. 9. The flow orifice associated with the "B" charging/HHSI pump at Unit 2 will be replaced prior to the end of the next refueling outage. REPORTABILITY The UFSAR for Unit 1 and Unit 2 assumes that a minimum of one charging/HHSI pump would be available to operate in the event of an accident. Generation and subsequent accumulation of gas bubbles in stagnant charging/HHSI piping could result in both trains of pumps being unable to operate. For certain design basis accidents and pump operating alignments, the potential existed for accumulated gas to create a condition in which the charging/HHSI pump flow may have been lost due to gas-binding. Under these conditions, it cannot be conclusively stated that flow would be re-established, or that damage to the charging/HHSI pump would not occur such that long term operation could be assured. Thus, the ability to provide emergency core cooling via the charging/HHSI pumps could have been adversely affected and the safety system may not have performed its' safety function for the postulated accident. On December 8, 1997 at 0412 hours, a one hour non-emergency notification of this condition at Unit 1 and Unit 2 was made pursuant to the requirements, of 10 CFR 50.72 (b)(1)(ii). The determination that a minimum of one HHSI/charging pump may not have been available to provide emergency core cooling as described in the UFSAR, is considered a condition outside the design bases of the plant and a condition that alone could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. Therefore, this condition is reportable in accordance with 10 CFR 50.72(b)(1)(ii), 10 CFR 50.73(a)(2)(ii), and 10 CFR 50.73(a)(2)(v). TEXT PAGE 5 OF 5 The original design of the charging/HHSI pump minimum flow recirculation line orifices did not minimize the generation of noncondensable gas bubbles. This design defect could have resulted in a loss of safety function necessary to mitigate the consequences of an accident. Therefore this condition could create a substantial safety hazard, and is reportable pursuant to 10 CFR 21 requirements. This report also constitutes a 10 CFR 21 notification. SAFETY IMPLICATIONS For certain design basis accidents and pump Operating alignments, the potential existed for accumulated gas to create a condition in which the charging/HHSI pump flow may have been lost due to gas-binding. Therefore, the ability to provide emergency core cooling via the charging/HHSI pumps could have been adversely affected and the safety system may not have performed its' safety function. However, emergency procedures are in place to respond to this condition. Based on the operating alignments for the Unit 2 charging/HHSI pumps, a conservative estimate indicates that approximate 2.7 percent of the total operating time since 1992 the potential existed for gas accumulation to occur at a level which would potentially cause gas binding of the pumps. This time could be further reduced as the charging/HHSI pumps were most vulnerable during the time interval until the initial vent following pump swap when gas accumulation in the stagnant pump suction piping was most significant. Evaluation of the loss of Unit 1 charging/HHSI pump flow during a large break loss of coolant accident with credit taken for the actual performance of the low head safety injection pumps and the resulting injection flows indicates that the peak clad temperature limits could be met when other margins are considered. These include such things as margin in peaking factors, and steam generator tube plugging levels. There were no actual concurrent failures of redundant charging/HHSI pumps at either unit. Therefore, there were no actual safety consequences to the health and safety of the public. PREVIOUS SIMILAR EVENTS A review of Licensee Event Reports for Beaver Valley Unit 1 and Unit 2 did not identify any similar events within the past two years. *** END OF DOCUMENT ***

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