Information Notice 1993-61, Excessive Reactor Coolant Leakage Following a Seal Failure in a Reactor Coolant Pump or Reactor Recirculation Pump
| ML031480397 | |
| Person / Time | |
|---|---|
| Issue date: | 08/09/1993 |
| From: | Grimes B K Office of Nuclear Reactor Regulation |
| To: | |
| References | |
| IN-93-061 | |
| Download: ML031480397 (3) | |
Information Notice No. 93-61 Index Site Map I FAQ I Help I Glossary I Contact Us A Search A... NuclearRRegulatoryyCommissionmHome Who We Ae Whai We Do Nuclear Nuclear Radioactive II PublicI Home ll Wh W Ae lWhi e o l Reactors Materials I Waste ll InvolvementHome > Electronic Reading Room > Document Collections > General Communications > Information Notices > 1993 > IN 9UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555August 9, 1993NRC INFORMATION NOTICE 93-61: EXCESSIVE REACTOR COOLANT LEAKAGEFOLLOWING A SEAL FAILURE IN A REACTORCOOLANT PUMP OR REACTOR RECIRCULATION PUMP
Addressees
All holders of operating licenses or construction permits for nuclear powerreactors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this informationnotice to alert addressees to the potential for excessive reactor coolantleakage following a seal failure in a reactor coolant pump or reactorrecirculation pump. It is expected that recipients will review theinformation for applicability to their facilities and consider actions, asappropriate, to avoid similar problems. However, suggestions contained inthis information notice are not NRC requirements; therefore, no specificaction or written response is required.
Description of Circumstances
Oconee Nuclear Station, Unit 1On May 24, 1992, the licensee commenced a reactor shutdown from 100 percentpower because of excessive leakage from the 1A2 Reactor Coolant Pump seal.The maximum leakage was approximately 23 liters per minute 6 gpm]. The sealfailed because of the premature degradation of obsolete seal parts that hadmistakenly been installed.Westinghouse supplied the Unit 1 reactor coolant pumps, incorporating a three-stage seal series arrangement to limit coolant flow up the pump shaft.Although Westinghouse had provided the necessary information on the designchange of the seal, the information was not properly communicated to plantpersonnel. As a result, the obsolete seal parts were not removed from thestation stock and appropriate maintenance procedures were not revised toreflect the change. These deficiencies contributed to the fact thatmaintenance personnel inadvertently installed the obsolete seal parts.Further details can be found in Licensee Event Report (LER) 50-269/92-09 andNRC Inspection Report No. 50-269/92-13.9308030200.IN 93-61August 9, 1993 Millstone Unit 1http://www.nrc.gov/reading-rmldoc-collections/gen-comm/info-notices/1 993/in9306 1 .html 03/13/2003 Information Notice No. 93-61 On May 25, 1989, the licensee started up Unit 1 for Cycle 13 operation.During plant heatup on May 27, 1989, operators noted indications ofintermittent seal failure for the A' Reactor Recirculation Pump inner seal.The licensee continued power escalation and cycle startup testing while makingplans to replace the seal. On May 29, 1989, while at full-power operation,the drywell leakage exceeded the Technical Specifications limit and thelicensee commenced plant shutdown. The leakage was approximately 34 litersper minute 9 gpm] at the start of the event and increased to about 174 litersper minute 46 gpm] during the shutdown process. After reviewing the failedseal and consulting with the pump manufacturer, Byron Jackson, the licenseestill did not identify the exact cause of the seal failure. However, thelicensee did determine that the pump seal had probably failed as a result ofone or more of the following causes: (1) improper seal handling prior toinstallation, (2) introduction of debris and corrosion products into the sealcavity, and (3) improper depressurization following hydrostatic testing of theseal assembly.Further details can be found in LER 50-245/89-14, Revision 1, and in NRCInspection Report No. 50-245/89-12.Clinton Unit 1On May 21, 1989, the licensee took the reactor to critical for Cycle 2operation. On May 25, 1989, the pressure in the seal outer cavity decreasedto approximately 414 kPa gauge [60 psig], indicating failure of the upper sealstage. Approximately 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> later, the seal appeared to reseal and operatedproperly. On June 1, 1989, with the reactor at about 42-percent power duringpower ascension, upon shifting the B- Reactor Recirculation Pump speed fromlow to high, the operators immediately noted indications that both the upperand lower seals in the pump had failed. The seal failures resulted inincreased flow from the drywell floor drain sump inlet; the leakage reached amaximum of 242 liters per minute [64 gpm]. The licensee then initiated plantshutdown. Although the exact cause of the seal failure was not determined,the licensee indicated that the probable cause was improper assembly orimproper installation.Further details can be found in the licensee special report submitted to NRCon June 30, 1989, and in NRC Inspection Report No. 50-461/89-21.DiscussionBoth reactor coolant pumps and reactor recirculation pumps use a series ofprimary and secondary seals to limit the reactor coolant leakage tocontainment. A loss-of-coolant accident (LOCA) can occur if leakage throughthe seals of reactor coolant pump or reactor recirculation pump exceeds thecapacity of the normal makeup systems. Thus, the failure of these seals canrepresent a significant degradation of the reactor coolant pressure boundary.IN 93-61August 9, 1993 The NRC has, over a period of years, evaluated the issue of reactor coolantpump and reactor recirculation pump seal-related problems (Generic Issue 23)and the need for additional licensing requirements to reduce the potentialcore-melt risk resulting from the failure of these pump seals. An evaluationprogram was initiated to resolve the generic issue and address several reactorcoolant pump seal leaks that occurred in the late 1970s and the early 1980s.Analysis performed in conjunction with the evaluation indicated that theoverall probability of core-melt due to small-break LOCAs could be dominatedby reactor coolant pump seal failures. The two conditions under which sealshave failed or could fail, normal operating conditions and off-normaloperating conditions, are addressed below:Seal performance under normal operating conditionsBased on the review of LERs and feedback from industry, some licensees appearto have recently made major improvements in reactor coolant pump and reactorrecirculation pump seal performance. This improvement is attributed to acombination of factors, including the following: enhanced seal qualityhttp://www.nrc.gov/reading-rmldoc-collections/gen-commlinfo-notices/1993/in93061 .html 03/13/2003 Information Notice No. 93-61 assurance programs, modified seal design, improved maintenance procedures andtraining, closer attention to detail, improved seal operating procedures, moreknowledgeable personnel involved in seal maintenance, reduction in frequencyof transients that stress the seals, and seal handling and installationequipment designed with the appropriate precision. However, not all plantshave implemented such measures, and some seal failures have caused substantialreactor coolant leakage (as described above).Development and implementation of appropriate procedures and training can helpassure correct operator action for operational conditions related to sealdegradation and can assist to identify seal degradation in a timely manner.This can thereby prevent or mitigate cascade failure of multistage sealassemblies.Section III of the ASME Boiler and Pressure Vessel Code has included specificexclusions for seal components under NB-3411.2 and NB-2121(b) relative todesign requirements. However, code exclusions by themselves do not relievelicensees from other pertinent regulatory requirements such as Appendix B to10 CFR Part 50, as applicable. For those items covered by Appendix B, asreflected in plant-specific licensing bases, a quality assurance program isrequired. The staff is considering additional generic action to addresswhether all licensees should treat certain seal components as safety-related.Seal performance under off-normal operating conditionsWith respect to off-normal operating conditions, particularly loss of all sealcooling water which can be caused by station blackout, loss of componentcooling water or loss of service water, the major concerns involve sealfailures due to adverse temperature effects on elastomer seal materials andperformance instabilities at the primary seal face related to coolant flashingand two-phase flow. The staff is proceeding with rulemaking to address issuesof ensuring reactor coolant pump seal function or compensating for sealfailure during loss of seal cooling events, including station blackout.IN 93-61August 9, 1993 This information notice requires no specific action or written response. Ifyou have any questions about the information in this notice, please contactone of the technical contacts listed below or the appropriate Office ofNuclear Reactor Regulation (NRR) project manager.ORIGINAL SIGNED BYBrian K. Grimes, DirectorDivision of Operating Reactor SupportOffice of Nuclear Reactor RegulationTechnical contacts: Jai Raj N. Rajan, NRR(301) 504-2788Peter C. Wen, NRR(301) 504-2832
Attachment:
List of Recently Issued NRC Information Noticeshttp://www.nrc.gov/reading-rmldoc-collections/gen-commlinfo-notices/l 993/in93061 .html 03/13/2003