ML17325B642

LER 99-013-00:on 990327,safety Injection & Centrifugal Charging Throttle Valve Cavitation During LOCA Could Have Led to ECCS Pump Failure.Caused by Inadequate Original Design Application of Si.Throttle Valves Will Be Developed

ML17325B642
Person / Time
Site:	Cook
Issue date:	06/01/1999
From:	Orourke B INDIANA MICHIGAN POWER CO.
To:
Shared Package
ML17325B641	List: ML17325B640 ML17325B642
References
LER-99-013, LER-99-13, NUDOCS 9906080148
Download: ML17325B642 (3)

Text

E NRC Form 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVEOSYOMB)IO.31~104 EXP)REB 06r30)2001 (6-1998) ESTSJATED SVRDEN PER RESPONSE TO COMPLY WITH TIES MANDATORY INFORMATIONCOLLECDON RECVESP. 50JJ HRS. REPORTED lESSONS lEARHED ARE INCORPORATED VITO THE UCENSINO PROCESS AND FED SACK 'TO INDVSTRY.

L(CENSEE EVENT REPORT (LER)

(See reverse for required number of 20MS P~

FORWARD COMNBPS REOARDVIO SVRDEN ESTSIATE TO THE INFORMATION AND RECORDS MANAOEMENT SRANCH IT% FSSL V.S. NVCLEAR REOVIATORY COLSSSSON, WASHSIOTON, DC 2N554NI. AND TO THE REDVCTION PRO/ECt O1100100, OFACE OF MANAOEMENT AJID SVOOET, WASISNOTON, DC digits/characters for each block)

FACIUTY NAME (I ) DOCKET NUMBER t2) PAOE IS)

Cook Nuclear Plant Unit 1 05000-315 1 of3 TITLE (4)

Safety Injection and CentNugat Charging Throttle Valve Cavitation During LOCA Could Lead to ECCS Pump Failure EVENT DATE {6) LER NUMBER {6) REPORT DATE (7) OTHER FACILITIES INVOLVED(6)

ACIUTY NAME SEQUENTIAL REVISION D.C. Cook, Unit 2 05000-316 MONTH DAY YEAR YEAR NUMBER NUMBER MONTH 'DAY YEAR FACIUTY NAM NUM R 03 27 1999 1999 013 00 06 01 1999 OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR li: (Check one or more) (11)

MODE (9) 20.2201 (b) 20.2203(a)(2)(v) 50.73(a)(2)(l) 50.73(a)(2)(viii)

POVlER 20.2203(a)(t) 20.2203(a)(3)(I) 50.73(a)(2)(6) 50.73(a)(2)(x)

LEVEL (10) 0 73.71 20.2203(a)(2N) 20.2203(a)(3)(9) 50.73{a)(2)(iii) 20.2203{a) {2)(ii) 20.2203(a)(4) 50.73(a)(2)(lv) OTHER 20.2203(a)(2)(ui) 50.36(c)(1) 50.73(a)(2)(v)

SPPCEF in AIJSVect IJeNAN 20.2203(a)(2)(iv) 50.36(c)(2) 50.73(a)(2)(vti) 01 n NRC Form MEA UCENSEE CONTACT FOR THIS LER (12)

TELEPHONE NUMBER 0IcIvde Area Code)

Ms. Brenda W. O'ourke, Compliance Engineer (616) 465-5901 x2604 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT {13)

REPORTABLE REPORTABLE TO COMPONENT MANUFACTURER TO EPIX COMPONENT MANUFACTURER EPIX SUPPLEMENTAL REPORT EXPECTED 14 EXPECTED YEAR X YES SUBMISSION 12 01 1999 IfYes, corn lete EXPECTED SUBMISSION DATE NO DATE 15 Abstract {Umitto 1400 spaces, I.e., approximately 15 single-spaced typewritten lines) (16)

On March 27, 1999, during development of an Emergency Core Cooling System (ECCS) thermal hydraulic flow analysis model, Engineering concluded that preliminary hydraulic flow analysis results indicated that the Unit 1 Safety Injection (Sl) and Centrifugat Charging (CC) throttle valves could potentially experience cavitation during a LOCA. Specifically, during a LOCA, large differential pressures created across the Sl and CC throttle valves would result in cavitation and possible mechanical erosion of the valves and downstream piping. During post-LOCA conditions, throttle valve erosion could lead to a pump runout condition and subsequent failure of the ECCS pumps. Radiography performed to determine valve position showed the valves to be less open than predicted by the hydraulic analysis. This condition could result in LOCA generated debris becoming lodged in the throttle valve, resulting in reduced flow to the reactor core. Similar conditions exist for the Unit 2 Sl and CC throttle valves. During continuing evaluation, plant personnel concluded that the conditions were reportable on May 4, 1999, and a 4-hour notification was made to the NRC in accordance with 10CFR50.72, for a condition that resutted in the plant being in an unanalyzed condition, and for any event or condition that alone could have prevented the fulfillment of the Sl or CC system safety function needed to mitigate the consequences of an accident.

The preliminary cause is inadequate original design application of the Sl and CC throttle valves. Because five of the six Unit 1 throttle valves were found to be less than 44 percent open, these vatves could be assumed to be partially or fully plugged with debris although the amount of plugging would depend on the nature and geometry of the debris. However, partial or full plugging could render the systems not capable of performing their intended safety function during the recirculation phase of a LOCA event. As part of the evaluation, a permanent solution for the cavitation-induced erosion and potential debris plugging will be devetoped to restore conformance with the plant design basis and to support ECCS operability. Possible modifications are currently under review and the method selected will be implemented prior to restart of each unit. A supplement to this LER will be provided upon completion of the condition evaluation.

9906080i48 'T)9060i PDR ADQCK 050003i5 8 PDR

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (6-1998)

LICENSEE EVENT REPORT (LER)

~ TEXT CONTINUATION FACILITYNAME (1) DOCKETNUMBER(2) LER NUMBER (6) PAGE (3)

YEAR SEQUENTIAL REVISION Cook Nuclear Plant Unit 1 05000-315 NUMBER NUMBER 2 of 3 1999 013 00 TEXT (Ifmore space is required, use additional copies of NRC Form (366A) (17)

Conditions Prior to Event Unit 1 was in Mode 5, Cold Shutdown Unit 2 was in Mode 5, Cold Shutdown Descri tion of Event On March 27, 1999, during development of an Emergency Core Cooling System (ECCS) thermal hydraulic flow analysis model, Engineering concluded that preliminary hydraulic flow analysis results indicated that the Unit 1 Safety Injection (Sl)[BQ] and Centrifugal Charging (CC)[SJJ throttle valves [FCVJ could potentially experience cavitation during a LOCA.

Specifically, during a LOCA, large differential pressures created across the Sl and CC throttle valves would result in cavitation and possible mechanical erosion of the valves and downstream piping. Throttle valve erosion during post-LOCA conditions could cause the valves to allow excess flow through the Sl and CC pumps, which could lead to a pump runout condition and subsequent failure of the ECCS pumps.

The preliminary flow analysis also predicted that the Sl and CC throttle valves would be less open than previously determined during evaluation of an industry event in 1995. In addition, actual valve stem measurements concluded that the Sl and CC throttle valves were less open than predicted by the preliminary flow analysis. As a result, radiography was performed on April 8, 1999, for two Sl throttle valves to better assess their position. Radiography results also concluded that the throttle valves were more closed than'predicted by the hydraulic analysis, and also showed indications of possible valve erosion. With the throttle valves found less open, debris generated in containment during a LOCA could potentially become lodged in the throttle valves, resulting in reduced flow to the reactor core. Similar conditions exist for the Unit 2 Sl and CC throttle valves. During continuing evaluation of the identified conditions, on May 4, 1999, plant personnel concluded that these conditions were reportable to the NRC.

Cause of Event The preliminary cause is inadequate original design application of the Sl and CC throttle valves. At the time of original plant design, this throttle valve design was commonly used throughout the industry for the same throttling application.

However, it was not recognized that these valves are susceptible to mechanical cavitation damage as a result of high differential pressures across the valve.

Opportunities to identify this potential condition were missed during previous evaluations of industry events regarding the potential for post-LOCA ECCS pump runout damage due to throttle valve degradation.

Anal sis of Event In accordance with the requirements of 10CFR50.72(b)(2)(i) and 10CFR50.72(b)(2)(iii)(D), a 4-hour notification was made to the NRC for a condition that resulted in the plant being in an unanalyzed condition, and for any event or condition that alone could have prevented the fulfillment of the Sl or CC system safety function needed to mitigate the consequences of an accident. As such, this LER is being submitted in accordance with the requirements of 10CFR50.73.

UFSAR Section 6.2 states that the primary function of the ECCS is to provide cooling water to the reactor core in the event of a LOCA. The ECCS system consists of six ECCS pumps: two Centrifugal Charging pumps, two Residual Heat Removal (RHR) pumps, and two Safety Injections pumps. The subject throttle valves are installed on each of the four CC pump lines to the reactor coolant loops and on the discharge side of each Sl pump.

NRC FORM 366A (6-1998)

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (6-1998)

LICENSEE EVENT REPORT (LER)

~ TEKF CONTINUATION DOCKETNUMBER(2) LER NUMBER (6) PAGE (3)

FACIUTYNAME (1)

YEAR SEQUENTIAL REVISION Cook Nuclear Plant Unit 1 05000-315 NUMBER NUMBER 3of3 1999 013 00 TEXT (Ifmore space ls required, use additional copies ofNRC Form (366A) (17)

The operation of the ECCS following a LOCA consists of two distinct phases: 1) the injection phase during which reactivity is controlled, initial cooling of the core is accomplished, and coolant lost from the primary system is replenished, and 2) the recirculation phase, in which long term core cooling is provided during the accident recovery period. Long term core cooling is provided, in part, by the Sl and CC systems that deliver coolant water from the containment recirculation sump back to the reactor coolant system. Following a LOCA event, any debris that was generated in containment will be filtered through a 0.25-inch square mesh screen [BP](SCN] prior to entering the containment sump to prevent foreign material from entering the ECCS system.

V The preliminary results of the ECCS hydraulic flow analysis were used to calculate the potential cavitation damage to the Sl and CC throttle valves under various ECCS system operating configurations. The most limiting operating configuration assumes the throttle valves to be wide open to simulate the maximum erosion condition. The flow analysis concluded that with the throttle valves wide open, cavitation would likely occur and the Sl pumps would be in a runout condition, while both CC pumps would remain available. For the case where only one CC pump is running, analysis results conclude that the single CC pump would go to the runout condition, in addition to both Sl pumps. For this configuration, calculations indicate that the Sl valves would be in a choked flow condition, which could cause cavitation-induced erosion of the valves and/or downstream piping. In addition, the CC throttle valves were calculated to be in the incipient damage range, which is the level at which cavitation is considered sufficient to result in erosion damage. Westinghouse letter NSAL-96-001, "Erosion of Globe Valves in ECCS Throttling Applications," indicates that even with throttle valve erosion, the CC and Sl pumps should remain operable long enough to allow the operators to use the RHR pumps for long term cooling. However, no formal analysis has been performed to validate the impact this condition has on long term ECCS post-LOCA operation.

Information received from the valve manufacturer requires the throttle valves to be approximately 44 percent open to ensure debris would not become lodged in the valve. Because five of the six Unit 1 throttle valves were found to be less than 44 percent open, these valves could be assumed to be partially or fully plugged with debris although the amount of plugging would depend on the nature and geometry of the debris. However, partial or full plugging of the Sl and CC throttle valves could render the systems not capable of performing their intended safety function during the recirculation phase of a LOCA.

Corrective Actions A permanent solution for the cavitation-induced erosion and potential debris plugging of the throttle valves will be developed to restore conformance with the plant design basis and to support ECCS system operability. Possible modifications are currently under review and the method selected will be implemented prior to,restart of each unit.

As part of the Restart effort, an improved operating experience program is being developed. In addition, the Expanded System Readiness Review project teams are evaluating operating experience information to assess whether applicable industry issues have been adequately addressed for applicability to D.C. Cook.

The adequacy of the systems to perform their safety and accident mitigation functions is being reviewed during the discovery process at D.C. Cook under a number of system assessments and programmatic assessments to support the Restart Plan. These include the Expanded System Readiness Review (ESRR) and the Licensing Basis Review. Identified deficiencies are being addressed under the Corrective Action Program.

The evaluation for this condition has not been completed. Based on the results of the completed evaluation, information regarding the safety significance, and corrective and preventive actions will be provided in an update to this LER.

Previous Similar Events None

~ NRC FORM 366A (6-1998)