05000423/LER-1996-029-02, :on 960830,functional Deficiency Was Noted in Setting of ECCS Throttle Valve Positions.Caused by Deficiency in Original Design Sys.Plant Mod Will Be Prepared to Resolve Pump Boost,Valve Clogging & Valve Erosion Issues

:on 960830,functional Deficiency Was Noted in Setting of ECCS Throttle Valve Positions.Caused by Deficiency in Original Design Sys.Plant Mod Will Be Prepared to Resolve Pump Boost,Valve Clogging & Valve Erosion Issues

ML20129E948
Person / Time
Site:	Millstone
Issue date:	09/29/1996
From:	Laudenat R NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20129E946	List: 05000423/LER-1996-029-02, :on 960830,functional Deficiency Was Noted in Setting of ECCS Throttle Valve Positions.Caused by Deficiency in Original Design Sys.Plant Mod Will Be Prepared to Resolve Pump Boost,Valve Clogging & Valve Erosion Issues
References
LER-96-029-02, LER-96-29-2, NUDOCS 9610010207
Download: ML20129E948 (5)

LER-1996-029, on 960830,functional Deficiency Was Noted in Setting of ECCS Throttle Valve Positions.Caused by Deficiency in Original Design Sys.Plant Mod Will Be Prepared to Resolve Pump Boost,Valve Clogging & Valve Erosion Issues

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(viii) 10 CFR 50.73(a)(2)(ii) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function
4231996029R02 - NRC Website

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NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY oMB NO.3160-0104 44g53 EXPIRES 04/30/98 l yow?#?!"#1#$?o"#s!.Vr%s*"*J"ob"t"?^Rs L'ta"9o^"l4&"I "'!'an'a ToA=* iafe'Jo ^22 LICENSEE EVENT REPORT (LER) 18!T'a'=fM&fte,t4#J"st"? 'n?"!,3M"ei l?nu;?htaandi?leiL" EiW!k??ol"?nks'"*

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FACirTY NAME (1)

DOCKET NUMBER (2)

PAGE (3)

Millstone Nuclear Power Station Unit 3 05000423 1 of 5 TITLE (4)

Functional Deficiency in the Setting of the Emergency Core Cooling System Throttle Valve Positions EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (7)

OTHER FACILITIES INVOLVED (8)

MONTH DAY YEAR YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACluTY NAME DOCKET NUMBER NUMBER 08 30 96 96 029 00 09 29 96 i

OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5: (Check one or more) (11)

MODE (9) 5 20.2201(b) 20.2203(a)(2)(v) 50.73(a)(2)(i) 50.73(a)(2)(viii)

POWER 20.2203(a)(1) 20.2203(a)(3)(i) 50.73(a)(2)(ii) 50.73(a)(2)(x)

LEVEL (10) 000 20.2203(a)(2)(il 20.2203(a)(3)(ii) 50.73(a)(2)(iii) 73.71 20.2203(a)(2)(ii) 20.2203(a)(4) 50.73(a)(2)(iv)

OTHER

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50.36(c)(1)

X Sc.i3(an2nv) 20.2203(a)(2)(iii) gc;v,. A strggelow g_

20.2203(a)(2)(iv) 50.36(c)(2) 50.73(a)(2)(vii)

LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBFR (include Area Codel R. T. Laudenat, Nuclee.: Licerising Supervisor (8601444-5248 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE

SYSTEM COMPONENT MANUFACTURER REPORTABLE

CAUSE

SYSTEM COMPONENT MANUFACTURER REPORTABLE To NPRDS To NPROS i

A SUPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY YEAR SUBMISSION

[

YES NO

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(If yes, Complete EXPECTED SUBMISSION DATE).

ABSTRACT (Limit to 1400 spaces, i.e.. approximately 15 single-spaced typewritten lines) (16)

On August 30,1996, with the plant in Mode 5 at 0-percent power, an engineering evaluetion determined that a functional deficiency in the setting of the Emergency Core Cooling System (ECCS) throttle valve positions was a condition which alone could result in the loss of the Charging (CHS) and High Head Safety injection (SlH) safety 2

functions in the recirculation phase.

The positions of the throttle valves was established during original start-up testing to ensure adequate injection flows and prevent pump runout. This testing was conducted under injection conditions and therefore, the suction boost provided by the Containment Recirculation Spray System (RSS) pumps during recirculation was not considered in the setting of the throttle valves. Additionally, Westinghouse identified that the ECCS pump runout margin e.ssociated with the impellers used at the plant may be less than that assumed in the original system design. Coupled with the RSS pump boost, this could result in pump flow rates in excess of maximum limits during the recirculation phise, potentially leading to pump damage and inoperability. This condition was reported at 1128 on August 30, 1996, pursuant to 10CFR50.72(b)(2)(iii)(D) as a condition which alone could have prevented the CHS and SlH

)

systems from performing their safety function to mitigate the consequences of an accident.

Plant modifications will be implemented to ensure that the ECCS pumps will provide adequate injection flows and not runout during recirculation with RSS pump boost. The plant will be maintained in Mode 5 or lower until such time as the modifications have been completed.

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9610010207 960927 PDR ADOCK 05000423 S

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.U.S. NUCLEAR REGULATORY COMMISSIONl L4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 3 05000423 NUMBER NUMBER 2 of 5 96 029 00 TEKT filmore space is required, use additional copies of NRC Form 366A) (17) 1.

Descriotion of Event On August 30,1996, with the plant in Mode 5 at 0-percent power, an engineering evaluation determined that a functional deficiency in the setting of the Emergency Cnre Cooling System (ECCS) throttle valve positions was a condition which alone could result in the loss of the Charging (CHS) and High Head Safety injection (SlH) safety functions in the recirculation phase. The current ECCS throttle valve positions were established without considering the suction boost pressure of the Containment Recirculation Spray System (RSS) pumps which would be present during the recirculation phase. The throttle valve position, coupled with a reduction in ECCS pump runout margin available relative to original design conditions, would result in CHS and SlH pump flow rates in excess of maximum limits during the recirculation phase, potentially leading to pump damage and inoperability. This condison was reported at 1128 on August 30,1996, pursuant to 10CFR50.72(b)(2)(iii)(D),

es a condition which alone could have prevented the CHS and SlH safety systems from performing their safety functions to mitigate the consequences of an accident.

The licensing basis of the plant requires that the recirculation phase be sustainable for post accident long term cooling. The RSS suction boost coupled with a reduction in CHS and SlH pump runout margin could cause the CHS and SlH pumps to become inoperable early in the recirculation phase, in this condition, the ability of the pitnt to maintain sufficier.t flow during the recirculation phase would be significantly reduced such that the j

d: cay heat boil-off rate vrould rapidly exceed the heat remaval rate and thus compromise core cooling.

i 11.

Cause of Event

The cause of this event is a deficiency in the original design of the system coupled with inadequate start-up testing. The positions of the throttle valves were established during original start-up testing in accordance with Westinghouse specifications to ensure that the ECCS pumps would provide adequate injection flows and ECCS pumps would not runout. However, this testing was conducted under injection conditions and therefore, the boost from the RSS pumps which would exist during recirculation was not considered in the setting of these velve positions.

Whereas pump boost as a condition is not of itself necessarily harmful to the operation of the ECCS, it is the cumulative affect of the following conditions that results in a pump boost magnitude that makes it deleterious:

RSS pumps are used for the ECCS recirculation phase in lieu of the lower head Residual Heat Removal (RHR) pumps.

The as-built performance of each RSS pump is approximately 7 percent higher than the original design

curve, The RSS heat exchangers have a flow limitation of 4600 gpm due to tube vibration concerns, even e

though the RSS pump maximum flow is 5100 gpm. To preclude RSS heat exchanger flows higher than 4600 gpm, direct RSS pump injection during cold leg recirculation and during hot leg recirculation was prohibited by plant procedures in 1986. While this action resolved the RSS heat exchanger flow limitation problem, it contibuted to the CHS and SlH suction boost problem.

Sand-cast type impellers are installed in the CHS and SlH pumps. Sand-cast impellers are generally more susceptible to cavitation at flows close to the specified runout flows. Based on information from Westinghouse, if an attempt is made to operate at or beyond the specified runout flow, cavitation will occur regardless of the Net Positive Suction Head Availible (NPSHA) at the suction of the pump.

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.U.S. NUCLEAR RE2ULA10.lY COMMISSION LICENSEE EVENT REPORT (LER) l l

TEXT CONTINUATION FACILITV NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE 13) f 2

YEAR SEQUENTIAL REVISION

(

j Millstone Nuclear Power Station Unit 3 05000423 NUMBER NUMBER 3 of 5 96 029 00 i

l TEKT (If more space is required. use additionalcopies of NRC Form 366A) it 7) l I

Therefore, the specified runout flows should be treated as maximum allowable flows for ccceptable j

pump operation.

I I

The ECCS, as currently configured, is vulnerable to pump runout conditions during the recirculation phase. The 1

system is vulnerable to pump damage and ultimately system inoperability.

Ill. Analysis of Event J

This is being reported as a condition which alone could result in the loss of the CHS and SlH safety functions in ths ECCS recirculation phase, pursuant to 10CFR50.73(a)(2)(v)(D). Specifically, the setting of the ECCS branch lins throttle valves during start-up testing was not properly coupled to the RSS pump boost which would be available during the recirculation phase. The RSS pump boost available is of sufficient magnitude to result in CHS and SlH pump flow rates in excess of maximum limits, potentially leading to pump damage and eventual inoperability.

The potential for the ECCS to fail to meet the long-term cooling requirements was being investigated in response to industry reports that the throttle valves were susceptible to clogging and erosion during the recirculation phase. Initially, it was postulated that debris from containment, smaller than the sump screen 3/32" opening but larger than the clearance provided in the subject throttle valves, could become lodged in the valve passages and sufficiently reduce flow to below the minimum allowed to prevent core uncovery. It was subsequently dstermined that while the high velocities and differential pressures in the vicinity of these throttle valves would e:rve to provent blockage, the same high velocities and differential pressures would result in valve cavitation and subsequent valve seat erosion to the po:nt where eventually the throttling capacity of the valves could decrease.

This potential decrease in throttling capacity could result in ECCS pump runout and subsequent inoperability.

While irrestigating the ECCS branch line throttle valve clogging and erosion issues, it was determined that the position of these throttle valves were established during original start-up testing under injection conditions and thtrefore, did not consider RSS pump boost which would be available during recirculation. Due to the substantial magnitude of the RSS pump boost during recirculation, the current valve positions would not preclude the runout of either of the ECCS pumps at any time, independent of valve erosion, and the ECCS pumps would experience pump damage and eventualinoperability. This pump boost condition had previously bun identified in Westinghouse Letter NEU-91-611 dated October 1,1991.

Th3 Westinghom Letter also identified an issue regarding :he amount of runout margin available for the ECCS pumps manufactured by Dresser / Pacific Pumps utilized within the plant. It was previously assumed that the runout margin for the SlH and CHS pumps was 15 gpm or more beyond the specified design maximum flow rate.' The following guidelines were established in 1991 to assess the impact on the unit:

If an attempt is made to operate at or beyond the critical flow capacity of the impeller, cavitation would occur regardless of the NPSHA at the suction of the pump.

j These flow rates should be treated as maximum allowable flows for acceptable pump operation. These j

flow rates are acceptable only if the NPSHA satisfies the identified pump requirements.

For the SlH pumps, Dresser Pumps has determined that the asymptote is located beyond 675 gpm and cavitation probably occurs on the suction impeller. The original design runout flow was 670 gpm for i

SlH pump "A" and 650 gpm for SlH pump "B" as specified in Technical Specification 4.5.2.h.

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.U.S. NUCLEAR REIULATORY COMMISSION 14-951 l

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 3 05000423 NUMBER NUMBER 4 of 5 l

96 029 00 l

TEXT (11more space is required. use additionalcopies of NRC form 366A) (17)

For the CHS pumps, Dresser Pumps has determined that the asymptota is located beyond 560 gpm.

l e

The original design runout flow was 560 gpm as specified in Technical Specification 4.5.2.h.

t l

Th:refore, the design objectives for an acceptable solution to ensure that the ECCS is capable of meeting its dzsign safety function are as follows i

l 1.

Meet or exceed the minimum injection flow requirements delineated in the technical specifications during the injection phase.

l 2.

Ensure maximum pump flows are maintained below those specified in Technical Specification 4.5.2.h during recirculation with boost from the RSS pumps considered.

3.

Ensure minimum throttle valve clearances are greater than 3/32".

l 4.

Ensure maximum throttle valve velocities are below that at which significant valve erosion would occur.

l Thz colution to both the clogging and valve erosion issues is to reduce the throttling burden on the valves such j

thtt the revised valve clearances would be larger than the sump screen mesh clearance and would have l

associated fluid velocities below which significant erosion will not occur. The required reductlcn in the throttling l

burden on the valves could be accomplished through the installation of flow orifices in series w?. each throttling vilva, thus maintaining the same net ECCS hydraulic resistance and original flow balance.

l In order to address the RSS pump boost issue, the ECCS hydraulic resistance must be increased; however, it cin only be increased in such a manner that does not result in deliverable flows less than the required injection flows for core recovery delineated in the technical specifications. To this end, the additional resistance required j

for pump boost may only be partially addressed by increasing the hydraulic resistance in the branch lines beyond that required to relieve the valve throttling burden to the extent allowed by the current system flow margin in tha injection phase. The balance of the resistance must be derived from supplemental resistance only in the r circulation flow path.

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IV. Corrective Action

A plant modification is being prepared to resolve the pump boost, valve clogging, and valve errosion issues.

Tha proposed modifications will:

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Install restrictive orifices in selected ECCS branch lines to relieve the throttling burden on the branch line I

throttling valves. This will permit the valves to be set in more open positions while still satisfying the minimum required deliverable flows during the injection phase.

Install restrictiva orifices in the suction supply headers to the CHS pumps from the RSS pumps to l

mitigate RSS pump boost.

l l

The plant will be maintained in Mode 5 or lower until such time as all modifications have been completed and j

l tha issues resolved.

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NRC.F6RM 366A U.S. NUCLEAR RE ULATORY COMMISSION (4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 3 05000423 NUMBER NUMBER 5 of 5 96 029 00 TEXT fif more space is required, use additional copies of NRC Form 366A) (17)

V.

Additional Information

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