05000298/LER-2020-003, Manual Reactor Scram Due to Digital Electro-Hydraulic Fluid Leak
| ML21014A446 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 12/28/2020 |
| From: | Dent J Nebraska Public Power District (NPPD) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NLS2020065 LER 2020-003-00 | |
| Download: ML21014A446 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| 2982020003R00 - NRC Website | |
text
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10 CFR 50.73 NLS2020065 December 28, 2020 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Subject: Licensee Event Report No. 2020-003-00 Cooper Nuclear Station, Docket No. 50-298, DPR-46
Dear Sir or Madam:
The purpose ofthis correspondence is to forward Licensee Event Report 2020-003-00.
There are no regulatory commitments contained in this request.
Sincerely,
~u;JU/rr1fl John Dent, Jr.
Vice President and Chief Nuclear Officer
/jo
Attachment:
Licensee Event Report 2020-003-00 cc:
Regional Administrator w/attachment USNRC - Region IV Cooper Project Manager w/attachment USNRC - NRR Plant Licensing Branch IV Senior Resident Inspector w/attachment USNRC-CNS' SRAB Administrator w/attachment NPG Distribution w/attachment INPO Records Center w/attachment via IRIS entry SORC Chairman w/attachment CNS Records w/attachment COOPER NUCLEAR STATION 72676 648A Ave/ P 0. Box 98 / Brownville, NE 68321 http://www.nppd.com
Abstract
On November 1, 2020, at 0534, Cooper Nuclear Station (CNS) Control Room Operators inserted a manual reactor scram due to low Digital Electro-Hydraulic (DEH) tank level. Further investigation revealed that a leak had developed at governor valve (GV) 1, and a pencil-sized stream of EH fluid was discovered leaking out of the lower, outboard 1nspecbon ~
on the GV1 actuator.
Investigation revealed one bolt on the actuator inspection port cover plate was bottomeck)Lrt and prevented appropnate bghtening for an adequate seal. The <Hing on this cover plate was ruptured, allowing excessive leakage.
The causes were on-srte vendor craftsmanship was unsatisfactory in the failure to recognize the bottomed-out bolt during actuator installation and inspector port cover plate holes on the new GV1 actuator body were not threaded deep enough to meet vendor specrlications. CNS will appropriately classify cnbcal Turbine Generator Fluid connecbons that are subject to additional verificabon of bolting and torqueing performance. In addition, CNS will review the final vendor evaluation of the actuator manufactunng defects/field craftsmanship and detenrnne if additional actions are warranted.
This event was not risk s1gnrf1cant.
PLANT STATUS YEAR 2020 SEQUENTIAL NUIIEER
- - 003 REV NO.
- - 00 Cooper Nuclear Station (CNS) was in Mode 1, Power Operation, at 100 percent pcmer at the time of the event on November 1, 2020.
BACKGROUND The power conversion systems at CNS are designed to produce electrical energy through conversion of a portion of thermal energy contained in the saturated steam supplied from the reactor, condense the b.Jrbine exhaust steam into water, and reb.Jm the water to the reactor as heated feec:lwater. The saturated steam produced by the reactor is passed through the high pressure b.Jrbine [EIIS:TRB] where the steam is expanded and then exhausted through the moisture separators [EIIS:MSR]. The moisb.Jre separators reduce the moisture content of the steam to close to zero percent. The steam is then passed through the low pressure b.Jrbines where the steam is again expanded. From the low pressure b.Jrbines, the steam is exhausted into the condenser [EIIS:COND] where the steam is condensed and de-aerated and then returned to the cycle as condensate.
The main b.Jrbine [EIIS:T A] consists of a high pressure section and a low pressure section comprised of two b.Jrbines in tandem.
Steam from the reactor is admitted to the high pressure b.Jrbine section through two main stop valve and governor valve assemblies [EIIS:PCV]. After expansion through the high pressure b.Jrbine section, steam flows to four moisb.Jre separators and reb.Jms to the low pressure b.Jrbine section by passing through four sets of combined intermediate valves (intercept valves and reheat stop valves combined into one assembly) [EIIS:ISV]. These intermediate valves, fully open during normal operation, limit or isolate steam flow from the moisb.Jre separators to the low pressure b.Jrbines under certain conditions. This action will prevent potential damage to the low pressure turbines.
The b.Jrbine utilizes a Digital Electro-Hydraulic (DEH) [EIIS:TG] system to control reactor pressure by positioning governor valves and condenser bypass valves. It consists of solid state governor devices, governor, startup control devices, emergency devices for b.Jrbine and plant protection (overspeed governor, master trip, vacuum trip, motoring protection, tlYust bearing wear trip, low bearing oil pressure trip) and special control and test devices. The control system operates the main stop valves, governor valves, bypass valves, reheat stop and intercept valves and other protective devices. DEH system oU pressure is maintained by two hydraulic pumps [EIIS:P] located at the DEH reservoir tank [EIIS:T].
EVENT DESCRIPTION
During Refueling Outage 30 (Fall 2018) three governor valve actuators were replaced with refurbished actuators. Upon testing, a fourth governor valve actuator had to be emergently replaced due to a leak. The preventative maintenance frequency was changed from every 6 years to every 2 years to replace the four governor valve actuators fNery cycle. Three spare governor valve actuators had previously been installed at CNS and were refurbished for use in Refueling Outage 31 (Fall 2020). A fourth governor valve actuator had to be purd,ased.
While staging the governor valve actuators for replacement of the existing installed actuators, it was noticed that the purchased actuator was missing inspection port cover plates. Per vendor recommendation, a decision was made to relocate covers from one of the actuators being removed to the new actuator prior to installation.
In-field work to relocate the inspection port cover plates commenced by the vendor and the four ad:uators were installed. The DEH system was successfully post-work tested and in service for approximately two weeks. Three days after CNS reached 100%
power, an on leak was discovered on the number one governor valve actuator inspecbon port cover plate. Page 2 of 3
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 08/31/2023 (08-2020)
Es!Jrnated
- 3. Lffi t<<.UBER YEAR Cooper Nuclear Station 05000-298 2020 SEQUENTIAL NUMBER
- - 003 REV NO.
- - 00 On November 1, 2020, the Control Room received an alarm for abnormal turbine DEH fluid reservoir level, resulting in a rapid power reduction per sl:atJon procedure and subsequent manual reactor scram.
Investigation of the number one governor valve actuator inspection port cover plate revealed that one bolt was bottomed-out and prevented appropriate tightening for an adequate seal, and the o-ring was ruptured, resulting in the oil leak A visual gap between the bolt head and the lock washer was apparent indicating the bolt was bottomed-out. This configuration was not noticed dunng installation. Measurements taken confirm that this bat could not have physically been installed correctly because the holes in the actuator body were not threaded deep enough to accept the length of specified bolt Two other bolts on the leaking CCNer were discovered to be loose enough to move with an Allen wrench.
To correct the condrbon, the bolts 'Nere cut short enough to achieve adequate tightening while maintaining necessary ttread engagement in the existing holes On November 4, 2020, at 1153, CNS reached 100 percent power.
BASIS FOR REPORT This event is reportable under 10 CFR 50.73(aX2Xiv)(A) as an event that resulted in actuation of systems listed in paragraph (aX2)(ivXB); specifically, (a)(2)(iv)(B)(1) for a reactor protection system actuation resulting in a reactor scram and (a)(2XivX6)(2) for a Group 2 isolation occumng due to reactor vessel level reaching the isolation setpoinl The event was reported as Event Notification 54976.
SAFETY SIGNIFICANCE
This event has negligible safety significance. The manual reactor scram removed the main turbine from service. Insufficient DEH system pressure prevented operation of the main turbine bypass valves
CAUSE
The drect cause of this event was at least one bolt on number one governor valve actuator inspection port cover plate was bottomed-out This was due to on-site vendor craftsmanship being unsatisfactory in the failure to recognize bottomed-out bolt upon actuator installation. In addition, the inspection port CCNer plate holes on the new number one governor valve actuator body were not threaded deep enough to meet vendor specifications.
CORRECTIVE ACTIONS
CNS will appropriately classify critical Turbine Generator Fluid connections that are subject to additional verffication of bolting and torqueing performance. In addition, CNS will review the final vendor evaluation of the actuator manufacb.Jring defects/field a-aftsmanship and determine rf additional actions are warranted.
PREVIOUS EVENTS On November 11, 2009, CNS control room operators inserted a manual reactor scram after a non-isolable DEH fluid leak developed This was reported under LER 2009-004-00, Manual Reactor Saam for DEH Flwd Leak.
On November 6, 2009, CNS control room operators inserted a manual reactor scram when vessel water level lowered quickly after a turbine trip. This was reported under LER 2009--002--00, Manual Saam on Low Water Level Caused by Turbine Trip from Hydraulic Fluid Leak. Page 3 of 3