05000530/LER-2018-001-01, Reactor Trip on Low Steam Generator Level
| ML18355A809 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 12/20/2018 |
| From: | Lacal M Arizona Public Service Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 102-07834-MLL/SPD LER 2018-001-01 | |
| Download: ML18355A809 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 5302018001R01 - NRC Website | |
text
10 CFR 50.73 102-07834-MLL/SPD December 20, 2018 MARIA L. LACAL Senior Vice President Nuclear Regulatory and Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tei 623 393 6491 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Reference:
Dear Sirs:
Arizona Public Service Company (APS) Letter No. 102-07782, dated August 23, 2018, Licensee Event Report 2018-001-00
Subject:
Palo Verde Nuclear Generating Station (PVNGS) Unit 3 Docket No. STN 50-530 / License No. NPF 74 Licensee Event Report 2018-001-01 Enclosed please find Licensee Event Report (LER) 50-530/2018-001-01 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER supplement provides the cause and corrective actions for an automatic actuation of the PVNGS Unit 3 reactor protection system previously reported in the referenced letter.
In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the Nuclear Regulatory Commission (NRC) Regional Office, NRC Region IV, and the Senior Resident Inspector.
Arizona Public Service Company makes no commitments in this letter. If you have questions regarding this submittal, please contact Matthew Kura, Department Leader, Nuclear Regulatory Affairs, at (623) 393-5379.
Sincerely, AC MLL/SPD Enclosure cc:
K. M. Kennedy S. P. Lingam C. A. Peabody NRC Region IV Regional Administrator NRC NRR Project Manager for PVNGS NRC Senior Resident Inspector PVNGS A member of the STARS Alliance LLC Callaway
- Diablo Canyon
- Palo Verde
- Wolf Creek
NRC FORM 366 (04-2018)
U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)
(See Page 2 for required number of digits/characters for each block)
'% w -* (See NUREG-1022, R.3 for instruction and guidance for completing this fomi http://www.nrc.qov/readinq-rm/doe-collections/nureas/staff/sr1022/r3/)
APPROVED BY OMB; NO. 3150-0104 EXPIRES: 03/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 1. Facility Name Palo Verde Nuclear Generating Station (PVNGS) Unit 3
- 2. Docket Number 05000530
- 3. Page 1 OF 4
- 4. Title Unit 3 Reactor Trip on Low Steam Generator Level
- 5. Event Date
- 6. LER Number
- 7. Report Date
- 8. Other Faciiities invoived Month Daw Year Year Sequential Rev Month Day Year Facility Name Docket Number uay Number No.
05000 HR 97 2018 2018
- - 001 niI 19 9n 2018 Facility Name Docket Number UD
^ /
- - U I I
I ^
05000 9, Operating Mode
Day Year Abstract (Limit to 1400 spaces, i.e., approximately 14 single-spaced typewritten lines)
On June 27, 2018, at approximately 2257 Mountain Standard Time, the PVNGS Unit 3 Control Room received a feedwater control system trouble alarm due to a steam generator feedwater economizer valve position/demand deviation Interaction with the malfunctioning economizer valve resulted in an unplanned movement of the economizer valve in the closed direction. This caused the main feedwater pump (MFWP) discharge pressure to increase tripping one of two operating MFWPs. This resulted in a rapid decrease in Steam Generator (SG) level that actuated a reactor protection system trip on low SG water level. Water level then rapidly rose to the SG high level trip set point because the malfunctioning economizer valve did not close. At that point an automatic main steam isolation signal occurred, requiring manual actuation of the auxiliary feedwater and essential spray pond (ultimate heat sink) systems.
The cause of the event was attributed to an auxiliary operator taking an action contrary to station requirements to block air venting from an economizer trip valve causing the valve to move in the closed direction.
Corrective actions include enhanced operations training on responding to economizer valve failures, specifically, and pre scripted events placing operators in a perceived sense of urgency. Additional guidance and operating experience will be incorporated into the alarm response procedure for the feedwater control system.
No similar conditions have been reported by PVNGS in the past three years.
NRC FORM 366 (04-2018)
All times are Mountain Standard Time and approximate unless otherwise indicated.
1. REPORTING REQUIREMENT(S)
This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73 (a)(2)(iv)(A) to report an automatic actuation of the PVNGS Unit 3 Plant Protection System (PPS) (EIIS: JC) that occurred as a result of an automatic low steam generator (SG) water level generated trip on all four channels of the PPS. This event is additionally being reported pursuant to 10 CFR 50.73 (a)(2)(iv)(A) as an automatic actuation of main steam isolation valves and manual actuation of auxiliary feedwater and essential spray pond (ultimate heat sink) system pumps.
This event was reported to the NRC pursuant to 10 CFR 50.72 (b)(2)(iv)(B) on June 28, 2018 at 0237, via the emergency notification system (EN 53477).
- 2. DESCRIPTICN CF STRUCTURE(S), SYSTEM(S) AND CCMPCNENT(S):
Reactor Protection System (RPS):
The RPS (EIIS; JC) ensures the reactor is rapidly and reliably shut down to protect the fission product barriers and assist the engineered safety features systems in accident mitigation. The RPS consists of four independent, redundant channels and includes a number of sensors, calculators, logic circuits, and supporting equipment that monitor nuclear steam supply system (EIIS: AB) parameters. When all four channels of RPS are in service, a reactor trip is actuated when two of four channels generate trip signals. Inputs to the RPS include both a high and a low steam generator (SG) level trip signal. The low level trip prevents exceeding reactor coolant system design pressure due to a loss of heat sink by tripping the reactor during a main feedwater (MFW) pipe break, reactor coolant pump rotor seizure, inadvertent opening of an atmospheric dump valve, or during a loss of MFW or condenser vacuum. The high level trip prevents moisture carryover from the SGs from damaging the main turbine generator. The high SG level reactor trip is coincident with a main steam isolation signal actuation.
Main Steam (MS) Isolation Signal (MSI5):
The MSIS, generated by the engineered safety features actuation system (EIIS: JE), ensures 10 CFR 100 limits are not exceeded in the event of a steam generator tube rupture by closing the MS isolation valves and the MFW Isolation valves. The MSIS also prevents the unaffected SG from feeding steam into a high energy line break. An MSIS can be generated by high containment pressure, low SG pressure, or high SG water level.
Feedwater Control System (FWCS):
The FWCS (EIIS: JB) coordinates the adjustment of the two MFW pumps (MFWPs), the downcomer valve, and the economizer valve to automatically regulate feedwater flow to control SG water level. The downcomer valve controls flow at low power levels and closes as flow is transferred to the economizer valve at approximately 15-percent power. The economizer valve then modulates to control flow up to 100 percent power. The downcomer valve reopens to its half-open position at 50-percent power and remains so through 100-percent power operations.
Feedwater flow is subsequently transferred back to the downcomer valve when power drops below 15-percent. A reactor trip causes a close signal to be sent to the economizer valve. The economizer valve is controlled by a digital valve controller and a pneumatic trip valve; designed to fail the economizer valve "as-is on a loss of instrument air.
Reactor Power Cutback System (RPCS):
The RPCS responds to large load rejections or failures of either MFWP by dropping pre-selected control element assemblies and initiating other necessary control actions to obtain a rapid reduction in reactor power. This allows the plant to remain on-line during these events and significantly reduces the requirements for steam bypass valve and MFWP capacity.Page 2 of 4(04-2018)
U.S. NUCLEAR REGULATORY COMMISSION
/^\\ LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.aov/r6adina-nTi/doc-collections/nureas/staff/sri022yr3/)
APPROVED BY OMB: NO. 3150-0104 EXPIRES: 3/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
- 3. LER NUMBER Palo Verde Nuclear Generating Station (PVNGS) Unit 3 05000-530 YEAR 2018 SEQUENTIAL NUMBER
- - 001 REV NO.
- - 01
3. INITIAL PLANT CONDITIONS
On June 27, 2018, PVNGS Unit 3 was in Mode 1 (Power Operation) at 100 percent power with the reactor coolant system at normal operating temperature and normal operating pressure. There were no other structures, systems, or components out of service that contributed to this event.
4. EVENT DESCRIPTION
On June 27th, 2018 at 22:57 the Unit 3 Control Room received a FWCS trouble alarm indicating a SGI economizer valve position/demand deviation. In accordance with the alarm response procedure, an attempt was made to take manual control of the economizer valve from the control room. The control room operator reduced the output demand but SO level continued to slowly rise with no indicated change in economizer valve position. An auxiliary plant operator was dispatched to investigate locally and reported that air was porting from somewhere in the location of the economizer valve. Without direction from the control room, the auxiliary operator placed his thumb sequentially over the two vent ports on the economizer trip valve. When his thumb was placed over the second port, the economizer valve responded by moving in the closed direction from the steady state position of approximately 70 percent open to approximately 30 percent open. Steam generator water level began to Ipwer and MFWP discharge pressure increased causing the B MFWP to trip on high discharge pressure. The MFWP trip initiated a reactor power cutback. SO level continued to lower and 28 seconds after the RPCS actuation, the SO low level reactor trip set point was reached and the reactor automatically tripped.
The control room staff entered the standard post-reactor trip actions procedure. During this time, the SO level began to rise rapidly because the economizer valve did not automatically close on the reactor trip signal but remained unresponsive at one quarter open. In approximately four minutes, SO water level reached its high level setpoint and initiated an MSIS which tripped the remaining A MFWP. Control room staff took manual control of steam pressure using atmospheric dump valves and started the essential motor-driven auxiliary feedwater pump (AFWP) to control SO level and the essential spray pond pump to support cooling the AFWP room.
It was later determined that a diaphragm in the economizer valve, Fisher Controls model 475-16-100, pneumatic control valve (EIIS: PCV) had failed, leaving the economizer valve in its as-is position and rendering it unresponsive to the FWCS.
5. ASSESSMENT OF SAFETY CONSEQUENCES
This event did not result in a challenge to the fission product barriers or result in the release of radioactive materials to the environment. There were no actual safety consequences as a result of this event and it did not adversely affect the health and safety of the public.
The RPS functioned as designed and initiated an automatic reactor trip that placed the plant in a safe condition. All CEAs fully inserted into the reactor core.
The Unit 3 reactor trip did not result in a transient more severe than those already analyzed. The primary system and secondary pressure boundary limits were not approached.
6. CAUSE OF THE EVENT
The cause of the event was attributed to an auxiliary operator taking an action contrary to station requirements to block air venting from an economizer trip valve causing the valve to move in the closed direction.Page 3 of 4
- 3. LER NUMBER Palo Verde Nuclear Generating Station 05000-530 YEAR SEQUENTIAL NUMBER REV NO.
(PVNGS) Units 2018
- - 001
- - 01(04-2018) m U.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET (See NUREG-1022, R.3 for instruction and guidance for completing this form http://www.nrc.qov/readinq-nn/doc-collections/nureQs/staff/sr1022/r3/)
APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 3/31/2020
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
7. CORRECTIVE ACTIONS
The malfunctioning economizer valve trip valve was replaced with a new trip valve and filter regulator. Positioner calibration, as-left diagnostics, and as-left loop checks were performed satisfactorily.
Corrective actions include enhanced operations training on responding to economizer valve failures, specifically, and pre-scripted events placing operators in a perceived sense of urgency. Additional guidance and operating experience will be incorporated into the alarm response procedure for the feedwater control system.
8. PREVIOUS SIMILAR EVENTS
The station reported a similar event on February 2, 1998, when Unit 1 experienced a low SO water level automatic reactor trip and an engineered safety feature actuation of auxiliary feedwater due to the spurious closure of an economizer valve. The cause was a failed dynamic compensator card in the FWCS master controller. (LER 528-1998-002-00).Page 4 of 4