05000528/LER-2023-001-02, Reactor Trip Following a Main Turbine Trip
| ML24026A298 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 01/26/2024 |
| From: | Harbor C Arizona Public Service Co |
| To: | Office of Nuclear Reactor Regulation, Document Control Desk |
| References | |
| 102-08712-CDH/TR LER 2023-001-02 | |
| Download: ML24026A298 (1) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 5282023001R02 - NRC Website | |
text
Cary D. Harbor Senior Vice President Site Operations
Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel: 623.393.7953 102-08712-CDH/TR January, 2024
U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Subject: Palo Verde Nuclear Generating Station Unit 1 Docket No. STN 50-528 /Renewed License No. NPF-41 Licensee Event Report Supplement 2023-001-02
Enclosed, please find the Licensee Event Report (LER) 50-528/2023-001-02 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER supplement provides clarification to a corrective action that was submitted to the Nuclear Regulatory Commission (NRC) under LER 2023-001-01 on September 13, 2023, for the Unit 1 reactor trip that occurred on April 8, 2023.
In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the NRC Regional Office, NRC Region IV, and the Senior Resident Inspector.
No new commitments are being made to the NRC by this letter. Should you need further information regarding this submittal, please contact Michael DiLorenzo, Department Leader, Nuclear Regulatory Affairs, at (623) 393-3495.
Sincerely, Harbor, Cary Digitally signed by Harbor, Cary (Z16762)
(Z16762) Date: 2024.01.26 10:02:21 -07'00'
CDH/TR/cr
Enclosure: Unit 1 Licensee Event Report 2023-001-02 cc: J. Monninger NRC Region IV Regional Administrator
A member of the STARS Alliance, LLC
Callaway
- Diablo Canyon
- Palo Verde
- Wolf Creek ENCLOSURE
Unit 1 Licensee Event Report 2023-001-02
Abstract
On April 8, 2023 at approximately 2144 MST, following a turbine trip due to the loss of control oil header pressure, both 13.8 kV non-class buses de-energized, which caused a loss of power to the Reactor Coolant Pumps and resulted in a reactor trip. The B auxiliary feedwater pump was started to feed both steam generators due to the de-energization also having led to the main feed pumps tripping on low suction pressure. A Main Steam Isolation Signal was manually actuated due to a loss of circulating water flow to the condenser, which isolated the main steam header and required the use of Atmospheric Dump Valves for heat removal.
The cause of the reactor trip was the result of the turbine generator primary and backup reverse power relays failing to actuate due to not reaching the trip setpoint timing. There was high reactive load on the system grid. This resulted in the reverse power relays not actuating. Corrective actions include enhancement of the excitation system, modification of the existing fast bus transfer logic upon a turbine trip, and a design change to replace the electro-mechanical reverse power relays with microprocessor-based relay equivalents for the main generator in all three units.
All times are in Mountain Standard Time.
1. REPORTING REQUIREMENT(S)
This Licensee Event Report (LER) is being submitted pursuant to 10 CFR 50.73(a)(2)(iv)(A), because the plant had a reactor trip due to a loss of Reactor Coolant Pumps (RCPs), manual actuation of the Main Steam Isolation Signal (MSIS) which affected multiple Main Steam Isolation Valves (MSIVs), and manual actuation of the B-train Auxiliary Feedwater Pump.
This event was reported to the NRC pursuant to 10 CFR 50.72(b)(2)(iv)(B) RPS Actuation as a 4-Hr Non-Emergency report and under 10 CFR 50.72(b)(3)(iv)(A) Specified System Actuation as an 8-Hr Non-Emergency report on April 9, 2023, via the Event Notification System (ENS#56459). An additional update to the ENS report was completed on May 3, 2023.
- 2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
Atmospheric Dump Valve (ADV) (EIIS: SB): The ADVs are pneumatically operated and may be opened/closed by the control room operator to control pressure or primary plant cooldown in the event the main condenser and/or steam bypass control system is not available.
Auxiliary Feedwater (AF) Action Signal (AFAS) (EIIS: BA): During normal power generation, the two essential AF pumps are in a standby condition. The system is provided with two channels of AFAS (AFAS-1 or -2) corresponding to each Steam Generator (SG). An AFAS will automatically start and align the essential pumps to feed the affected SG upon receipt of a low steam generator water level signal. As long as the affected SG is intact, as evidenced by the existing SG differential pressure (relative to the other SG), the auxiliary feed system will automatically maintain SG level within a prescribed wide range indication band. AFAS is an Engineered Safety Features Action Systems (ESFAS) actuation.
Control Oil System (CO) (EIIS: TG): The main turbine control oil is the high-pressure fluid which comprises the hydraulic portion of the electro-hydraulic control system used to control turbine operation. The high-pressure fluid is provided to the main turbine stop and control valves, combined intercept valves, as well as the trip devices in the trip and overspeed protection circuits. There are two main hydraulic fluid pumps that are 100 percent capacity, motor-driven, variable-delivery piston pumps. They are designed to maintain a constant pressure through the entire delivery range using a pressure compensator provided for each pump.
During normal operations, only one pump operates with the second pump maintained in standby. If a pump trips and the standby pump does not auto-start, CO header pressure will continue to slowly drop. Main CO low header pressure acts as an input to the main turbine trip logic.
Main Generation System (MA) (EIIS: TB): The MA system consists of the main generator, an isolated phase bus, a bank of main step-up transformers, and a unit auxiliary transformer (UAT). The main generator converts mechanical power received from the main turbine to electrical power. The MA system is started by pressurizing the generator with hydrogen, starting the isophase bus and generator stator cooling, bringing the turbine up to speed, applying field excitation, and synchronizing the generator to the system.
The main generator is excited using a separate system known as the excitation system, i.e., EX2100e. The excitation system provides the necessary electrical field to the generator's rotor, allowing it to generate electrical power. The excitation system ensures that the main generator in the nuclear power plant operates at the desired voltage and maintains stability during various operating conditions. It is crucial for efficient power generation and grid synchronization.
Fast Bus Transfer (FBT): With the plant in normal plant operations, during a turbine trip or loss of supply from the unit auxiliary transformer, (not involving an electrical fault or under frequency), the sequence of events includes an FBT. A FBT would be initiated when its control logic is satisfied, which includes a trip of the unit auxiliary transformer output breakers, resulting in the alternate supply breakers closing within a few cycles to connect the 13.8 kV buses to the startup transformers. Typically, the startup transformers supply buses during plant startup or at other times when the turbine generator or unit auxiliary transformer is out of service. Transfers of these buses can also be initiated by a plant operator from the control room.
Turbine Generator Reverse Power Relays (EIIS: TL): The generator reverse power relays, one primary and one backup, are extremely sensitive, three-phase, directional power relays used for turbine protection against motoring. This condition could occur if the steam supply to the turbine is lost while the generator is connected to the grid. Both relays are interlocked such that they are inoperative unless the generator is connected to the grid.
In the event of a turbine trip during normal plant operations, not involving an electrical fault or under frequency, the turbine generator will remain synchronized to the high voltage transmission network until residual energy in the turbine is dissipated. The generator will motorize for a short period of time and will not trip until a sustained reverse power condition exists and the reverse power relay actuates. The reverse power relay actuation will simultaneously trip the generator exciter, the 525 kV breakers and the unit auxiliary transformer output breakers, thereby initiating a fast bus transfer.
Power Factor (PF): In an electrical generation system, PF refers to the ratio between the real power and the apparent power consumed by the system. It measures the efficiency of power utilization and indicates the extent to which the current is effectively used for useful work. PF is influenced by the type of load connected to the system. There are two types of loads: resistive loads, such as incandescent lights and electric heaters, which consume only real power, and reactive loads, such as induction motors and transformers, which consume both real power and reactive power.
Main Steam Isolation Valves (MSIV) (EIIS: SB): Each of the main steam lines is equipped with one quick acting MSIV. The MSIVs close on a MSIS generated by either low steam generator pressure, high steam generator level or high containment pressure. The MSIVs fail closed on loss of control or actuation power. Closing the MSIVs isolates each steam generator from the other, and isolates the turbine, Steam Bypass Control system, and other auxiliary steam supplies from the steam generators.
Reactor Coolant System (RCS) (EIIS: AB): The RCS is comprised of two main flow loops each of which includes two Reactor Coolant Pumps (RCPs) and one SG. The primary function of the RCPs is to provide the necessary head to maintain forced circulation of reactor coolant through the PCS during normal operations. The reactor coolant leaving the core of the reactor vessel enters two hot legs, one per loop, and flows to the SG. Critical operation of the reactor requires all four RCPs to be in operation to ensure adequate RCS flow. The RCPs are powered from non-class 1E 13.8 kV buses with two RCPs per bus.
Reactor Protection System (RPS) (EIIS Code: JC): The system's functions are to protect the core Specified Acceptable Fuel Design Limits and RCS pressure boundary for incidents of moderate frequency, and to provide assistance in limiting initial conditions for certain infrequent events and limiting faults. The RPS consists of four independent, redundant channels and includes a number of sensors, calculators (including the core protection calculators (CPCs) (EIIS: JC)), logic circuits, and supporting equipment that monitor nuclear steam supply system (EIIS: AB) parameters. The RPS 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. When all four channels of RPS are in service, a reactor trip is actuated when two of four channels generate trip signals.
Reactor Power Cutback System (RPCS) (EIIS:JD): The RPCS is a safety feature that is designed to automatically reduce the reactor's power output in the event of an abnormal event. At Palo Verde, this occurs following a loss of a feed pump or the main generator. The system is designed to then drop selected subgroups of CEAs to reduce reactor power to match secondary power. This prevents the power mismatch from creating a heat-up event in the primary and causing a reactor trip.
Steam Bypass Control System (SBCS) (EIIS:JI): The SBCS is a control system to regulate the flow of steam and maintain optimal operating conditions. At Palo Verde, the system is designed to automatically bypass the main turbine and send steam from the main steam header directly to the condenser, atmosphere, or a combination of both, while controlling steam header pressure at setpoint. Valves open on a loss of the turbine, helping to maintain secondary heat removal.
3. INITIAL PLANT CONDITIONS
On April 8, 2023, PVNGS Unit 1 was in Mode 1 (Power Operation) at 100 percent power with the RCS at normal operating temperature and normal operating pressure. There were no SSCs that were inoperable at the start of the event that contributed to the event.
At the time of the event Unit 3 was in Mode 1 at 100 percent power and Unit 2 was in a planned refueling outage.
4. EVENT DESCRIPTION
On April 8, 2023, at 2141, Unit 1 A Control Oil (CO) pump tripped and the B CO pump automatically started. The running amperage for the B CO pump indicated low, and the CO system pressure continued to degrade until it reached the main turbine trip setpoint and tripped the main turbine at 2143. This resulted in a reactor power cut back (RPCB), but an automatic reverse power relay actuation did not occur to trip the generator switchyard output breakers.
At 2144, operators manually opened the generator switchyard output breakers per the applicable procedural guidance. A loss of power to the 13.8 kV non-class buses, NAN-S01 and NAN-S02, caused the Reactor Coolant Pumps (RCPs) to trip, which resulted in a RPS actuation and a reactor trip. The non-class loads were de-energized due to the loss of power to the non-class 13.8 kV buses, which included condensate pumps. The loss of the condensate pumps caused the main feedwater pumps to trip on low suction pressure. The B train Auxiliary Feed Pump, AFB-P01, was started manually to feed both steam generators.
At 2202, Unit 1 completed Standard Post Trip Actions (SPTA) and the Loss of Forced Circulation Emergency Operating Procedure (EOP) was entered. No Emergency Plan classification was required.
At 2204, operators manually actuated MSIS per the Loss of Forced Circulation EOP, due to the loss of the circulating water flow to the main condenser. This isolated the main steam header and prevented the use of the steam bypass control system, requiring the use of ADVs for heat removal.
At 2308, 13.8 kV power was restored to NAN-S01 through NAN-S03 from offsite power.
At 0124 on April 9, 13.8 kV power was restored to NAN-S02 through NAN-S04 from offsite power.
At 0310, RCP 1A and RCP 2A were restarted, restoring forced flow circulation of the RCS.
5. ASSESSMENT OF SAFETY CONSEQUENCES
Given the initial turbine trip and the subsequent associated response of Probabilistic Risk Assessment (PRA) modeled plant equipment, the conditional core damage probability (CCDP) for this event is 3.21E-06 and the conditional large early release probability (CLERP) for this event is 1.21E-07. The nuclear safety risk significance of the event was small, but greater than minimal due to the lack of fast bus transfer. These results do not take credit for any potential realignment of available offsite power to the non-class buses and recovery of main feedwater or alternate feedwater via the condensate pumps.
There was no radiological or industrial safety significance involved in this event.
6. CAUSE OF THE EVENT
The direct cause of the reactor trip was the turbine generator primary and backup reverse power relays failing to actuate due not reaching the trip setpoint timing. The lack of reverse power relay actuation, due to the transient highly reactive loading conditions, precluded a fast bus transfer (FBT) which ultimately led to a reactor protection system actuation and reactor trip. A reverse power actuation signal still had not been received when the generator switchyard breakers were opened, which again precluded the initiation of an FBT.
The root cause was the transient highly reactive, i.e., VAR, loading condition following the turbine trip that was outside the reverse power relays ability to respond. The large transient reactive load was a result of the aggregate effects of exciter, switchyard, and grid changes that have occurred over time, including series reactor installations in the switchyard, a switchyard line out of service for maintenance, a Palo Verde unit in a refueling outage, and light seasonal loading. The large transient reactive load caused the PF to be near zero which reduced the sensitivity of the reverse power relays.
A contributing cause is the ground fault on the B-phase cable splice connection that led to the trip of the A CO pump.
The fault occurred because there was insufficient insulation for the application. Insulation degradation occurred due to internal interference and normal system vibrations over time. The investigation concluded that there was inadequate guidance in site specifications for 480 VAC splice connections.
An additional contributing cause is the low level of hydraulic fluid that led to the B CO pump delay in developing pressure which caused the main turbine trip on low CO header pressure. The insufficient oil level was due to the decision to jumper out the emergency level switch alarm.
7. CORRECTIVE ACTIONS
IMMEDIATE INTERIM CORRECTIVE ACTIONS:
All three units will be in a split-bus operation, as operational risk allows, until the design modification to address the root cause is implemented. In a split-bus arrangement, one 13.8 kV non-class bus is aligned to the UAT, and the other is aligned to offsite power. This action will preclude the possibility of a complicated reactor trip should the reverse power relays fail to actuate.
The degraded cable splice connection on the A CO pump was repaired.
Addition of hydraulic fluid to the CO reservoir, replacement of the normal level indicating switch, removing the jumper for hydraulic fluid reservoir alarm on panel B06B, and retrieval/reinstallation of the missing float parts from the CO reservoir for the normal level indicating switch were completed for the B CO pump hydraulic fluid low level condition.
CORRECTIVE ACTIONS IDENTFIED DURING EVALUATION:
The corrective actions to prevent recurrence include tracking the implementation of a design change in each unit to zero the reactive power (VARs) post turbine trip and to conditionally remove the line drop compensation. The design change will also enhance the excitation system, i.e., EX2100E, to provide a fast bus transfer to offsite power (through NAN-S03 and NAN-S04) upon verification of main turbine stop or control valves and intercept or intermediate stop valves being closed. A separate design change to replace the electro-mechanical reverse power relays with microprocessor-based relay equivalents for the main generator is in progress.
The site specification for 480 VAC connections will be updated to add adequate guidance to prevent degradation of cable splice connection insulation in junction boxes.
A design change will be implemented to address the level float arm assembly set screw in the CO reservoir and maintenance instructions will be updated to ensure normal level indicating switch functionality during normal system operations and during testing. Additional actions are to revise site documents/procedures to ensure process gaps with identifying, prioritizing, and correcting deficient equipment conditions are addressed as well as implementing training and learnings for the use of troubleshooting and CO system operating experience.
8. PREVIOUS SIMILAR EVENTS
No similar events have been reported by PVNGS in the last three years due to the same direct cause.