05000416/LER-2001-003
| Docket Number | |
| Event date: | 08-07-2001 |
|---|---|
| Report date: | 10-04-2001 |
| 4162001003R00 - NRC Website | |
{{#Wiki_filter: A. Reportable Occurrence On August 7, 2001 at approximately 2017 hours, while operating at steady state conditions, a reactor scram occurred (10CFR50.73 (a) (2) (iv)(A)). The scram was the result of a Reactor Protection System (RPS)[JC] actuation from a turbine control valve fast closure signal. This signal was the result of a generator load transient caused by failure of a 500 kV disconnect located in an offsite switchyard. The switchyard where the failed disconnect was located directly feeds the GGNS switchyard.
A generator load reject was sensed due to the grid transient, causing generator load demand and valve rate limiters to automatically switch off. A turbine control valve fast closure signal was sensed as Turbine Control Valves (TCVs) [TA] began to close in response to a sudden drop in load. The reactor scrammed but the End of Cycle Recirculation Pump Trip (EOC-RPT) did not occur (10 CFR 50.73 (a) (2) (vii)).
No major equipment was out of service at the time of the scram. The Reactor Core Isolation Cooling System (RCIC)[[BN] was inoperable due to the performance of corrective maintenance, however the maintenance had been completed and the system was functional at the time of the scram. All control rods inserted and all other equipment responded as expected, except for the Condensate/Feedwater System which was lost after the scram occurred due to hotwell level problems. RCIC was manually initiated and began injecting into the vessel. The manual initiation of RCIC is not reportable under 10CFR50.73 (a) (2) (iv) because feedwater remained in service long enough to restore level to near the normal band. RCIC was started as an option for level control. No ECCS initiated as a result of this event.
Notification was made to the NRC's Emergency Notification System (ENS) reporting this condition pursuant to 10CFR50.72(b)(2)(iv) (B) — "any event or condition that results in actuation of the reactor protection system (RPS) when the reactor is critical except when the actuation results from and is part of a pre-planned sequence during testing or reactor operation" (RPS) [JC].
An update to this ENS report was made reporting the EOC-RPT failure. This condition was reported pursuant to 10CFR50.72(b)(3)(v)(A) — "Any event or condition that at the time of discovery could have prevented the fulfillment of the safety function of a structure or systems that are needed to shut down the reactor and maintain it in a safe shutdown condition". Upon further review and based on engineering judgement, it is expected that the EOC-RPT function to down-shift the recirculation pumps would have occurred under the design basis limiting condition i. e., full generator load rejection with failure of the main turbine bypass valves.
The partial load rejection that occurred during this event is a operational transient. This event is being reported as a single cause or condition (design flaw), that caused two independent channels to become inoperable in a single system designed to mitigate the consequences of an accident (10 CFR 50.73(a) (2) (vii) (D)).
B. Initial Conditions At the time of the event the reactor was in OPERATIONAL CONDITION 1 with reactor power of approximately 100 percent. Moderator temperature was approximately 540 degrees F and reactor water level approximately 36 inches.
C. Description of Occurrence On August 7, 2001, at approximately 2017 hours, while operating at steady state conditions, a reactor scram occurred. The scram was the result of RPS actuation from a TCVs fast closure signal. This signal was the result of a generator load transient caused by failure of a 500 kV disconnect located in an offsite switchyard.
This offsite switchyard where the disconnect failed directly feeds the GGNS switchyard. As a result of the TCV fast closure, reactor pressure increased to approximately 1040 psig.
Turbine bypass control valves responded properly and no safety relief valves lifted. Reactor water level dropped immediately after the scram due to void collapse, and both recirculation pumps downshifted on reactor water level 3. Reactor water level dropped to about —35 inches wide range on the initial transient. Feedwater responded properly and restored level.
While feedwater was being placed on startup level control, a lowering level trend made it necessary to increase reactor feed pump discharge pressure to feed the vessel. The feedwater system adjustment resulted in an inadvertent overfeed and reactor water level 8 (53.5 inches) and reactor water level 9 (56.0 inches) set points were exceeded briefly. Reactor water level peaked at about 58 inches wide range. Subsequently, hotwell level control malfunctioned causing condensate pumps to trip on low hotwell level and loss of feedwater. Reactor water level dropped to about 15 inches wide range and was recovered after RCIC was manually initiated as an option to maintain reactor water level until hotwell level was restored and the condensate/feedwater system was returned to service. No ECCS initiation or Primary/ Secondary Containment isolation set points were reached and all safety systems functioned properly, except for the EOC-RPT failure. All other equipment operated as expected. No plant conditions or evolutions in progress at the time of the scram had an effect on the events leading to the scram or on the consequences of the scram.
D. Apparent Cause Based on the post trip analysis, the cause of the reactor scram was TCV fast closure as sensed by secondary fluid pressure of less than 46.0 psig as load demand turned off and rate limiter was bypassed due to a sensed load reject signal. The initiating event was a phase-to-phase fault at an offsite switchyard that directly feeds the GGNS switchyard and a subsequent transient on the 500 kV feeder. The fault on the feeder line resulted in generator load fluctuations, which were sensed by the turbine EHC logic as a generator load reject.
The root cause for failure of the EOC-RPT function to actuate was determined to be installation of an inadequate design, where a turbine control system failure could impact the NSSS system.
FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET (2) Attachment 2 to GNRO-2001/00073 E. � Corrective Actions
Immediate Corrective Actions:
1. CR-GGN-2001-1371 was initiated to investigate the initiating event for the scram and the failure of EOC- RPT to actuate.
2. An on-shift post trip analysis was performed which provided an adequate basis for restart based on identification of the initiating event and assessment of safety system performance. Restart was allowed but restricted to less than 40 percent Core Thermal Power and slow speed recirculation pumps until the EOC- RPT Technical Specification was changed to allow operation above 40 percent power via emergency Technical Specification Amendment 148.
Long Term Corrective Actions:
3. ER 2001-0285 is being developed for the EOC-RPT logic.
F. Previous Occurrences
15, 2000. This scram was the result of a turbine control valves fast closure signal. The EOC-RPT function did not occur during this event. The EOC-RPT failure was reported in LER 2000-006-00, dated October 13, 2000.
G. Safety Assessment The safety significance of this event concerns the failure of the EOC-RPT to function in response to a partial generator load reject and the associated TCV fast closure. The purpose of this function is to trip the recirculation pumps to slow speed in anticipation of a pressurization transient in the reactor vessel. The EOC-RPT is activated whenever main turbine electro-hydraulic control (EHC) secondary fluid pressure drops below the trip setpoint. The conditions necessary to achieve the trip set point are governed by the turbine control system. The EOC-RPT function is credited in the cycle-specific transient analysis in the development of the MCPR operating limit.
As described in UFSAR Section 15.2.2, the generator load rejection and the generator load rejection with failure of the bypass valves are classified as incidents of moderate frequency or anticipated operational occurrences. Events in this category are analyzed to prevent fuel cladding failures. In this case, operating limits are established, such that if the event occurs, the Technical Specification 2.1.1.2 MCPR safety limit and Linear Heat Generation Rate (LHGR) overpower criteria are protected. During this event, all control rods inserted as expected on the RPS scram associated with the Turbine Control Valve fast closure.
G. Safety Assessment (Continued) Additionally, the TCVs did not completely close and the bypass control valves responded properly. As such, the vessel pressurization for this event was significantly less severe than that of the current analyses, which assumes full closure of the TCVs and failure of the bypass system. Therefore, the actual event is bounded by the current analyses ensuring that the MCPR safety limit and LHGR overpower criteria were protected and the health and safety of the public were not compromised.
H. Additional Information
Energy Industry Identification System (EIIS) code are identified in the text within brackets [ ].
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