05000260/LER-2010-003
| Browns Ferry | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2602010003R00 - NRC Website | |
I. PLANT CONDITION(S)
Prior to the events, Browns Ferry Nuclear Plant (BFN) Units 1, 2, and 3 were operating in Mode 1 at 100 percent thermal power (approximately 3458 megawatts thermal). The Unit 2 Reactor Protection Systems (RPS) [JC] A and B were being provided electrical power from their normal supply.
II. DESCRIPTION OF EVENT
A. Event:
On June 9, 2010, at approximately 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> Central Daylight Time (CDT), outboard Main Steam Isolation Valve (MSIV) A [SB] closed while transferring the Reactor Protection System (RPS) 120 VAC power from the normal to the alternate power supply in preparation for a planned activity. At approximately 0331 hours0.00383 days <br />0.0919 hours <br />5.472884e-4 weeks <br />1.259455e-4 months <br /> CDT Unit 2 received a Primary Containment Isolation Signal (PCIS) Group 1 [JE] isolation signal resulting in the closure of all of the MSIVs and automatic reactor scram.
During and following the performance of the RPS power transfer several actions took place rapidly leading to the automatic reactor scram. The sequence leading to the scram, shown below, was obtained from the Unit 2 event recorder starting at approximately 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> CDT.
6.7 seconds: The power to RPS B lost power during the manual transfer and subsequent expected half scram.
8.0 Seconds: The steam flow began to drop in Main Steam Line (MSL) A as the outboard MSIV A closes. The steam flow in MSLs B, C and D began to rise.
10.0 Seconds: The steam flow in MSL A is less than 2 million pounds per hour (Mlb/hr).
The flow in MSLs B, C, and D increased to greater than 4 Mlb/hr.
10.3 Seconds: Unit 2 receives a PCIS Group 1 initiation signal on high steam flow. All of the MSIVs go closed.
10.5 seconds: Unit 2 auto scram contactor A indicates alarm. The Auto scram contactor B is already tripped due to the signal from the B RPS lost power during power transfer. This leads to the full reactor scram.
During the MSIV scram, all automatic functions occurred as expected. All control rods [AA] inserted. Operations personnel briefly entered Emergency Operating Instruction, 2-E0I-001, "Reactor Pressure Vessel Control," controlling both reactor vessel pressure and reactor vessel water level.
The Group 1 PCIS initiation signal was the only isolation signal that was received prior to the reactor scram. The A Control Room Emergency Ventilation (CREV) [VI] system auto initiated.
Standby Gas Treatment (SGT) [BH] subsystems A, B, and C were in service prior to the event and continued to operate through the event. Operations personnel manually initiated High Pressure Coolant Injection (HPCI) [BJ] and Reactor Core Isolation Cooling [BN] (RCIC) systems to control reactor water level. Reactor pressure vessel pressure was controlled by manually opening one safety relief valve and the MSL drain valves.
At approximately 0335 hours0.00388 days <br />0.0931 hours <br />5.539021e-4 weeks <br />1.274675e-4 months <br /> CDT, Operations personnel reset the PCIS Group 1 Isolation Signal, and by approximately 0341 hours0.00395 days <br />0.0947 hours <br />5.638227e-4 weeks <br />1.297505e-4 months <br /> the reactor scram was reset. By 0405 hours0.00469 days <br />0.113 hours <br />6.696429e-4 weeks <br />1.541025e-4 months <br /> CDT the MSIVs were reopened, Operations personnel then controlled the reactor pressure with the turbine bypass valves [JI]. A heat rejection path was established using the main condenser [KE]. HPCI and RCIC were removed from service and the reactor water level was being maintained with the condensate [SG] and feedwater [SJ] systems.
At 0408 hours0.00472 days <br />0.113 hours <br />6.746032e-4 weeks <br />1.55244e-4 months <br /> CDT, with the reactor in Mode 3, BFN received a second RPS actuation which was invalid. Intermediate Range Monitor (IRM) [IG] F experienced a momentary signal increase with concurrent spikes on the IRM C. All systems responded as designed. All required safety functions were previously completed; therefore, no safety system actuations occurred. The invalid RPS actuation and reactor scram was reset at 0431 hours0.00499 days <br />0.12 hours <br />7.126323e-4 weeks <br />1.639955e-4 months <br /> CDT.
TVA is submitting this report in accordance with 10 CFR 50.73(a)(2)(iv)(A). An event that resulted in a manual or automatic actuation of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B) (i.e., reactor protection system including reactor scram or trip, and general containment isolation signals affecting containment isolation valves in more than one system or multiple main steam isolation valves (MSIVs)).
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Major Occurrences:
June 9, 2010, 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> CDT Unit 2 receives a PCIS Group 1 initiation signal June 9, 2010, 0331 hours0.00383 days <br />0.0919 hours <br />5.472884e-4 weeks <br />1.259455e-4 months <br /> CDT Unit 2 automatically scrammed on closure of the MSIVs.
June 9, 2010, 0341 hours0.00395 days <br />0.0947 hours <br />5.638227e-4 weeks <br />1.297505e-4 months <br /> CDT Operations reset the MSIV scram.
June 9, 2010, 0408 hours0.00472 days <br />0.113 hours <br />6.746032e-4 weeks <br />1.55244e-4 months <br /> CDT Unit 2 receives an invalid RPS actuation and automatic reactor scram on when IRMs F and C concurrently spiked.
June 9, 2010, 0431 hours0.00499 days <br />0.12 hours <br />7.126323e-4 weeks <br />1.639955e-4 months <br /> CDT Operations reset the invalid RPS actuation and scram.
June 9, 2010, 0658 hours0.00762 days <br />0.183 hours <br />0.00109 weeks <br />2.50369e-4 months <br /> CDT TVA made a four hour non-emergency report per 10 CFR 50.72(b)(2)(iv)(B) and an eight hour non-emergency report per 10 CFR 50.72(b)(3)(iv)(A).
D. Other Systems or Secondary Functions Affected
None
E. Method of Discovery
The automatic reactor scram from the MSIV closure was immediately apparent to Operations personnel through numerous alarms and indications in the main control room.
F. Operator Actions
Operations personnel responded to the reactor scram and MSIV closure according to Abnormal Operating Instruction, A01-100-1, "Reactor Scram," as required. Operations momentarily entered Emergency Operating Instruction, EOI-1, "Reactor Pressure Control.
G. Safety System Responses
Following the Group 1 RPS scram, all control rods inserted, CREV A auto started and SGT A, B, and C were in service prior to the scram and continued to operate through the event. No Emergency Core Cooling System setpoints were reached during the MSIV scram.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of the automatic scram was the closure of MSIV A which resulted in MSL high flow and PCIS Group 1 isolation and subsequent closure of all of the remaining MSIVs resulting in completing the actuation of the RPS.
B. Root Cause
The root cause for the MSIV A closure was indeterminate. TVA's analysis identified two possible causes: 1) Debris from the control air system in the Direct Current (DC) solenoid plunger area or 2) DC power system [El] electrical circuitry issues. TVA's post scram testing and evaluation did not find any evidence of a failure due to debris. However, Units 1 and 3 have a filter installed on the control air supply to the MSIVs. Unit 2 does not.
There is currently insufficient data to establish whether the DC power system may have played a role in the event.
C. Contributing Factors
None
IV. ANALYSIS OF THE EVENT
TVA's evaluation of the event indicates that MSIV A closed during the RPS power transfer resulting in an unanticipated PCIS Group 1 isolation and full reactor scram. As the MSIV closed, the reactor pressure and steam flow through the remaining steam lines increased until the PCIS Group 1 isolation setpoint was reached and all of the MSIVs closed. The Reactor automatically scrammed, as designed on MSIV closure.
The loss of RPS power was a planned evolution controlled by an existing plant procedure. By procedure the power to the 120-V AC was lost to RPS B during the power transfer and resulted in a half scram on Unit 2. Plant control air [LD] is used as a motive force for the outboard MSIVs.
Regardless of the status of the AC power, the MSIV air control is designed to keep the MSIV open with the DC powered solenoid and the subsequent full reactor scram should have not occurred. The procedure in use during the event requires a verification of the DC solenoid integrity by measuring voltage drop through the circuit. This verification was performed satisfactorily. However, during RPS power transfer, the DC solenoid failed to maintain the air supply to the A MSIV and as a result it closed.
With the exception of the DC-powered solenoid, all plant equipment and logic performed as expected during the event.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of the reactor scram from the MSIV closure was not significant. A reactor scram from 100 percent power is analyzed by the BFN Updated Final Safety Analysis Report. The MSIV closure scram is bounded by a generator load reject without bypass valves.
Following the reactor scram all engineered safety features functioned as designed. All control rods fully inserted. The reactor water level was controlled by the Operator using both HPCI and RCIC in the manual mode. The peak reactor pressure during the event reached 1052 psig therefore no relief valve setpoints were challenged. Once the MSIVs were reopened, the feedwater and condensate system were used for reactor vessel water level control. The reactor pressure vessel pressure was controlled with the turbine bypass valves. A heat rejection path was established using the main condenser. Therefore, WA concludes that the event did not affect the health and safety of the public.
VI. CORRECTIVE ACTIONS
A.� Immediate Corrective Actions The MSIV solenoid manifold which includes the three solenoids, AC solenoid, DC solenoid and a test solenoid, was replaced. The manifold that was in place during the event was sent to an offsite site facility for diagnostic evaluation.
To validate the performance of the replacement manifold, the MSIV was successfully fast stroked. RPS B was transferred back to the normal power to validate the AC, DC, and test solenoid coils functioned properly.
B. Corrective Actions to Prevent Recurrence - (The corrective actions to prevent recurrence are being managed by TVA's Corrective Action Program.) WA plans to Install a filter in the control air system upstream of the of the outboard MSIV control air header similar to Units 1 and 3. WA also plans to install transient monitoring equipment on the DC solenoid circuit and evaluate the results. If the results of the monitoring change the root cause and corrective actions, WA will provide a revised report.
VII. ADDITIONAL INFORMATION
A. Failed Components
None B. Previous LERs on Similar Events None
C. Additional Information
Corrective action documents for this report are Problem Evaluation Reports 233981 and 234151.
D. Safety System Functional Failure Consideration:
This event is not a safety system functional failure in accordance with NEI 99-02.
E. Scram With Complications:
This event was not a complicated scram according to NEI 99-02.
VIII. COMMITMENTS
None