05000260/LER-2010-003, Regarding Reactor Scram Due to Closure of the Main Steam Isolation Valves and Subsequent Invalid RPS Scram from the Intermediate Range Monitoring System

Regarding Reactor Scram Due to Closure of the Main Steam Isolation Valves and Subsequent Invalid RPS Scram from the Intermediate Range Monitoring System

ML102240258
Person / Time
Site:	Browns Ferry
Issue date:	08/09/2010
From:	Polson K Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 10-003-00
Download: ML102240258 (7)

LER-2010-003, Regarding Reactor Scram Due to Closure of the Main Steam Isolation Valves and Subsequent Invalid RPS Scram from the Intermediate Range Monitoring System

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(1) 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(iv)(B), System Actuation
2602010003R00 - NRC Website

text

Tennessee Valley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 August 9, 2010 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D. C. 20555-0001 10 CFR 50.73 Subject: Licensee Event Report 50-26012010-003-00 The enclosed Licensee Event Report (LER) provides details of a valid reactor scram due to closure of the Main Steam Isolation Valves (MSIV) and a subsequent invalid actuation of the Reactor Protection System (RPS) from the Intermediate Range Monitoring (IRM) system that occurred following completion of the safety functions.

The initial verbal notification of the invalid RPS actuation from the IRMs stated that it was reportable per 10 CFR 50.72(b)(3)(iv)(A) and required an LER within 60 days per 10 CFR 50.73(a)(2)(iv)(A). However, the Tennessee Valley Authority (IVA) has since determined that this event is not reportable per 10 CFR 50.72(b)(3)(iv)(A) or 10 CFR 50.73(a)(2)(iv)(A) since the actuation was from an invalid signal and occurred after the safety function had already been completed. Discussion of the IRM related RPS actuation in the enclosed LER is for completeness of the event description.

The TVA is submitting this report in accordance with 10 CFR 50.73 (a)(2)(iv)(A) as any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B)(1) and (2), i.e., the Reactor Protection System, including reactor scram or reactor trip, and general containment isolation signals affecting more than one system.

There are no commitments contained in this letter. Should you have any questions concerning this submittal, please contact D. W. Williamson acting Site Licensing and Industry Affairs Manager at (256) 729-2636.

K. J. Poison Vice President cc: See page 2

U.S. Nuclear Regulatory Commission Page 2 August 9, 2007 Enclosure cc (w/ Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 EXPIRES 08/31/2010 (9-2007)

, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. PAGE Browns Ferry Unit 2 05000260 1 of 5

4. TITLE: Reactor Scram Due to Closure of the Main Steam Isolation Valves and Subsequent Invalid RPS Scram From The Intermediate Range Monitoring System

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED SEQUENTIAL REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUNE R

EV MONTH DAY YEAR None NUMBER NO.

NoeN/A FACILITY NAME DOCKET NUMBER 06 09 2010 2010 -

003 00 08 09 2010 None N/A

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 0l 20.2201(b)

[I 20.2203(a)(3)(i) 0l 50.73(a)(2)(i)(C)

El 50.73(a)(2)(vii) 0l 20.2201(d)

E] 20.2203(a)(3)(ii)

[I 50.73(a)(2)(ii)(A)

[I 50.73(a)(2)(viii)(A)

[I 20.2203(a)(1)

El 20.2203(a)(4)

El 50.73(a)(2)(ii)(B)

[I 50.73(a)(2)(viii)(B)

El 20.2203(a)(2)(i)

El 50.36(c)(1)(i)(A)

[I 50.73(a)(2)(iii)

El 50.73(a)(2)(ix)(A)

10. POWER LEVEL El 20.2203(a)(2)(ii)

El 50.36(c)(1)(ii)(A) 0 50.73(a)(2)(iv)(A)

[I 50.73(a)(2)(x)

[I 20.2203(a)(2)(iii)

El 50.36(c)(2)

El 50.73(a)(2)(v)(A)

[1 73.71(a)(4)

E] 20.2203(a)(2)(iv)

E] 50.46(a)(3)(ii)

El 50.73(a)(2)(v)(B)

El 73.71(a)(5) 100 [E

20.2203(a)(2)(v)

El 50.73(a)(2)(i)(A)

El 50.73(a)(2)(v)(C)

El OTHER El 20.2203(a)(2)(vi)

El 50.73(a)(2)(i)(B)

[E 50.73(a)(2)(v)(D)

SpFcity3nAbstractbels...inNRC

12. LICENSEE CONTACT FOR THIS LER NAME TELEPHONE NUMBER (Include Area Code)

Steve Austin, Licensing Engineer 256-729-2070MANU-REPORTABLE MANU-REPORTABLE

CAUSE

SYSTEM COMPONENT FACTURER TO EPIX

CAUSE

SY FACTURER TO EPIX

14. SUPPLEMENTAL REPORT EXPECTED

15. EXPECTED MONTH DAY YEAR SUBMISSION El YES (If yes, complete 15. EXPECTED SUBMISSION DATE)

[

NO DATE N/A N/A N/A ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)

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 closed while transferring the Reactor Protection System (RPS) 120 V-AC 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 isolation signal resulting in the closure of all of the MSIVs and automatic reactor scram. During the scram, all automatic functions occurred as expected. All control rods inserted. Operations personnel briefly entered Emergency Operating Instruction, 2-EOI-001, "Reactor Pressure Vessel Control," controlling both reactor vessel pressure and reactor vessel water level. 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. The root cause for the MSIV closure event is 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 electrical circuitry issues. TVA plans to Install a filter in the control air system upstream of the of the outboard MSIV control air header. TVA 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, TVA will provide a revised report.

NRC FORM 366 (6-2004)

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 V-AC 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 MSIVA closes. The steam flow in MSLs B, C and D began to rise.

10.0 Seconds: The steam flow in MSLA 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-EOI-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 Mamor Occurrences:

June 9, 2010, 0330 hours0.00382 days <br />0.0917 hours <br />5.456349e-4 weeks <br />1.25565e-4 months <br /> CDT June 9, 2010, 0331 hours0.00383 days <br />0.0919 hours <br />5.472884e-4 weeks <br />1.259455e-4 months <br /> CDT June 9, 2010, 0341 hours0.00395 days <br />0.0947 hours <br />5.638227e-4 weeks <br />1.297505e-4 months <br /> CDT June 9, 2010, 0408 hours0.00472 days <br />0.113 hours <br />6.746032e-4 weeks <br />1.55244e-4 months <br /> CDT June 9, 2010, 0431 hours0.00499 days <br />0.12 hours <br />7.126323e-4 weeks <br />1.639955e-4 months <br /> CDT June 9, 2010, 0658 hours0.00762 days <br />0.183 hours <br />0.00109 weeks <br />2.50369e-4 months <br /> CDT Unit 2 receives a PCIS Group I initiation signal Unit 2 automatically scrammed on closure of the MSIVs.

Operations reset the MSIV scram.

Unit 2 receives an invalid RPS actuation and automatic reactor scram on when IRMs F and C concurrently spiked.

Operations reset the invalid RPS actuation and scram.

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, AOI-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, CREVA auto started and SGTA, 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 MSIVAclosure 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 MSIVA 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, TVA 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.)

TVA 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. TVA 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, TVA 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