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LER 98-002-00:on 980816,personnel Observed Both Channels of Rod Block Monitor Sys Bypassed for Cr.Caused by Failure of Gene to Properly Document & Specify Adjustments.Will Implement Necessary Procedure Controls
	ML18039A517
Person / Time
Site:	Browns Ferry
Issue date:	09/15/1998
From:	Rogers A; TENNESSEE VALLEY AUTHORITY
To:	;
Shared Package
ML18039A516	List: ML18039A515; ML18039A517;
References
	LER-98-002-04, LER-98-2-4, NUDOCS 9809220041
	Download: ML18039A517 (12)
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NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 EXPIRES 04/30/Se (4.96) ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH THIS MANDATORY INFORMATION COLLECTION REQUEST) 50.0 HRS. REPORTED LESSONS LICENSEE EVENT REPORT (LER) LEARNED ARE INCORPORATED INTO THE LICENSINO PROCESS AND FED BACK TO INDUSTRY. FORWARD COMMENTS REOARDINO BURDEN ESTIMATE TO THE INFORMATION AND RECORDS MANAGEMENT BRANCH IT-e F33), U.S.

(See reverse for required number of. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC 20555+001, AND TO THE PAPERWORK REDUCTION PROJECT (3150%)04), OFFICE OF digits/characters for each block) MANAGEMENTAND BUDGET. WASHINGTON. DC 20603.

FACIUl'Y NAME l1) DOCKET NUMSEI( l2) PAO ERE l3)

Browns Ferry Unit 2 05000260 1OF6 Rod Block Monitor Inoperable During Control Rod Movement EVENT DATE l5) LER NUMBER (6I REPORT DATE (7) oTHER FAGILITIEs INYQLYED le)

MONTH DAY YEAR SEQUENTIAL REVISION MONTH DAY FACIUTY NAME DOCKET NUMBER I/UMBER NUMBER FACIUTY NAME DOCKET NUMBER 08 16 1998 1998 002 00 09 15 1998 NA oPERATING THIS REPORT IS SUBMITTED PURSUA NT To THE REQUIREMENTS OF 10 CFR 5: (Check one or moro)

MODE (9) 20. 2201 (b) 20.2203(a)(2) (v) X 50.73(a)(2)(i) 50.73 (a) (2) (viii)

PQWER 20.2203(a)(1) 20.2203(a)(3) li) 50.73(a) (2) lii) 50.73(a) (2)(x)

LEVEL (10) 20.2203(a) l2) (i) 20.2203(a) (3) (ii) 50.73(a) (2)(iii) 73.71 100

20. 2203 (a) (2) lii) 20.2203la)l4) 50.73(a) (2) liv) oTHER 20.2203(a)(2)(iii) 50.36(c)(1) 50.73(a) (2) (v) Specify in Abstract below

20. 2203(a) (2) (iv) 50.36(c)(2) 50.73(a) (2) (vii) or In NRC Form 366A LICENSEE CONTACT FOR THIS LER (12)

TELEPHONE NUMBER Ilnclude Ates Code)

A. T. Rogers, Senior Licensing'Project Manager (256) 729-2977 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE To CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE NPRDS TO NPRDS SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED MONTH DAY YEAR YES NO SUBMISSION DATE (15)

(If yes, complete EXPECTED SUBMISSION DATE).

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16)

During the weekly surveillance procedure to exercise fully withdrawn control rods on August 16, 1998, Unit 2 Operations personnel observed both channels of the Rod Block Monitor System (RBMS) bypassed for a control.rod which should not have caused this condition. Operations personnel entered the appropriate Technical Specification (TS) Limiting Condition for Operation (LCO) for the RBMS being inoperable and initiated corrective maintenance.

Troubleshooting on the RBMS identified that the system failed to operate correctly for selected control rods, only under a specific set of circumstances, due to lack of adjustment of a previously unidentified potentiometer. Engineers at General Electric Nuclear Energy (GENE) failed to properly document and specify the adjustments necessary in the newly installed Nuclear Measurement Analysis and Controls (NUMAC) Power Range Neutron Monitoring System (PRNMS) and RBMS.

Because this condition could have existed under limited circumstances since the installation of the current RBMS in October, 1997, TVA is reporting this event pursuant to 10 CFR 50.73(a)(2)(i)(B), as any operation or condition prohibited by the plant's Technical Specifications based on both channels of the RBMS being inoperable without completion of applicable required actions.

'))80922004i 980'3)25 PDR ADOCK 05000260 8 PDR

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NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION I4.96)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGENE 3 YEAR SEQUENTIAL REVISION NUMBER 2OF6 Browns Ferry Unit 2 05000260 1998 002 00 TEXT (if more space is required, use addirional copies ot NRC Form 366AJ I17)

I. PLANT CONDITIONS At the time of the event, Units 2 and 3 were in Mode 1 at 100 and 87 percent power respectively. Unit 1 was shutdown and defueled.

II. DESCRIPTION OF EVENT A. Event:

On August 16,1998, during the weekly performance of Unit 2 Surveillance Requirement (SR) 3.1.3.2, Control Rod Drive (CRD) [AA] Exercise for Fully Withdrawn Control Rods, the operator noticed that both RBMS channels were bypassed following selection of a non-edge control rod. This condition should not have caused either channel to bypass. The surveillance procedure was stopped and the appropriate TS LCO was entered for both RBMS channels being inoperable. Unit 3 does not have this type of RBMS design and it was verified that the current Unit 3 RBMS does not exhibit the problem.

Interim corrective actions were established in order for the RBMS to operate properly and control rod exercising continued on August 17, 1998. Troubleshooting and corrective maintenance revealed that a potentiometer was not properly adjusted. On August 21, 1998, potentiometer adjustments were made and post-maintenance testing verified that the RBMS was operating satisfactorily.

Because this condition could have existed under limited circumstances since the installation of the current RBMS in October, 1997, TVA is reporting this event pursuant to 10 CFR 50.73(a)(2)(i)(B), as any operation or condition prohibited by the plant's Technical Specifications based on both channels of the RBMS being inoperable without completion of applicable required actions.

B. Ino erable Structures'om onents orS stemsthatContributedtothe Event:

None.

C. Dates and A roximate Times of Ma or Occurrences:

October, 1997 NUMAC PRNMS and RBMS installed during Unit 2 refueling outage.

August 16, 1998 1029 hours0.0119 days 0.286 hours 0.0017 weeks 3.915345e-4 months CDT Operations personnel declared the RBMS inoperable and entered a TS LCO August 17, 1998 1813 hours0.021 days 0.504 hours 0.003 weeks 6.898465e-4 months CDT Administrative measures incorporated to comp'ensate for the RBMS problem and control rod exercising continued.

August 21, 1998 0855 hours0.0099 days 0.238 hours 0.00141 weeks 3.253275e-4 months CDT Potentiometer adjusted and post-maintenance testing completed on RBMS.

NRC FORM 366 I4-95I

0 II NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION I4.96)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAIVIE 1 DOCKET LER NUMBER 6 PAGENE 3 YEAR SEQUENTIAL REVISION NUMBER 3OF6 Browns Ferry Unit 2 OS000260 1998 002 00 TEXT iifmore spece is required, use eddirionel copies ot NRC Form 366A j I 17)

D. Other S stems or Seconda Functions Affected:

,None.

E. Method of Discove During performance of weekly control rod exercise, Operations personnel observed the RBMS not functioning properly.

F. 0 erator Actions:

The operators noticed the RBMS was not functioning properly, entered the appropriate TS LCO for the RBMS being inoperable, and initiated corrective maintenance. The TS LCO was met by placing both channels of the RBMS in trip.

'G. Safet S stem Res onse:

None.

III. CAUSE OF THE EVENT A. Immediate Cause:

The RBMS automatically bypassed during a certain sequence of control rod selection and failed to initiate a null sequence to adjust RBMS setpoints when required.

B. Root Cause:

GENE failed to properly include in its non-proprietaiy documentation sent to purchasers of its NUMAC PRNMS and RBMS the information necessary to ensure that Reactor Manual Control System (RMCS) and RBMS signal conditioning interface circuit potentiometer R77 adjustments were properly implemented.

C. Contributin Factors:

None.

IV. ANALYSIS'OF THE EVENT The RBMS is provided to prevent control rod withdrawal when reactor power should not be increased at the existing reactor coolant flow rate by initiating a control rod block at certain setpoints. By initiating a control rod withdrawal block, the system will prevent local fuel damage as a result of a single rod withdrawal erro'r under the worst. permitted condition of RBM bypass. The RBMS has two channels,'ach of which uses input signals from a number of Local Power Range Monitor (LPRM) channels. A trip signal from either RBMS channel can initiate a'rod block. One RBMS channel may be bypassed without loss of function. The RBMS is automatically bypassed if a peripheral control rod is selected or reactor power is below 30%. The RBMS si nalis eneratedb avera in a redeterminedsetofLPRMsi nals. TheLPRMsi nalsusedde endson NRC FORM 366 I4-SS)

41 0

NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (4.9SI LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGENE 3 YEAR SEQUENTIAL REVISION NUMBER 4OF6 Browns Ferry Unit 2 05000260 1998 002 00 TEXT (Il more space is required, use additional copies ot NRC Form 366A/ I17I the control rod selected for withdrawal or insertion. After selection of a control rod, a null sequence is initiated to reinitialize each RBMS channel and calculate the average of the related LPRM detectors and a gain factor that will adjust the average to 100%. Thereafter, until another rod is selected, the gain factor is applied to the LPRM average to obtain the RBMS signal value. The RBMS signal value is compared to the RBMS trip setpoints.

On August 16,1998, during the weekly performance of 2-SR-3.1.3.2, CRD Exercise for Fully Withdrawn Control Rods, the operator noticed that both RBMS channels were bypassed following selection of a non-edge control rod which should not have caused a bypass condition. Operations personnel immediately stopped the CRD exercise, entered the appropriate TS LCO and initiated corrective maintenance. The TS LCO was met by placing both channels of the RBMS in trip.

Site Engineering and Instrument Maintenance personnel commenced troubleshooting activities with assistance from GENE personnel. Further investigation revealed that the system was not initiating a RBMS null sequence upon selection of a control rod in the same RBMS rod group. The problem was traced to a signal conditioning circuit potentiometer adjustment. GENE provided TVA personnel with instructions to make the necessary potentiometer adjustment. TVA personnel implemented the instructions in a work order and completed the potentiometer adjustment on August 21, 1998. Post-maintenance testing verified that the RBMS was functioning properly.

TVA replaced the existing Average Power Range Monitor (APRM) and RBMS with a NUMAC PRNMS during the Unit 2 Cycle 9 refueling outage. The potentiometer (R77) adjustment was not identified by GENE to personnel involved in'preparation and approval of the modification PMT package at the site during the system installation. Therefore, the PMT scope did not include testing of this part of the RBMS internal circuitry.

Specifically, the PMT scope did not include selecting control rods within the same RBMS rod group to ensure that the null sequence initiation signal was being properly generated. Instead, the RMCS null initiate signal was simulated by performing a wire lift and reland. This resulted in satisfactory operation of the'RBMS null sequence but did not account for the actual RMCS null initiate signal pulse width which is considerably smaller than the pulse width generated from a wire lift and reland. The R77 potentiometer was found to be set to detect 7 millisecond pulses from the RMCS.which typically produces pulses in the 4 to 8 millisecond range. Consequently, not all RMCS null initiation pulses were being detected. The R77 potentiometer was readjusted to detect RMCS null initiate signals to less than 4 milliseconds to ensure that all such signals are processed.

The null sequence functions to reinitialize the setpoints applied for a rod block. The RBMS null sequence can be initiated from two sources. Selecting a control rod generates a unique rod group contact closure which is scanned by the RBMS and results in a null sequence being performed. Also, a null sequence can be initiated through the RMCS and RBMS signal conditioning circuit upon control rod selection. Therefore, selecting a control rod in a different control rod group would initially cause the null sequence to be performed via the unique control rod group selection logic path. Selecting a control rod within the same group does not result in a change in the unique control rod group contact which is closed thus leaving only the RMCS and RBMS signal conditioning circuit path which was not functional in certain circumstances because of the potentiometer setting. This resulted in erroneous RBMS setpoints being applied only during certain control rod manipulations.

NRc FQRM 366 I4-95)

0 ~P NRC FORM 366A U.S. NUCLEAR REGULATORY COMM)SS)ON i4-96)

LlCENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGENE 3 YEAR SEOUENTIAL REVISION NUMBER 5OF6 Browns Ferry Unit 2 05000260 1998 002 00 TEXT flfmore space is required, use additional copies of NRC Form 366Af )17)

V. ASSESSMENT OF THE SAFETY CONSEQUENCES The RBMS is designed to limit control rod withdrawal if localized neutron flux exceeds a predetermined setpoint during control rod manipulations. It serves to block further control rod withdrawal to preclude a Minimum Critical Power Ratio (MCPR) Safety Limit violation. Troubleshooting revealed that the system was automatically bypassing only during a unique set of conditions. During normal control rod withdrawals for power ascension, the system performed satisfactorily. However, during the control rod exercise, the control rods were being selected in a sequential pattern row by row. Therefore, the null sequence was failing to recalculate the rod block signal when a rod adjacent to the selected rod was in the same RBMS rod group.

During this unique pattern of control rod selection, the control rod is typically moved a single notch either in or out. Therefore, no significant changes in power would occur during this set of circumstances. Furthermore, at no time was a safety limit exceeded while the RBMS was not operable.

The RBMS is not considered a safety system but is an operational system for the purpose of backing up the operator to prevent a single operator error or equipment malfunction from causing fuel damage. The RBMS is a local power control system and is not used to control bulk power. As stated in the Final Safety Analysis Report, analysis has shown that even with multiple operator errors concurrent with equipment malfunctions a 10 CFR 20 dose event is not credible.

Based on the above, and considering that the RBMS would have failed to operate only during a very limited set of circumstances, it is concluded that there is no adverse impact on safety as a result of this event.

VI. CORRECTIVE ACTIONS A. Immediate Corrective Actions:

Operations implemented an administrative control to ensure the RBMS performed a null sequence setpoint calculation when required and did not improperly bypass the RBMS function for any applicable control rod.

B. Corrective Action to Prevent Recurrence:

TVA will review the resolution to the GENE internal Corrective Action Report to assess whether the potential for future events of this nature have been addressed.'VA will review PRNMS and RBMS design documents including a visual inspection of the system to verify that all potential analog adjustments have been identified and assist GENE in documentation and revision of vendor documents.'VA will implement the necessary procedural controls to periodically verify proper PRNMS and RBMS analog adjustments.

'TVA does not consider this corrective action a regulatory commitment. The completion of this item will be tracked in TVA's Corrective Action Program.

NRC FORM 366 (4-95)

O. Jl ll I%

< 4 NRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION I4-eel LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME 1 DOCKET LER NUMBER 6 PAGENE 3 YEAR SEQUENTIAL REVISION NUMBER 6OF6 Browns Ferry Unit 2 05000260 1998 002 00 TEXT ilfmore spaceis required, use addirional copies of NRC Form 366A/ I17)

VII. ADDITIONALINFORMATION 0

A. Failed Com onents:

None.

B. Previous Similar Events:

LER 296/97005 was similar in that it identified a condition prohibited by the plant's Technical Specifications reported under 10 CFR 50.73(a)(2)(i)(B). The root cause was that the Operations crew lacked a questioning attitude regarding an inoperable primary containment isolation valve. No corrective actions in the previous LER would have prevented the occurrence of the condition described in this report.

No other LERs have been issued which involve a involve a failure to enter an LCO.

LER 260/96006 was similar in that the General Electric provided BFN with non-conservative information regarding the Safety Limit Minimum Critical Power Ratio (SLMCPR). The root cause of the event was an error in the application of the methodology used to determine SLMCPR. This condition was reported under 10 CFR 50.73(a)(2)(v) as any event or condition that alone could have prevented the fulfillment of the safety function of the structures or systems required to mitigate the consequences of an accident. No corrective actions in the previous LER would have prevented the occurrence of the condition described in this report.

No other LERs have been issued which involve vendor supplied information.

VIII. COMMITMENTS None.

NRC FORM 366 {4-95)

Cl 4l li II I

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ML18039A517

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