ML20113D494

Application for Amend to License NPF-12,revising Tech Spec 3/4.1.3, Moveable Control Assemblies, to Permit 72 H for Performance of Diagnostic Procedures to Determine Source of Inoperability of Control Rods.Fee Paid

ML20113D494
Person / Time
Site:	Summer
Issue date:	04/09/1985
From:	Dixon O SOUTH CAROLINA ELECTRIC & GAS CO.
To:	Harold Denton Office of Nuclear Reactor Regulation
Shared Package
ML20113D497	List: ML20113D494 ML20113D507
References
NUDOCS 8504150323
Download: ML20113D494 (8)

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SOUTH CAROLINA ELECTRIC & GAS COMPANY post OFFtCE 764 CotuustA. south CAnouNA 29218

o. w. om oN.;.. April 9, 1985 VsCE PRESIDENT NUCLEAn OPEmanows Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, D.C. 20555

Subject:

Virgil C. Summer Nuclear Station Docket No. 50/395 Operating License No. NPF-12 Rod Control System

Dear Mr. Denton:

South Carolina Electric and Gas Company (SCE&G) hereby requests a revision to the Virgil C. Summer Nuclear Station Technical Specification 3/4.1.3, " Movable Control Assemblies."

The description of the proposed change and the significant hazards determination is forwarded in the Attachment.

This change has been reviewed and approved by both the Plant Safety Review Committee and the Nuclear Safety Review Committee. A check in the amount of one hundred fifty dollars

($150.00) is enclosed to initiate the processing of this request.

If you have any questions, please advise.

ruly ours, C s P f f*v Q . ,FV. Di n, Jr.

MDB/OWD/gj

Attachment:

cc: V. C. Summer C. A. Price T. C. Nichols, Jr./O. W. Dixon, Jr. C. L. Ligon (NSRC)

E. H. Crews, Jr. K. E. Nodland E. C. Roberts R. A. Stough W. A. Williams, Jr. G. Percival D. A. Nauman C. W. Hehl J. N. Grace J. B. Knotts, Jr.

Group Managers gh H. G. Shealy O. S. Bradham 40 NPCF File 8504150323 050409 ,00k k 6 \

PDR ADOCK 05000395 ihgo\

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• l ATTACHMENT Description of the Proposed Change to the Rod Control System Technical Specification I. Proposed Technical Specification Change It is proposed that Technical Specification 3.1.3.1 be revised to permit 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> for the performance of diagnostic procedures to determine the source of inoperability of the control rod (s) due to electrical problems and repair of the Rod Control System prior to orderly shutdown.

II. Justification for the Proposed Change A. Rod Control System Description

1) Arrangement of Mechanisms The Virgil C. Summer Nuclear Station is a three loop plant with forty-eight (48) full length rod drive mechanisms arranged into banks and groups.

A typical bank and group arrangement is shown in Figure 1. A group consists of four mechanisms that are electrically paralleled to step simultaneously. A bank of mechanisnm consists of two (2) groups that are moved in staggered fashion such that the groups are always within one step of each other. The arrangement includes two (2) shutdown banks (A and B) and four (4) control banks (A, B, C and D). Control banks are moved in overlap in the following withdrawal s equence: When Control Bank A reaches a predetermined height in the top half of the core, Control Bank B starts to move with A. Control Bank A stops at the top of the core and Bank B continues until it reaches a predetermined height in the top half of the core when Control Bank C starts to move out with B. This sequence continues until all rods are withdrawn. The insertion sequence is the opposite of the withdrawal sequence.

2) Main Control Room Controls Controls for the Rod Control System located in the Main Control Room are listed in Figure 2.

The In-Hold-Out lever is used for manual rod motion and is located on the Main Control Board.

Also, on the Main Control Board is a Bank Selector Switch with eight (8) positions. In the manual position, control banks are moved from the

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Attachment Description of the Proposed Change to'the. Rod Control. System Technical. Specification Page~2

2) Main Control Room Controls (cont' d )

In-Hold-Out. lever in: overlap. Control. banks are moved in overlap by the Automatic.Tavg Control System with the switch in the auto position. Six.

(6) additional positions are provided for individual bank movement.

Step counters, one for each group, are , located on the Main Control Board to display demanded rod position. A Digital Rod Position Indication System, not-connected to the Rod Control System, is used to display actual rod position and is used in conjunction with the step counters to determine deviation between demanded and actual position.

In-out lights'show the request for rod motion from either the In-Hold-Out lever or the Automatic Tavg Control System. A startup'pushbutton is provided to reset the step counters and all internal system counters such as the bank-overlap counter'on startup. An alarm reset pushbutton resets internal' system failure detectors and alarms which include a seal-in feature. Lift coil disconnect switches, one for each mechanism, are provided to assist in retrieval of a~ dropped rod.

Two (2) annunciators are located on the control board: a Rod Control Urgent Failure and a Rod Control Non-Urgent Failure Alarm. The Rod Control '.

Urgent Failure Alarm indicates that a control system failure has occurred that'would affect the' ability of the control system to move rods. A Rod Control Non-Urgent Failure Alarm indicates failure of one-or more redundant power supplies'that feed the system printed circuit cards.

3) Basic Thyristor Bridge Control Circuit Three-(3) thyristors forming a half wave phase

. controlled bridge supply current to four (4) mechanism coils (either lift, movable, or stationary gripper) as shown in Figure 3. Current feedback signals from shunts in series with each coil are used to regulate the current commanded by a slave cycler located in the system logic cabinet.

l Attachment Description of the Proposed Change

to the Rod Control System

. Technical Specification

~Page 3

4) Power-Cabinet Power Circuits Five (5) thyristor bridges form one system power cabinet as shown in Figure 4. Four (4) power cabinets are used in the system. The power cabinet amplifies low level command signals from a slave cycler in the logic cabinet. One (1) power cabinet drives three (3) groups of four (4) mechanisms and is capable of moving one (1) group- while holding the other two (2) in position. The selection as to which group is to move is made with multiplexing thyristors, one (1) for each group of movable coils and one (1)-for each lift coil. The lift coil multiplexing thyristors also serve as lif t disconnect switches for retrieving a dropped rod.

5) System _ Block Diagram Four (4) power cabinets are supplied with power from two motor-generator sets normally operating in parallel through two reactor trip breakers in series as shown in Figure 5. The logic cabinet includes a pulser, master cycler, bank overlap unit, and f our (4) slave cyclers. The pulser determines-the speed of rod motion as directed by the reactor Tavg control system when automatic operation is selected or by a preset speed when manual operation is selected. The master cycler directs pulses from the pulser alternately to the slave cyclers for the two (2) groups in a bank. Selection of which bank or banks are to move is done by the bank overlap unit.

The slave cycler sequences the mechanism coils through one (1) step, either -in~ or out for each "go" pulse-from the master cycler. k DC hold cabinet is

.provided to allow replacement of printed circuit cards 'in the power cabinet while the plant is in operation.

B. Rod Control System Diagnostic Features

1) Failure Detection and Alarms A Rod Control Urgent Failure Alarm is actuated by five (5) failure detectors in each power cabinet or by three (3) failure detectors in the logic cabinet as shown in Figure 6. A Rod Control Urgent Failure

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? Description of the Proposed Change to the Rod Control System TechnicalL Specification Page 4:

1) Failure Detection and Alarms (cont'd)

Alarm stops automatic rod motion and permits manual-movement of. a selected bank if the logic cabinet and the two (2) power cabinets associated with the selected bank are not in urgent alarm.

Detection of a failure by a failure detector results in the following indications:

-a. A failure detector lamp, one for each type'of f ailu re, located on the edge of a printed circuit card in the failed cabinet is energized.

b. A red urgent failure lamp on the front of the failed cabinet is energized.

c. A " Rod Control Urgent Failure Alarm" annunciator in the Main Control Room is actuated.

A Rod Control Non-Urgent Failure Alarm indicates fai' lure of one (1) of a number of redundant power supplies and does not effect the' operation of the system.

2) Ef fect of Mechanism Mechanical Failure on Control System The Rod Control System operates independently of the rod drive mechanisms. Nothing_has been included in the system, either by design or inadvertantly, to allow it to see movement of the mechanism mechanical parts, the drive shaf t or rod control clusters; therefore, mechanical binding would not be detected by or cause a Rod Control System alarm. This has

.bc2n verified many times during tactory checkout of completed systems where the test loads consist only of simulated mechanism coils with iron pipes in the

center-to approximate the magnetic properties of the coils. Checkout of the. Rod Control System at the site was done during Hot Functional Testing with only the mechanism coils connected and no reactor core, rods, or drive shaf ts in place.

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' Attachment' Description of the Proposed Change to the Rod Control System-

--Technical 1 Specification Page 5~

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-3 ) -How to Distinguish Between Control' System and Mechanical Problems Based on the previous discussion and Figure 7, a Rod

' Control Urgent Failure Alarm must be the result of a control-system' failure-and cannot be related to a mechanically inoperable rod or rods. There are failures that do not result in a Rod Control Urgent Failure Alarm that could prevent one or more rods

-from moving. In this case, the problem can be traced to either the control system or mechanism by monitoring the mechanism coil currents. Built-in test points are located in the power cabinets for this purpose. If the control system will not vary the currents'to-the mechanism coils, the problem must-be in the control system.and not the mechanisms. If the control system varies currents to .the coils, then the mechanism may be suspect.

Grossly abnormal currents would indicate control system problems and mildly abnormal currents would indicate mechanism problems. Recordings of the currents would have to be studied in this event.

Whatever the cause of the inability to move control rods, repair or shutdown consistent with existing technical specifications would be initiated.

C. -Process for Identifying Root Cause of Rod Control System Malfunctions

1) Troubleshooting.and repair of Rod Control system malfunctions-require adequate time to systematically implement diagnostic procedures to identify the malfunction and its root cause as well as repair the

. system.

Replacement of many, but not all, components in the logic and power cabinets is possible while maintaining all rods in a fixed position; however, determination of which' components can be replaced takes a thorough analysis of that component's functions in the system and should not be done hurriedly. The following typical sequence of events should occur for a major rod control problem in which no rods are dropped but a ROD CONTROL URGENT FAILURE ALARM is received:

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' Attachment Dascription of the. Proposed Change to'the Rod Control: System

Tachnical . Specification

.Pcge 6 C .' Process for Identifying Root Cause of Rod Control System Malfunctions (cont' d)

a. Initiate planning including review of technical information and troubleshooting procedures.

Notify the appropriate IEC supervisory personnel and technicians familiar with the Rod Control System. Allow time for travel to site and receive briefing on initial findings. Estimated time is 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

b. Implementation of detailed troubleshooting procedures and determination of problem area.

Estimated time is 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

c. Westinghouse rod control experts are contacted for technical assistance based on the results of the troubleshooting efforts of step "b" above.

Experts' travel to. site and anticipated repair efforts are. planned. Estimated time is 24 h our s.

d. Repair parts, if not available on site, are placed on emergency order and flown to site.

This time will include, as necessary,-time for part preparation (i.e., conditioning of process cards). Estimated time is 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

e. Repair is made and system is functionally tested. Estimated time is 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

D. Significant Hazards Determination The proposed change is considered to increase the margin of safety. Seventy two hours to diagnose and repair electrical problems will prevent hurried, and therefore

' less safe, diagnosis of malfunctions, other than rod misalignment or mechanical binding in the non-safety related Rod Control System. Insufficient time (i.e.,

six hours permitted by existing technical

j. specifications) to find the root cause of a malfunction necessitates-treating l

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4 Attachment Description of the Proposed Change to the Rod Control System Technicsl Specification Page 7 D. Significant Hazards Determination (cont'd) symptoms as opposed to implementing permanent solutions.

This change would minimize the potential for dropped or misaligned rods or any further Rod Control System malfunctions as a result of inadequate time for proper diagnostic test procedure performance. The additional time will also permit the operating personnel more flexibility in voluntary investigations of suspected rod control system problems.

The proposed change does not increase the probability or consequences of accidents previously evaluated because contr ~ rods will remain trippable, thereby maintaining shutdo'm margin. In addition, the power distribution will continue to be regulated by other existing technical specifications during the proposed 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> allowed for testing and repair of the Rod Control System.

The proposed change does not create the possibility of a new or different kind of accident previously evaluated; rather, it allows sufficient time for licensee personnel to perform orderly maintenance, thereby minimizing the potential for human error in the diagnosis and repair of rod control malfunctions. The 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> period permitted for troubleshooting and repair does not constitute a decrease in plant safety.