ML20205H622
| ML20205H622 | |
| Person / Time | |
|---|---|
| Site: | Fort Saint Vrain  |
| Issue date: | 08/15/1986 |
| From: | PUBLIC SERVICE CO. OF COLORADO |
| To: | |
| Shared Package | |
| ML20205H603 | List: |
| References | |
| EE-12-0013, EE-12-0013-IA, EE-12-13, EE-12-13-IA, TAC-62198, NUDOCS 8608200121 | |
| Download: ML20205H622 (21) | |
Text
{{#Wiki_filter:- EE-12-0013 Issue A INTEGRATED SYSTEMS STUDY (ENGINEERING EVALUATION) 0F THE CONTROL R00 DRIVE MECHANISM R00 POSITION INDICATION INSTRUMENTATION FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO Prepared by: $;W h Reviewed By: Y 2'/I'O Nuclear Design Manager: D .MA[ L l.4f2 7h 6'- r 5~ ~ 8 6 l D 0K 5 0 67 ; P PDR
FORT ST. VRAIN NUCLEAR GENERATING STATION PUBLIC SERVICE COMPANY OF COLORADO @@GibOM O Service ~ CHECK LIST OF DESIGN VERIFICATION sy w. msw I PAGE QUESTIONS FOR DESIGN REVIEW METHOD YES NO.N/A 3 1. Were the inputs correctly selected and incorporated into design? g 2. Are assumptions necessary to perform the design activity adequately described and reasonable? Where necessary, are the assumptions identified for subsequent re-verifications when the detailed design activities are completed? 3 3. Are the appropriate quality and quality assurance requirements specified? g 4. Are the applicable codes, standards and regulatory requirements includ: 1 ssue properly identified and ara their requirements for design met? i and addenda dC 5. Have applicable construction and operating experience been considered? I 6. Have the design interface requirements been satisfied? d 7. Was an appropriate design method used? dU 8. Is the output reasonable compared to inputs? 3 9. Are the specified parts, equipment, and processes suitable for the required application? g 10. Are the specified materials compatible with each other and the design environmental conditions to which the material will be exposed? d 11. Have adequate maintenance features and requirements been specified? g 12. Are accessibility and other design provisions adequate for performance of needed maintenance and repair? g 13. Has adequate accessibility been provided to perform the in-service inspection expected to be required during the plant life? bb 14. Has the design properly considered radiation exposure to the public and plant personnel?
- 15. Are the acceptance criteria incorporated in the design documents sufficient to allow verification that design requirements have been satisfactorily accomplished?
] g 16. Have adequate pre-operational and subsequent periodic test requirements been appropriately specified?
- 17. Are adequate handling, storage, cleaning and shipping requirements specified?
W 18. Are adequate identification requirements specified? 3 19. Are requirements for record preparation review, approval, retention, etc., adequately specified? NOTE: If the answer to any question is no, provide additionalinformation and resolution below. RESOLUTION OF DESIGN DEFICIENCIES UNCOVERED DURING THE DESIGN VERIFICATION PROCESG W de ff s e.n d o w e e t. 4,waA ds;g N rw; t s o4 Wa E3;avig E *\w d;.a . K.%. M etw/n . Form ( A) 344 226
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EE-12-0013 Issue A TABLE OF CONTENTS Page No.
1.0 INTRODUCTION
AND
SUMMARY
1 2.0 SCOPE 1 3.0 APPROACH 2 4.0 EVALUATION 2
5.0 CONCLUSION
7 APPENDIX A: INDEX 9 APPENDIX A: FIELD DATA SHEETS 6 pages APPENDIX B: ANALYSIS OF CAM SLOPE 1 page APPENDIX C: SECTION CUT OF CONTROL R0D DRIVE MECHANISM 1 page APPENDIX D: SPECIFICATION FOR PROTOTYPE POTENTI0 METERS 1 page 8 ~__- ~-_ _ _ _ - - - - - - - -
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4 EE-12-0013 Issue A
1.0 INTRODUCTION
AND
SUMMARY
This is an integrated systems study - engineering evaluation (EE) of the Fort St. Vrain (FSV) Control Rod Position Indication (RPI) system. The evaluation was performed to determine if design deficiencies existed in the RPI system and the effect of these deficiencies on the reliability of the system. The RPI system consists of control rod-pair in-out limit switches, slack-cable limit switches, and control rod-pair position potentiometers, located on the control rod drive mechanisms. It also includes the associated lights and meters in the control room. The EE determined that deficiencies in the RPI system do exist and it makes recommendations for modifications to correct the deficisncies in a selected number of control rod drive mechanisms during the next refueling. The recommendations are included in the Conclusion section of this evaluation. These modified control rod drive mechanisms will be monitored for a set time period. If the result is favorable, it will serve as guidance for orderly upgrading of the remainder of the drive mechanism in a manner consistent with the overall control rod drive preventive maintenance program. Appendix A contains the field inspection data sheets and photographs of various new and used limit switches and position potentiometers. Appendices B, C, and D contain a sketch of the control rod drive mechanism and the cam actuation mechanism, and requirements for prototype potentiometers. 2.0 SCOPE This EE investigated several active and passive electrical components in the RPI system to identify any potential design deficiencies. These components included the control rod-pair full-in and full-out limit switches, slack-cable limit switches and rod-pair position potentiometers. The associated lights and meters in the control room were also evaluated. The evaluation consisted of reviewing FSV operational and taintenance records for corroded, worn, or malfunctioning parts. It included a field inspection of several identical RPI system switches, potentiometers, and ancillary mechanisms located in control rod drives having past operational history. The results were compared'with suppliers' maintenance drawings to determine physical ar.d electrical degradation. Suppliers were contacted for concurrence with the conclusions.
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. j EE-12-0013 Issue A 3.0 APPROACH 3.1 Define Operational Deficiencies The first step in evaluating the RPI system was to determine if deficiencies exist and their specific nature. This was achieved through interviews with field personnel who were responsible for correcting equipment problems or resolving documentation conflicts. Plant trouble reports, station service requests and nonconformance reports were reviewed to confirm the data gathered during the interviews.
3.2 Evaluate System Components As a result of deficiencies identified by the procedure described in Section 3.1, the components which make up the RPI system were evaluated. Some of these components were active, some passive. This evaluation was based on field inspections and review of applicable drawings, specifications and suppliers' catalog data, and preventive maintenance recommendations. 3.3 Review of Components Within Their System Folloting the evaluation of the deficient components in the RPI system, each was reviewed to determine its effect on other components in the RPI system. These combined effects were used to determine the total reliability of the system. This procedure was also used to locate potential single point failures. 3.4 Proposed Modifications Modifications were then proposed which would be expected to eliminate design deficiencies. These modifications should improve the RPI system reliability and eliminate the known failure modes. 4.0 EVALUATION 4.1 Evaluation of Components 4.1.1 Limit Switches 4.1.1.1 Full-In/ Full-Out Limit Switches The evaluation started by researching the application of the limit switches. These limit switches provide the indicating
EE-12-0013 Issue A signals for full-in or full-out position for each control rod pair. The most common problem found was the binding of the switch, which results in an inaccurate indication in the control room. Disassembly of the switches revealed pitting and erosion of the switch shafts, which resulted in the binding , of the switches. Further investigation I revealed that the pitting and erosion of the shafts, and the deposition of the shaft metal on the switch housings, was due to excessive lateral forces exerted on the limit switch shafts. Subsequent review and testing by the switch manufacturer confinned the analysis to be correct. That is "the slope of the cam is too steep for reliable long term operation of the switch." (See the sketch in Appendix B for clarification). 4.1.1.2 Slack-Cable Limit Switches The slack-cable limit switches provide the indicating signals for the control rod pairs in the event of a broken cable or a damaged rod. The review indicated no known failures of these switches. The principal of operation is very simple. When the rods are suspe.nded by the drive mechanism the limit switch is actuated, resulting in no signal in the control room. In the event of a broken cable or other failure in which weight is removed from the drive, a precision calibrated spring provides a force upward on the actuath g plate. This action results in the indication in the control room due to the change in state of the switch. Review of this application by the switch manufacturer confirmed this to be an acceptable application of the switch. 4.1.2 Position Potentiometers The position potentiometers provide control room indication of control rod position. When three position potentiometers were disassembled, an oxide
EE-12-0013 Issue A was found on the resistive element part of the potentiometer. The color of this resistive element relative to that of a new potentiometer was different. The brightness of the visible portion of the copper core on which the resistive element is wound was greater on an installed potentiometer than that of a new unit. These observations led to the following conclusions: 1) when moisture is introduced into the potentiometer, a voltage potential develops between the copper core and the resistive element in areas where the copper core is not completely sealed from the environment; 2) this results in the transfer of copper to the resistive element in the form of an oxide; 3) changes in the resistivity of the resistive element would account for some of the measurement inaccuracies. Subsequently, the manufacturer concurred that this analysis is credible. In other potentiometers inspected, broken bodies and/or drive gears were found. These potentiometers remained operational to some degree, but lost accuracy due to the damage sustained. 4.1.3 Indicating Hethods The methods utilized to indicate rod position in the control room were reviewed. They include: 1) 37 sets of full-in/ full-out and slack-cable indicating lights; 2) 37 analog gauges for position indication; and 3) a series of selectable digital meters for redundant position indication. The indicating lights are low voltage incandescent units wired to energize when the condition is to be indicated. The analog and digital gauges are voltmeters scaled to indicate l position. All position indication methods use a l current source to provide a varying voltage dependent on potentiometer resistance. All switches, potentiometers and rod position indicators are redundant. The limit switch indicating lights are not redundant, but are tested frequently. Although no significant concerns were identified relative to indication, it should be noted that these control room indicators were reviewed during the I-9303/04 human factors control board review (P-85454, 12 85). EE-12-0013 Issue A 4.2 Review of the Components Within Their Systems The movement of the control rod pair (by its drive motor in the outward direction or by gravity with motor control, in the inward direction) is by a cable connected to a rotating drum. As the drum rotates, the rod pair is moved. This drum, through gearing, is coupled to a shaft which drives a cam wheel for full-in/ full-out indication and to a potentiometer for position indication. The apparent deficiency in the system is this comon shaft. A review of failures indicates that prior to administrative limits (see Note 1) being established, this comon shaft, in some instances, has broken, resulting in loss of all indication. However, subsequent to the establishment of administrative limits, no known drive shaft failure has occurred. (See Appendix C for clarification.) NOTE 1: Administrative control has been extablished in P-85242, 7-10-85 (Interim Rod Indication Tech Spec) and G-85294, 7-23-85 (Supplemental Safety Evaluation). 4.3 Proposed Modifications 4.3.1 Limit Switches 1 The ootimum method of mechanical failure prevention is the use of non-contact (proximity) sensing to replace the full-in/ full-out cam-actuated limit switches. It is planned to replace the existing switches with proximity sensors specifically designed for FSV's application and environment. " Targets" for the sensors will be provided by replacing the existing cam (shown in Appendix B) with stainless steel metallic cubes. These targets are the same size as the existing cams and will not cause any new mechanical interference problems. The method of operation is as follows: 1) upon movement of the control rod pair, the cam wheel is rotated; 2) as the rod pair reaches full-in or full-out, the target will be drawn under the sensor, exactly as the present cam l is drawn under the switch (F3 in Appendix B); 3) when the target covers approximately fifty percent of the i sensor face, the sensor will indicate the condition to a central control unit which will switch the state of an electronic switch. The control unit is redundant for reliability. Failure of either control unit will sound an alarm in I
EE-12-0013 Issue A the control room. The balance of the system will function precisely as it was originally designed. 4.3.2 Position Potentiometers Several areas of technology were reviewed in position instrumentation designs and concepts. The first was that of shaft encoders. These included optical encoders, magnetic encoders, counting units, multi-turn, single turn, bi-directional and dual phase units. All these units perform the similar function of converting rotary motion to a signal (which varies in nature from unit to unit). The manufacturers in all cases indicated that the available technology is insufficient for this application. The primary concern was thermal effects on electronic components within the encoders. Review was also conducted in_the field of resolver technology (selsyns). Selsyn technology was determined to be unsuitable for use in RPI at FSV. This is due to the very high cost of modifications and is not a viable solution when other technologies can be used. (In order to use selsyns, the present Control Rod Drive Mechanism would require a full scale redesign and remanufactu.re. The electrical penetrations into the reactor core would require replacement. Also the control room modifications would be very extensive because the present indication s is incompatible with selsyn technology.) ystem Given the shortcomings of these design concepts, investigations into the technology surrounding potentiometer design was initiated. Various manufacturers were contacted, including the manufacturer of the potentiometers currently used. Several design requirements were established, which if met, could provide a highly reliable device. These requirements are shown in Appendix D. They are the result of the following primary system operating goals: 1) eliminate the potential for potentiometer or drive a high shaft damage degree due resistance; of moisture to overdriving;)2) provide 3 seal all copper away from the resistive element. If all these goals were met, the past failure modes would be eliminated. EE-12-0013 Issue A
5.0 CONCLUSION
S 5.1 Operational Deficiencies Operational deficiencies of the RPI system are known to exist. However, after installation of the modifications presented in this EE, the proven components are expected to eliminate these concerns. 5.2 System Components Component failures have occurred as discussed in section 4.0 of this EE. They have various causes which are associated primarily with the original design of the Control Rod Drive Mechanism. They inclue ; 1) switches damaged by cams with slopes which are te steep; 2) potentiometers with broken bodies and shafts due to being overdriven and;
- 3) potentiometers having materials in them which when exposed to a humid environment develop an oxide resulting in inaccurate output.
The new (designed and manufactured per Appendix D)offerpotentiometers a viable solution to the potentiometer concerns in that they are highly resistant to moisture, and all copper is sealed from the effsets of moisture. The new sensors for full-in/ full-out position indication cannot be damaged by the environment (unit is sealed) or the mechanism as they have no moving parts, are non-contact and are capable of operation at several hundred degrees Fahrenheit above the requirements of the control-rod drive environment. l 5.3 Components Within Their Systems l l The review of the components within their systems revealed a high degree of interaction which is accomplished accurately as long as the signal sensors are operational. l Only one area of concern was revealed. In the control rod
- l drive mechanism, one shaft drives the cam gear (which
! provides for full-in/ full-out switch actuation) and drives the 2-gang potentiometer (which provides the position signals). In the event of shaft breakage, all position sensing is lost. To prevent actual drive shaft failure, administrative controls provide for the movement of the control rod drive mechanism such that defined limits are not exeeded. These limits do not allow operators to " drive" the rods past the full-in or full-out position in an attempt to
EE-12-0013 Issue A actuate the position switches if they appear to be non-functional. Exceeding - these limits can jam the potentiometer and potentially break the drive shaft. To further preclude this loss of indication, the new potentiometers have fifty percent more turns than the existing potentiometers. This will make it virtually impossible to jam the potentiometer due to overdriving. 5.4 Modifications The modifications presented in this EE will be installed for a selected number of control rod drive mechanisms during the next refueling outage. These components and their performance will be monitored for a set time. The length of time will be based on failure frequency from historical reports. If the results are favorable, an action plan will be developed for upgrading the balance of the control rod drive mechanisms in a manner consistent with the overall control rod drive maintenance program. On the basis of this EE, it is determined that the existing RPI system is operational within the administrative limits discussed in Section 5.3 above. It is also shown that the installation of i newly developed components could result in significantly improved reliability and extended life for the system. l
Appendix A: Field Data Sheet Index Number Item #/ Component Title Supplier Data Sheet Originator Date Page 1 #1/ Limit Switch Microswitch /3-/4 -8/ Page 2 #2/ Limit Switch Microswitch ~ / 4 -M6 Page 3 #3/ Limit Switch Microswitch m m-- _ 8-/4 -86 Page 4 #4/ Potentiometer Beckman _
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EE-12-0013 - Issue A SPECIFfCAT10N FOR PROTOTYPE POTENTI0 METERS Appendix D, page 1 0 PublicService 2420 W. 26th Avenue Suite 100-0 Denver, CO 80211 Company of Colorado P.O. 8cx 84o Denver. CO 8o201 -0840 (303) 571 - 7511 February 8, 1985 Fort St. Vrain Unit No. 1 NDG-85-0081 Beckman Industrial Corp. 901 0xford Street Toronto, Ontario Canada M8Z ST2 Attn: Dan McBride
Dear Mr. McBride:
Below listed are our revised requirements for the quantity . l of potentiometers shown on the attached requisition. Please t note this letter is specification for construction of required potentiometers. Requirements: 1 - 10 Turn 3600* Elect. 5400* Mech. , Elec. Turns , Centered in Mechanical Turns l 2 - Mounting Same as Bechman 7603 3 - 2 Gang - Electrically Independent 4 - 1000 OHMS t 5% 5 - Linearity 91/10 of 1% Error 6 - High Reliability Seal - Per Item 10 7 - High Temperature Reliability - 300'F Min. 8 - Power Rating, Watts 5 9 70*F, Derating to 0 9 Maximum Temperature Rating 9 - Taps - Conductors or Terminals Penetrating "Can" (May Be Axial) 10 - Ofrect Water Ingress Resistance to Maximum Level ! Reasonably Attainable Maintaining l Torque < 10 In. Oz. l 11 - Seismic Qualification - STD for Series 7600 l 12 - Shock - - - - - - - - - STD for Series 7600 l 13 - Standard Tests Not Named Above i 14 - Hydrolizable Chlorides Subject to Item 16 15 - Copper may be used only if it is sealed , l completely away from other parts. 16 - Due to the atmosphere involved, all materials require approval prior to their application. 17 - Pressure = 845 P.S.I. 18 - Voltage Tracking Sect. I to Sect. 2 = .25%.
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Should you have any questions or desfre further information, l please do not hesitate to contact me at (303) 571-7134. ry truly yo s, Wry w ames G. Henderson JGH/dh Attachment _}}