ML20195J227
| ML20195J227 | |
| Person / Time | |
|---|---|
| Issue date: | 05/25/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20195J225 | List: |
| References | |
| PROJECT-692 NUDOCS 9906180143 | |
| Download: ML20195J227 (6) | |
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e SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION CE OWNERS GROUP TOPICAL REPORT CE NPSD-951. REVISION 1 JUSTlFYING " REACTOR TRIP CIRCUlT BREAKERS SURVEILLANCE FREQUENCY EXTENSION" 1.0 - ' INTRODUCTION By letter dated July 17,1996 (Reference 1), Energy Operation Inc., (E.l.), the licensee of Waterford Steam Electric Station, Unit 3, requested changes tp olant Technical Specifications i(TSs). The request constituted a lead-plant submittal, based on the Combustion Engineering Owners. Group (CEOG) Topical Report CE NPSD-951 Revision 1, " Reactor Trip Circuit Breakers Surveillance Frequency Extension." The licensee stated in the letter that to support -
the proposed changes to p' ant TSs; the CEOG will send its topical report for NRC review and approval under separate cover. By letter dated September 18,1996 (Reference 2), the CEOG submitted the topical report and stated that the'CEOG endorsed the proposed change to the
- Waterford TSs as a lead-plant submittal for the CEOG This report was prepared by ABB Combustion Engineering (ABB-CE) and presents the technical basis to justi'y increasing the reactor trip circuit breakers (RTCBs) functional surveillance test interval (STI) from the current 1-month to a 3-month STI, This report examines the operating history and failure rate of the l
RTCBs in the CE-designed nuclear steam supply system (NSSS) power plants to justify the STI l
extension for the RTCBs. The purpose of the topical report is to justify a reduction in testing of l-the RTCBs and thereby increase their reliability while minimizing the potential for an inadvertent
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. plant trip. The topica.1 report also recommended extendir,g the allowed outage time (AOT), for closing an inoperable RTCB to permit testing of other RTCBs, from the current 1-hour AOT, allowed by the Improved Standard Technical Specifications (STSs) (NUREG-1432) for CE plants, to a 2-hour AOT. Following review of the subject topical report, the staff requested additional information, by letter dated September 8,1998 (Reference 3). The licensee responded to the staff request by letters dated October 22,1998 (Reference 4), January 12,1999 (Reference 5), and February 5,1999 (Reference 6). The licensee further withdrew the
'TSs change request to extend the AOT from'1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> for closing an inoperable RTCB to permit testing of other RTCBs. Subsequently,' a licensee letter dated March 16,1999 (Reference 7) submitted a summary of telephone calls between ABB-CE, NRC staff, and the licensee during which ABB-CE agreed that additional information would be required to support the.'AOT extension and it would be acceptable to deny an extension of AOT at this time, given a
- need for additional justification.
The evaluation that follows addresses the acceptability of the topical report justification for the STI extension and denial of the AOT extension because of insufficient justification in the topical report.
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. The evaluation that follows addresses the acceptability of the topical report justification for the STI extension and denial of the AOT extension because of insufficient justification in the topical report.
2.0 EVALUATION in the topical report, CEOG made the following recommendations to change the TS requirement of RTCB surveillance testing and provided justification for those recommendations:
I 1.
The STI for manual functional test of RTCBs should be extended from monthly to quarterly. This should be staggered with the automatic functional test of the reactor
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protection system (RPS) so that all the RTCBs are tested once every 6 weeks.
2.
The AOT for closure of an inoperable RTCB to permit testing M other RTCBs should be extended from 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
1 3.
A licensee applying for an extension of this STI and AOT, should review its maintenance j
frequency to ensure that (a) the appropriate recommendations are being followed and (b) the frequency of such maintenance is appropriate.
The staff's review of the topical report justification for Recommendations 1 and 2 follows:
STI Extension for RTCB Manual Functional Test The RPS in all operating CE power plants, except for Palisades and Fort Calhoun, utilizes RTCBs to interrupt power to the control element drive mechanisms (CEDMs) to trip the reactor (Palisades and Fort Calhoun use contactors instead of circuit breakers). Power is interrupted to CEDMs when an RTCB is tripped by either de-energizing a normally energized under-voltage j
(UV) trip device or energizing a normally de-energized shunt trip device by a manual or an automatic RTCB trip signal. The UV trip device is one major concern with an RTCB operation. In the topical report, the CEOG stated that the UV trip device is more complex than the shunt trip device because the UV trip devices are designed to be normally energized. Being normally energized means that they operate at relatively higher temperatures and that they must rely on a spring for opening the device. An adjustment of this spring and the settings at which the coilis energized or de-energized, requires special attention. Being spring-actuated, the UV device is more sensitive to the torque required to rotate the trip shaft and its lubrication. Problems of sluggish operation of these devices are generally remedied by cleaning and lubrication. Various bulletins and service advice from the circuit breaker vendors (GE and Westinghouse) and several NRC generic letters and information notices identified the UV trip device problems, made recommendations, and specified corrective actions. In the topical report, the CEOG stated that the CE plant licensees have implemented those recommendations and taken the specified corrective actions.
The monthly RTCB test is performed manually by actuating the breaker's associated trip pushbutton The CE plant TSs also require an independent verification of RTCB UV and shunt trip capability and functional testing of the RPS manual trip channels during the plant refueling outage. Each of these tests results in tripping the associated RTCB. The TS-required functional
, test of the RPS logic also causes tripping of the associated RTCB. Therefore, an RTCB experiences a minimum of 39 test related trips during a refueling cycle (18 trips because of the breaker monthly tests plus 18 trips because of the RPS logic monthly tests plus 3 trips - one each due to the refueling outage functional tests of the RTCB UV and shunt trip devices and the associated reactor manual trip channel).
The underlying problem being addressed in the CEOG topical report is the excessive testing of RTCBs. A review of the Institute of Nuclear Power Operation's (INPO's) Nuclear Plant Reliability Data System (NPRDS) identified 238 failures out of roughly 60.000 actuations of the RTCDs at CEOG power plants. The majority were failure to close following testing or maintenance. Topical Report CEN-327-A, Supplement 1, dated January 1998, "RPS/ESFAS Extended Test Interval Evaluation," approved the extension of RPS component functional test STI from monthly to quarterly for all CE power plants using RTCBs. On the basis of CEN-327-A, Supplement 1, the functional STI for bistables and RPS logic was changed from monthly to quarterly in all CE power plant TSs except for the RTCB STl, which was recommended in the topical report to remain monthly. The new improved STSs (NUREG-1432, SR 3.3.3.1) include a bracketed value of a 92-day STI for channel functional t'st on RPS logic and RTCB. The bracket indicates that a justification is required for change from the current monthly to a quarterly (92 days) STI. It is, however, noted that a justification is needed only for the extension of RTCB STI and not for the extension of the RPS STi, as it is already justified in Topical Report CEN-327, Supplement 1. For those CE power plants at which the RPS logic functional test STI is quarterly, the RTCB test-related trips are reduced to 27 in a refueling cycle. However, even 27 test-related trips of an RTCB during a refueling cycle is excessive; this is the subject of the CEOG topical report for further reducing the number of these test-related RTCB trips.
Tcpical Report CE NPSD-951, Revision 1, justified an RTCB STI extension from monthly to quarterly based on the breaker unreliability calculation using 39 test-related trips of an RTCB per refueling cycle. In this calculation, the topical report used NPRDS data of RTCB failures on demand at various CE power plants to ca!culate RTCB unreliability. The CEOG evaluated 90 months of data (from January 1986 to mid-1993), which showed only one RTCB failure to open on demand. The topical report methodology for calculating an RTCB unreliability (uncertainty to open on demand) follows:
RTCB unreliability to open is equal to the number of its failures to open divided by the number of demands. As stated in the topical report, there are 92 RTCBs used in 13 operating CE power plant units (10 units of 8 RTCB design and 3 units of 4 RTCB design). Each RTCB experienced a total of 39 test-related trips during a typical refueling cycle of 20 months (18 months plus a 2-month outage). The NPRDS data indicated one RTCB failure to open on demand during a period of 90 months, and for conservatism, the topical report considered all RTCB trip demands to be test related during the 90-month period, as follows:
The RTCB unreliability = {1 failure]/ ((92 breakers)(39 tests per breaker /20 months)(90 months))
= [1 failure]/ [16,146 test-related demands]
= 0.62Ed This calculated unreliability value is conservative as it did not include RTCB openings due to scrams and maintenance, which would cause further reduction of the unreliability value. The
- topical report compared this unreliability value to the one in NUREG/CR-4639, " Nuclear Computerized Library for Assessing Reactor Reliability (NUCLARR). The NUCLARR system developed by the NRC includes hardware component failure data and provides a repository of reliability data that can be used to support a variety of risk-assessment activities. The NUCLARR value for the RTCB unreliability (failure to open on demand) from four different data sources is 5.3Ed Equipment with smaller unreliability values is subject to less frequent tests. A comparison of the calculated and the NUCLARR RTCB unreliability values indicates that the RTCB unreliability at CE power plants is less than the value in NUREG/CR-4639 (by a factor of
- 13) and, therefore, the RTCB STI extension is acceptable. Additionally, the RTCB reliability to trip on demand will be verified every 6 weeks instead of every 3 months when the 3-month STI RTCB manual functional Jest is staggered with the 3-month STI RPS logic functional test.
AOT Extension for an Inocerable RTCB Closure in the CE NSSS design, the RTCBs provide reactor trip in a one-out-of -two-taken-twice logic scheme. This trip scheme reduces the potential of an inadvertent reactor trip from a spurious trip of an RTCB (Westinghouse NSSS design has a one-out-of-two RTCB reactor trip scheme where a spurious opening of an RTCB causes a reactor trip). Power input to the reactor trip switchgear (RTSG) comes from two full-capacity motor generator (MG) sets operated in parallel and cross-connected in such a way that the loss of either MG set does not de-energize the CEDMs. There are two independent power supply buses, each bus powering half of the CEDMs. Power is supplied from the MG sets to each bus through two redundant trip paths.
l Each trip path has two RTCBs in series (this design is used in the three Palo Verde units) or two parallel sets of two RTCBs in series (this design is used in all other CE units except for Palo Verde). Both of these RTCB arrangements ensure that a spurious or test-related opening of an RTCB will not interrupt power to the CEDM buses. When an RTCB, its manual trip channel, or its trip initiating logic is found inoperable, the new, improved STSs require opening the associated RTCB within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, even if the RTCB is operable. With one RTCB (or a set of two RTCBs) open, the redundancy requirement is met and the plant can operate in this configuration for an extended period of time. However, in this configuration, a surveillance test of an operable RTCB, RPS logic, or manual trip channel cannot be performed without a reactor trip unless the inoperable open RTCB is closed, in order to avcid a reactor trip, the new, improved STSs allow closing the affer,ted (open) RTCB for 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to perform functional test on the above-stated component of an operable RPS channel. Newer CE power plants, such as Waterford Unit 3 and Palo Verde, have implemented this 1-hour AOT in their plant TSs. The topical report recommended that those CE plants that do not use this allowance iwarporate it in their plant TSs. The topical report also recommended increasing this AOT from the currently allowed 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />.
To justify the AOT extension, the CEOG stated in the topical report that it is not often that a plant needs to invoke this 1-hour AOT, but when time is needed,1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> is insufficient for testing the two RTCBs in each of the other trip legs without rushing the work. After this 1-hour period expires, the plant must be shut down. The CEOG also stated that extension of the AOT would allow an inoperable RTCB to remain closed for a short period. During this time, the plant is relying on the single operable RTCB in that trip path to open if required. However, this has been judged acceptable for 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, at least at four CE plants. Its acceptability is based c.; the
l alternative of possibly forcing the plant to shut down for missed surveillance versus the unlikely event of a trip occurring with the single RTCB failing on demand during this short period.
The staff found the preceding justification insufficient for granting the AOT extension, as no data were included in the topical report to indicate insufficiency of the current 1-hour AOT. ABB-CE, in Reference 7, agreed with the staff's finding that additicnal information was needed to support the AOT extension. ABB-CE also agreed that it would be acceptable to deny the AOT extension at this time, given the need for additionaljustification. Additionally, the 1-hour AOT is found acceptable at four CE power plants and in the new improved CE STSs. Therefore, the AOT extension from the currently allowed 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> is not acceptable.
3.0 CONCLUSION
On the basis of our review of Topical Report CE NPSD-951, Revision 1 and ABB-CE's telecommunication summary, the staff concludes the following:
The RTCB " failure on demand" data and the calculated unreliability support the STI 1
extension for the RTCB manual functional test from a monthly to a quarterly schedule and is, therefore, acceptable.
2.
The topical report did not sufficiently justify extending the AOT for an inoperable closed RTCB to permit testing of other RTCBs from the currently allowed 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and, therefore, the staff denies the AOT extension.
4.0 REFERENCES
1.
M. B. Sellman (E.I.) letter to NRC dated July 17,1996, "Waterford Unit 3 TS Change Request."
2.
CEOG Topical Report CE NPSD-951, " Reactor Trip Circuit Breakers Surveillance Frequency Extension," dated September 1994, transmitted to NRC by D. F. Pilmer (ABB-
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CE for Waterford Unit 3 ) letter dated September 18,1996.
i 3.
C. P. Patel (NRC) letter to Charles M. Dugger (E.I.), dated September 8,1998, " Request for Additional Information on Waterford Unit 3 TS Change Request and CEOG Topical Report."
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'4.
E. C. Ewing (E. l.) letter to NRC, dated October 22,1998, " Response to NRC Request for Additio.1al Information on TS Change Request."
5.
E. C. Ewing (E.I.) letter to NRC, dated January 12,1999, 'TS Change Request for RTCB STI Extension."
6.
E. C. Ewing (E. l.) letter to NRC, dated February 5,1999, "TS Change Request for RTCB STI Extension."
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l 7.
E. C. Ewing (E.I.) letter to NRC, dated March 16,1999," Documentation of Telephone Calls Relating to TS Change Request."
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