ML20132D351
| ML20132D351 | |
| Person / Time | |
|---|---|
| Site: | Clinton  |
| Issue date: | 12/13/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20132D348 | List: |
| References | |
| NUDOCS 9612190335 | |
| Download: ML20132D351 (8) | |
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UNITED STATES i
NUCLEAR REGULATORY COMMISSION 2
WASHINGTON, D.C. 2066H001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION I
RELATED TO AMENDMENT N0.111 TO FACILITY OPERATING LICENSE N0. NPF-62 ILLIN0IS POWER COMPANY. ET AL.
CLINTON POWER STATION. UNIT N0. 1 DOCKET NO. 50-461 f
1.0 INTRODUCTION
By letter dated February 22, 1996, and supplemented by letters dated July 4 and September 20, 1996, Illinois Power, the licensee, requested changes to Facility Operating License No. NPF-62, Appendix A Technical Specifications, for Clinton Power Station (CPS). The requested changes would revise CPS Technical Specification 3.3.4.1, "End of Cycle Recirculation Pump Trip (E0C-RPT) Instrumentation," by deleting Surveillance Requirement 3.3.4.1.6 which requires the RPT breaker interruption time to be determined at least once per 1
60 months.
2.0 BACKGR0'LND By letter dated February 22, 1996 (Ref 1), the licensee proposed the elimination of the reactor recirculation pump (RPT) breaker interruption time testing requirements from the Technical Specifications (TS) for CPS.
The E0C-RPT instrumentation initiates a RPT in the event of a turbine trip or generator load rejection transient. This is intended to reduce the peak reactor pressure and power, and provides additional margin to core thermal Minimum Critical Power Ratio Safety Limits for these events.
The E0C-RPT system sensors detect initiation of closure of the main turbine stop valves (TSV) and fast closure of the main turbine control valves (TCV).
When the sensor channel set point is exceeded, switches close to provide a signal to the E0C-RPT trip logic. When the E0C-RPT breakers trip open, the recirculation pumps downshift to slow speed. Actuation of the E0C-RPT logic trips both recirculation pumps from fast speed operation. There are two E0C-RPT breakers in series per recirculation pump.
Both E0C-RPT breakers for each recirculation pump trip upon actuation of the E0C-RPT system.
Surveillance Requirement (SR) 3.3.4.1.5 of the CPS Technical Specifications (TS), requires that E0C-RPT SYSTEM RESPONSE TIME tests be conducted on an 18-month STAGGERED TEST BASIS on a per Function basis. As described in the TS Bases, all channels of one Function are tested every 18 months on an alternating basis such that both Functions are tested every 36 months.
Response times cannot be determined with the reactor at power because the reactor recirculation pumps must be tripped.
9612190335 961213 PDR ADOCK 05000461 P
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!* The CPS TS defines EOC-RPT SYSTEM RESPONSE TIME as the time interval from initial movement of the associated TSV or TCV to complete suppression of the 4
electric are between the fully open contacts of the recirculation pump circuit i
breaker. The RPT breaker interruption time portion of the E0C-RPT RESPONSE TIME is required to be determined at least once per 60 months for each pair of pump breakers in accordance with SR 3.3.4.1.6 of the CPS TS and is defined in 3
the Bases for SR 3.3.4.1.6 as breaker response time plus arc suppression time.
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Breaker response time is defined as the time from application of voltage to the trip coil until the main contacts separate. Arc suppression time is defined as the time from main contact separation until the complete j
suppression of the electrical arc across the open contacts.
I In their submittal of February 22, 1996, the licensee stated that RPT breaker j
interruption time testing is unnecessr.ry and requested elimination of the o
requirement. They stated that the test necessitates the use of temporarily installed equipment and requires extensive planning and preparation.
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addition, the nature of the test (i.e., the very short time periods associated l
with the breaker arc suppression time and the delay introduced by the current j
transducers) presents a challenge to both the equipment used to record the i
test response time and to the capability of precisely determining the actual t
breaker arc suppression time. The licensee also stated that discussions with 1
the breaker manufacturer have confirmed that measurement of the arc i
suppression time is unnecessary and that actual arc suppression times are not
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subject to change for properly maintained breakers.
IP and the manufacturer believe that the robust design of the E0C-RPT breakers provides assurance of continued satisfactory performance, and that any degradation of the breaker that could cause significant degradation of the arc suppression time is prevented by performance of recommended preventive maintenance or is detected i
by other required testing. The licensee stated that actual measurement of breaker interruption time does not provide any additional assurance of safety 1
and proposed using 95 milliseconds as an assumed arc suppression time.
This value would be used in lieu of an actual measured arc suppression time when j
determining E0C-RPT SYSTEM RESPONSE TIME.
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In their submittal, the licensee proposed the following changes to CPS TS j
3.3.4.1, "End of Cycle - Recirculation Pump Trip (E0C-RPT) Instrumentation":
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Deletion of SR 3.3.4.1.6 which requires periodic verification of RPT l
breaker interruption time.
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Deletion of Note 1 to SR 3.3.4.1.5 which references SR 3.3.4.1.6.
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2 to SR 3.3.4.1.5 will remain as an unnumbered note.
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The staff had several questions on the original submittal, and these were i
contained in a request for additional information (RAI) dated April 24, 1996 i
(Ref 2). These questions were answered in Reference 3, Illinois Power Company letter "Clinton Power Station Response to Request for Additional Information Related to Proposed Amendment of Facility Operating License No. NPF-62 (LS !
004)" dated July 4, 1996.
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' On September 20, 1996, CPS submitted a modification to their original request to include a modification of the wording within the TS Bases for Section SR 3.3.4.1.5. (Ref 4). The wording was revised to acknowledge that a mechanical timing check of the RPT breakers is performed as part of the preventive maintenance performed to validate or maintain the assumed EOC-RPT breaker response time. The TS Bases are also being revised to specify the acceptance criterion for the mechanical timing check. The new wording will read:
SR 3.3.4.1.5 This SR ensures that the individual channel response times are less than or equal to the maximum values assumed in the accident analysis. The EOC-RPT SYSTEM RESPONSE TIME acceptance criteria are included in applicable plant procedures. The EOC-RPT SYSTEM RESPONSE TIME, includes an assumed RPT breaker interruption time of 95 milliseconds. The assumed RPT breaker interruption time is validated by the performance of periodic mechanical timing checks, contact gap measurements and high potential tests on each breaker in accordance with plant procedures at least once per 36 months. The acceptance criterion for the RPT breaker mechanical timing check shall be $34 milliseconds.
E0C-RPT SYSTEM RESPONSE TIME tests are conducted on an 18 month STAGGERED TEST BASIS. The Note requires STAGGERED TEST BASIS frequency to be determined on a per Function bases. This is accomplished by testing all channels of one Function every 18 months on an alternating basis such that both Functions are tested every 36 months. This Frequency is based on the logic interrelationships of the various channels required to produce an E0C-RPT signal.
Response times cannot be determined at power because operation of final actuated devices is required. Therefore, this frequency is consistent with the typical industry refueling cycle and is based upon plant operating experience, which shows that random failures of instrumentation components that cause serious response time degradation, but not channel failure, are infrequent occurrences.
3.0 EVALUATION
IP has requested that response time. testing of breaker interruption time be discontinued, and that for the purpose of determining response time of the EOC-RPT system, an assumed breaker interruption time of 95 milliseconds be used in lieu of an actual measured time. The assumed value of 95 milliseconds is based on:
1.
A maximum time value of 95 milliseconds has been substantiated by IP during past surveillance testing at CPS, 2.
The vendor specified breaker interruption time is 50 milliseconds, which is much less than the proposed assumed value, and 3.
Testing of the circuit breaker during equipment qualification testing confirmed an actual breaker interruption time of 24 to 34 milliseconds.
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i The staff agrees with the above justification and, therefore, finds that 95 i
milliseconds is a conservative value for breaker interruption time for a l
properly operating breaker.
IP has stated that the assumed breaker interruption time 9111 be validated through measurement of the contact gap separation and the contact gap resistance. These measurements are required by CPS procedures at least once per 36 months, and provide assurance of proper electrical performance of the i
breakers. Adequate mechanical performance of the breakers is demonstrated by a logic functional test at least once every 18 months. This test, SR l
3.3.4.1.3, includes actuation of the breakers.
l IP has provided additional justification to rely on an assumed breaker
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interruption time:
1.
The manufacturer's recommended maintenance cycle for the breakers is based on an expected 2,000 cycles of the circuit breaker. The E0C-RPT breakers are subjected to less than 1% of this amount of cycling during the current 36 month maintenance cycle.
In fact, the EOC-RPT breakers are not expected to be subjected to 2,000 cycles during the life of the plant. Because the utilization of the E0C-RPT breakers at CPS is much less than the utilization for which the breakers are designed, the possibility of degraded performance is reduced, i
2.
The design of the breaker is such that there is seldom failure of the breaker to open within the vendor specified time limits that does not also result in a failure of the breaker to operate. Problems with the j
mechanism of the breaker would most likely cause mechanical failures, not a degradation of performance that would cause the breaker to open in j
a time greater than the vendor specified time limit. Thus, while j
degradation of the breaker mechanism that would impact the mechanical j
opening time of the breaker may be possible, the breaker mechanism would-i be expected to fail to operate rather than fail in a manner that would be difficult for operators and maintenance personnel to recognize.
3.
Proper setting of the contact gaps ensures the air gap separation will j
be sufficient to extinguish the arc and halt current flow. This does not involve disassembly of the breaker and thus will not cause degraded performance of the breaker. Verification of proper contact gap settings ensures that there will be sufficient air gap during opening of the i
breaker. The air gap is one of the most significant parameters in j
extinguishing the arc.
4.
The performance of a high potential test verifies that the breaker insulation is adequate to eliminate the possibility of stray paths that would allow current flow after the breaker has opened. Therefore, the 1
only path for current is through the breaker contacts. This provides assurance that the current flow through the breaker will be halted when the breaker is called upon to open for an EOC-RPT trip.
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. 5.
The proposed change to eliminate interruption time testing has been discussed with the breaker manufacturer and the intended treatment of response time testing is consistent with the manufacturer's recommendations for testing and maintenance of the breakers.
(As discussed in Reference 3, the breakers are Westinghouse Class 1E, safety-related, type 75DVP500.) The vendor has stated that breaker interruption time testing is not recommended because breaker interruption time does not change. The vendor further states that i
maintenance and testing that includes high potential testing and measuring of the contact gap for each breaker (as presently being performed at CPS) will provide assurance that the breaker interruption time will continue to be within assumed limits.
To substantiate the assertion that any failure which could cause significant degradation of the arc suppression time is prevented or detected by performance of recommended preventive maintenance, the licensee was asked to submit a failure modes and effects analysis (FMEA) (Ref.2).
IP submitted the following FMEA in response (Ref 3):
Applicable failure modes that would render the E00-RPT breakers inoperable (with respect to opening of the breaker on demand) are.a catastrophic failure and a degraded type of failure:
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L (1)
A catastrophic failure of the vacuum breaker to open on demand would be caused by failure of the operating mechanism or vacuum interrupters. To l
maintain high performance and reliability with respect to these potential failure modes, maintenance is performed on the breakers in accordance with vendor recommendations.
Preventive maintenance tasks include checking _the interrupter for vacuum and contact wear gap, checking and adjusting the mechanism latches, lubrication, and testing that includes megger and ductor testing.
(2)
Degraded failure of the vacuum breaker to open, i.e., a slow trip, wa.ld l
be caused by degradation of the operating mechanism. Again, proper maintenance (the preventive tasks described above) prevents such failures. Westinghouse Instruction Bulletin (I.B.) 32253-3B includes a section on the mechanical timing of breakers which states, "The breakers are checked at the factory for contact speed and contact bounce. These values do not change appreciably during the mechanical life of the breaker and are not considered as a part of the regular inspection and maintenance program."
While this is not a complete FMEA, as the components within the breaker are not analyzed for failure modes and effects, it is sufficient to show that the two primary failures, failure of the operating mechanism or of the vacuum l
interrupters have been considered and addressed.
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. IP discussed failure probability and history as follows:
There is no failure probability data available specifically for the Westinghouse DVP-type breakers such as the E0C-RPT breakers used at CPS.
Industry data is available for failure rates of the DHP metal-clad drawout type breakers. This data can provide some perspective on the reliability of the DVP breakers considering that the DVP breakers can be expected to be more reliable.
(The DVP breakers utilize fewer moving parts due to the vacuum technology employed.
Further, although CPS is only in its sixth operating cycle, performance of the DVP breakers has been excellent as no failures or degradation has occurred to date.) The IEEE failure rate data for the DHP breakers includes catastrophic, degraded and incipient modes of failure, but does not include a failure mode specifically identified as " failure to open within specified time limits." Nevertheless, the data can serve to provide a gauge of reliability.
Per IEEE 500-1984, the expected failure rate for the composite of all failure modes of metal clad drawout circeit breakers (rated at greater than 600 volts) is 0.4 failures per one million hours. The IEEE failure rate number is conservative as it includes data from Class 1E and non-1E equipment supplied by a large number of manufacturers.
The staff asked IP to provide a copy of the manufacturer's recommended breaker maintenance practices, including recommended maintenance intervals.
In addition, IP was asked to provide a copy of the CPS required maintenance
- schedule, In response, IP provided Section 6 of Westinghouse DVP Circuit Breaker Manual I.P.32-253-3B, " Instructions for Porcel-line Type DVP Vacuum Circuit Breakers," as Enclosure 1 to Reference 3.
Proper maintenance, according to the manufacturer, consists of the following:
1.
Check contact wear gap 2.
Check vacuum Interrupter assembly 3.
Mechanical timing test i
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Clean insulation 5.
Lubrication IP also provided copies of the applicable CPS Preventive Maintenance (PM) Task Descriptions. These PMs trigger performance of CPS Electrical Maintenance Procedure No. 8410.07, which was also provided. The staff review of this information verified that the CPS maintenance program for the breakers is consistent with the manufacturer's recommendations.
Based on our review of the information provided, the staff concurs that use of an assumed 95 milliseconds for breaker interruption time will provide a conservative value when performing routine breaker performance surveillances.
The staff also agrees that a properly maintained breaker does not appear to have failure modes which will affect response time or arc suppression time which would not be detected by existing routine surveillances and maintenance
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procedures. The staff has also reviewed the CPS maintenance procedures and concurs that they are consistent with the manufacturers recommendations. The staff, therefore, concludes that the proposed changes are acceptable.
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4.0 STATE CONSULTAIl0N f
l In accordance with the Commission's regulations, the Illinois state official j
was notified of the proposed issuance of the amendment. The state official had no comments.
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5.0 ENVIRONMENTAL CONSIDERATION
This amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 or changes a surveillance requirement. The NRC staff has determined that.the amendment involves no significant increase in the amounts, and no l
significant change in the types, of any effluents that may be released i
offsite, and that there is no significant increase in individual or cumulative i
occupational radiation exposure. The Commission has previously issued a proposed finding that the amendment involves no significant hazards 4
l consideration and there has been no public comment on such finding (61 FR 18169). Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).. Pursuant to 10 CFR i
51.22(b), no environmental impact statement or environmental assessment need j
be prepared in connection with the issuance of the amendment.
6.0 CONCLUSION
j The staff has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public
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will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common l
defense and security or to the health and safety of the public.
t Principal Contributor:
Paul Loeser 4
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Date:
December 13, 1996 i
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References:
1.
Illinois Power Company letter "Clinton Power Station Proposed amendment of Facility Operating License No. NPF-62 (LS-94-004)" dated February 22, 1996 2.
NRC letter " Request for Additional Information Concerning Proposed Change Eliminating the End Of Cycle Recirculation Pump Trip Breaker Interruption Time Testing Requirements - Clinton power Station, Unit No i
1 (TAC No. 94888)" dated April 24, 1996.
3.
Illinois Power Company letter "Clinton Power Station Response to Request i
for Additional Information Related to Proposed Amendment of Facility Operating License No. NPF-62 (LS-94-004)" dated July 4, 1996.
4.
Illinois Power submitted "Clinton Power Station Submittal on Additional Changes to the Technical Specification Basis in Support of Proposed Amendment of Facility Operating License No. NPF-62 (LS-94-004)" dated September 20, 1996.
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