ML20245D082
| ML20245D082 | |
| Person / Time | |
|---|---|
| Site: | Callaway  |
| Issue date: | 04/14/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20245D076 | List: |
| References | |
| NUDOCS 8904280102 | |
| Download: ML20245D082 (11) | |
Text
- -____-__
[f
\\
UNITED STATES
+
g NUCLEAR REGULATORY COMMISSION r,
- l WASHINGTON, D. C. 20555 j
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 44 TO FACILITY OPERATING LICENSE NO. NPF-30 UNION ELECTRIC COMPANY I
CALLAWAY PLANT, UNIT 1 l
DOCKET NO. STN 50-483 i
1.0 INTRODUCTION
{
A recent Westinghouse Owners Group (WOG) survey of Westinghouse operating plants found that a high percentage of inadvertent plant trips were initiated by either low-low steam generator level or low feedwater flow trip signals. A Trip Reduction and Assessment Program (TRAP) was established by the WOG to investigate methods and design modifications to reduce the frequency of unnecessary reactor trips occurring in Westinghouse plants. As a result of this assessment, the WOG issued two generic topical reports and proposed system modifications. One of the reports, Reference 1, proposed a trip donal design change to add a Trip Time Delay (TTD) circuit. The other Environmental Allowance Modifier (p signal design change to implement an report, Reference 2 proposed a tri EAM). These topical reports were reviewed and approved by the staff in Reference 3.
These generic topical reports were reissued as References 4 and 5, respectively, indicating staff approval, by the inclusion of Reference 3.
By Reference 6, the licensee requested an amendment to Facility Operating License No. NPF-30 for Callaway Unit 1.
The licensee also submitted clarifying information by References 8, 9, 10, 11, and 12. The purpose of the proposed modification is to reduce the frequency of unnecessary feedwater-related reactor trips. This amendment proposed to modify the plant by implementing an EAM and TTD circuit for the steam generator low-low level trip instrumentation. The licensee also proposed to revise the Technical Specifications and FSAR in conjunction with the plant modifica-3 tion. The development of the plant modification and safety analysis for Callaway Unit I are in accordance with the above topical reports.
2.0 DISCUSSION 1
The Westinghouse reactor is designed to trip if a low-low level condition is detected in any one of the four steam generators. The reactor trip derived from the steam generator low-low level protects the reactor from loss of heat sink in the event of sustained steam /feedwater flow mismatch resulting from loss of normal feedwater or feedwater system pipe break inside or outside containment. In the event of loss of feedwater, the reactor would be tripped on low-low steam generator level. Therefore, a low-low steam I
generator level trip circuit is provided for each steam generator to ensure that sufficient initial heat removal capability (inventory) is available in the steam generator at the start of a transient.
1 j
8904280102 890414 PDR ADOCK 05000483 P
PNU 1
g
. The licensee' proposed to modify the plant steam generator low-low level reactor trip circuitry by adding an EAM and TTD. The EAM would select a setpoint for the steam generator low-low level trip, including an environ-
. mental (containment atmosphere) uncertainty associated with the actual plant environmental conditions. The TTD would delay a steam generator low-low level trip actuation based upon the power level.
In the event'that the onset of an adverse containment environment is sensed by the EAM via containment pressure, a higher steam generator low-low level trip setpoint would be automatically selected,to account for larger environ-mental uncertainty due to a feedwater line. rupture inside containment. A lower setpoint would be used by the.EAM during normal operation when there would be a larger steam generator level operating band in order to permit feedwater inventory control. The trip actuation may then be delayed by the TTD depending on power level at the time that the low-low level signal is sensed. The licensee stated that this steam generator level reactor trip modification would provide more operational flexibility and at.the same time, ensure adequate protection during normal and accident conditions.
The licensee provided a plant-specific Westinghouse Topical Report, Reference 7, which established plant-specific safety analysis limits for the steam generator low-low level trip time delay, normal and adverse contain-ment trip setpoints, and the EAM containment pressure setpoint, for Callaway Unit 1.
3.0 EVALUATION 3.1 INSTRUMENTATION AND CONTROL SYSTEMS EVALUATION DESIGN CRITERIA The licensee provided logic diagrams and schematic drawings describing the proposed circuit modifications.
]
The applicable' regulatory Instrumentation and Control (180) system design criteria for the proposed modification was listed in Section 3.7 of Reference 7.
The listing included appropriate General Design Criteria (from 10 CFR Part 50, Appendix A), Regulatory Guides and IEEE Standards. The plant-specific topical report was submitted as an attachment to Reference 6.
Reference 7 was, therefore, considered to be an integral part of the licensee's request.
Section 3.7 of Reference 7 included subsections describing how each of the important elements of the design criteria would be accomplished.
The proposed modification was designed as an extension of the existing Class 1E plant protective system. The design criteria requires that the same Class IE design requirements be maintained for the proposed modification as are applicable to the plant protective system.
In this review, the proposed modification was compared with the established requirements for Class 1E 4
systems. Important requirements such as the single' failure criteria,
{
channel independence and separation were found to be accommodated in the 1
J I
s
?
. proposed modification. Other requirements such as operating bypasses are not relevant in this review because the proposed modification does not include a requirement for any new operating bypasses and does not impact any existing bypasses.
The design features of the proposed modification were also compared to the I&C design requirements identified in the staff-approved References 4 and 5.
While there are variations in the two designs, the design proposed by the licensee is consistent in detail with that recommended in the generic topical reports. The plant-specific design also reflects a response to commentr by the staff regarding power level setpoints.
FSAR REVISIONS Reference 6 included marked up pages from Chapter 7 of the Callaway FSAR indicating the changes that would be necessary to implement the proposed modification. These changes were reviewed and found to be consistent with the proposed modification and adequately meet the I&C requirements of the staff.
SETPOINTS AND ALLOWABLE VALUES Information on setpoints and allowable values relating to the proposed modification was provided in the licensee's request and in the proposed revision to the plant Technical Specifications.
The power levels for which the time delays would be imposed are below 10% of Rated Thermal Power (RTP) for a 232-second delay and between 10 and 20% of l
RTP for a delay of 122 seconds. These power levels are lower than was l
indicated in Section 7 of Reference 4.
These more conservative power level setpoints acconnodate a staff concern stated in Reference 3.
l An adverse environmental condition has been defined in Reference 5 to i
existwhenthesurfacetemperatureofthestgamgeneratorleveltransmitter (a differential pressure device) exceeds 180 F.
Pipeline breaks are the only identified cause for this condition. The licensee has interpreted the adverseenvironmentalconditigntoexistwhenthesurfacetemperatureofthe
)
level transmitter exceeds 180 F.
By analysis this condition wguld only occur after the containment atmosphere temperature exceeds 230 F.
This condition is identified by measurement of containment pressure. The licensee proposed to use a setpoint of 1.5 psig in the containment building to imply an adverse environmental condition.
q During normal environmental conditions the steam generator low-low level setpoint would be set to trip when the level drops below 14.8% of the narrow range instrument span. This corresponds to a level of 112.9 inches above the top of the highest tube bundle.
If an adverse environmental condition is detected, the trip level would be increased to 20.2% of the narrow range instrument span which corresponds to a level of 119.9 inches above the top of the highest tube bundle.
. TESTING Reference 6 included a description of a program for surveillance testing of the proposed circuit modification. Additional information was ccntained in the revisions to the plant Technical Specifications and in the revisions to the FSAR.
The current capability to test the reactor trip system, including the steam generator low-low level trips, would not be changed by the proposed plant modification. The current capability for periodic testing of the analog channel portion of the reactor trip system during power operation would also continue to be maintained. The process analog out)ut signals to the logic circuitry would be interrupted during individual c1annel tests by test switches which when thrown place any EAM or TTD function into a conservative state.
On-line testing of the steam generator low-low level, EAM and TTD circuits would be conducted in overlapping stages.
In the first stage a test circuit would provide test signals to the steam generator low-low level circuits to determine if the circuits are functioning correctly. -The level channels would be tested one at a time to verify one-out-of-four operation and with various combinations of two at a time to verify two-out-of-four operation.
The second stage would be a test circuit that provides test signals to the EAM circuits to determine if the circuits are functioning correctly. The EAM system design includes a test interlock that would latch-in the adverse containment environment signal.
The third stage would be t test circuit that provides test signals to the TTD circuits to determine if the circuits are functioning correctly. The TTD system design includes a test interlock that would set the TTD circuit in a conservative state.
The containment pressure monitors and Delta-T power monitors would be tested by removing the transmitters from service and injecting test signals into the circuitry. During the test period a (conservative) tripped condition would prevail. All testing would be conducted according to written procedures.
i Reference 2, Appendix C, pages C-10 and C-19 recommended monthly testing of the steam generator low-low level and the containment pressure EAM monitors.
The proposed revision of Technical Specification Table 4.3-1 indicated that quarterly testing and Table 4.3-2 indicated monthly testing would be performed for this instrumentation. The reason for this discrepancy is that after Reference 2 was written, Callaway Unit I was approved for testing of
5 c...
., Reactor Trip' System (RTS) instrumentation on a quarterly basis. Callaway's Technical Specification requirements for Engineered Safety Features Actuation System (ESFAS) is for nonthly testing. Since the ESFAS surveillance requirements are more restrictive than those for RTS, the steam generator level, Delta-T power, containment pressure, EAM, and TTD instrument-ation would be tested on a monthly basis.
Testing of the TTD timers would be required on a monthly basis but is not identified in the Technical Specifications. However, the licensee has committed in Reference 6 to test the TTD timers on a monthly basis. Based on this review, the staff finds the surveillance testing program identified for the proposed modification consistent with Regulatory Guide 1.118 and acceptable.
INSTRUMENT UNCERTAINTY
.The licensee's request included a section on instrument loop uncertainty in Section 4.0, of Reference 7.
A listing of calculated and measured statistical variances were provided to identify the sources and magnitude of the uncertainty for each of the monitoring instruments included in the
)roposed modification. These included the effects of instrument drift, cali> ration error and environment.
Instrument response times were also identified. The totsi ccmponent uncertainty factors were statistically combined to provide an overall channel allowance which was incorporated into the proposed Technical Specification revision. This approach and the results are consistent with those used in previous evaluations and are acceptable to the-staff.
ALARMS,' ANNUNCIATORS, INDICATORS AND STATUS LIGHTS The proposed modification included the additions of alarms, annunciators, indicators and status lights to provide the operator with adequate information regarding the status of the EAM and TTD instrumentation. Presently, there is an alarm and annunciator for each steam generator to indicate when at least one level. channel has dropped below its trip setpoint. A "first out" alarm and reactor trip occurs if more than one level channel drops below the low-low setpoint for any steam generator. However, with the proposed modification, the signal to the current alarm and annunciator would not be actuated until after all applicable time delays were expired. Each level channel (16 channels) j would be provided with a status light to indicate when the bistable trip point had been reached.
Additional alarms and annunciators would be provided with the proposed modification. A new low-low level alarm would be provided for each steam generater to indicate when the level in at least one channel had dropped below the low-low level setpcint in that steam generator. The operator may then observe individual steam generator indicators to determine epnropriate action.
A new single alarm and annunciator would be provided to indicate the presence of an adverse environment. The staff finds these arrangements acceptable, J
s 4
I
. IMPLEMENTATION OF THE TTD The logic units for the TTD would act on a combination of two power levels
]
(10% and 20% of RTP), two time delay values (122 seconds and 232 seconds) and the number of steam generator low-low level trips signals (one or more
(
thanone). The logic would function according to the descriptions in j
References 6 and 7.
At power levels below 10% of RTP a low-low level trip i
signal from any steam generator or from more than one steam generator would j
be inhibited for a period of 232 seconds. At power levels more than 10% but i
less than 20% of RTP, the trip signals would be inhibited for a period of 122 seconds. Above 20% of RTP there would be no time delay. Power level signals for the TTD circuits would be derived from existing Delta-T signals from the overtemperature and overpower protection channels. Redundant sensors, processors and logic units would be utilized to meet the Class IE
{
requirements.
The licensee has provided diagrams of the logic system to implement the previously-descri)ed features. These diagrams have been reviewed in concept and application and found to apply the appropriate principles of redundancy, independence and testability required in Class 1E circuitry. The staff finds the basic design and conceptual application of the proposed modification acceptable.
IMPLEMENTATION OF THE EAM The EAM would distinguish between a normal or adverse containment environment as detected by atmosphere pressure in the containment building in excess of 1.5 psig. A signal indicating an adverse environment would enable a higher 14.8%) generator low-low level trip setpoint (20.2% of narrow range span vs steam The adverse environmental level setpoint would be higher due to the inclusion of instrument uncertainties related to the adverse environment.
The EAM circuit design is essentially the same as was described in Reference 5.
The EAM circuitry has been designed with a latch-in feature that remains enabled until manually reset. This feature prevents the circuit from i
reverting back to a normal mode without manual intervention once an adverse environment has been detected. The EAM would be employed redundantly to meet the requirements of Class 1E circuitry.
l The EAM manual reset would be located in the 7300 Series Process Protection System cabinets, located behind the control board. The EAM annunciator will l
stay lit until the EAM is reset. Based on the licensee's description, the staff finds the circuit acceptable.
SUMMARY
The staff's review effort concentrated on plant specific design aspects of the steam generator low-low level reactor trip modification. Based on the l
l
, above evaluation, the staff finds the proposed modification design and methodology to be consistent with the previously-approved generic design.
The Callaway Unit 1 steam generator low-low level reactor trip modification is in conformance with the instrumentation and control systems design criteria (IEEE-279)andisthereforeacceptable.
13.2 REACTOR TRIP SYSTEM EVALUATION The steam generator low-low level signal wi'l trip the reactor and actuate the auxiliary feedwater system. These acN as are necessary to protect the reactor core and to maintain an adequate heat sink for decay heat removal.
The licensee, in WCAP-11883 (Reference 7 ) submitted by its letter dated August 30, 1988, provided safety analyses to support its proposed modification of the steam generator low-low level trip circuitry for the Callaway Plant. These reports provide:
1.
Basic functional description of the Callaway Plant EAM and TTD design.
2.
Results of calculations performed, consistent with the WCAP-11325-P-A (Reference 4) approved methodology, to develop the Safety Analysis Limits (SAL's) for the steam generator low-low level, power. level dependent trip time delays.
3.
Results of calculations performed to develop the SAL's for the steam generator low-low level normal and harsh containment environment trip setpoints.
4.
Evaluation of the impacts of the SAL's specified above on the FSAR non-LOCA safety analysis design bases.
j i
5.
Evaluation of the impact of the SAL's specified above on the FSAR LOCA-related safety analysis design bases.
J The Callaway TTD design is based on the introduction of two unique nominal power bistable setpoints of 10% and 20% Rated Thermal Power. Callaway Plant specific loss of normal feed ater analyses have been performed w
assuming a reactor trip on steam generator low-low level setpoint at 0%
span to provide the SAL's for 1/4 and 2/4 logic time delays at specified power levels. These analyses were performed consistent with the WCAP-11325-P-A safety analysis methodology. The results of these analyses
)
indicated the following.
1.
For 10% Rated Thermal Power with 1/4 or 2/4 steam generator logic, the SAL TTD is 240 seconds.
i 2.
For 20% Rated Thermal power with 1/4 or 2/4 steam generator logie, the SAL TTD is 130 seconds.
A plant-specific feedline break analysis (with offsite power available) at 20% rated thermal power was also performed assuming a reactor trip on steam generator low-low level setpoint at 0% span. The results of the analysis indicate that with a SAL TTD of 130 seconds, the acceptance criteria of
l l
3 I
! this Condition IV event is met. This provides confirmation that introduction of the steam generator low-low level time delay at part-power does not invali-date the conclusions presented in the FSAR for the feedline break transient.
This analysis is, therefore, a confirmation of the generic conclusions presented in WCAP-11325-P-A.
The licensee also performed an evaluation to demonstrate that the proposed TTD values are acceptable relative to other design basis transients which are affected by the steam generator low-low level trip.
SUMMARY
The staff has evaluated the above-stated licensee's analyses and concluded that the assumptions of these analyses are conservative and the methodology is consistent with the approved safety analysis methodology of WCAP-11325-P-A.
3.3 CONTAINMENT SYSTEM EVALUATION The licensee provided Westinghouse Topical Report WCAP-11883 (Reference 7) which established plant-specific safety analysis limits for the steam generator low-low level trip time delay, normal and adverse containment trip setpoints, and the EAM containment pressure setpoint. The licensee analyzed various accident conditions for the applicability of the EAM/TTD concepts to the Callaway plant and developed limits for the steam generator low-low level trip setpoint used in the determination of post-accident containment temperature, and the EAM containment pressure setpoint. The licensee also evaluated the impact of the specified limits on design basis LOCA and non-LOCA containment analyses. The licensee performed containment analyses for the feedwater line rupture inside containment consistent with previously-approved Westinghouse Topical Report, WCAP-11342-P-A (Reference 5) and determined the most limited mass and energy release rates (from feedwater line breaks) through the contain-ment pressure and temperature analyses. The licensee calculated a minimum
)
pressure for which the containment environment is assumed normal and a maximum containment temperature rise for heat-up of the steam generator level transmitter reference leg.
The staff has reviewed the licensee's submittals and applicable Westinghouse topical reports and finds that the containment analyses to establish limits for the steam generator low-low level trip setpoints and the adverse containment condition trip setpoint were within the design l
basis containment temperature and pressure analysis limits in the FSAR.
1 The methodology and computer codes used for the analysis are consistent with those used in the FSAR and the Westinghouse topical reports which were l
l i
e
.N
s
~,
-9 previously approved by the staff. The staff, therefore, finds the licensee's safety analyses for the plant modification to be acceptable.
SUMMARY
I Based on the above, the staff concludes that the licensee's safety analyses and evaluations for the containment to support proposed changes in the steam generator low-low level trip setpoints are within the previously-approved containment design limits, and therefore, are accetpable.
4.0 ENVIRONMENTAL CONSIDERATION
Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact has been prepared and published in the Federal Register on April 12,1989(54 FR14713 ).
Accordingly, based upon the environmental assessment, the Commission has determined that the issuance of this amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
The staff has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the will'not be endangered by operation in the proposed manner; and (2) public such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
i j
. REFERENCES 1.
Licensing Topical Report, Westinghouse Owners Group. WCAP-11325, " Steam Generator Low Water Level Protection System Modifications to Reduce feedwater-Related Trips," November 1986.
2.
Licensing Topical Report, Westinghouse Owners Group, WCAP-11342,
" Modification of the Steam Generator Low-Low Level Trip Setpoint to Reduce Feedwater-Related Trips," November 1986.
3.
Letter, NRC (A. C. Thadani) to Westinghouse Owners Group (R. A. Newton),
" Acceptance for Referencing of Licensing Topical Reports WCAP-11325,
" Steam Generator Low Water Level Protection System Modifications to Reduce Feedwater-Related Trips," and WCAP-11342, " Modification of the Steam Generator Trip Setpoint to Reduce Feedwater-Related Trips"," January 7, 1988.
4.
Licensing Topical Report, Westinghouse Owners Group, WCAP-11325-P-A Revision 1, " Steam Generator Low Water Level Protection System Modifications to Reduce Feedwater-Related Trips," February 1988.
Licensing Top' Modification of the Steam Generator Low-Low Level Trip ical Report, Westinghouse Owners Group, WCAP-11342-P-A 5.
Revision 1, Setpoint to Reduce Feedwater-Related Trips," February 1988.
6.
Letter, Union Electric (D. F. Schnell) to NRC (Document Control Desk),
" Steam Generator Level Reactor Trip Modification," August 30, 1988, ULNRC-1822, 7.
Licensing Topical Report, Westinghouse Electric Company, WCAP-11883,
" Implementation of the Steam Generator Low Low Level Reactor Trip Time Delay and Environmental Allowance Modifier in the Callaway Plant,"
August 1988.
8.
Letter, Union Electric (D. F. Schnell) to NRC (Document Control Desk),
" Steam Generator Level Reactor Trip Modification," November 18, 1988, ULNRC-1863.
9.
Letter, Union Electric (D. F. Schnell) to NRC (Document Control Desk),
" Steam Generator Level Reactor Trip Modification," December 28, 1988, ULNRC-1884.
- 10. Letter, Union Electric (D. F. Schnell) to NRC (Document Control Desk),
" Steam Generator Level Reactor Trip Modification," February 7,1989, ULNRC-1905.
l
i
' ty.. i s.
- 11.-
- 11. Letter, Union Electric (D. 'F. Schnell) to NRC (Document Control Desk),-
" Steam Generator Level Reactor Trip Modification," February 10, 1989, ULNRC-1911.
- 12. Letter, Union Electric (D. F. Schnell) to NRC (Document Control Desk),
" Steam Generator Level Reactor Trip Modification," February 15, 1989, ULNRC-1913.
Principal Contributors:
B. Marcus, SICB-C. Liang, SRXB J. Guo, SPLB Dated:
April 14, 1989 l
1 l
_--____m.m
___m.
m..-__...__.___m
_ - _ - _ _ _. _ -. _ _