ML20082M364
| ML20082M364 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 04/17/1995 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20082M342 | List: |
| References | |
| NUDOCS 9504240197 | |
| Download: ML20082M364 (4) | |
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UNITED STATES j
,j NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 2006H001 i
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION i
RELATED TO AMENDMENT NOS. 39 AND 25 TO FACILITY OPERATING LICENSE NOS. NPF-87 AND NPF-89 TEXAS UTILITIES ELECTRIC COMPANY COMANCHE PEAK STEAM ELECTRIC STATION. UNITS 1 AND 2 DOCKET NOS. 50-445 AND 50-446
1.0 INTRODUCTION
By application dated August 9, 1994, (LAR 94-013, TXX-94211), Texas Utilities Electric Company (TU Electric /the licensee) requested changes to the Technical Specifications (TSs) (Appendix A to Facility Operating License Nos. NPF-87 and NPF-89) for the Comanche Peak Steam Electric Station (CPSES), Units 1 and 2.
The proposed changes would delete the high negative neutron flux power range reactor trip function from the CPSES TSs.
The affected TSs are: TS 2.2.1, " Reactor Trip System Instrumentation Setpoints," and TS 3/4.3.1, " Reactor Trip System Instrumentation."
In Tables 2.2-1, 3.3-1, and 4.3-1, change the FUNCTIONAL UNIT column entries for Item 4 from " Power Range, Neutron Flux, High Negative Rate" to "Not used" and delete the entries in the remaining columns for these items. Also affected is BASES Section 2.2.1.
The second paragraph under the subsection entitled " Power Range, Neutron Flux, High Rates" is deleted.
2.0 BACKGROUND
The requirement for the high negative neutron flux rate reactor trip function
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was based on an early Westinghouse analysis methodology which took credit for this trip function to protect against localized departure from nucleate boiling (DNB) following the inadvertent drop of one or more rod cluster i
control assemblies (RCCAs).
Through the use of more advanced analytical j
techniques, it has been shown that this protection is not required to prevent DNB.
In the current design basis for CPSES, Units 1 and 2, this trip function is not credited in any of the accident analyses nor is it relied upon to provide a diverse reactor trip function.
The inclusion of this function in the reactor trip system requires that periodic surveillance be performed to ensure the operability of the trip function.
In addition to the expenditure of plant resources to perform the surveillance, the risk of an inadvertent reactor trip is increased.
Current procedures require that a channel be put in the tripped condition during testing, thereby reducing the effective trip logic from 2-out-of-4 to 1-out-of-3 coincident logic. The 1-out-of-3 coincident logic is significantly more kND
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By removing this trip function from the reactor trip system, the potential for inadvertent plant trips can be reduced with no impact on plant safety.
3.0 EVALUATION The high negative neutron flux rate reactor trip function was originally provided to protect against.the dropped RCCA events described in Final Safety i
Analysis Report (FSAR) Section 15.4.3.
For these events, it was assumed that the reactor was at full power with the RCCAs inserted to the power-dependent insertion limits.
Following the drop of one or more RCCAs from the same group, the reactor power rapidly decreases due to the negative reactivity insertion of the dropped RCCA(s).
If the negative reactivity insertion was sufficiently large, a reactor trip would be initiated on the high negative flux rate reactor trip function.
If no reactor trip on the high negative flux rate occurred, the reactor coolant system (RCS) would cool down and depressurize due to the turbine / core power mismatch. DNB was not expected to occur at this point due to the lower power and reduced RCS temperatures; thus the specified fuel design limits were not exceeded. The original Westinghouse analysis methodology (WCAP-10297-P-A, " Dropped Rod Methodology for Negative Flux Rate Trip Plants," June 1983) credited the use of the high negative neutron flux rate trip function.
For the analysis of scenarios using the the new analytical methods, the high negative flux rate trip is not credited.
Two advanced analytical methodologies that are applicable to CPSES are:
WCAP-11394-P-A, " Methodology for the Analysis of the Dropped Rod Event," January 1990 and RXE-91-002-A,
" Reactivity Anomaly Events Methodology for Comanche Peak Steam Electric Station Licensing Applications," TU Electric, October 1993.
In the case of WCAP-ll394-P-A the staff safety evaluation (SE) dated October 23, 1989, found that no credit was taken for any direct reactor trip due to dropped RCCAs.
That SE also stated that further review (for each cycle) was not necessary, given the utility assertions that the analysis described by Westinghouse has been performed and the required comparisions have been made with favorable results. The staff SE for the licensee's methodology (RXE-91-002-A) dated January 19, 1993, found the topical report acceptable for reload licensing analyes for CPSES, subject to the certain applicable limitations and restrictions.
The issue of high negative flux rate trip was not explicitly addressed as a limitation or restriction in the SE.
Further review of the original RXE-91-002 submittal indicates that no credit was taken for the high negative flux rate trip.
Westinghouse and licensee methodologies do not take credit for the high negative flux rate reactor trip function.
If the negative reactivity insertion following a dropped rod is sufficiently large, a reactor trip will be generated on low pressurizer pressure.
For this case, it has been shown that no DNB occurs.
In addition, it has been shown that no DNB occurs during the return to full power conditions due to the actions of the automatic rod control system and moderator temperature reactivity feedback mechanisms.
Thus, for scenarios that do not result in a low pressurizer pressure trip, a reactor trip is unnecessary.
i Y Through'the deletion of this trip function from the reactor trip system, the potential for inadvertent plant trips.is reduced. The deletion of the trip l
function removes the potential for spurious reactor trips generated within the
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high negative neutron flux rate circuitry.
In addition, the current TSs i
require that each of the four channels be tested quarterly to ensure operability. During the period of surveillance testing, the affected channel is placed in the " TRIP" condition, thereby reducing the coincident logic required for plant trip from two-out-of-four to one-out-of-three. The one-out-of-three coincident logic is much more susceptible to inadvertent actuation due to spurious signals or spikes, excessive signal noise, or personnel errors. Through the deletion of th':, trip function from the TSs, the need to perform the surveillance testing is deleted.
L In summary, the high negative neutron flux rate reactor trip function is not credited in any of the CPSES accident analyses. All potential dropped rod events have been shown to either result in a low pressurizer pressure reactor trip or do not require any automatic protective action from the reactor protection system.
Further, the deletion of this trip function will decrease i
the potential for inadvertent or unnecessary reactor trips by removing a source of potential spurious signals and eliminating the possibility of-inadvertent trips during the performance of required surveillance testing.
Therefore, the staff concludes that the proposed TS changes do not adversely 1
affect plant safety, will result.in a net benefit to the safe operation of the facility, and are acceptable.
4.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Texas State official was notified of the proposed issuance of the amendments. The State official had i
no comments.
5.0 ENVIRONMENTAL CONSIDERATION
TM amendments change a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20. The NRC staff has determined that the amendments involve no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increase in individual or cumulative occupational radiation exposure. The Commission has previously issued a proposed finding that the amendments involve no significant hazards consideration, and there has been no public comment on such finding (59 FR 49438). 'Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of the amendments.
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6.0 CONCLUSION
I The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the 12 public 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 amendments will not be inimical to the common 3,
defense and security or to the health and safety of the public.
Principal Contributor: Timothy Polich Date:
April 17, 1995 e
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