ML20059J302
| ML20059J302 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 11/03/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059J300 | List: |
| References | |
| NUDOCS 9311120159 | |
| Download: ML20059J302 (5) | |
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NOS. 20 AND 6 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 April 30, 1993, as supplemented by letter dated July 30, 1993, Texas Utilities Electric Company (the licensee) requested changes to the Technical Specifications (Appendix A to Facility Operating License No. NPF-87 and NPF-89) for the Comanche Peak Steam Electric Station, Unit Nos. I and 2 (CPSES). The July 30, 1993, supplemental letter was clarifying in nature-and thus, within the scope of the initial notice and did not affect the NRC staff's proposed no significant hazards considerations determination. The proposed changes would replace the Technical Specification (TS) requirements associated with the boron dilution mitigation system (BDMS) with alarms, indicators, procedures and controls to allow proper resolution of potential boron dilution events. The BDMS was developed to detect and mitigate a boron dilution event in Modes 3, 4, and 5 prior to a complete loss of shutdown margin (criticality). The original analysis of the inadvertent boron dilution event for Modes 3, 4, and 5, described in the CPSES Final Safety Analysis Report (FSAR) Section 15.4.6, was based on the operation of the BDMS.
However, TU Electric subsequently identified concerns about the inverse count rate ratio and the flux multiplication setpoint used by the BDMS in the boron dilution analyses which render the system of little use from an accident analysis standpoint.
This prompted NRC Information Notice 93-32.
The proposed changes would revise TS 3/4.3.1, " Reactor Trip System Instrumentation," by removing reference to the source range boron dilution flux doubling instrumentation and its associated action statement, surveillance and implementation footnotes. The specific changes are listed below:
(1) Table 3.3-1, Reactor Trip System Instrumentation - Delete reference to item 6b, concerning the operability of the boron dilution flux doubling instrumentation and its related footnote.
(2) Table 3.3-1, Table Notations - Delete Table Notation "h" concerning blocking flux doubling signals during startup and its related footnote.
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' 1 (3) Table 3.3-1, Action Statements - Delete Action Statement 5.2 and its related footnote.
1 (4) Table 4.3-1, Reactor Trip System Instrumentation Surveillance l
Requirements - Delete source range, neutron flux trip actuating device t
operational test R(12) and its related footnote.
(5) Table 4.3-1, Table Notations - Delete the portion of Table Notation (9) concerning the quarterly verification of boron dilution alarm setpoint.
Also delete Table Notation (12) concerning the 18-month surveillance requirement of the boron flux doubling circuitry and the baron dilution footnote.
2.0 EVALUATION As stated above, the BDMS alone could not be shown to prevent CPSES from returning to critical following a boron dilution event. Therefore, the following changes are proposed to the plant hardware and operations:
(1) Two additional high level alarms on the volume control tank (VCT) will be installed. The alarm setpoints will be lower than the current high-high VCT level alarm. This change is acceptable since it improves instrumentation reliability and provides redundancy of alarms. Credit for these alarms has been taken in the revised boron dilution analyses for Modes 3, 4, and 5, as evaluated below.
(2) The normal operating mode of the letdown divert valve will be revised from "AUT0" to "VCT".
This change is acceptable since it should enhance operator awareness during planned dilution operations.and, thus, reduce the potential for inadvertent dilution during routine plant operations.
(3) An alarm will be installed on the letdown divert valve to annunciate when the valve is not in the "VCT" position. This is acceptable as it will serve to heighten operator awareness of the potential for a dilution event during and following planned plant evolutions.
(4) Plant operating procedures will be revised to require the operation of at least one reactor coolant pump (RCP) in Modes 3, 4, and 5 or have a valve in the flow paths of potential boron dilution sources closed or under administrative control. Operation in Modes 3, 4, and'5 with at least one RCP operating is allowed by the TS.
Isolating the dilution sources reduces the possibility of an inadvertent dilution. These changes are, therefore, acceptable.
l In addition, the BDMS flux multiplication alarm, although not used as the primary signal, will be retained as a plant design feature to provide the plant operators a diverse method for. identifying a potential dilution event.
j The licensee's April 30, 1993, submittal provided a detailed safety analysis l
of the postulated inadvertent boron dilution event in Modes 3, 4, and 5 using i
the hardware and operational changes proposed above. The event is assumed to
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be initiated through a malfunction in the reactor makeup control system (RMCS) or by operator error.
Since all sources of unborated water are locked out during refueling, the boron dilution event is not analyzed from Mode 6 initiation.
The event has been reanalyzed to demonstrate compliance with the Standard Review Plan (SRP), NUREG-0800, Revision 2.
SRP 15.4.6 states that, if operator action is required to terminate a dilution event, the following minimum time intervals must be available between the time when an alarm announces an unplanned moderator dilution and the time of total loss of shutdown margin (criticality):
(1) During refueling:
30 minutes (2) During startup, cold shutdown, hot standby, and power operation:
15 minutes TU Electric has modified these definitions to require the above operator action time intervals to extend from the time when an alarm announces an unplanned moderator dilution to the time when the corrective actions must be initiated in order to prevent a complete loss of shutdown margin. The event is not considered to be terminated (i.e., criticality prevented) until the borated water from the refueling water storage +.ank (RWST) purges the diluted water in the charging lines and enters the reactor coolant system (RCS).
Thus, the reanalysis must show that the operators have at least 15 minutes from the alarm in which to perform the actions required to terminate an inadvertent boron dilution event before the shutdown margin is completely eroded. This is referred to as the net operator response time, or t0P15.
This is consistent with, and, in fact, more conservative than the SRP 15.4.6 requirements and is therefore acceptable.
System delays for valve manipulations and purging diluted pipes with boric' acid are considered and further reduce the net operator response time, as shown below.
The net operator response time (t0P15) is calculated by the following relationship:
10P15 - tCRIT - tFILL - tSWAP - tPURG where tCRIT is the time required to deborate the volume which becomes diluted during the transient from the shutdown margin concentration to the critical concentration.
tFILL is the time required to increase the net volume of the RCS and CVCS by the amount required to actuate the high VCT water level alarm.
tSWAP is the sum of the opening time of the RWST isolation valves and the closing time of the VCT isolation ~ valves.
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tPURG is the period of time from the end of th: valve swapover to the time that borated water from the RWST enters the RCS.
This revised methodology was used to analyze the inadvertent boron dilution events for CPSES, Unit 1 (Cycles 3 and 4) and Unit 2 (Cycle 1).
In all cases, it was verified that the reactor operators have greater than 15 minutes after an alarm in which to perform the actions required to terminate the event before the inadvertent criticality could occur, thereby meeting the SRP requirements.
There may be other dilution events which are not obviously bounded by the analysis described above.
For example, for small dilution flow rates, the time required to fill the VCT to the high VCT water level setpoint may be greater than the time required to dilute the RCS to the critical condition.
However, the time intervals involved in this case are relatively long and other alarms such as the concentrated boric acid flow or total makeup flow deviation alarms would alert the operator to a potential inadvertent boron dilution. The alarms generated by the nuclear instrumentation system, or the available trend recorders would also be available to alert operators for these slow dilution cases.
For these relatively slow transients, TU Electric has proposed an alternate event acceptance criterion. This criterion requires that the time between the start of the event and the complete loss-of shutdown margin (inadvertent criticality) be at least 30 minutes.
The staff has determined that this provides reasonable assurance that the above-mentioned other indications would alert the reactor operators to an inadvertent boron o
dilution event and allow initiatiation of timely corrective actions.
i SRP 15.4.6 also requires redundancy of alarms that would alert the operator to an unplanned dilution.
The licensee has committed to the installation of redundant alarms for high VCT water level with a lower setpoint (70 percent j
span) than the current high-high VCT water level alarm (98 percent span) and has taken credit for this alarm in the reanalysis of the dilution events. -In addition, the licensee has committed to installation of an alarm on the letdown divert valve (LCV-112A) which will annunciate when the valve is not in the "VCT", or normal, mode. These alarms, in addition to the available boric acid flow indication and deviation alarm, the total makeup blended flow indication and deviation alarm, the VCT high pressure alarm, and the alarms provided by the nuclear instrumentation system, provide an acceptable level of redundancy and diversity to alert operators to an ongoing inadvertent boron dilution event.
The staff has reviewed the proposed TS changes for CPSES that remove reference to the source range boron dilution flux doubling instrumentation.
In addition, we have evaluated the proposed changes to the plant hardware and operations as well as the revised boron dilution event methodology.
Based on the above safety evaluation, we find these proposed changes acceptable, i
3.0 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 no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The 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 and changes surveillance requirements. 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 l
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 (58 FR 43933). Accordingly, the amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR-SI.22(b) no environmental impact statement or environmental assessment need be prepared in connectinn with the issuance of the amendment.
5.0 CONCLUSION
The Commission has concluded, based on the consideration discussed above, that:
(1) there is reasonable assurance that the health and safety of the i
public will not be endangered by operation in the proposed manner, (2) such activitios will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendments will not be inimical to the common defense and security or to the health and safety of the public.
1 Principal Contributor: L. Kopp, SRXB/NRR Date:
November 3, 1993 l