ML20046B459
| ML20046B459 | |
| Person / Time | |
|---|---|
| Site: | Comanche Peak  |
| Issue date: | 07/30/1993 |
| From: | William Cahill, Walker R TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| TXX-93280, NUDOCS 9308040235 | |
| Download: ML20046B459 (3) | |
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.r MM Log # TXX-93280
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F File # 10010 1
C 915.4
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Z Ref. # 10CFR50.71(a)
TUELECTRIC 10CFR2.790(b)
William J. Cahill, Jr.
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July 30, 1993 U. S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, DC 20555
SUBJECT:
COMANCHE PEAK STEAM ELECTRIC STATION (CPSES)
DOCKET NOS. 50-445 AND 50-446 REQUEST FOR ADDITIONAL INFORMATION REANALYSIS OF INADVERTENT BORON DILUTION EVENT I
(TAC NOS. M86356 AND M86357)
REF.
TU Electric letter TXX-93098, Appendix A, dated April 30, 1993, from W. J. Cahill, Jr., to the NRC concerning submittal of License Amendment Request 93-001, " Reanalysis of Inadvertent Boron Dilution Event."
Gentlemen:
In the referenced letter, TU Electric submitted a revised analysis of the Inadvertent Boron Dilution Event.
By letter dated July 15, 1993, the NRC
]
requested additional information concerning the submittal. The following i
information is provided in response to the request:
NRC Question 1 Standard Review Plan (SRP) 15.4.6 requires that at least 15 minutes be available from initiation of an alarm to complete loss of shutdown margin (criticality).
The new analysis requires 15 minutes be available from initiation of an alarm to initiation of operator action. Justify this deviation from the SRP and explain how inadvertent criticality is precluded.
NRC Question 2 Why isn't the SRP 15.4.6 type of analysis, which shows that 15 minutes are available from an alarm to the complete loss of shutdown margin, used in conjunction with the proposed CVCS alarms for the new Comanche Peak boron dilution analysis?
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TXX-93280 Page 2 of 3 TU Electric Response to NRC Questions 1 and 2 For the design basis scenario, the reactor operators will terminate the event by isolating the dilution source and re-borating the system. The event is not i
considered to be terminated (i.e., criticality prevented) until the borated water from the RWST purges the dilute water in the charging lines and enters the RCS. As described on Page 25 of the reference, the Net Operator Response Time (t0P15) is the difference between the time that an alarm annunciates the event and the time at which operator action could still prevent an inadvertent criticality. The Net Operator Response Time is required to be greater than or equal to 15 minutes. Following receipt of an alarm annunciating the event, the time to criticality is required to be greater than the Net Operator Response Time by, at a minimum, the valve stroke times plus the charging line purge time.
Whereas the SRP requires that 15 minutes be available from the time an alarm annunciates an inadvertent boron dilution event until the shutdown margin is lost (i.e., the occurrence of an inadvertent criticality), the acceptance criterion used by TU Electric requires that 15 minutes must be available between the time the alarm annunciates and the time at which the reactor operator must initiate action to prevent criticality. Thus, the acceptance criterion used with the revised analysis of the inadvertent boron dilution event exceeds the requirements described in the SRP.
NpC Question 3 What is the reason for the additional criterion requiring at least 30 minutes between the initiation of the event and the loss of shutdown margin? Is this tCRIT? How is this requirement assured for all boron dilution events?
HRC Question 4 For small dilution flow rates in which the time required to fill the VCT to the high VCT water level alarm setpoint may be greater than the time required to dilute the RCS to the critical condition, how is the total time available for operator action shown to be greater than 30 minutes and how does this preclude inadvertent criticality?
TU Electric Response to NRC Questions 3 and 4 As described on Pages 4 and 5 of the reference, one of the two event acceptance criteria listed in Section 11.5 is required to be satisfied for inadvertent boron dilution events initiated in the shutdown operational modes.
i Consistent with the SRP, the maximum dilution rate is assumed in the design basis scenario.
For that scenario, it is required that at least 15 minutes be available between the time of alarm and the time at which operator action is j
required to prevent an inadvertent criticality. The credited alarm is generated on high VCT water level.
l
P TXX-93280 Page 3 of 3 In the development of this revised analysis, all potential failure modes, dilution paths and plant evolutions were considered.
Some failures, dilution j
paths and/or plant evolutions were identified in which the dilution rate could i
be less than the maximum rate. These scenarios were addressed through consideration of operating procedures which preclude certain dilution paths, explicit allowances in the analysis for those effects which result in a charging / letdown flow mismatch (see page 23 of the reference), enhanced operator awareness during plant evolutions, or by crediting alarms other than the high VCT water level. Those scenarios in which other alarms are credited involve relatively slow transients.
For these relatively slow transients, the i
primary alarm credited in the design basis scenario may not be actuated prior to a return to criticality; however, other indications are available to alert the reactor operators of a potential dilution incident.
These other i
indications are described on pages 12 and 30 of the reference.
In order to provide some reasonable assurance that the reactor operators have time to detect an inadvertent boron dilution event and initiate corrective actions, an i
alternate event acceptance criterion is proposed. This criterion requires i
that the time between the start of the event and the complete loss of shutdown t
margin (inadvertent criticality) be at least 30 minutes.
In other words, tCRIT, as shown on Page 25 of the reference, must be greater than or equal to 30 minutes.
To summarize, for the design basis, maximum dilution rate scenario prescribed in the SRP, the high VCT water level alarm is credited for annunciation of an inadvertent boron dilution occurring at the maximum dilution rate. The applicable event acceptance criterion is that at least 15 minutes must be available for the reactor operator to initiate corrective actions to prevent an inadvertent criticality following the receipt of the high VCT water level alarm.
For dilution events which involve a lower dilution rate, the proposed event acceptance criterion is that the time between the initiation of the event and the complete loss of shutdown margin must be at least 30 minutes.
Should you have questions concerning this submittal, please contact Bob Dacko at (214) 812-8228.
Sincere y h'
)
William J. Cahill, Jr.
By:
M Roger D. Walker Manager of Regulatory Affairs BSD c-Mr. J, L. Milhoan, Region IV Resident Inspectors, CPSES (2)
Mr. T. A. Bergman, NRR Mr. L. A. Yandell, Region IV