ML20126J634
| ML20126J634 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 04/16/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20126J633 | List: |
| References | |
| NUDOCS 8105010521 | |
| Download: ML20126J634 (5) | |
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i Io UNITED STATES Ig J. j e g
NUCLEAR REGULATORY COMMISSION r,,. '
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WASHINGTON, D. C. 20555 j
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l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION
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SUPPORTING AMENDMENT N066 TO FACILITY OPERATING LICENSE NO. DPR-29, AND AMENDMENT NO. 60 TO FACILITY OPERATING LICENSE NO. DPR-30 i
COMMONWEALTH EDISON COMPANY AND IOWA-ILLINDIS GAS AND ELECTRIC COMPANY l
QUAD CITIES STATION UNIT NO. 1 QUAD CITIES STATION UNIT NO. 2 l
DOCKET NOS. 50-254, 50-265
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1.0 Introduction l
i By letters dated November 7,1976 and Fberuary 21, 1 978, Commonwealth Edison 1
, Company (the licensee) requested amendments to the Technical Specifications for Quad Cities Units 1 and 2.
Additional information was provided by j
licensees letters dated May 31, 1978, and February 14 and April 25, 1979.
The requested amendments to the Quad Cities Units 1 and 2 Technical Speci-fications involve five proposed changes to instrument setpoints associated j
with the main steam, condensate and emergency core cooling systems. The
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setpoint changes involve the following four trip functions and alarm:
Turbine Condenser-Low Vacuum Scram Setpoint l
Main Steamline-low Pressure Isolation Setpoint i
Main Steamline-High Flow Isolation Setpoint l
ECCS-ADS Interlock Setpoint i
ECCS-High Pressure Alarm Setpoint l
l The licensee has proposed changes to the above instrument setpoints to l
reduce the number of nuisance alarms and spurious trips caused by drift i
of the present instrument settings. This has resulted in reportable occurrences having no safety significance.
2.0 Evaluation The staff has evaluated the effects of the proposed changes on core and
_l system transient performance and on postulated accident consequences, i
We also have evaluated the setpoint change ~s relative to Regulatory Guide 1.105, " Instrument Setpoints." This provides guidelines in the selection of j
8105010 D
. instrument setpoints, considering instrument accuracy, drift, margin,
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range adjustment mechanism, and assumptions used in selecting setpoints.
2.1 Turbine Condenser-Low Vacuum Scram Setpoint The licensee has proposed that the turbine condenser low vacuum scram setpoint be lowered from >23 inches to >21 inches Hg. vacuum. Loss of main condenser vacuum results in closure of the turbine stop valves and disables the turbine bypass function.
Closure of the stop valves without bypass in turn causes a pressure transient, neutron flux rise and increase in fuel cladding surface heat flux. However, closure of the turbine stop valves results in a reactor scram signal as soon as the valve position reaches 90% open. Thus for a loss of condenser vacuum, the condenser low vacuum scram anticipates the_ subsequent stop valve closure and thereby results in a less severe reactor pressurization and fuel thermal heat flux rise.
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An instantaneous and total loss of vacuum represents the most severe loss of condenser vacuum which _can be postulated. Such an event would simulta-neously cause fast closure of the turbine stop valves and prevent bypass i
function while minimizing the beneficial effects of the anticipatory low condenser vacuum scram function. The consequences of this postulated event (instantaneous loss of condenser vacuum), are bounded by the turbine trip without bypass (TT w/o BP) transient, which assumes no credit for the anti-cipatory (low condenser vacuum) scram.
For the TT w/o BP analysis, credit is taken for the direct stop valve closure scram. Thus, the consequences of the loss of condenser vacuum event is not explicitly analyzed since it is bounded by the more limiting TT w/o BP event. Since the more severe licensing basis TT w/o BP event does not take credit for the subject low vacuum scram, a change of the scram setpoint value will not affect the con-sequences for this limiting event. Thus, although the consequences of the (slower) loss of vacuum could be expected to be somewhat more severe because of a later scram, the safety margins associated with the licensing basis event (i.e., TT w/o BP)will not change.
t The proposed vacuum scram setpoint change has also been reviewed for j
consistency with the quidelines in Regulatory Guide 1.105.
The range, accuracy, drift, margin and span of the four turbine condenser low' vacuum pressure switches satisfy the recommendations of the applicable guide.
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In view of the foregoing considerations, the proposed low vacuum setpoint changes are acceptable.
2.2 Main Steamli'ne-Low Pressure Isolation Setpoint The licensee has proposed to lower the main steamline low pressure isolation setpoint from >850 psig to >825 psig. The purpose of the low pressure isolation setpoint is to give protection against an excessive reactor de-pressurization which would result in rapid cooldown of the vessel and to assure that reactor power operation at pressures lower than that specified ln the basis for_ the thermal-hydraul.ic safety limit does not occur.
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. l The most limiting event, which takes credit for the main steamline low pressure isolation setpoint is.the pressure regulator failure transient.
For this event the regulator is assumed to fail in the fully open position, j
Vessel pressure drops rapidly until steamline pressure falls to the low l
pressure isolation setpoint, which initiates closure of the main steamline isolation valves. The resulting pressurization and power increase transient l
is quickly terminated when the MSIVs reach 10 percent closed position j
causing a reactor scram.
r Lowering the setpoint from 850 psig to 825 psig will permit a somewhat lower l
pressure to be attained during the subject transient which will result in I
increased voiding prior to the effects of repressurization resulting from l
MSIV closure. The incremental increase on negative reactivity caused by the somewhat larger void content will effectively result in the pressuriza-
.l tion phase of the transient initiating at a lower power level. Since I
sensitivity studies show pressurization transients are milder when initiated at lower power levels the proposed change will effectively lessen the decrease in Critical Power Ratio (CPR) caused by a pressure regulator failure. No l
other potentially limiting transients or accidents take credit for this safety setting, and the reduced setting provides adequate protection against violation of the lowest pressure specified in the basis for the thermal-hydraulic safety limit.
i The proposed main steamline low pressure isolation setpoint changes have
'l been reviewed for consistency with guidance in Regulatory Guide 1.105, i
The accuracy, drift, margin, range and span of the four main steamline low pressure switches satisfy the recommendations of the applicable guidance.
On the basis of the foregoing considerations regarding the proposed main steamline low pressure isolation setpoint. changes, the proposed changes are acceptable.
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2.3 Main Steamline-High Flow Isolation Setpoint i
The licensee has proposed to increase the main steamline high flow isolation setpoint from <.120% of rated steam flow to <140% of rated steam fl ow. The licensee states that the higher value should allow the reactor to operate at full power during MSIV closure testing withoGt i
initiating a total isolation which would result in a reactor scram. Tha t t
is, the remaining three steam lines will be able to pass 133% of rated steam flow without initiating a high flow isolation.
i The purpose of the main steamline high flow isolation function is to provide protection'against pipe breaks in the main steamline outside the drywell. The licensee states that the consequences of a main steam-line break, as evaluated in the FSAR, will remain unchanged with the high _ flow setpoint increased from 120%.to 140% of rated steam flow.
The basis for this conclusion is that for a complete severence of one main steamline, steam flow almost instantaneously increases to a maximum j
t of 200% of rated steam flow as limited by the flow r.estrictors. Thus,
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the present and proposed setpoint would be attained virtually at the same time. The licensee further states that for this reason the FSAR j
does not explicitely consider a particular high flow isolation setpoint.
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i We agree with the licensee's conclusion that for a complete severence break the consequences will remain unchanged. However, we requested that the licensee show that the proposed setpoint change would not alter the concl.usion that the complete severence break is the worst break within the spectrum of postulated steamline breaks occurring outside of conta inment, In response to our request the licensee reports that the plant would have to be operated at 140 percent steam flow conditions for several minutes before the doses associated for the DBA steamline break would be equaled.
However, the licensee further states that the plant would operate no longer than 10 seconds in this mode.
This conclusion is based on the results of the pressure regulator failure transient analysis which shows that at a steam flow rate of only 115%, vessel pressure would drop by about 100 psi in the first 10 seconds. Thus closure of the MSIV would be rapidly initiated.
For a steam flow rate of 140% MSIV closure would initiate even sooner due to the faster depressurization. Thus even though the steam flow associated with a somewhat larger steam line break could avoid being terminated by the raised high flow isolation setpoint, the break would rapidly be isolated by a low reactor pressure condition.
The proposed main steamline high flow isolation setpoint changes have been reviewed also for consistency with guidance in Regulatory Guide 1.105.
The sixteen differential pressure flow switches used for the main steamline high flow isolation function are found to satisfy the recommendations of the applicable regulatory guide.
On the basis of the foregoing considerations regarding the proposed main steamline high flow isolation setpoint changes, the proposed changes are acceptable.
2.4 ECCS-ADS Interlock Setpoint and ECCS-High Pressure Alarm The licensee has proposed to increase the Low Pressure Coolant Injection (LPCI) pump discharge pressure interlock setpoint of the Emer Cooling System - Automatic Depressurization System (ECCS-ADS)gency Core from its present range of 75 psig to 100 psig to a new range of 100 psig to 150 psig.
Additionally, the licensee has proposed that the high pressure alarm set-point of the ECCS fill system be increased from < 74 psig to < 90 psig.
The purpose of the ECCS-ADS interlock is to ensure that, prior to ADS initiation, following a small break LOCA in which the high pressure core spray system fails, the LPCI pumps are running with sufficient discharge pressure to adequately reflood the core. The licensee has requested the interlock setpoint pressure range be increased to ensure that the low end of the pressure range cannot be satisfied by the ECCS fill system pump.
1 The shutoff head of the low pressure coolant injection pumps is approxi-mately 350 psi. Thus the increased ECCS-ADS interlock pressure can easily
_ l be achieved and exceeded by any normally running LPCI pump.
In response to our request, the licensee provided an acceptable basis for their position l
that the LPCI pumps could reach the higher interlock pressure within the
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. time period assumed in the most recent LOCA-ECCS analysis involving ADS initiation.. The maximum allowable time for the LPCI pumps to achieve full
. speed (discharge pressure) after receiving the ECCS actuation signal is t
43 seconds.
In addition ADS blowdown initiates after a 120 second time period has elapsed from a coincident high drywell pressure and low water level signals with LPCI discharge side pressure satisfying the ECCS-ADS l
interlock pressure. Thus at least a 77 second margin is available for ADS t
operation. Additionally the startup sequence for the LPCI pumps given in Section 6.2.7.3 of the FSAR indicates that all LPCI pumps would be expected to reach full speed (discharge pressure) 20 seconds after high drywell pressure is reached. Thus the staff agrees that there is no additional delay in ADS initiation which would affect peak cladding temperatures for LOCA-ECCS analyses involving ADS initiation.
I The proposed ECCS-ADS interlock and ECCS-high pressure alarm setpoint changes have been reviewed also for consistency with Regulatory Guide 1.105. The ECCS-ADS interlock Technical Specification limit and setpoint changes include coo.rdinating and changing eight RHR pressure switches, four core spray switches and one ECCS fill system high pressure alarm switch. The i
thirteen pressure switches affected by the proposed changes satisfy the recommendations of Regulatory Guide 1.105.
On the basis of the foregoing considerations regarding the proposed ECCS-ADS system interlock setpoint changes, the proposed changes are j
accepta ble.
i 3.0 Environmental Consideration We have determined that these amendments do not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this j
determination, we have further concluded that these amendments involve an action which is insignificant from the standpoint of environmental l
impact, and pursuant to 10 CFR Section 51.5(d)(4) that an environmental
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impact statement, or negative declaration and environmental impact i
appraisal need not be prepared in connection with the issuance of these j
amendments.
i 4.0 Conclusion We have concluded, based on the considerations discussed above, that:
I (1) because the amendments do not involve a significant increase in the i
probability or consequences of accidents previously considered and do not involve a significant decrease in a safety margin, the amendments do not j
. involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered i
by operation in the proposed manner, and (3) such activities will be l
conducted in compliance with the Commission's regulations and the issuance i
of these amendments will not be inimical to the common defense and security j
or to the health and safety of the public.
Da ted: April 16,1981 l
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