ML19253C169
| ML19253C169 | |
| Person / Time | |
|---|---|
| Site: | Big Rock Point File:Consumers Energy icon.png |
| Issue date: | 11/02/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19253C159 | List: |
| References | |
| NUDOCS 7911300082 | |
| Download: ML19253C169 (5) | |
Text
[ [N UNITED STATES
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NUCLEAR REGULATORY COMMISSION j
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SAFETY EVALUATION BY THE OFFICE OF' NUCLEAR REACTOR REGULATION SUPPORTING AMENDMENT NO. 31 TO LICENSE N0. DPR-6, CONSUMERS POWER COMPANY BIG ROCK POINT POWER PLANT DOCKET N0. 50-155 1.0 Introduction By letter dated October 23, 1979 and supplement dated October 31, 1979, Consumers Power Company (CPC) requested changes to the Technical Specifications for the Big Rock Point Plant. The changes would revise the specifications for low reactor water level and low steam drum water level setpoints.
This safety evaluation also addresses the acceptability of the proposed changes and the acceptability of modifications.made to the water level instruments to correct a reported deficiency.
2.0 Discussion
2.1 Background
By Licensee Event Report 79-22 submitted to the NRC by letter dated September 22, 1979, CPC reported a potential deficiency in the water level instrumentation used for reactor scram and initiation of engineered safety features. The problem was identified as 'a result of a generic design review conducted by the General Electric Company, GE, the reactor vendor. At the time the problem was identified, the Big Rock Point Plant'was shutdown for repairs.
CPC stated they would take corrective action prior to startup. By letter dated October 23, 1979 and supplement dated October 31, 1979 CPC described the problem and corrective action to be taken. These letters also requested changes to technical specifications to specify the reactor vessel water level and steam drum water level setpoints in a manner more appropriate for the modified instruments.
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. 2.2 Description of Problem Reactor vessel water level and steam drum wa'.;er level signals are obtained from remote monitoring instruments manufactured by the Yarway Corporation.
These instruments measure water level by measuring a pressure differential between a reference water leg whose height is independent of vessel water level and a variable water leg whose height is a function of vessel water level. The measurements are affected by water density. The instrument at Big Rock Point was the type that had the reference leg heated by heat transfer from the variable leg. The purpose of using a heated leg was to minimize water temperature and density differences between the variable lea and reference leg which occur as reactor coolant conditions change.
The instruments were calibrated so that indicated level would be the same as actual level at nonnal ambient containment temperatures and normal reactor operating conditions. The problem would occur during a loss of coolant accident which would depressurize the reactor coolant system and depressurize the water legs in the level instrument.
If the temperature of the reference leg is too high, water in the reference leg would flash to steam and cause the indicated water level to be above actual water level. As a consequence, the automatic initiation of reactor scram, containment isolation and core spray actuation caused by low water level would not occur at the time assumed in the safety analyses.
2.3 Corrective Action To correct the problem, CPC has modified the level instruments by removing the heat clamps which aided heat transfer from the variable leg to the reference leg and removed insulation to allow greater heat loss from the instrument.
The purpose of the modification is to reduce the reference leg temperature sufficiently so that under any postulateo loss of coolant accident conditions there would be no flashing in the reference leg. CPC anticipates that the corrective action will reduce the reference leg temperature from a previous calculated temperature of about 4140F to a temperature less than 2500F. CPC has also informed us that thermocouples will be placed on the level instrument to verify that the reference leg temperature is below 2500F. The modification removes the temperature compensating feature of the instrument and causes a difference between indicated and actual water level under certain operating conditions.
Therefore CPC has proposed a revision to the technical specifications for t he purpose of clarifying the required setpoints and to assure that actuation occurs at the required water level under all operating conditions.
These changes are:
(1)
Section 6.1.2 - Change the scram setting and tolerance for low reactor water level from 610'6" + 1" to 610'10" (3'1" above top of fuel).
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. (2) Section 6.1.2 - Add " Calibrated for Operating Temperature and Pressure" to the low steam drum water level setting of 8.0" below steam drum centerline.
(3) Section 11, subsection 3.1.4 - Change the core spray initiation setpoint from 610'6" + 1" to 610'10" (3'1" above the top of active fuel).
(4) Section ll, subsection 3.1.5 - Change the Reactor Depressurization System (RDS) initiation setpoint from -25" to -17" from steam drum centerline.
3.0 Evaluation Modifying the level instruments by removing the, jacket and heat clamps that currently bxist around and between the variable leg and the reference leg will, according to the licen ee, reduce the temperature in the reference leg to less than 2500F, and thus prevent erroneous high-level readings. CPC states during a rapid depressurization of the prinary system, which results in the most severe containment temperature effect on the level instrument: reference leg, the reactor vessel will not depressurize to the saturation pressure of the reference leg (about 29.5 psia at 2500F) until after the scram and ECCS setpoints have been reached. The heat transfer time constant of the instrument reference leg has been calculated to be 20 to 30 minutes; therefore, containment temperature effects on the level instrument during a rapid depressurization are not significant with regard to setpoint actuation, ile concur with this analysis. Thus even the combination of the worst depressurization event and instrument heatup (LOCA) will not result in reference leg flashing or erroneous hih level indications prior to safety system actuation. These level instrument systems have no posi, accident monitoring functions.
As a result nf the proposed modifications, the temperature compensation ability of the level instruments will be significantly reduced. The low reactor vessel level setpoint will be calibrated for operating temperature and pressure (hot calibration), thus as primary system temperatures decrease, the actual level in the reactor vessel will be slightly lower than the indicated level. To compensate for this difference, between indicated and actual reactor vessel water level, the licensee has proposed to raise the reactor vessel low level scram setpoint from 610'6" + 1" to 610'10".
CPC has submitted an analysis which shcws that with this setpoint change, that under the worst conditions of system temperature and pressure, a scram will be initiated prior to the actual vessel level falling to 61G'6".
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. This correspono; to an indicated level of 610'10" with saturated water at at :ospheric pressure. For all other conditions, the actual level at the time of scram will be in the conservative direction, i.e., higher than the required 610'6".
This narrcw range level instrument covers the range from 609'9" to 610'15" with a new scram setpoint of 610'10". The uncertainty of this instrument has been detennined to be about 1".
Therefore, this modification is consistent with the assumption used in the plant safety analysis for a low reactor vessel water level of 610'5".
Since this proposed modification to the low vessel level setpoint is at the lower limit cf the previous specification and bounds this setpoint in the conservative direction, we find the proposed modification and technical specification changes acceptable.
Similar modifications will be performed to the steam drum level instruments.
These instruments have a range of 60" (+30" from the steam drum centerline) with a RDS actuation setpoint at -25".
The licensee proposes to raise this setpoint to -17" from steam drum centerline.
CPC's analysis of worst case conditions on this similar level measurement instrument shows that the setpoint will be reached when actual steam drum level is -20".
These wide range instruments have an uncertainty of approximately 1-1/2".
"p CPC has provided data which shows that this setpoint change is likewise in the conservative direction with respect to the current specification and bounds the potential system pressure and temperature effects, and instrument uncertainty. On these bases, we find this modification to be acceptable.
In addition, CPC proposes to calibrate the steam drum low level scram setpoint (8.0" below the steam drum centerline) at operating temperature and pressure. This is because operating restraints do not allow establishment of a setpoint high enough to accommodate the effects of loss-in-temperature compensation while meeting the present specification under all operating conditions (specifically during primary system heatup and cooldown). CPC states that while the actual level will be lower at other temperatures, the minimum primary system liquid inventory assumed in the accident analysis will be maintained. The uncertainty of this instrument is covered in the analysis. We concur with CPC's analysis, and therefore find this proposed equipment modification and technical specification change to be acceptable.
4.0 Environmental Consideration We have detennined that the amendment does not involve 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 determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact and, pursuant to 10 CFR 951.5(d)(4), that an environmental impact statement or negative d0claration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
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5.0 CONCLUSION
We have concluded, based on the considerations discussed above, that:
(1) because the amendment does not involve a significant increase in the proba-bility or consequences of accidents previously considered and does act involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities wil' be conducted in compliance with the Comission's regulations and the issuance of this amendment will not be inimical to the comon defense and security or to the health and safety of the public.
Date:
November 2, 1979
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