ML20112J336
| ML20112J336 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 06/17/1996 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20112J329 | List: |
| References | |
| NUDOCS 9606200112 | |
| Download: ML20112J336 (7) | |
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21 UNITED STATES g-y j
NUCLEAR REGULATORY COMMISSION WASHINGTON. D.C. 20566 4 001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION 4
RELATED TO AMENDMENT NO.215 TO FACILITY OPERATING LICENSE NO. DPR-53 AND AMENDMENT NO.192 TO FACILITY OPERATING LICENSE NO. DPR-69 BALTIMORE GAS AND ELECTRIC COMPANY CALVERT CLIFFS NUCLEAR POWER PLANT. UNIT NOS. 1 AND 2 DOCKET NOS. 50-317 AND 50-318
1.0 INTRODUCTION
By letter dated May 28, 1996, as supplemented May 31 and June 5, 1996, Baltimore Gas and Electric Company (BGE) requested an exigent amendment for Calvert Cliffs Nuclear Power Plant, Unit Nos. I and 2 to allow installation of a proposed modification to the alosed loop tervice water (SRW) system.
This request concerns a change to reflect the irstallation of a variable flow controller for the service water inlet control valves for the containment air coolers (CACs) that is not within the current licensing basis of Calvert Cliffs Nuclear Power Plant Unit Nos. I and 2.
The proposed amendment would replace the mechanical stops from the SRW inlet control valves of the CACs with a variable flow controller for each of those same inlet control valves.
The licensee determined that this proposed modification constituted an unreviewed safety question as described in 10 CFR 50.59(c).
The May 31 and June 5, 1996 letters provided clarifying information that did not change the initial proposed no significant hazards consideration determination.
No changes to the Technical Specifications are required by these amendments.
The purpose of the proposed modification is to reduce the potential heat load on the SRW heat exchangers (cooled by the saltwater system) during the early phases of a design basis accident (DBA). Without the proposed modification, either the existing water temperature limits on the ultimate heat sink (VHS) would have to be lowered (resulting in reduced power or shutdown) or the SRW heat exchanger would have to be cleaned at intervals no longer than every 14 days when the UHS (Chesapeake Bay) temperature becomes elevated during the summer months.
The licensee became aware of this problem following the evaluation of data from a recently installed side sweam monitor used to measure the rate of microfouling of the SRW heat exchangers on the saltwater system side. This' data indicated that the microfouling rate was significantly greater than previously estimated.
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, 2.0 EVALUATION The SRW is a closed loop system which uses demineralized water with a corrosion inhibitor added.
The system removes heat from secondary side plant components, blowdown recovery heat exchangers, CACs, spent fuel pool cooling heat exchangers, and three of-the four emergency diesel generators (EDGs).
The fourth EDG is air cooled. During response to a DBA, SRW provides cooling water to the CACs and the three EDG heat exchangers only. Therefore, heat removed from the containment via the CACs raises SRW temperature and under maximum heat removal conditions can approach the design temperature limits of the EDGs cooled by SRW.
The SRW supply line for each CAC currently has an air-operated control valve (CV) which is normally open (solenoid de-energized) and designed to fail open.
On receipt of a safety injection actuation signal (SIAS), the CVs move to a preset throttled position set.by a mechanical stop. This preset position is set to cover a high range of uncertainties in the actual flow conditions.
Following the proposed modification, the CVs will still move to a preset throttled position on receipt of an SIAS but the position will be based on a smaller range of uncertainties thereby reducing the maximum potential flow.
The preset position will now be determined by the position of the flow controller which will be set to provide flow based on actual measured flow conditions.
To cover all possible flow conditions and to allow for SRW pump wear, the present mechanical stop is set (at least 1500 gallons per minute
[gpm] under the maximum allowed pump wear conditions) such that the SRW flow to the CACs will be above the minimum required (1400 gpm used in analyses) regardless of the SRW pump conditions. Therefore, actual flow could be as high as 2100 gpm under ideal pump conditions and taking into account instrument inaccuracies and other uncertainties.
Under these maximum possible flow conditions the high heat transfer rate (via the CACs) occurring after a DBA could result in SRW temperatures that exceed the analyzed limit for EDG operation whenever the VHS is at seasonably high temperatures.
The proposed modification would limit the maximum SRW flow to the CACs following an SIAS thereby maintaining SRW temperature within the limits for EDG operation.
The transient heat analysis used to predict the SRW temperature profile and, hence, the maximum SRW temperature reached following a DBA uses an SRW heat exchanger fouling factor that includes a certain amount of microfouling. The microfouling factor is based on the resistance to heat transfer created by the formation of a slime and silt layer on the inside (salt water side) of the heat exchanger tubes.
The licensee's recent determination of the actual rate of microfouling shows that the amount of microfouling assumed in the transient heat analysis can be reached in as little as 14 days.
By reducing the amount of potential heat transfer into the SRW system via the CACs it is possible to increase the amount of microfouling allowed (assumed in the analysis) while maintaining SRW temperatures within design limits for the EDGs even with elevated UHS temperature. Therefore, the more accurate SRW flow control (reduced flow setpoint uncertainty band) resulting from the proposed modification will allow a larger microfouling factor to be assumed in the
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- transient heat' analysis. This will result in less frequent cleaning of the
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heat exchanger tubes.
In fact, the licensee's revised calculations based on
- actual flow conditions show that equilibrium microfouling will occur without exceeding the conditions ~ assumed in the transient heat analysis. The reduced
- cleaning frequency will increase the availability-of the EDGs and the CACs 1
- since they are inoperable whenever their respective SRW heat exchanger. is i
taken out-of service, i
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The proposed modification would add a safety-related flow control loop that i
would modulate the CV to obtain a predetermined flow rate.. On receipt of the
' recirculation actuation signal (RAS) the CV will return to the full open j
. position just as it does in the current design. The revised design also i
provides the operator with the option of adjusting or isolating SRW flow to j
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the CACs 'if necessary.. The new design uses a feedback loop based on the j
actual.SRW flow..'It is the same design of flow equipment used in the auxiliary feedwater (AFW). system to-provide post-accident flow control to the steam generators. The equipment uses proven analog electronic technology and the licensee' indicated that the history of. operation on the AFW system shows l
that it has been very reliable. The new equipment will be safety grade Class
.1E and redundant by virtue of the redundancy provided by the SRW trains.
Those portions of.the system located in a harsh environment will be i
environmentally qualified in accordance with 10 CFR 50.49.
Existing technical specifications for the engineering safety-features actuation system (ESFAS)
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. and the:SRW system, require monthly testing and refueling interval testing of the operation of the CVs to verify proper o)eration of the valves and the i
control lonp system. Therefore, no new tecinical specifications are required.
By letter dated May 31, 1996, the licensee, in response to a staff request, confirmed that the modification will have no effect on the containment post-accident pressure response. The staff also requested the licensee to confirm that the post-accident containment environmental conditions (EQ) would not be L
adversely affected (telecon W. Long/P. Furio, on 5/31/96).
The. licensee stated that the EQ temperature profiles would also not be affected since the l
throttled service water flow to the CACs would remain within analytical limits.. Therefore, no new containment analyses were required. The most recent containment' peak pressure and temperature analyses (both pre-RAS and e
- post-RAS) bound the new operating conditions with the proposed modification 4
installed. The staff also verified that the proposed modification will not affect containment integrity since the SRW system containment isolation valves are not'being modified by this change. Additionally, by letter dated June 5, 1996, the licensee confirmed that with the revised design, SRW flow through the cooler, and'at the low pressure areas downstream, will remain subcooled 4-throughout the planned flow control. band.
Based'on its evaluation as described above, the staff concludes that the i
proposed modification is an improvement over the current design because it
- reduces the CV flow setpoint uncertainty band, will not adversely affect the i
containment pressure and temperature response to design basis accidents and will not affect containment integrity.
Further, the proposed modification will conform to the' current-design in that the new equipment is Class.lE and
- environmentally qualified in_ accordance with 10 CFR 50.49 where necessary.
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j The proposed modification should' also result in a higher availability of the I
i EDGs and CACs because of less frequent cleaning of the SRW heat exchangers.
j The staff, therefore, concludes that the proposed modification is acceptable.
'3.0 EXIGENT' CIRCUMSTANCES i
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Pursuant to 10 CFR 50.91(a)(6), the licensee requested the proposed amendments-on an exigent basis. The proposed change would permit the Calvert Cliffs i
Nuclear Plant, Unit Nos. I and 2, to install a variable flow controller for i
the service water inlet' control valves for the containment air coolers. The j
licensee states that they could not have foreseen the need for this request prior to this time. This modification is the result of a substantial proactive effort in dealing with the concerns that BGE have with their SRW.
~ The history of BGE's activities' concerning the SRW System is given in a
i Attachment (1) of the proposed amendment. This particular modification was i
determined to be necessary after BGE obtained data from a side stream monitor i
that BGE had installed to measure the rate of microfouling in the SRW heat c
i exchangers..The' data from the side stream monitor was not analyzed and 2
available to BGE until January 17, 1996.
By mid-February, BGE had determined that the installation of flow controllers on the CAC inlet valves was necessary to offset the effects of the larger.than expected microfouling.
BGE has comitted the'necessary money and resources-to install this modification i
before the sumer.
Design and procurement activities were done in parallel, About mid-April, the engineering was to the stage that work could begin on the o
j' safety evaluation (SE) required by 10 CFR 50.59. Refinements to the engineering continued even as the SE was being' developed. On May 24, 1996, the Plant General Manager determined that an unreviewed safety question e
l existed for this modification.
This request has been-submitted as soon as practical after the determination was made.
i It is important for BGE to perform this modification on the schedule set out a i
number of months ago. To prevent operational and safety impacts, this i
modification must be installed before the hot sumer weather causes the i
l Chesapeake Bay water temperature to exceed the SRW temperature limit.
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Historically, the Chesapeake Bay water. temperature has approached or exceeded
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the current limit by the last week in June. As noted above, whenever the SRW heat exchangers are removed from service for cleaning, some safety-related equipment is rendered inoperable.
It is important to minimized the amount of j
time BGE is in these more vulnerable conditions (with some safety-related equipment out-of-service). Additionally, BGE believes that reducing the power
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-output from both units significantly during a time of high demand (high sumer temperatures) is not in the best interest of the public.
Therefore, the Commission finds that-there is a need to act quickly and that the licensee has used its best efforts.to make a timely application and did i
not create the exigency.
4.0 FINAL NO SIGNIFICANT HAZARDS CONSIDERATION
DETERMINATION s
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The~ Commission has made a final determination that the amendments involve no significant hazards consideration.
Under the Commission's regulations in 10 i
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. l CFR 50.92, this means that operation of the facility, in accordance with the proposed amendments, would not (1) Involve a significant increase in the probability or consequences of an accident previously evaluated; or (2) Create the possibility of a new or different kind of accident from any accident previously evaluated; or (3) Involve a significant reduction in a margin of safety.
Evaluation of these standards is presented below:
1.
Would not involve a significant increase in the probability or 4
consequences of an accident previously evaluated.
The proposed modification is the result of [the licensee's] need to reduce the peak post-accident heat load on the service water (SRW) heat exchangers.
It will replace the mechanical stops currently on the control valves which admit SRW into the containment air coolers (CACs) with a flow controller loop.
By throttling the SRW to the CACs, the heat load on the SRW heat exchangers is reduced during the early phases of an accident. The increased accuracy of throttling would allow the SRW system to perform its safety function during periods of high ultimate heat sink temperatures.
During the summer months, the Chesapeake Bay water (the ultimate heat sink for the units) heats up substantially during some parts of the day. At times, these high temperatures could exceed the current expected limits for the heat exchanger operation. With the more accurately throttled valves, the effect of high ultimate heat' sink temperatures is reduced.
The modification will ensure that the SRW heat exchangers are capable of meeting their intended safety function up to the maximum expected bay water temperature.
The safety function of the SRW System is to provide cooling to the CACs and the Emergency Diesel Generators (EDGs) following a design basis accident.
With this proposed modification in place, the SRW System will continue to meet this safety function. All of the failure mechanisms for this modification have previously been evaluated and were found acceptable. However, because the proposed modification may have a higher probability of malfunction for which compensatory 1
actions may not adequately control the consequence of failure, the probability of a malfunction of systems important to safety may be slightly increased, and this modification nas been determined to be an unreviewed safety question.
j The single failure of the flow controllers would not be an initiator to an accident.
The system provides cooling to safety-related equipment ft,llowing an accident.
It supports accident mitigation funt.tieiis.
Therefore, this proposed modification does not significantly increase the probability of an accident previously 4
evaluated.
The proposed modification will enhance the ability of the SRW system to respond to accident conditions under a wider range of environmental conditions (i.e., higher ultimate heat sink temperatures).
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. Malfunctions of the flow controller have been evaluated and determined
-to result in consequences that are no more severe than those previously approved. ~ A failure of the flow controller could allow the valve to fail in a position that does not allow the SRW System to perform its safety function. Since the SRW System is redundant on each. unit, a single failure of one of the flow controllers would not prevent the.other redundant portion of the system from performing its safety function..The consequences of a single failure of the SRW System have been previously analyzed and these consequences do not change due to this modification.
Therefore, any increase in the probability or consequences of an accident previously evaluated is not significant.
2.
Would not create the possibility of a new or different type of accident from any accident previously evaluated.
Tht; SRW System provides cooling water to the CACs and EDGs.. The purpcse of the components that are affected by this modification is to mitigate accidents. ~ The single failure of the flow controllers would not be an initiator to an accident. This modification does not change the equipment's function,-or significantly alter the method of operating the equipment to be modified. The system will continue to operate in essentially the same manner as before the modification was done.
Therefore, the proposed change does not create the possibility of a new or different type of accident from any accident previously evaluated.
3.
Would not involve a significant reduction in a margin of safety.
The margin of safety is reduced for this proposed modification, but not significantly.
If the CAC inlet valve fails open, the CAC on that train would continue to perform its safety function. However, the EDG on that train _would receive cooling water above the design temperature and may fail to perform its safety function..The redundant EDG would i
provide adequate electricity to continue to perform its safety i
function.
If the CAC inlet valve fails in the closed position, the EDG would continue to function; however, the affected CAC would not receive adequate cooling water.
The other three CACs would provide adequate cooling for the containment. Also, the Containment Spray System provides additional containment cooling as a backup to the i
CACs.
If the CAC inlet valve fails to throttle properly, the consequences are bounded by the other two cases discussed above.
Adding a more. complex component that could fail and result in a failure of. the.SRW System does reduce the margin of safety, but not significantly because:
- 1) the proposed flow controller is very reliable and not likely to fail; 2) the other redundant CAC and EDG l
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- are available to mitigate the consequence of an accident should there be a single failure of the flow controller.
Therefore, this modification does not significantly reduce the margin of safety.
5.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Maryland State official was notified of the proposed issuance of the amendments. The State official had no comments.
6.0 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. 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 made a final finding that the amendments involve no significant hazards consideration. 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.
7.0 CONCLUSION
The Commission has concluded, based on the considerations 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 activities will be conducted in compliance with the Commission's regulations,
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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.
Principal Contributors:
W. LeFave l
W. Long H. Garg Date: June 17, 1996
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