ML20078N016
| ML20078N016 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 02/09/1995 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20078M746 | List: |
| References | |
| NUDOCS 9502150051 | |
| Download: ML20078N016 (4) | |
Text
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S UNITED STATES 5
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NUCLEAR REGULATORY COMMISSION j
wAssiwoTon, o.c. aoss5-oooi SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMEN 0 MENT NO. 103 TO FACILITY OPERATING LICENSE NO. NPF-49 NORTHEAST NUCLEAR ENERGY COMPANY. ET AL.
MILLSTONE NUCLEAR POWER STATION. UNIT NO. 1 3
DOCKET NO. 50-423
1.0 INTRODUCTION
By letter dated August 16, 1994, as supplemented January 10, 1995, the Northeast Nuclear Energy Company (the licensee), submitted a request for l
changes to the Millstone Nuclear Power Station, Unit No. 3 Technical Specifications (TS). The proposed changes would add a footnote to TS 3.5.2 that extends the outage time for each residual heat removal (RHR) pump and the associated RHR heat exchanger. This exception may only be used one time per pump and is not valid after April 30, 1995.
Nuclear industry experience has indicated that the mechanical seals on RHR i
pumps, similar to those on the Millstone Unit No. 3 RHR pumps, have.l the a history of failing after approximately 5 to 7 years of operation. Therefore i
licensee plans to replace the RHR mechanical seals prior'to the start of its next refueling outage which is scheduled to begin in April 1995.
The RHR pumps are a part of the RHR system whose primary function is to i
transfer heat from the reactor coolant system (RCS) to the Component Cooling Water System. During normal plant operations, the RHR system is used to remove decay heat from the reactor core and reduce the temperature of the reactor coolant to the cold shutdown temperature. This cooldown is referred to as the second phase of cooldown where the first phase of cooldown is 4
accomplished by the Auxiliary Feedwater System and the steam' generators. Once the plant is at cold shutdown the RHR system will maintain this temperature until the plant is started up again.
Following a loss of coolant accident (LOCA), the RHR system serves as the low pressure injection portion of the Emergency Core Cooling System (ECCS). The RHR system also is used to transfer refueling water between the refueling water storage tank (RWST) and the refueling cavity before and after refueling operations. When the reactor is in Mode 1 (operating), each RHR pump is in a standby mode of operation until it is needed by the ECCS.
950215oost 950209 0iM 03 DR ADOCK O
> The licensee plans to replace the mechanical seals on the RHR pumps while the plant is in Mode 1.
A significant reduction in exposure to radiation can be achieved if the seal replacement is done while the reactor is in operation because the RHR pumps are used during shutdown.
Subsequently, the exposure rate increases in the vicinity of the pumps during periods shortly after plant shutdown.
In addition, one of the primary functions of the RHR system is heat removal during plant shutdown so it is desirable to have both trains available during that time.
Therefore, it is proposed to make this seal replacement while the plant is in Mode 1.
This will require a one time extension of the allowable outage time from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br />.
The mechanical seal replacement will be made on one pump at a time.
2.0 EVALUATION The replacement of the mechanical seals on the RHR pumps is a preventive maintenance activity.
Performance of the seal replacement while at power will be made when the RHR system is not needed to perform its shutdown cooling function. This on line maintenance activity will a'1so be performed under i
conditions that result in reduced exposure of personnel to radiation. The one time extension of the allowable outage time from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> increases pump unavailability during operation. The licensee indicates that a net safety improvement results, however, because the increased allowable outage time permits the modifications to be performed while the plant is operating.
This on-line maintenance activity reduces the pump unavailability during plant shutdown and the radiation exposure to workers.
The requested time of 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> is based on the licensee's estimate _gf the amount of time needed to perform the replacement (67 hours7.75463e-4 days <br />0.0186 hours <br />1.107804e-4 weeks <br />2.54935e-5 months <br />), assuming no failures, plus 25% to account for potential delays.
Given the uncertainty associated with the actual task completion time, the proposed increase in the allowable outage time to 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> seems reasonable.
The ECCS consists of three separate subsystems: centrifugal charging, safety injection (SI), and RHR.
Each subsystem consists of two redundant, 100%
capacity trains. The ECCS accumulators and the RWST are also part of the ECCS, but are not considered part of an ECCS flow path as described by this limiting condition for operation (LCO) in TS 3.5.2.
The ECCS flow paths consist of piping, valves, heat exchangers, and pumps such that water from the RWST can be injected into the RCS following a LOCA.
Each of the three subsystems consists of two 100% capacity trains that are interconnected and redundant such that either train is capable of supplying 100% of the flow required to mitigate the accident consequences. This interconnecting and redundant subsystem design provides the reactor operators with the ability to utilize components from opposite trains to achieve the required 100% flow to the core.
In Mode 4 (hot shutdown), the required ECCS train consists of two separate subsystems:
centrifugal charging and residual heat removal. Due to the stable conditions associated with operation in Mode 4 and the reduced probability of occurrence of a Design Basis Accident (DBA), the ECCS
! operational requirements are reduced.
It is understood in these reductions that certain automatic SI actuation is not available.
In Mode 4, only one train of the two independent and redundant ECCS trains is required to be operable to ensure that sufficient ECCS flow is available to the core following a DBA.
In this mode, sufficient time exists for manual actuation of the required ECCS to mitigate the consequences of a DBA.
i During the time each train is made inoperable for the mechanical seal replacement, the redundant ECCS train will remain operable and be available to perform its safety function. The effects of having one RHR pump out of service for 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> instead of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> was evaluated. The licensee examined the core melt frequency (CMF) increase when one RHR pump is unavailable. The licensee calculated the instantaneous CMF increase due to one RHR pump out of service at power to be 4E-08/ year (based on the RHR pump being unavailable for an entire year). The proposed amendment would grant a one time extension of the allowed outage service time from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 120 hours0.00139 days <br />0.0333 hours <br />1.984127e-4 weeks <br />4.566e-5 months <br /> rather than for an entire year. The licensee indicates that even accounting for potential uncertainties, this is an insignificant increase.
The change results in a small one time increase in CMF, which is not considered to be significant. The licensee also concluded that the increase in pump reliability and the decrease in radiation exposure during the mechanical seal replacement justify the performance of the seal replacement i
during Mode 1.
The safety function of the RHR system will continue to be provided by the redundant ECCS train.
Licensee analyses indicates that the additional 48.
consecutive hours over the current allowable outage time of 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, for this one time, does not result in a significant increase in the CMF.
In Mode 1, the RHR pumps are in standby; however, at least one RHR pump will remain operable when the plant is shut down. Therefore, performing the modifications in Mode 1 will allow both ECCS trains to be available for decay heat removal while the plant is shut down.
The staff finds the licensee's conclusions and proposed TS change, based on their analyses, to be reasonable. Therefore, we find this one-time extension of the TSs acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Connecticut State official was notified of the proposed issuance of the amendment.
The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
Pursuant to 10 CFR 51.21, 51.32, and 51.35, an environmental assessment and finding of no significant impact have been prepared and published in the Federal Reaister on February 9,1%6(60 FR 800). Accordingly, based upon the 7
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environmental assessment, the staff has determined that the issuance of the amendment will not have a significant effect on the quality of the human environment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that:
(1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributor:
M. Griggs Date:
February 9, 1995 sqw *