ML20195C891
| ML20195C891 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 11/11/1998 |
| From: | Hammer M NORTHERN STATES POWER CO. |
| To: | |
| Shared Package | |
| ML20195C883 | List: |
| References | |
| NUDOCS 9811170235 | |
| Download: ML20195C891 (8) | |
Text
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l UNITED STATES NUCLEAR REGULATORY COMMISSION f
NORTHERN STATES POWER COMPANY MONTICELLO NUCLEAR GENERATING PLANT DOCKET NO. 50-263 SUPPLEMENT NO. 2 TO REQUEST FOR AMENDMENT TO OPERATING LICENSE DPR-22 LICENSE AMENDMENT REQUEST DATED NOVEMBr, a,1997 Northern States Power Company, a Minnesota corporation, hereby submits a supplement to License Amendment Request dated November 25,1997 for the Monticello Nuclear Generating Plant.
This letter contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY By M
Michael F Hammer Plant Manager Monticello Nuclear Generating Plant On this // day of[qrgsg, /977 before me a notary public in and for said Country, personally appeared Michael F Hammer, Plant Manager, Monticello Nuclear Generating Plant, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, information, and belief the statements made in it are true and that it is not interposed for delay.
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MARCUS H. VOTH Marcus H. Voth Notary Public - Minnesota
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Wright County My Commission Expires January 31,2000 9811170235 981111F PDR ADOCK 05000263 P
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U S Nuclear Regulatory Commission Page 1 of 7 November 11,1998 Atta'chment' Supplement No. 2 to License Amendment Request Dated November 25,1997 j
Condensate Storage Tank Low Level HPCl/RCIC Suction Transfer
Background
High Pressure Coolant injection (HPCI) System and Reactor Core isolation Cooling (RCIC) System pump suctions are normally aligned to the two,230,000-gallon capacity, parallel-connected Condensate Storage Tanks (CSTs) installed at Monticello (Figure 1). In the event these systems are initiated for any reason, reactor grade demineralized water will be injected into the reactor vessel. The CSTs are not safety related. There are no Technical Specification requirements related to operability or use of the CSTs.
Sections 6.2.4.2.2 and 10.2.5.2 of the Monticello Updated Safety Analysis Report (USAR) discuss the automatic HPCl/RCIC suction transfer feature. The safety grade, seismically designed, source of water for HPCI and RCIC is the suppression pool. Safety related automatic circuitry is provided to transfer HPCI and RCIC pump suction from the CSTs to the suppression pool on sensing low level in the CSTs.
Level switches, LS 23-74 and LS 23-75 are used in a one-out-of-two logic scheme to open redundant HPCI (MO 2061 & MO 2062) and RCIC (MO 2100 & MO 2101) suction valves from the suppression pool. CST suction valves for HPCI (MO 2063) and RCIC (MO 2102) automatically close when their j
respective suppression pool suction valves are fully open.
An increase in the Technical Specification setpoint of LS 23-74 and LS 23-75 has been requested to allow for differential pressure between the reactor building and the recombiner building, vortexing at the CST suction pipe, stroke time of the suppression pool suction isolation valves, and adoption of approved setpoint methodology.
Description of Level Switch Cross Connection Change The Technical Specifications currently require both LS 23-74 and LS 23-75 to be operable to provide redundancy in the HPCl/RCIC suction transfer logic. In the event one or both switches are inoperable, continued plant operation is permitted for up to 30 days provided HPCI and RCIC pump suctions are aligned to the suppression pool. If both level switches are not restored to operability within 30 days, the plant must be shutdown.
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l U S Nuclear Regulatory Commission Page 2 of 7 l
November 11,1998 Atta'chment' I
QQNDENSATE SIOBAGE l
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h REACTOR BLOG 1f ECCS PUMP SUCTION Figure 1 Simplified Drawing of CSTs, CST Level Columns, Instrumentation, and Suction Piping l
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U S Nuclear Regulatory Commission Page 3 of 7 November 11,1998 Attichment' For periods of extended maintenance, a temporary cross connection between level switches LS 23-74 and LS 23-75 will be installed. The temporary cross connection will consist of tubing l
and fittings. An increase in the level switch setpoints is also necessary to accommodate l
additional vortexing that results from one CST operation. Changes to the configuration of LS 23-74 and LS 23-75 will be controlled as a temporary modification in accordance with plant directives.
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Evaluation of Failure of Level Switch Piping The temporary cross connection configuration will be evaluated for seismic loading before initial use, j
Since the cross connect design will meet seismic Class I requirements, there is no potential for failure of the temporanj cross connection to present an interior flooding concem. Even if failure of the tubing or fittings is postulated, functioning of the level switches would not be affected.
Failure of the top cross connect tubing will result in both evel switch housings being vented to the l
reactor building when level falls below the top level colun n connection. This does not affect the l
performance of the switches. Failure of the bottom cross connect tubing would facilitate actuation of l
the level switches.
l Evaluation of Freezing or Plugging of Level Switch Piping Cross connecting LS 23-74 and LS 23-75 with one CST out of service could potentially increase the l
probability of failure of the level transfer logic. Freezing or plugging of a section of tubing common to both level switches could prevent both switches from actuating. This would defeat automatic transfer of the HPCI and RCIC pump suctions from the operable CST to the suppression pool. Manual operator action would be required in the event of low CST level and HPCI or RCIC actuation.
Any increase in probability of failure of the automatic transfer logic is, however, believed to be very small for the following reasons.
Increased Potential for Freezing of Level Switch Piping The CST instrumented level columns and all tubing associated with LS 23-74 and LS 23-75 are located in the Reactor Building, including the tubing associated with the proposed level switch cross connection. The probability of reaching freezing temperatures in the Reactor Building during plant operation is extremely small.
l Each instrumented level column is connected to its associated CST by a relatively large 4-inch pipe.
l This pipe lies below grade between the Reactor Building and the concrete dike surrounding both l
CSTs. Within the dike the pipe is electrically heat traced and insulated. Proper operation of the heat l
tracing is verified periodically. Water in the CSTs is normally maintained at about 100 F.
Convection currents would be expected to transfer heat from water in the tank to the 4-inch pipe if the electrical heat tracing failed for any reason. A low CST temperature alarm (90 F) is provided in the
U S Nuclear Regulatory Commission Page 4 of 7 November 11,1998 Atta'chmenf i
Control Room to alert operators well before freezing could occur. Freezing of the outdoor piping is very unlikely.
Therefore we believe that the potential for freezing which disables the automatic transfer logic is very small, either in the normal configuration of the level switches or with the level switches temporarily cross connected.
Increased Potential for Plugging of Level Switch Piping Relatively large piping is used between each CST and its associated instrumented level column. Any plugging in the 4-inch connecting piping that could restrict flow to the extent that the level column would not iespond to changes in tank level is extremely unlikely.
Tubing connects the level switches to each level column in the normal switch configuration. Tubing of a similar diameter is proposed for operating with the level switches temporarily cross connected during operation with one CST. Plugging of the instrument tubing to the degree that level switches would be disabled is believed to be unlikely for the following reasons:
Water quality in the CSTs meets specifications for reactor makeup. All water admitted to the CSTs passes through demineralizers and other treatment and filtration equipment before entering the tanks.
Piping associated with the condensate storage system is stainless steel.
Foreign material exclusion practices are applied for maintenance of the condensate storage system.
CSTs are periodically inspected. Tanks are taken out of service, one at a time, for maintenance when necessary. Corrosion particles and other foreign material resulting from degradation of the tank interiors are effectively prevented by this practice.
Plugging has not been a problem in the past Therefore we believe that the potential for plugging by foreign material of the piping and tubing associated with the automatic transfer logic is very small, either in the normal configuration of the level switches or with the level switches temporarily cross connected.
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U S Nuclear Regulatory Commission Page 5 of 7 November 11,1998 Atta'chmenf NO SIGNIFICANT HAZARDS CONSIDERATIONS The evaluation of No Significant Hazards Considerations contained in our November 25, 1997, License Amendment Request is to be replaced with the following expanded evaluation.
The Commission has provided standards (10 CFR Section 50.92) for determining whether a significant hazards consideration exists. A proposed amendment to an operating license for a i
facility involves no significant hazards consideration if operation of the facility in accordance with
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the proposed amendment would not (1) involve a significant increase in the probability or consequences of an accident previously evaluated;(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 i
margin of safety.
After reviewing this proposed amendment we have concluded that:
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The proposed amendment will not involve a significant increase in the probability or consequences of an accident previously evaluated The proposed setpoint change and temporary level switch cross connection will not affect the way the suction transfer equipment functions, introduce new failure modes, or significantly increase the probability of failure of this equipment.
A slight increase in the probability of failure of the CST suction low level automatic transfer function may result, however, during plant operation with one CST in l
service and the CST low level transfer switches temporarily cross connected.
1 This temporary modification preserves the redundancy of the automatic level transfer logic and allows HPCI and RCIC to remain aligned to the condensate l
storage system.
When the switches are cross connected, sections of piping and instrument tubing will be shared by both level switches. The probability that freezing or plugging of l
a common section of piping or tubing will disable both switches will be slightly higher than during two CST operation with the level switch piping in its normal configuration.
l The level switches would be cross connected at infrequent intervals to permit prudent and timely CST preventive maintenance and at the same time continue to provide HPCI and RCIC with a source of reactor makeup quality water. In the unlikely event of a spurious actuation of either system, only high quality water would be injected into the reactor vessel.
Overall, the possibility of freezing or plugging of piping and tubing associated with the automatic transfer level switches has been shown to be very small, with or 1
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U S Nuclear Regulatory Commission Page 6 of 7 November 11,1998 Atta'chment l
without the temporary level switch cross connection in place. During periods of operation with one CST, we believe the small additional opportunity for level instrument failure due to freezing or plugging is more than compensated for by the benefits of maintaining a high quality source of water to the HPCI and RCIC pumps.
l The proposed level switch cross connection will not affect the way the suction transfer equipment functions. The cross connection tubing will be evaluated for seismic loads equivalent to the existing instrument piping. Rupture of the tubing will not prevent the function of the level switches from being accomplished and no other equipment important to safety is impacted by these changes.
1 Technical Specification and other specified margins of safety are effectively increased by the proposed changes. The HPCl/RCIC low CST level suction l
transfer level is being adjusted upward in the conservative direction.
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The changes do not present the opportunity for a new release path for radioactive material.
These changes have no impact on the protection of the health and safety of the public.
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The proposed amendment will not create thep_ossibility of a new or different kind of accident from any accident previously analyzed No system, structure, or component (SSC) described in the USAR as important to safety is affected by these changes except for the low level CST HPCl/RCIC suction transfer function. Postulated malfunctions related to the proposed changes to the low level switches are bounded by the failure of the HPCI system, which has been previously evaluated in the USAR. The RCIC system is not relied upon to mitigate any USAR design basis accident.
No new types of credible events could be identified which could be created by the l
proposed setpoint change and level switch cross connection. No new failure modes are associated with the proposed changes 3)
The proposed amendment will not involve a significant reduction in the margin of l
safety No margin of safety is reduced. Technical Specification and other specified margins of safety are effectively increased by the proposed activities. The HPCl/RCIC low CST level suction transfer setpoint is being adjusted upward in the conservative direcion. Cross connecting the level switches associated with this transfer will presave the redundancy built into the logic during extended l
.3 U S Nuclear Regulatory Commission Page 7 of 7 November 11,1998 Atta'chment outages of one CST. A small additiorial reduction in the reliability of the automatic transfer logic due to possible freezing or plugging of common instrument piping results when the level switches are temporarily cross connected during infrequent periods of operation with one CST in service. This small l
reduction in reliability of the automatic transfer function is fully compensated for l
by the ability to perform necessary and prudent preventive maintenance on the
- CSTs while at the same time supplying the HPCI and RCIC systems with water from the preferred high quality source.
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