L-MT-12-051, License Amendment Request: Remove Degraded Voltage Transfer to the 1AR Transformer
| ML12150A169 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 05/25/2012 |
| From: | O'Connor T Northern States Power Co, Xcel Energy |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-MT-12-051 | |
| Download: ML12150A169 (24) | |
Text
Monticello Nuclear Generating Plant 2807 W County Road 75 Monticello, MN 55362 May 25, 2012 L-MT-12-051 10 CFR 50.90 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Monticello Nuclear Generating Plant Docket 50-263 Renewed Facility Operating License No. DPR-22 License Amendment Request: Remove Degraded Voltage Transfer to the I A R Transformer Pursuant to 10 CFR 50.90, the Northern States Power Company - Minnesota (NSPM),
a Minnesota corporation, requests U.S. Nuclear Regulatory Commission (NRC) approval of a proposed change to the Monticello Nuclear Generating Plant (MNGP)
Updated Safety Analysis Report (USAR) and Technical Specification (TS) Bases to remove degraded voltage transfer capability to the Auxiliary Reserve Transformer (1AR).
MNGP is currently operating with the I A R Transformer removed from service, to address an issue identified during the recent 2012 NRC Component Design Basis Inspection. This issue involves the time delay assumed in the safety analyses for the degraded voltage transfer logic associated with the I A R Transformer, which is governed by Specification 3.3.8.1, "Loss of Power (LOP) Instrumentation." Removing the degraded voltage transfer capability to the I A R Transformer will remedy the TS compliance issue identified by NRC Region Ill and allow the I A R Transformer to be returned to service.
A modification has been prepared to remove the capability to automatically transfer to the I A R Transformer as a source of power to the essential buses on degraded voltage and instead directly transfer to the Emergency Diesel Generators (EDGs). This modification addresses the TS compliance issue while allowing the IAR Transformer to be returned to service. NRC approval of the proposed USAR and TS Bases changes is required since the required revisions involve a change in the plant licensing basis. The proposed change will significantly increase plant safety by providing additional defense-in-depth with the return of a qualified off-site source available to power the safety related essential buses at the plant. provides a description of the proposed change and includes the technical evaluation and associated no significant hazards determination and environmental evaluation. Enclosure 2 provides a marked-up copy of the TS Bases pages showing
Document Control Desk L-MT-12-051 Page 2 of 2 the proposed changes. Enclosure 3 provides a marked-up copy of the USAR pages showing the proposed changes.
The MNGP Plant Operations Review Committee has reviewed this application. In accordance with 10 CFR 50.91, a copy of this application, with enclosures, is being provided to the designated Minnesota Official.
Summaw of Commitments This letter proposes no new commitments and does not revise any existing commitments.
MNGP has reviewed the impact on reliability and increased potential for essential bus challenges due to the inability to perform preventative maintenance activities on the 2R and 1R Transformers. This impact is also reflected in the Probabilistic Risk Assessment evaluation.
Approving this license amendment request allowing restoration of the I A R Transformer to service will significantly improve defense-in-depth by restoring a qualified off-site power source to the safety related essential buses, substantially increasing nuclear safety. NSPM requests approval of the proposed license amendment within a time frame commensurate with the safety significance of the amendment, with a 30 day implementation period. The basis for requesting this amendment reflects NSPM1s desire to restore a qualified source of off-site power to the essential buses at the station in an expeditious manner based on the additional risks associated with operating in this condition during the summer season when electrical demands are greatest and storm activity affecting grid conditions are more likely. In summary, approval of this request in an expeditious timeframe is prudent from a defense-in-depth perspective and an enhancement to public safety.
perjury that the foregoing is true and correct.
President, Monticello Nuclear Generating Plant any - Minnesota Enclosures (3) cc: Administrator, Region Ill, USNRC Project Manager, Monticello, USNRC Resident Inspector, Monticello, USNRC Minnesota Department of Commerce
L-MT-12-051 Page Iof I TABLE OF CONTENTS SECTION TITLE PAGE
1.0 INTRODUCTION
1 2.0 DISCUSSION OF THE PROPOSED CHANGES 2 3.0 IMPACTS OF I A R TRANSFORMER REMOVED FROM SERVICE 2 3.1 Risk Significance of the Removal of the I A R Transformer from 3 Service 3.2 Times when the I A R Transformer was Relied Upon in the Last 4 Few Years 3.3 Impact on Preventative Maintenance Activities 5 3.4 Summarv of Pertinent Licensing Correspondence 6 4.0
SUMMARY
DESCRIPTION OF PLANT SWITCHYARD AND 7 TRANSFER LOGIC
5.0 PROPOSED CHANGE
S 8
6.0 TECHNICAL ANALYSIS
8
7.0 REGULATORY ANALYSIS
7.1 No Significant Hazards Determination 7.2 Applicable Regulatory Requirements 8.0 ENVIRONMENTAL EVALUATION 15
9.0 REFERENCES
16
L-MT-12-051 Enclosure 1 Page 1 of 16 DESCRIPTION OF CHANGES LICENSE AMENDMENT REQUEST TO REMOVE DEGRADED VOLTAGE TRANSFER TO THE IAR TRANSFORMER I.O INTRODUCTION Pursuant to 10 CFR 50.90, the Northern States Power Company - Minnesota (NSPM),
a Minnesota corporation, requests U.S. Nuclear Regulatory Commission (NRC) approval of a proposed change to the Monticello Nuclear Generating Plant (MNGP) licensing basis. It is proposed to revise the MNGP Updated Safety Analysis Report (USAR) and Technical Specification (TS) Bases to reflect removal of the degraded voltage automatic transfer capability to the Auxiliary Reserve Transformer (IAR), once approved.
The 2R Transformer (preferred source) and 1R Transformer (alternative source) are designed to carry the full plant loads and supply the 4.16 kV system. Normally, one of the two transformers is in service to supply non-safety plant loads and both essential (safety related) 4.16 kV Buses 15 and 16. MNGP has an additional off-site source, the I A R Transformer, sized to supply the two essential buses. In the event of degraded or loss of voltage to Buses 15 and 16, an Essential Bus Transfer (EBT) separates the essential buses from their normal non-safety supply and automatically switches them preferentially to the I A R Transformer (if there is acceptable voltage on the bus), and if not, to the associated Emergency Diesel Generator (EDG) (described in more detail herein).
Currently, the MNGP is operating with the I A R Transformer removed from service, to address a TS compliance issue associated with degraded voltage transfer to the I A R Transformer identified by Region Ill during the recent 2012 NRC Component Design Basis Inspection (CDBI), associated with Specification 3.3.8.1, "Loss of Power (LOP)
Instrumentation." This action, while necessary, has a negative consequence in that it has eliminated a qualified source of off-site power to the essential buses at the station.
Note, a loss of power transfer to the I A R Transformer is unaffected by the TS compliance issue with the transformer in service. To restore a qualified off-site source, a modification has been prepared to remove the capability to automatically transfer to the I A R Transformer as a source of power to the essential buses on degraded voltage and instead directly transfer to the EDGs. This action significantly improves plant defense-in-depth by restoring a qualified source of off-site power to the safety related essential buses at the plant, thereby, increasing nuclear safety.
To implement this modification, prior NRC approval of the proposed changes to the TS Bases and USAR is required per 10 CFR 50.59 (i.e., removing the automatic transfer to the I A R Transformer on degraded voltage and instead transferring the essential buses directly to the EDGs for a degraded voltage condition). This change to the licensing basis will allow the I A R Transformer to be returned to service while a
L-MT-12-051 Enclosure I Page 2 of 16 permanent design modification for the degraded voltage transfer to the I A R Transformer issue is developed.
2.0 DISCUSSION OF THE PROPOSED CHANGES The current design for EBT includes a five second time delay within the degraded voltage transfer scheme for each essential bus. This time delay logic was provided to assess the capability of the I A R Transformer to provide acceptable voltage to the respective essential bus(') and to transfer to the respective EDG, for that essential bus, if the essential bus voltage has not been restored to an acceptable voltage once the relay times out. This time delay is not explicitly described within the TS Bases for the loss of power specification.
The proposed modification removes this time delay and the I A R Transformer degraded voltage automatic transfer feature from these circuits. The EBT loss sf voltage functions, including transfer to I A R Transformer, and the degraded voltage EBT functions to the respective EDG's are unaffected by these changes.
The automatic transfer to the I A R Transformer on degraded voltage is reflected in the MNGP TS Bases and the USAR licensing bases and as such NRC prior approval is required. Enclosure 2 provides the proposed changes to the TS Bases for Specification 3.3.8.1, "Loss of Power (LOP) Instrumentation." Enclosure 3 provides the revised USAR pages reflecting the necessary revisions to the licensing basis due to this change.
3.0 SAFETY IMPACTS OF I A R TRANSFORMER REMOVED FROM SERVICE A review of the safety impacts of the removal from service of the I A R Transformer is provided in the following sub-sections. The areas considered include:
e risk determined by Probabilistic Risk Assessment techniques, e actual situations where the I A R Transformer has been relied upon, and e impact on reliability and increased potential for essential bus challenges; due to non-performance of preventative maintenance activities on the 2R and 1R Transformers.
The following illustrates the real effects of the I A R Transformer being removed from service at the MNGP to address the TS compliance issue.
- 1. The essential buses are the safety related 4.16 kV Buses 15 and 16. They are normally supplied by non-safety related 4.16 kV Buses 13 and 14, which are in turn supplied by the 2R Transformer (preferred source) or the 1R Transformer.
L-MT-12-051 Page 3 of 16 As discussed below, removal of the I A R Transformer from service is estimated to result in a continuous estimated Core Damage Frequency (CDF) increase of approximately 1E-07lyear. Also, as a Maintenance Rule component removal of the I A R Transformer from service has substantial impact on plant safety. Additionally, the desirability of having an additional off-site source to the 4.16 kV essential buses is clearly indicated considering these factors and is also indicated in previous NRC correspondence.
3.1 Risk Significance of the Removal of the I A R Transformer from Service NSPM is not proposing this license amendment on the basis of risk-informed decision-making as described in the Regulatory Guide series 1.I 74 - 1.178.
Rather, NSPM is proposing this license amendment using risk insights derived from Probabilistic Risk Assessments (PRAs) in combination with traditional engineering (deterministic) analyses to focus licensee and regulatory attention on issues commensurate with their importance to safety in accordance with the NRCs internal guidance, LIC-101, Revision 3, "License Amendment Review Procedures."
As determined by PRA, the rapid timing logic associated with loading the low pressure ECCS pumps from the essential buses under degraded or loss of offsite power conditions is important for large break LOCA events up to and including the design basis accident (DBA) LOCA. This logic timing is not critical for non-LOCA transients or medium to small break LOCA events since there will be much more time available to provide reactor vessel makeup for adequate core cooling in these cases.
Therefore, the risk associated with the degraded voltage timing logic is limited by two fundamental conditions, the frequency of a large break LOCA event, and the probability of experiencing a degraded voltage condition on the essential buses concurrent with the large break LOCA. A large break LOCA event(2)is estimated to occur at a frequency of 1.64 E-04lyear. The probability of degraded voltage conditions existing on the essential buses at any given time can be conservatively estimated by assuming the following information related to large motor starts over a 2 year cycle; this estimation leads to a probability of 2.7E-05.
The frequency of a concurrent large LOCA under degraded voltage conditions then is conservatively estimated as 4.4E-91year. This suggests that the degraded voltage timing for loading ECCS pumps onto the essential buses is not risk significant. A search of License Event Reports generated over the past 20 years, reveals no case where degraded voltage logic persisted for 9 seconds leading to a bus transfer to the I A R Transformer.
- 2. A large break LOCA event is defined as a pipe break with greater than 3 inch diameter by PRA analysis.
L-MT-12-051 Page 4 of 16 To satisfy the TS compliance issue related to the degraded voltage logic timing, it was necessary to remove the I A R Transformer from service. This reduction in offsite AC power redundancy leads to a continuous estimated CDF increase of approximately 1E-07lyear. The risk increase is approximately 20 times more significant than the conservative risk potential associated with the degraded voltage timing logic issue.
Additionally, under 10 CFR 50.65, the I A R Transformer is a component covered under the Maintenance Rule, and by definition, as such, the transformer's extended removal from service has a substantial impact on plant safety.
This moderate yet indefinite-in-length risk increase provides incentive to re-establish the ability to have the I A R Transformer available as a backup AC power supply to the essential 4.16 kV buses.
Times When the I A R Transformer was Relied Upon in the Last Few Years For each of the following events the I A R Transformer has played an essential role in plant response, and as such, it's availability to provide power to the essential buses is vitally important, in that it is a pre-installed alternative source providing additional defen~e-in-de~th.(~)
On September II , 2008, the plant was operating at 100 percent power with the IR Transformer isolated and tagged out for planned maintenance. Off-site power was being supplied by the 2R Transformer with the I A R Transformer and EDGs as backup power sources. At approximately 2248 hours0.026 days <br />0.624 hours <br />0.00372 weeks <br />8.55364e-4 months <br /> on September 11, 2008, a lockout of the 2R Transformer occurred due to a fault on the 2RS to 2R Transformer cable causing a loss of normal off-site power. The essential buses, i.e., Bus 15 and 16, were automatically powered from the I A R Transformer following this event (Reference 1).
On September 17, 2008, the plant was shutdown in a forced outage with the 2R Transformer isolated and tagged out for repair. Off-site power was being supplied by the 1R Transformer with the I A R Transformer and EDGs as backup power supplies. On September 17, 2008, at 0934 the site experienced a loss of normal off-site power when a man-lift being serviced by a vendor came into contact with a 115 kV line causing the 1R Transformer to trip while the 2R Transformer was out-of-service for repair (Reference 2). The essential buses, i.e., Bus 15 and 16, were automatically powered from the I A R Transformer following this event.
- 3. As the events at Fukushima illustrate, having additional sources of electrical power to the essential buses is fundamentally important for unusual conditions.
L-MT-12-051 Page5of 16 On October 21,201 1, a lockout of 2R Transformer occurred due to a ground fault on the " A phase of 2RS to 2R Transformer cable disconnecting the 2R source from off-site power (Reference 3). Off-site power automatically shifted from the 2R to the 1R Transformer. Both the 11 and 12 EDGs auto started but were not needed as safety-related Bus 16 automatically transferred to the 1R Transformer and safety-related Bus 15 automatically transferred to the I A R Transformer when the breaker supplying Bus 13 from the 1R Transformer failed to close.
In each of these events the I A R Transformer played an essential role in the plant response to the events. For the September 2008 events, the I A R Transformer was the source of power to the site for several days. From a safety perspective the I A R Transformer has been demonstrated to provide the MNGP with an additional valuable asset that provides defense-in-depth and enhances public safety.
3.3 lmrsact on Preventative Maintenance Activities While not quantifiable, removal from service of the I A R Transformer has a major impact on the preventative maintenance. As summarized below, many preventative maintenance (PM) activities, corrective maintenance, and testing is normally performed when the 2R and 1R Transformers are removed from service in the spring. Note that performance of these tasks has a significant impact on off-site power reliability by reducing the potential for challenges to the AC power supply to the essential buses due to issues with the 2R or 1R Transformers.
For example, in the spring, cottonwood seedsldebris are found impinged on the cooling fins of the 2R Transformer, reducing cooling capacity. PM's to remove these cottonwood deposits are scheduled for performance in June 2012. With the I A R Transformer removed from service the inherent risk in performing this activity increases, since removing either the 2R or 1R Transformers from service for maintenance now requires entering Specification 3.8.1, "AC Sources -
Operating," Required Action A, for one required off-site circuit inoperable, with a 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> Completion Time. Plant operation with the I A R Transformer out-of-service, therefore, substantially increases plant risk if either the 2R or 1R Transformers has to be removed from service for any reason, e.g.,
maintenan~e.(~)
- 4. Availability of a third off-site source (IAR Transformer) has allowed many preventative maintenance activities to be scheduled in the spring. Unlike most utilities, who only meet the Limiting Condition for Operation requirement for off-site sources and therefore plan these type of activities for shutdown conditions, e.g., refueling outages, this situation has a disproportionate impact on current plant risk for MNGP.
L-MT-12-051 Page 6 of 16 The following major PM activities for the 2R and 1R Transformer cannot be performed if the transformers are not isolated.
Replace ammeter switch whose open circuit resulted in lockout of Bus 11 during transfer from the 1R to 2R Transformer in November 201 1.
Megger Test cable that provides power to the 2R Transformer coolers.
Test cables in B105 conduit prior to the next occurrence of supplying power from the 1R Transformer (water leaked through the conduit).
lnspect 2R Transformer 4.16 kV Bus inside 4kV Switchgear Rooms.
0 Calibrate 2R Transformer winding temperature indicators.
e lnspect 34.5 kV circuit switch 3N5. (Power to 2R Transformer via 3N5.)
Test relaying associated with 3N5 circuit switch and the CLIP fuses.
Take oil sample from 34.5 kV circuit breaker 3N4 for oil property testing, Replace circuit breaker 3N4. Crack in primary side jumper terminal.
Adjustlrepair 3N4-B-A disconnect - difficult to operate vs. other phases.
Perform doble testing of the 2RS Transformer (feeds 2R Transformer).
Seasonal cleaning of the 2R Transformer cooler to remove cottonwood debris. The cooler fans have in the past tripped on thermal overload when coolers became too plugged to pass sufficient air.
The above activities require the respective transformer (1R or 2R Transformer) to be removed from service. Performance of these activities significantly improves off-site power reliability by reducing potential challenges to the AC power supply to the essential buses.
3.4 Summarv of Pertinent Licensing Correspondence The removal of the ability to first attempt to transfer automatically to the I A R Transformer on a degraded voltage condition returns the system to a condition previously determined acceptable by the NRC, in the original degraded voltage transfer scheme. The proposed modification will restore the degraded logic transfer scheme similar to the scheme installed in 1978. Later NRC correspondence (Reference 4 - the Safety Evaluation for the event), indicated that when degraded voltage was detected from any one of the sources of off-site
L-MT-12-05 1 Page 7 of 16 power, transferring directly to the onsite EDG without considering the other off-site sources was undesirable.
The NRC stated on page 5 of their Safety Evaluation that:
A design that inherentlv precludes access to the other sources of offsite power is not consistent with a design obiective to provide redundant access circuits to the grid. We believe that, just as on a main generator trip and on loss of voltage, it may be appropriate for degraded voltage protection to consider other access circuits to the grid that mav have adequate power before resorting to the onsite EDGs.
This concept, that a design that inherently precludes access to other sources of off-site power is not consistent with the design objective to provide redundant access circuits to the grid, applies to the ability to switch to the I A R Transformer on a loss of voltage condition.
The ability to automatically transfer to the I A R Transformer in a loss of voltage situation has been lost with the removal of this transformer from service to address the TS compliance issue. As discussed previously the ability to transfer to the I A R Transformer on a loss of essential bus voltage provides significant safety benefits.
4.0
SUMMARY
DESCRIPTION OF PLANT SWITCHYARD AND TRANSFER LOGIC There are 7 (345 kV and 115 kV) transmission line connections from the MNGP switchyard to the NSP interconnected transmission grid. Points of connection to the grid are arranged by routes and intra-right-of-way spacing to minimize multiple line outages while delivering power to where required. The 345 kV and 115 kV transmission lines connect via the switchyard to the No. 1 Reserve Transformer (1R), the primary station Auxiliary Transformer (2R), and the Auxiliary Reserve Transformer (IAR). The 1R and 2R Transformers are designed to carry the full plant loads and supply the 4.16 kV system as described below.
The plant 4.16 kV system consists of six in-plant 4.16 kV buses which can be supplied from either of two sources. The normal source is 2R Transformer. The alternate source is the 1R Transformer supplied from the 115 kV Substation. Protective relaying, if activated, de-energizes 2R Transformer, and initiates an open circuit transfer to the 1R Transformer. 4.16 kV essential Buses 15 and 16 are normally supplied by 4.16 kV Buses 13 and 14, respectively, which in turn are supplied by the 2R Transformer (preferred source) or the 1R Transformer.
In the event of degraded or loss of voltage to the two essential (safety related) 4.16 kV Buses 15 and 16, an EBT separates them from non-safety related 4.16 kV Buses 13
L-MT-12-051 Page 8 of 16 and 14 and switches them to an alternate source. Each division, i.e., Bus 15 and Bus 16, is currently provided with a transfer scheme that controls automatic transfer to the I A R Transformer and another that controls automatic transfer to the associated essential bus EDG.
5.0 PROPOSED CHANGE
S The following changes are proposed to the Monticello TS Bases for Specification 3.3.8.1, "Loss of Power (LOP) Instrumentation." provided in Enclosure 2:
Remove discussion about transferring to I A R if the 4.16 kV essential bus degraded voltage is present for approximately 5 seconds (bottom of second paragraph on page B 3.3.8.1-1).
The following revisions are proposed to the Monticello USAR provided in Enclosure 3:
Revise USAR Section 8.4.1.3, "Performance Analysis," page 7, second paragraph, to indicate that transfer of the essential buses to the EDGs will occur due to degraded voltage on the essential buses.
Revise USAR Section 8.4.1.3, page 7, third paragraph, to eliminate reference to degraded voltage values in a sentence referring to the I A R Transformer.
0 Revise USAR Section 8.4.1-3, page 7, third paragraph, remove reference to values for degraded voltage for the I A R Transformer (no longer applies) and add a sentence indicating that the essential buses will transfer to the EDG for degraded voltage conditions. provides revised draft page(s) proposed to be added to the Monticello TS Bases. Enclosure 3 provides revised draft page(s) proposed to be added to the Monticello USAR.
6.0 TECHNICAL ANALYSIS
MNGP has an additional off-site source, the I A R Transformer, sized to supply the two essential buses. In the event of degraded or loss of voltage to Buses 15 and 16, an EBT separates the essential buses from their non-safety supply and automatically switches them preferentially to the I A R Transformer (if there is acceptable voltage on the bus), and if not, to the associated essential bus EDG. The current design for EBT includes a five second time delay in the degraded voltage transfer scheme for each essential bus. This time delay logic was provided to assess the capability of the I A R Transformer to provide acceptable voltage to the respective essential bus, and to transfer to the respective EDG for that essential bus if the essential bus voltage has not
L-MT-12-051 Page 9 of 16 been restored to an acceptable voltage once the relay times out. This additional time delay is not explicitly described in Specification 3.3.8.1, "Loss of Power Instrumentation."
Currently, the MNGP is operating with the I A R Transformer removed from service, to address a TS compliance issue associated with degraded voltage transfer to the I A R Transformer. This action has eliminated a qualified source of off-site power to the essential buses at the station. To restore this qualified source, a modification has been prepared to remove the capability to automatically transfer to the I A R Transformer on degraded voltage and instead directly transfer to the EDGs. The proposed modification will remove the 5 second time delay and the I A R Transformer degraded voltage automatic transfer feature. The EBT loss of voltage functions including the transfer to I A R Transformer will be unaffected by the proposed changes. The EBT degraded voltage functions for transfer of the essential bus to the EDG's will eliminate the 5 second time delay which currently occurs.
This EBT transfer scheme, including the 5 second time delay for degraded voltage sensed from the IAR Transformer, is reflected in the USAR and is part of the current licensing basis of the plant. This time delay for degraded voltage conditions is not explicitly described or part of the current TSs for 4.16 kV essential bus degraded voltage. However, the transfer to the I A R Transformer on degraded voltage is described within the TS Bases. Therefore, modifications are required to the USAR and TS Bases to reflect the elimination of this capability to automatically transfer to the I A R Transformer on degraded voltage.
A reduced voltage condition on a 4.16 kV essential bus indicates that, while offsite power may not be completely lost to the respective essential bus, available power may be insufficient for starting large ECCS motors without risking damage to the motors that could disable the ECCS function. In accordance with the present design, in the event of degraded voltage on a 4.16 kV essential bus (Bus 15 or 16), an EBT separates the bus from the respective non-safety related 4.16 kV bus (Bus 13 or 14) and switches them to an alternate source. Two transfer schemes are provided, one controlling automatic transfer to the I A R Transformer and the other that controls automatic transfer to the associated essential bus EDG.
This license amendment request removes automatic transfer to the I A R Transformer on degraded voltage, but retains the automatic transfer to the associated essential bus EDG. Hence, while the option to first attempt a transfer to another off-site source (IAR) will be removed, automatic transfer to the associated essential bus EDG still occurs for degraded voltage.
Following installation of the modification, the power supply to the essential buses will be directly transferred from off-site power (2R, IR and I A R Transformers) to the onsite EDG(s) when the voltage on the bus drops below the 4.16 kV essential bus degraded voltage TS function Allowable Values. This ensures that adequate power will be
L-MT-12-051 Page 10 of 16 available to the required equipment. The 4.16 kV essential bus degraded voltage TS Allowable Values are low enough to prevent inadvertent power supply transfer, but high enough to ensure that sufficient power is available to the required equipment.
The removal of the ability to automatically transfer to the I A R Transformer for a degraded voltage condition returns the system to a condition that was previously determined acceptable by the NRC in the original degraded voltage transfer scheme.
As discussed above, transferring directly to the EDGs for a degraded voltage condition preserves the assumptions of the safety analysis and is safe.
L-MT-12-051 Page 11 of 16
7.0 REGULATORY ANALYSIS
7.1 No Significant Hazards Determination In accordance with the requirements of 10 CFR 50.90, the Northern States Power Company - Minnesota (NSPM) requests an amendment to facility Renewed Operating License DPR-22, for the Monticello Nuclear Generating Plant (MNGP) to revise the MNGP Updated Safety Analysis Report (USAR) to reflect removing the automatic transfer to the I A R Transformer on degraded voltage.
The NSPM has evaluated the proposed amendment in accordance with 10 CFR 50.91 against the standards in 10 CFR 50.92 and has determined that the operation of the MNGP in accordance with the proposed amendment presents no significant hazards. NSPM1sevaluation against each of the criteria in 10 CFR 50.92 follows.
- 1. Does the proposed amendment involve a significant increase in the probability or consequences of an accident previously evaluated?
Response: No.
The Emergency Diesel Generators (EDGs) provide a source of emergency power when offsite power is either unavailable, or is degraded below a point that would allow safe unit operation. Undervoltage protection currently will generate a loss of power (LOP) EDG start if a loss of voltage or degraded voltage condition occurs on the 4.16 kV vital bus.
The proposed licensing basis change conservatively transfers directly from the source to the EDG, skipping the I A R Transformer with respect to the degraded voltage function. The undervoltage relays will continue to meet their required function to transfer 4.16 kV buses to the EDGs in the event of insufficient offsite power voltage. This transfer will ensure that the Class 1E equipment is capable of performing its function to meet the requirements of the accident analysis. The proposed change does not affect any accident initiators or precursors.
There are no changes to the initial conditions contributing to accident severity or consequences. The safety analyses continue to meet all applicable acceptance criteria. The proposed amendment will not degrade the performance of SSCs important to safety.
Therefore, the proposed change does not involve a significant increase in the probability or consequences of an accident previously evaluated.
L-MT-12-051 Page 12 of 16
- 2. Does the proposed amendment create the possibility of a new or different kind of accident from any accident previously evaluated?
Response: No.
No new accident scenarios, transient precursors, failure mechanisms, or limiting single failures are introduced as a result of the proposed change.
Revising the degraded voltage transfer logic capability to transfer directly to the EDG instead of first attempting to transfer to the I A R Transformer, was previously licensed for the plant, is typical for the industry, and as such does not significantly reduce plant reliability, i.e., plant safety will be maintained. The delay time associated with the degraded voltage transfer logic is not an initiator of any accident and does not create any new system interactions or failure modes of any structures, systems or components. Equipment important to safety will continue to operate as designed. The proposed change does not result in adverse conditions or result in a significant increase in the challenges to safety systems.
Operation of the MNGP in accordance with the proposed change will not create the possibility of a new or different type of accident from any accident previously evaluated.
- 3. Does the proposed amendment involve a significant reduction in a margin of safety?
Response: No.
The EDGs provide emergency electrical power to the essential buses in support of equipment required to mitigate the consequences of design basis accidents and anticipated operational occurrences, including an assumed loss of all offsite power. The LOP Technical Specification (TS) parameters (loss of voltage and degraded voltage) instrumentation channels will respond to measured parameters within the necessary range and accuracy. The proposed amendment corrects nonconservative values assumed in the analyses for the degraded voltage protection function. The proposed change assures that the design requirements of the emergency electrical power system continue to be met.
There are no changes to the initial conditions contributing to accident severity or consequences. The safety analyses continue to meet all applicable acceptance criteria. The proposed amendment will not degrade the performance of SSCs important to safety.
Therefore, the proposed change does not involve a significant reduction in a margin of safety.
L-MT-12-051 Page 130f 16 Based on the above, the NSPM has determined that operation of the facility in accordance with the proposed change does not involve a significant hazards consideration as defined in 10 CFR 50.92(c), in that it does 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.
7.2 Applicable Regulatory Requirements The MNGP was designed largely before the publishing of the 70 General Design Criteria (GDC) for Nuclear Power Plant Construction Permits proposed by the Atomic Energy Commission (AEC) for public comment in July 1967, and constructed prior to the 1971 publication of Appendix A, "General Design Criteria for Nuclear Power Plants", to 10 CFR Part 50. As such, the MNGP was not licensed to the Appendix A, General Design Criteria (GDC).
The MNGP USAR, Section 1.2, lists the principal design criteria (PDCs) for the design, construction and operation of the plant. USAR Appendix E provides a plant comparative evaluation to the 70 proposed AEC design criteria. It was concluded that the plant conforms to the intent of the GDCs. The applicable GDCs and PDCs are discussed below.
- a. 10 CFR 50, Appendix A, General Design Criterion 17 -- Electric power svstems An onsite electric power system and an offsite electric power system shall be provided to permit functioning of structures, systems, and components important to safety. The safety function for each system (assuming the other system is not functioning) shall be to provide sufficient capacity and capability to assure that (1) specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and (2) the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents.
The onsite electric power supplies, including the batteries, and the onsite electric distribution system, shall have sufficient independence, redundancy, and testability to perform their safety functions assuming a single failure.
Electric power from the transmission network to the onsite electric distribution system shall be supplied by two physically independent circuits (not necessarily on separate rights of way) designed and located so as to
L-MT-12-051 Page 14 of 16 minimize to the extent practical the likelihood of their simultaneous failure under operating and postulated accident and environmental conditions. A switchyard common to both circuits is acceptable. Each of these circuits shall be designed to be available in sufficient time following a loss of all onsite alternating current power supplies and the other offsite electric power circuit, to assure that specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded. One of these circuits shall be designed to be available within a few seconds following a loss-of-coolant accident to assure that core cooling, containment integrity, and other vital safety functions are maintained.
Provisions shall, be included to minimize the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by the nuclear power unit, the loss of power from the transmission network, or the loss of power from the onsite electric power supplies.
MNGP PDC 1.2.6 Plant Electrical Power Sufficient normal and standby auxiliary sources of electrical power are provided to attain prompt shutdown and continued maintenance of the plant in a safe condition under all credible circumstances. The capacity of the power sources is adequate to accomplish all required engineered safeguards functions under all postulated design basis accident conditions.
- c. 10 CFR 50, Appendix A, General Design Criterion 18 -- Inspection and testing of electric power svstems Electric power systems important to safety shall be designed to permit appropriate periodic inspection and testing of important areas and features, such as wiring, insulation, connections, and switchboards, to assess the continuity of the systems and the condition of their components. The systems shall be designed with a capability to test periodically (1) the operability and functional performance of the components of the systems, such as onsite power sources, relays, switches, and buses, and (2) the operability of the systems as a whole and, under conditions as close to design as practical, the full operation sequence that brings the systems into operation, including operation of applicable portions of the protection system, and the transfer of power among the nuclear power unit, the offsite power system, and the onsite power system.
L-MT-12-051 Enclosure I Page 15 of 16
- d. PDC Criterion 38 - Reliabilitv and Testabilitv of Engineered Safety Features All engineered safety features shall be designed to provide high functional reliability and ready testability. In determining the suitability of a facility for proposed site, the degree of reliance upon and acceptance of the inherent and engineered safety afforded by the systems, including engineered safety features, will be influenced by the known and the demonstrated performance capability and reliability of the systems, and by the extent to which the operability of such systems can be tested and inspected where appropriate during the life of the plant.
The NSPM has evaluated the proposed changes against the applicable regulatory requirements and acceptance criteria. The technical analysis concludes that the changes associated with the proposed licenses amendment will continue to assure that the design requirements and acceptance criteria of MNGP ECCS LOCA analysis and single failure analysis are met. Based on this, there is reasonable assurance that the health and safety of the public, following approval of this license amendment, is unaffected.
8.0 ENVIRONMENTAL EVALUATION The NSPM has determined that the proposed amendment would not change a requirement with respect to installation or use of a facility or component located within the restricted area, as defined in 10 CFR 20, nor would it change an inspection or surveillance requirement. The proposed amendment does not involve (i) a significant hazards consideration, or (ii) authorize a significant change in the types or a significant increase in the amounts of any effluent that may be released offsite, or (iii) result in a significant increase in individual or cumulative occupational radiation exposure. Accordingly, the proposed amendment meets the eligibility criterion for a categorical exclusion set forth in 10 CFR 51.22(~)(9).Therefore, the NSPM concludes pursuant to 10 CFR 51.22(b), no environmental impact statement or environmental assessment need be prepared in connection with the proposed amendment.
L-MT-12-051 Page 16 of 16
9.0 REFERENCES
- 2. MNGP LER 2008 - 006 - 00, "Loss of Normal Offsite Power due to Equipment Contact with 115 kV Lines."
- 4. NRC Letter to NSP, "Monticello Operating Event of August 1, 1983: Degraded Voltage Relay Trip and Unexpected Loss of Offsite Power," dated September 8, 1983.
ENCLOSURE 2 MONTICELLO NUCLEAR GENERATING PLANT LICENSE AMENDMENT REQUEST REMOVE DEGRADED VOLTAGE TRANSFER TO THE I A R TRANSFORMER MARKED-UP TECHNICAL SPECIFICATION BASES PAGES (2 pages follow)
LOP Instrume,ntation B 3.3.8.1 B 3.3 INSTRUMENTATION B 3.3.8.1 Loss of Power (LOP) lnstrum6ntation BASES .
BACKGROUND Successful operation of the required safety functions of the Emergency Core Cooling Systems (ECCS) is dependent upon the availability of adequate power sources for energizing the various components such as pump motors, motor operated valves, and the associated control components. The LOP instrumentation monitors the 4.16 kV essential buses. Offsite power is the preferred source of power for the 4.16 kV essential buses. If the monitors determine that insufficient power is available, the buses are disconnected from the offsite power sources and connected to the onsite emergency diesel generator (EDG) power sources.
Each 4.1 6 kV essential bus has its own independent LOP instrumentation and associated .trip logic: The voltage for each bus is monitored at two levels, which can be considered as two different undervoltage Functions:
4.16 kV Essential Bus Loss of Voltage and 4.16 kV Essential Bus Degraded Voltage (Ref. 1). Each Function causes various bus transfers and disconnects. The 4.1'6 kV Essential Bus Loss of Voltage Function is monitored by four undervoltage relays for each emergency bus, whose outputs are arranged in a one-out-of-two twice logic configuration (i.e.,
one channel in each of two trip systems must trip for LOP actuation). The 4.1 6 kV Essential Bus Degraded Voltage Function is monitored by three undewoltage relays (with its associated time delay) for each emergency bus, whose outputs are arcanged in a two-out-of-three logic configuration.
Both LOP Functions provide an automatic start signal to both EDGs.
However, only the automatic start signal to the associated EDG (the EDG in the same division) is required. If the 4.16 kV Essential Bus Loss of.
Voltage signal is present for approximately 5 seconds, it will trip the supply breaker (from bus 13 or 14, as applicable) to the associated essential bus and provide a transfer,signal to the reserve auxiliary transformer (1AR). If the 4.16 kV Essential Bus Loss of Voltage signal is present for approximately 10 seconds (i.e., the transfer to I A R fails or I A R is deenergized) it will trip.all supply breakers to the essential bus (from bus 13 and 14, as applicable and from IAR) and provide a close signal to the EDG output breaker. If the 4.16 kV'Essential Bus Degraded Monticello B 3.3.8.1-1 Revision No. 0
LOP Instrumentation B 3.3.8.1
('- BASES BACKGROUND (continued) a close signal to the EDG output'breaker, 'The channels include
, . electronic equipment (e.g., relays) that compares measured input signals with pre-established setpoints. When the setpoint is exceeded, the channel output relay actuates, which then outputs a LOP trip signal to the trip logic.
APPLICABLE The LOP instrumentation is required for Engineered Safety Features to SAFETY function in any accident with a loss of offsite power. The required ANALYSES, LCO, channels of LOP instrumentation ensure that the ECCS and other and APPLICABILITY assumed systems powered from the EDGs, provide plant protection in the event of any of the Reference 2, 3, and 4 analyzed accidents in which a loss of offsite power is assumed. The initiation of the EDGs on loss of offsite power, and subsequent initiation of the ECCS, ensure that the fuel peak cladding temperature remains below the limits of 10 CFR 50.46.
cident analyses credit the loading of the EDG based on the loss of ite power during a loss of coolant accident. The diesel starting and ing times have been included in the delay time associated with each ty system component requiring EDG supplied power following a loss The LOP Instrumentation satisfies Criterion 3 of 10 CFR 5OZ36(c)(2)(ii).
The OPERABILITY of the LOP instrumentation is dependent upon the' OPERABILITY of the individual instrumentation channel Functions specified in Table 3.3.8.1-1. Each Function must have a required number of OPERABLE channels per 4.16 kV essential bus, with their setpoints within the specified Allowable Values. A channel is inoperable if its actual trip setpoint is not within its required Allowable Value. The actual setpoint is calibrated consistent with applicable setpoint methodology assumptions.
The ~llowableValues are specified for each Function in the Table.
Nominal trip setpoints are specified in the setpoint calculations. The nominal setpoints are selected to ensure that the setpoints do not exceed the Allowable Value between CHANNEL CALIBRATIONS. Operation with a trip setpoint less conservative than the nominal trip setpoint, but within the Allowable Value, is acceptable. Trip setpoints are those predetermined values of output at which an action should take place. The setpoints are compared to the actual process parameter (e.g., degraded voltage), and when the measured output value of the process parameter exceeds the setpoint, the associated device (e.g., trip unit) changes state.
The analytic limits are derived from the limiting values of the process parameters obtained from the safety analysis. The Allowable Values and C nominal trip setpoints (NTSP) are derived, using the General Electric setpoint methodology guidance, as specified in the Monticello setpoint Monticello B 3.3.8.1-2 Revision No. 0
ENCLOSURE 3 MONTICELLO NUCLEAR GENERATING PLANT LICENSE AMENDMENT REQUEST REMOVE DEGRADED VOLTAGE TRANSFER TO THE I A R TRANSFORMER UPDATED SAFETY ANALYSIS REPORT PAGES (1 page follows)
MONTICELLO UPDATED SAFETY ANALYSIS REPORT USAR 8.4 Revision 27 Page7of 17 If, prior to or subsequent to the transfer to the EDGs, the essential 41 60 Vac motors for emergency core cooling are required to operate, further load
. The essential bus transfer schemes ensure that a malfunction of any single component will only prevent the automatic re-energization of one division's essential bus by its associated EDG or the 1AR transformer.
Transfer of the essential buses to the 1AR transformer will normally occur on loss of voltage (nominally 2625 Vac for five . . seconds) 0 rn If the 1AR no-load voltage is unacceptable for an additional five seconds,or if the essential buses are being supplied from the 1AR transformer when the loss of voltage ewbgmdd wkage condition occurs, a transfer to Concurrently these relays initiate a 15-cycle delayed closure of the EDG supply breaker. Additional logic also assures that the EDG breaker is not closed until the diesel generator is ready to accept load.
The IAR transformer and the EDGs are limited in capacity and are primarily intended to supply safeguard loads, Certain other loads may be supplied if the situation permits. To prevent overloading these limited capacity sources, and to avoid excessive voltage drop during motor acceleration periods, load shedding and load application sequencing circuits are provided as previously mentioned.
The following discusses in detail the features of these systems.
For the situation where the full capacity reserve transformer (1 R) and the primary station auxiliary transformer (2R) are not available and loads are to be transferred to the 1AR transformer or to the EDGs, but which is not accompanied by a need for emergency core cooling, source capacity is available to supply a number of auxiliaries, the operation of which is desirable but not necessary from a safeguard standpoint. Such auxiliaries include the service water pumps, reactor building closed cooling water pumps, control rod drive pumps, fuel pool cooling water pumps, drywell cooling units, instrument and service air compressors and off-gas stack dilution fans. The RHR Service Water pumps are manually started when required.