ML20059J249
| ML20059J249 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 01/14/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20059J216 | List: |
| References | |
| NUDOCS 9401310415 | |
| Download: ML20059J249 (6) | |
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NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO.159 TO FACILITY OPERATING LICENSE NO. OPR-40 OMAHA PUBLIC POWER DISTRICT FORT CALHOUN STATION. UNIT NO.1 DOCKET NO. 50-285
1.0 INTRODUCTION
By letter dated October 7, 1992, as supplemented July 12, 1993, Omaha Public Power District (OPPD) submitted a request for changes to the Fort Calhoun Station (FCS), Unit No. 1 Technical Specifications (TSs). The requested changes would change the setpoint limit of the degraded-voltage protection system referred to as the offsite-power low signal (OPLS) in TS Section 2.14, Table 2-1, Item 6.b. At FCS, the electrical distribution system is designed with four separate 4160 V buses, two of which are designated as safeguards buses. These 4160 V safeguards buses are normally fed from the 161 kV system which constitutes the offsite power supply. The two safeguards buses, IA3 and 1A4, and their associated 480 V buses supply power to engineered safety feature (ESF) components that are required for safe reactor shutdown and removal of decay heat. To ensure that adequate voltage exists from the offsite power system to these buses during an accident, the electrical distribution system has been equipped with a degraded voltage protection system which is the second level of undervoltage protection. The degraded voltage protection system is referred to as the OPLS. The OPLS, composed of solid state undervoltage relays combined in 2-out-of-4 logic per bus, provides protection to safeguard equipment when an undervoltage condition is sensed on the 4.16 kV emergency buses IA3 or lA4 in the presence of a safety injection i actuation signal (SIAS). When a degraded voltage condition exists concurrent with a SIAS, the OPLS signal isolates buses lA3 and 1A4 from their offsite power supply (161 kV) and initiates automatic actions to load the safeguards equipment onto the emergency diesel generators. The original 0PLS setpoints
{ 2 3825.52 V (4.8 .5) seconds on bus lA3 and 1 3724.08 V (4.8 .5) seconds on bus lA4 } were originally designed to ensure the voltage being supplied during an accident does not drop below 90% of the motor nameplate rating after a 4.8 second time delay. The motor manufacturer recommends that terminal voltage be maintained at a minimum of 90% of motor nameplate rating to assure long-term motor protection from excessive current.
The proposed change is the result of an engineering analysis that concluded that the present OPLS setpoint limit for degraded grid protection was inadequate and outside the design basis. The primary cause of the OPLS setpoint being outside its design basis was an inadequate design modification l
9401310415 940114 PDR ADOCK 05000285 P PDR
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at the time the original analysis was performed. The assumptions made in the original analysis were nonconservative and had propagated throughout subsequent revisions to the analysis. A contributing cause.was the lack of procedural guidance for the performance and documentation of assumptions used
- in the calculations. Another contributing cause was the lack of comprehensive i design basis documentation prior to July 1989. This lack resulted in inadequate documentation of the original 0PLS design criteria, leading to credit being taken in subsequent modifications for non-automatic protection of safeguards loads. The proposed TS change for the OPLS actuation limit is necessary to ensure that the limit is consistent with the more restrictive ;
engineering analysis EA-FC-92-081 and will ensure acceptable voltages at the 480 V and 120 V levels during sustained degraded grid voltage conditions.
The July 12, 1993, letter provided clarifying information that did not change the initial proposed no significant hazards consideration determination.
2.0 EVALUATION OPPD has been involved in a Design Basis Reconstitution Project to verify or reconstruct the design basis calculations and analysis of FCS systems, including the electrical distribution systems. In 1989, the first revision of the design basis document for the electrical distribution system was issued.
An open item in this document identified the lack of supporting documentation to verify that certain loads could be assured of operating above their minimum voltage rating. In an effort to resolve this open item, a computer software program, " Electrical Load Honitoring System" (ELMS), was utilized to model the plant electrical distribution system for the purpose of reconstituting the original Degraded Voltage Analysis. The ELMS revealed that, during an accident in which safeguards equipment would be required to operate, the voltage supplied to some 480 V safeguard loads could be as low as 87.5% of rated voltage without OPLS being actuated. Since the possibility existed for voltage to be lower than the recommended 90% of rated voltage for certain 480 V safeguards loads without an OPLS actuation, the Plant Review Committee (PRC) determined in February 12, 1991, that the plant was outside of its design basis. This situation was reported to the NRC in License Event Report 50-285/91-01, dated March 21, 1991, which was subsequently revised on June 21, !
1991. Interim corrective actions included the following:
- 1. Engineering analysis EA-FC-017 was performed to derive new OPLS setpoints for current bus loading configurations. The new setpoints were more conservative than the setpoints that were currently listed in the TSs. The OPLS actuation relays have been recalibrated with the new setpoints determined by EA-FC-017. The new OPLS setpoints in conjunction with the loading configuration specified in the analysis will ensure safeguards motors are provided adequate voltage so that they I satisfy their design function during accident conditions. l l
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- 2. As part of the interim actions, a dedicated operator was stationed in the control room to monitor voltage levels on 4160 V buses IA3 and 1A4 and manually actuate OPLS if voltages -
dropped below the new OPLS alarm setpoints installed on the plant computer.
- 3. Administrative restrictions were placed on equipment rotation and bus-loading configurations through Operation Memorandum 91-01. The loading restrictions were necessary to ensure bus alignment is consistent with that assumed in the analysis for the new setpoints. ;
Administrative bus loading restriction remained in effect until '
long-term corrective action had been completed. These restrictions are referenced in the Safety Analysis for Operability Report, SAO- '91-002, "0PLS Actuation for Protection of 480 V Safeguard Motors."
If an electrical configuration arises due to plant operating >
requirements which are outside the permitted restrictions, plant staff will contact Engineering, evaluate the situation, and determine operability and reportability. ,
In order to remove the operational restrictions of the interim actions a ,
modification was implemented during the 1992 refueling outage. The 1992 modification altered existing logic circuitry in such a way that, upon receipt of a SIAS, the large 4.16 kV motors and equipment (i.e., fee &ater, heater drain and condensate pumps) not required to mitigate the consepences of an accident were automatically load shed. The modification also raised the OPLS i setpoints so that the actuation occurs (assuming a concurrent SIAS) at a higher level of decreasing voltage on the 4.16 kV safeguaros buses IA3 and 1A4 i based on Engineering Analysis EA-FC-90-057.
Subsequent to the installation of the 1992 modification described above, it has been determined that it is possible to permit the operation of a single main feedwater/ condensate pump train following a SIAS by establishing additional load shedding and modifying the OPLS setpoints. This conclusion is documented in Engineering Analysis EA-FC-92-081. Although it is not necessary to operate any main feedwater or condensate pumps to mitigate the consequences l of any design basis accident, it is desirable to provide the option of I operating a single main feedwater train in certain SIAS scenarios in order to ;
provide more operational flexibility during the response to an accident. '
Therefore, another modification is planned for the 1993 outage which will ,
provide this flexibility and, as in the case of the 1992 modification, will ,
require the OPLS setpoint to be changed due to the load change. This proposed {
change to the TS limit for the OPLS setpoints will be necessary to ensure the !
TS limit is consistent with the more restrictive and most recent OPLS setpoint i limit specified in Engineering Analysis EA-FC-92-081. Additionally, the l proposed TS change will provide additional assurance that the safety related loads that are sequenced on the emergency buses will have adequate voltage, b
There were a number of concerns identified by the NRC with regards to the OPLS setpoint change. The first concern was the type of engineering analysis that was performed to justify the OPLS calculation. OPPD explained that the OPLS calculations assumed that the ESF motors would accelerate their respective loads to full speed at 2 70% of motor nameplate voltage and that the motor voltage would recover to 2 90 % of motor nameplate voltage after motor acceleration. The plant bus loading was based on measurements taken during power operations. These load measurements were then used to calculate the worst case ESF starting voltage for the motors.
The second concern by the NRC staff was whether the new calculation analyzed down to 120 V level. The OPPD stated that a calculation was performed at the 120 V motor control center (MCC) contactor level to verify that automatically operated motor-operated valve contactors would pickup and hold in during the starting voltage transients. Another concern was the type of field verification that was done to validate the data used for both initial and revised OPLS calculation. The field verification conducted for the original OPLS analysis involved starting several ESF motors and measuring the voltage transients. This was used to verify the OPLS methodology and model accuracy.
OPPD considered the present electrical distribution model used to derive current OPLS setpoints to be valid based on the Diesel Generator Transient Analysis. This analysis was verified by comparing the diesel generator automatic operation data taken during the 1992 refueling outage to the computer generated transient load profile. The results showed acceptable agreement between the model and the actual equipment operation.
The third concern was how the new calculation would be controlled and updated with future load changes. OPPD has in place procedural controls which govern modifications to the facility and which require that an Electric System Interaction Evaluation be performed. This interacting evaluation is in the form of checklists which prompt that an analysis be performed if bus loading ,
is impacted. Bus loading is a critical input of the OPLS analysis, and '
accordingly the procedural controls address possible future changes.
The fourth concern was related to maintaining 2 70% of nameplate voltage at the motor terminals during acceleration. In response to question one in the licensee's amendment request dated July 12, 1993, OPPD stated that the OPLS calculation assumed that the ESF motors would accelerate to full speed at i 2 70% of motor nameplate voltage. The NRC staff questioned how this 70% value I was justified. The licensee stated that maintaining 70% of motor nameplate voltage during acceleration was supported by the manufacturer and provided the NRC staff with copies of the motor curves supplied by the manufacturer.
The final concern was related to the setpoint value contained in Table 2.1 for the degraded voltage. In Table 2.1 the degraded voltage relay setpoint is 3988.8 V for bus 1A3 and 3990.6 V for bus 1A4. The staff questioned why there were no minimum and maximum ( ) values and if the new values in the table provided adequate margin for equipment inaccuracies (instrument errors, setpoint drift, etc.). The licensee stated that the new value in Table 2.1 is
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I the minimum setpoint value that is allowed for the secondary undervoltage
. protection relays and that the calibration procedures used provide actual j setpoints that encompass equipment inaccuracies. However, the licensee has j selected not to put minimum and maximum values based on instrument 4 uncertainties in Table 2.1 of the Technical Specifications, l The proposed changes to increase the OPLS setpoint provides additional conservatism for the 4160 V vital buses to provide safety-related equipment with adequate voltage at the 4160 V, 480 V and 120 V levels and therefore reduce the risk of tripping or damaging the vital equipment associated with accident mitigation. For example, the proposed change will cause the OPLS to actuate at a higher level of voltage (concurrent with a SIAS), which will insure that a minimum 90% nameplate voltage rating is maintained to protect the 480 V motors from overheating due to excessive current during an accident.
Maintaining this protection of the 480 V motors and other safety-related loads is consistent with the assumptions in the FCS design basis and meets the intent of NRC Branch Technical Position PSB-1, " Adequacy of Station Electric Distribution System Voltages." Therefore, the NRC finds that the proposed revision of the OPLS setpoints from 1 3825.5 V (4.8 .5 sec) to 3988.8 V (4.8
.5 sec) on bus 1A3 and from 2 3724.08 V (4.8 .5 sec) to 2 3990.6 V (4.8
.5 sec) on bus 1A4 is acceptable.
3.0 STATE CONSULTATION
In accordance with the Commission's regulations, the Nebraska State official was notified of the proposed issuance of the amendment. The State official had no comments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes 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 amendment involves 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 l significant increase in individual or cumulative occupational radiation )
exposure. The Commission has previously issued a proposed finding that the i amendment involves no significant hazards consideration, and there has been no l public comment on such findirg (57 FR 55584). Accordingly, the amendment meets 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 amendment.
5.0 CONCLUSION
The Commission has concluded, based on the considerations discussed above, that: (1) there is reasonable a::surance that the health and safety of the j 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: Mark Pratt Date: January 14, 1994 I 1
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