ML25119A288
| ML25119A288 | |
| Person / Time | |
|---|---|
| Site: | Technical Specifications Task Force |
| Issue date: | 05/05/2025 |
| From: | Office of Nuclear Reactor Regulation |
| To: | Technical Specifications Task Force |
| References | |
| EPID L-2024-PMP-0000 | |
| Download: ML25119A288 (8) | |
Text
FINAL SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION TECHNICAL SPECIFICATIONS TASK FORCE TRAVELER TSTF-599, REVISION 1 ELIMINATE PERIODIC SURVEILLANCE TEST OF SIMULTANEOUS START OF REDUNDANT DIESEL GENERATORS USING THE CONSOLIDATED LINE ITEM IMPROVEMENT PROCESS (EPID: L-2024-PMP-0000)
1.0 INTRODUCTION
By letter dated April 3, 2025 (Agencywide Documents Access and Management System (ADAMS) Accession No. ML25093A204), the Technical Specifications Task Force (TSTF) submitted Traveler TSTF-599, Revision 1, Eliminate Periodic Surveillance Test of Simultaneous Start of Redundant Diesel Generators (TSTF-599), to the U.S. Nuclear Regulatory Commission (NRC). TSTF-599 proposed changes to the Standard Technical Specifications (STS) for boiling-water reactor (BWR) and pressurized-water reactor (PWR) designs under the consolidated line item improvement process (CLIIP). Upon approval, this Traveler will be made available for adoption and the changes will be incorporated into future revisions of the following NRC STS1:
NUREG-1430, Standard Technical Specifications, Babcock and Wilcox Plants, Volume 1, Specifications, and Volume 2, Bases, Revision 5, September 2021 (ML21272A363 and ML21272A370, respectively).
NUREG-1431, Standard Technical Specifications, Westinghouse Plants, Volume 1, Specifications, and Volume 2, Bases, Revision 5, September 2021 (ML21259A155 and ML21259A159, respectively).
NUREG-1432, Standard Technical Specifications, Combustion Engineering Plants, Volume 1, Specifications, and Volume 2, Bases, Revision 5, September 2021 (ML21258A421 and ML21258A424, respectively).
NUREG-1433, Standard Technical Specifications, General Electric BWR/4 Plants Volume 1, Specifications, and Volume 2, Bases, Revision 5, September 2021 (ML21272A357 and ML21272A358, respectively).
NUREG-1434, Standard Technical Specifications, General Electric BWR/6 Plants Volume 1, Specifications, and Volume 2, Bases, Revision 5, September 2021 (ML21271A582 and ML21271A596, respectively).
1NUREG-1433 provides the STS for BWR/4 plant designs, but is also representative of the BWR/2, BWR/3, and, in this case, of the BWR/5 plant design.
NUREG-1434 provides the STS for BWR/6 plant designs, but is also representative in some cases of the BWR/5 plant design.
The proposed change would delete a surveillance requirement (SR) in STS 3.8.1, AC
[alternating current] Sources - Operating, to verify that diesel generators (DGs) achieve a specific frequency and voltage within a specified time period when started simultaneously.
Associated changes were also proposed for STS 3.8.2, AC Sources - Shutdown, and Bases for STS 3.8.1 and STS 3.8.2.
1.1
System Description
The Class 1E AC electrical power distribution systems onsite standby power sources provide independence and redundancy to ensure an available source of power to the engineered safety feature (ESF) systems when the offsite or preferred power sources are unavailable. Specifically, the onsite standby power source consists of emergency DGs to power ESF buses. A DG starts automatically on a safety injection (SI) signal or on a loss of offsite power (i.e., an ESF bus degraded voltage or undervoltage) signal. The DG will start and automatically tie to its respective ESF bus after offsite power is tripped due to an ESF bus undervoltage or degraded voltage, independent of or coincident with an SI signal. The DGs will also start and operate in the standby mode without tying to the ESF bus on an SI signal alone. Following the trip of offsite power, nonpermanent loads are stripped from the ESF bus. When the DG is tied to the ESF bus, emergency loads are then sequentially connected to their respective ESF bus by the automatic load sequencer. The sequencing logic controls the permissive and starting signals to motor breakers to prevent overloading the DG by automatic load application.
In the event of a loss of preferred (offsite) power, the ESF electrical loads are automatically connected to the DGs in sufficient time to provide for safe reactor shutdown and to mitigate the consequences of a design basis accident, such as a loss of coolant accident.
1.2 Proposed Changes to Standard Technical Specification 3.8.1, AC Sources - Operating The proposed changes revise STS 3.8.1 by deleting SR 3.8.1.20 from NUREG-1430 through NUREG-1434. The current requirements differ slightly by NUREG and are as follows:
For NUREG-1430, SR 3.8.1.20 states:
NOTE: All DG starts may be preceded by an engine prelube period.
Verify, when started simultaneously from standby condition, each DG achieves, in
[10] seconds, voltage [3740] V and [4580] V, and frequency [58.8] Hz and
[61.2] Hz.
Frequency: [10 years OR In accordance with the Surveillance Frequency Control Program]
For NUREG-1431, SR 3.8.1.20 states:
NOTE: All DG starts may be preceded by an engine prelube period.
Verify when started simultaneously from standby condition, each DG achieves:
- a. In [10] seconds, voltage [3740] V and frequency [58.8] Hz and
- b. Steady state voltage [3744] V and [4576] V, and frequency [58.8] Hz and
[61.2] Hz.
Frequency: [10 years OR In accordance with the Surveillance Frequency Control Program]
For NUREG-1432, SR 3.8.1.20 states:
NOTE: All DG starts may be preceded by an engine prelube period.
Verify, when started simultaneously from standby condition, each DG achieves:
- a. In [10] seconds, voltage [3740] V and frequency [58.8] Hz and
- b. Steady state voltage [3740] V and [4580] V, and frequency [58.8] Hz and
[61.2] Hz.
Frequency: [10 years OR In accordance with the Surveillance Frequency Control Program]
For NUREG-1433, SR 3.8.1.20 states:
NOTE: All DG starts may be preceded by an engine prelube period.
Verify, when started simultaneously from standby condition, [each] [2A and 2C] DG achieves:
- a. In [12] seconds, voltage [3740] V and frequency [58.8] Hz and
- b. Steady state voltage [3740] V and [4580] V and frequency [58.8] Hz and
[61.2] Hz.
Frequency: [10 years OR In accordance with the Surveillance Frequency Control Program]
For NUREG-1434, SR 3.8.1.20 states:
NOTE: All DG starts may be preceded by an engine prelube period.
Verify, when started simultaneously from standby condition, [each] [Division 1, 2, and 3]
DG achieves:
- a. In [10] seconds, voltage [3744] V and frequency [58.8] Hz and
- b. Steady state voltage [3744] V and [4576] V and frequency [58.8] Hz and
[61.2] Hz.
Frequency: [10 years OR In accordance with the Surveillance Frequency Control Program]
1.3 Proposed Changes to Standard Technical Specification 3.8.2, AC Sources - Shutdown For NUREGs -1430, -1431, and -1432, STS SR 3.8.2.1 would be modified as follows (emphasis added):
NOTE------------------------------
The following SRs are not required to be performed:
SR 3.8.1.3, SR 3.8.1.9 through SR 3.8.1.11, SR 3.8.1.13 through SR 3.8.1.16, and [SR 3.8.1.18].
For AC sources required to be OPERABLE, the SRs of Specification 3.8.1, "AC Sources - Operating,"
except SR 3.8.1.8, SR 3.8.1.12, SR 3.8.1.17, and SR 3.8.1.19, and SR 3.8.1.20, are applicable.
Frequency: In accordance with applicable SRs 1.4 Proposed Changes to Standard Technical Specification Bases The associated STS Bases are affected in this manner:
The STS SR 3.8.1.20 Bases discussion would be deleted.
In the STS 3.8.1 Bases References section, brackets would be added to the referenced revision number to Regulatory Guide (RG) 1.9, Revision 3, Selection, Design, Qualification, and Testing of Emergency Diesel Generator Units Used as Class 1E Onsite Electric Power Systems at Nuclear Power Plants, to indicate that the revision is plant-specific. Brackets would also be added to the entire reference to RG 1.108, Revision 1, Periodic Testing of Diesel Generator Units Used as Onsite Electric Power Systems at Nuclear Power Plants.
In STS SR 3.8.2.1 Bases, a sentence would be deleted that describes the exclusion of this SR for the DGs that are not required to be operable per LCO 3.8.2.
1.5 Reason for Proposed Change Currently, STS SR 3.8.1.20 requires the simultaneous fast start2 of all redundant DGs and to verify that the DG starting independence has not been compromised. The SR is required to be performed in 10-year intervals or in accordance with the Surveillance Frequency Control Program (SFCP), with an initial frequency of every 10 years.
As stated in TSTF-599, at least four sites have extended the frequency to 20 years. The TSTF members stated that past successful performance during surveillances, actual events, and the other tests performed on the DGs outweigh the potential detrimental effects on plant safety and DG reliability. Therefore, the TSTF members have deemed the performance of the SR is not 2 A fast start test requires that a DG starts and achieves the required voltage and frequency in the required time, typically 10 seconds.
warranted by the safety benefit of the test. As a result, they proposed to eliminate SR 3.8.1.20 from the STS in TSTF-599.
2.0 REGULATORY EVALUATION
As described in the Commissions Final Policy Statement on Technical Specifications Improvements for Nuclear Power Reactors (58 FR 39132, dated July 22, 1993), [t]he new STS should include greater emphasis on human factors principles in order to add clarity and understanding to the text of the STS, and provide improvements to the Bases Section of the Technical Specifications which provides the purpose for each requirement in the specification.
The improved vendor-specific STS were developed and issued by the NRC in September 1992.
The Summary section of the Final Policy Statement on Technical Specifications Improvements for Nuclear Power Reactors states, in part:
Implementation of the Policy Statement through implementation of the improved STS is expected to produce an improvement in the safety of nuclear power plants through the use of more operator-oriented Technical Specifications, improved Technical Specification Bases, reduced action statement induced plant transients, and more efficient use of NRC and industry resources.
Section IV, The Commission Policy, of the Final Policy Statement on Technical Specifications Improvements for Nuclear Power Reactors states, in part:
The purpose of Technical Specifications is to impose those conditions or limitations upon reactor operation necessary to obviate the possibility of an abnormal situation or event giving rise to an immediate threat to the public health and safety by identifying those features that are of controlling importance to safety and establishing on them certain conditions of operation which cannot be changed without prior Commission approval.
[T]he Commission will also entertain requests to adopt portions of the improved STS [(e.g., TSTF-599)], even if the licensee does not adopt all STS improvements. The Commission encourages all licensees who submit Technical Specification related submittals based on this Policy Statement to emphasize human factors principles.
In accordance with this Policy Statement, improved STS have been developed and will be maintained for each NSSS [nuclear steam supply system] owners group. The Commission encourages licensees to use the improved STS as the basis for plant-specific Technical Specifications. [I]t is the Commission intent that the wording and Bases of the improved STS be used to the extent practicable.
The regulations in Title 10 of the Code of Federal Regulations (10 CFR) paragraph 50.36(a)(1) require that:
Each applicant for a license authorizing operation of a utilization facility shall include in his application proposed technical specifications in accordance with the requirements of this section. A summary statement of the bases or reasons for such specifications, other than those covering administrative controls, shall also be included in the application, but shall not become part of the technical specifications.
The regulations in 10 CFR 50.36(b) require that:
Each license authorizing operation of a utilization facility will include technical specifications. The technical specifications will be derived from the analyses and evaluation included in the safety analysis report, and amendments thereto, submitted pursuant to [10 CFR] 50.34 [Contents of applications; technical information]. The Commission may include such additional technical specifications as the Commission finds appropriate.
The categories of items required to be in the TS are listed in 10 CFR 50.36(c). The regulation at 10 CFR 50.36(c)(3) requires that TS include SRs, which are requirements relating to test, calibration, or inspection to assure that the necessary quality of systems and components is maintained, that facility operation will be within safety limits, and that the limiting conditions for operation will be met.
The NRC staffs guidance for the review of TS is in NUREG-0800, Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR [Light-Water Reactor]
Edition (SRP), Chapter 16.0, Technical Specifications, Revision 3, dated March 2010 (ML100351425). As described therein, as part of the regulatory standardization effort, the NRC staff has prepared STS for each of the LWR nuclear designs.
STS Bases for SR 3.8.1.20 reference guidance to perform simultaneous start DG testing in RG 1.108, Revision 1. RG 1.108, Revision 1, Regulatory Position C.2.b states:
Testing of redundant diesel generator units during normal plant operation should be performed independently (nonconcurrently) to minimize common failure modes resulting from undetected interdependences among diesel generator units. However, during reliability demonstration of diesel generator units during plant preoperational testing and testing subsequent to any plant modification where diesel generator unit interdependence may have been affected or every 10 years (during a plant shutdown), whichever is the shorter, a test should be conducted in which redundant units are started simultaneously to help identify certain common failure modes undetected in single diesel generator unit tests.
RG 1.108 was withdrawn in 1993 and superseded by RG 1.9. RG 1.108 continues to be referenced in the STS Bases because the majority of operating plants, which were licensed before 1993, reference this guidance in their plant-specific Bases and have commitments based on its use.
RG 1.9, Revision 3, incorporated the simultaneous start test. RG 1.9, Revision 3, Regulatory Position C.2.2.14 (Redundant Unit Test) states:
Demonstrate that, by starting and running both redundant units simultaneously, potential common failure modes that may be undetected in single emergency diesel generator unit tests do not occur.
Revision 3 of RG 1.9 is referenced in the STS Bases due to a majority of operating plants currently holding commitments to this revision. Subsequently, RG 1.9, Revision 4, Application and Testing of Safety-Related Diesel Generators in Nuclear Power Plants was issued in 2007 (ML070380553). As stated in section 1.3 of this safety evaluation, the licensee will have the option to reference the revision of RG 1.9 that is applicable to its plant.
3.0 TECHNICAL EVALUATION
The NRC staff reviewed Traveler TSTF-599, which proposed changes to NUREGs-1430, -1431,
-1432, -1433, and -1434. The regulatory framework the NRC staff used to determine the acceptability of the proposed changes consists of the requirements and guidance listed in Section 2.0 of this safety evaluation. The NRC staff reviewed the proposed changes to the STS to determine whether they meet the standards for TS in 10 CFR 50.36(c)(3) and conform to the Final Policy Statement on Technical Specifications Improvements for Nuclear Power Reactors.
Depending on the reactor type, two or more redundant DGs are required to be operable by STS limiting condition for operation (LCO) 3.8.1.b. Several SRs in STS 3.8.1 are used to establish the operability of the DGs to meet the LCO. SR 3.8.1.20 requires a simultaneous start of the redundant DGs to be performed every 10 years or in accordance with the SFCP. This surveillance test is consistent with the guidance in RG 1.108 (withdrawn in 1993) and RG 1.9 to test for common cause failure.
The STS 3.8.1 DG SRs that test for achieving the required frequency and voltage ranges within a specified time are the following:
Related Diesel Generator Surveillance Requirements Frequency 3.8.1.7 (each DG starts from standby) 184 days (6 months) 3.8.1.11 (each DG auto-starts on loss of offsite power signal) 18 months 3.8.1.12 (each DG auto-starts on SI signal from standby) 18 months 3.8.1.19 (each DG auto-starts on loss of offsite power signal in conjunction with SI signal from standby) 18 months 3.8.1.20 (simultaneous start of DGs from standby) 10 years These SRs also have the option to modify the frequency in accordance with the SFCP. During the performance of SR 3.8.1.20, no components are tested that are not already covered by one or more of the other related SRs. The DGs are designed to start on either a loss of offsite power signal, SI signal, or both signals concurrently, which is verified by SR 3.8.1.11, SR 3.8.1.12, and SR 3.8.1.19, respectively. The main focus of the performance of SR 3.8.1.20 is to test the independence of the starting logic and mechanisms of the redundant DGs.
With respect to DG operating experience, the TSTF members stated in TSTF-599 that they conducted a survey of licensees and determined that, for each unit, plants entered the condition of one inoperable DG an average of 27 times per year. When in this condition, the STS require the licensee to evaluate for a common cause failure. This is to determine whether or not the cause of the inoperable DG exists on the other DGs. Additionally, for the 104 units operating between 2005 and 2010, the TSTF determined that licensees performed around 14,000 common cause failure evaluations with no common cause failure mechanisms identified on the other DGs. These common cause failure evaluations covered various DG failure mechanisms.
Specific to simultaneous DG starts, the NRC staff reviewed Licensee Event Reports from January 1, 2000, through August 29, 2021, and identified 43 loss of offsite power events at 30 sites (ML25051A225). Each event resulted in the redundant DGs starting as required with no instance in which a redundant DG was found to be inoperable due to a common cause failure.
Performance of SR 3.8.1.20 at many plants requires rendering the DGs inoperable during their surveillance, which includes test preparation, performance, and recovery. This would result in reduced availability of the DGs because during the performance of SR 3.8.1.20, the automatic DG functions (e.g., starting, load rejection, load sequencing, etc.) are rendered inoperable.
Accordingly, if one or more DGs were rendered inoperable during preparation or performance of this SR and an accident were to occur, the plant might not be able to meet the accident analysis assumptions; most accident analyses assume a concurrent loss of offsite power and a limiting single failure (e.g., failure of one DG to start). The performance of SR 3.8.1.20 does not enhance safety over the increased risk of equipment inoperability, considering common cause failure is a very low likelihood event. Therefore, NRC staff finds the proposed deletion of STS SR 3.8.1.20 acceptable.
The proposed changes to STS 3.8.2, AC Sources - Shutdown, are editorial in nature to reflect the deletion of SR 3.8.1.20. Therefore, the NRC find the proposed changes to STS 3.8.2 acceptable.
4.0 CONCLUSION
The NRC staff reviewed Traveler TSTF-599, Revision 1, which proposed changes to STS NUREGs-1430, -1431, -1432, -1433, and -1434. The NRC staff determined that the proposed changes to the STS continue to meet the Commissions Final Policy Statement on Technical Specifications Improvements for Nuclear Power Reactors and 10 CFR 50.36(c)(3) because the remaining STS 3.8.1 DG SRs provide assurance that the necessary quality of systems and components are maintained, and that STS LCO 3.8.1.b will be met. This is in combination with the fact that in recent decades, industry operating experience during loss of offsite power events has demonstrated consistent performance of the DG simultaneous start function. Additionally, the NRC staff found the changes to the STS to be technically clear and consistent with customary terminology and format in accordance with SRP Chapter 16.0. In summary, the NRC staff reviewed the proposed deletion of STS SR 3.8.1.20, along with the associated changes to STS 3.8.2 and the STS Bases, and concludes that the STS continue to provide reasonable assurance of adequate protection of the health and safety of the public. Therefore, the NRC staff concludes that the proposed STS changes are acceptable.
Principal Contributors:
K. West, NRR/DSS/STSB V. Goel, NRR/DEX/EEEB Date: May 5, 2025