NMP2L2742, Response to Request for Additional Information by NRR to Support Review of Nine Mile Point Nuclear Station, Unit 2, License Amendment Request Re Risk Informed Categorization & Structures, Systems and Components

Response to Request for Additional Information by NRR to Support Review of Nine Mile Point Nuclear Station, Unit 2, License Amendment Request Re Risk Informed Categorization & Structures, Systems and Components

ML20241A047
Person / Time
Site:	Nine Mile Point
Issue date:	08/28/2020
From:	David Gudger Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NMP2L2742
Download: ML20241A047 (11)

Text

200 Exelon Way Kennett Square, PA 19348 www.exeloncorp.com 10 CFR 50.90 NMP2L2742 August 28, 2020 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Nine Mile Point Nuclear Station, Unit 2 Renewed Facility Operating License No. NPF-69 NRC Docket No. 50-410

Subject:

Response to Request for Additional Information by the Office of Nuclear Reactor Regulation to Support Review of Nine Mile Point Nuclear Station, Unit 2, License Amendment Request Regarding Risk Informed Categorization and Treatment of Structures, Systems and Components

References:

1. Letter from D. Gudger (Exelon Generation Company, LLC) to U.S. Nuclear Regulatory Commission, "Application to Adopt 10 CFR 50.69, 'Risk-informed Categorization and Treatment of Structures, Systems and Components for Nuclear Power Reactors,'" dated December 26, 2019

2. Letter from M. Marshall (Senior Project Manager, U.S Nuclear Regulatory Commission) to R. Reynolds (Exelon),"Nine Mile Point Nuclear Station, Unit 2-Request for Additional Information RE: Review of License Amendment Requests Regarding Risk Informed Categorization and Treatment of Structures, Systems and Components (L-2019-LLA-0290),"

dated July 30, 2020 By letter dated December 26, 2019 (Reference 1), Exelon Generation Company, LLC (Exelon) requested to change The Nine Mile Point Unit 2 (NMP2) Technical Specifications (TS). The proposed amendment request would modify NMP2 TS License Amendment

U.S. Nuclear Regulatory Commission Response t o Request for Additional Information Adopt 10 CFR 50.69 Docket No. 50-410 August 28, 2020 Page 2 Request to Adopt 10 CFR 50.69, "Risk-informed Categorization and Treatment of Structures, Systems and Components for Nuclear Power Reactors."

On July 28, 2020, the U.S. Nuclear Regulatory Commission (NRC) identified a draft request for additional information necessary to complete the review. On July 30, 2020 (Reference 2),

the NRC issued to Exelon a formal request for additional information.

to this letter contains the NRCs request for additional information immediately followed by Exelons response.

Exelon has reviewed the information supporting a finding of no significant hazards consideration and the environmental consideration provided to the NRC in Reference 1. The additional information provided in this response does not affect the bases for concluding that the proposed license amendment does not involve a significant hazards consideration.

Furthermore, the additional information provided in this response does not affect the bases for concluding that neither an environmental impact statement nor an environmental assessment needs to be prepared in connection with the proposed amendment.

There are no commitments contained in this response.

If you should have any questions regarding this submittal, please contact Ron Reynolds at 610-765-5247.

I declare under penalty of perjury that the foregoing is true and correct. Executed on the 28th day of August 2020.

Respectfully, David T. Gudger Sr. Manager, Licensing Exelon Generation Company, LLC

Response to Request for Additional Information cc:

USNRC Region I Regional Administrator w/attachments USNRC Senior Resident Inspector - NMP USNRC Project Manager, NRR - NMP A. L. Peterson, NYSERDA

ATTACHMENT 1 Response to Request for Additional Information

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 1 of 8 Docket No. 50-410 REQUEST FOR ADDITIONAL INFORMATION

RAI 1

1) NEI 00-04, Section 7.1 provides guidance related to categorizing components supporting multiple functions. The guidance states: "the SSC, or part thereof, should be assigned the highest risk significance for any function that the SSC or part thereof supports." Section 4 of NEI 00-04 addresses categorization of SSCs that serve an interface between two or more components, stating:

[] there may be circumstances where the categorization of a candidate low safety-significant SSC within the scope of the system being considered cannot be completed because it also supports an interfacing system. In this case, the SSC will remain uncategorized until the interfacing system is considered.

Section 4 of NEI 00-04 also addresses system boundary assignment stating:

All components and equipment within the defined boundaries of the chosen system should be included. However, care should be taken in extending beyond system boundaries to avoid the introduction of new systems and functions. []

The system boundary should be defined such that any components from another system only support the safety function of the primary system of interest.

The NEI guidance states that the interfacing system should be considered. The LAR does not explain how the interfacing system will be considered. In this context, the definition of system boundaries may become important for the categorization. Given these observations, address the following:

a. Describe, with examples, how system boundaries are defined to ensure that the interfacing SSCs are assigned to the right system to support a correct categorization outcome.

b. Describe and justify with examples, how the licensee intends to address the guidance in NEI 00-04 for interfacing SSCs. If an interface SSC is categorized before categorizing all the systems that it interfaces address the following:

i.

Describe how it is ensured that the sub-set of all the interfacing system functions associated with the interfacing component can and will be correctly and comprehensively identified without fully categorizing the interfacing system.

ii.

Confirm that interface components will only be categorized if interface component failure cannot prevent performance of any interface system functions.

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 2 of 8 Docket No. 50-410 Exelon Response to RAI 1 Exelon interprets the NEI 00-04 guidance involving interfacing systems to allow for categorization of any functions/SSCs that serve as an interface between two or more systems as long as all impacts to the interfacing system(s) are considered. Functions/SSCs that serve as an interface between two or more systems do not necessarily have to remain uncategorized prior to completing the categorization of all the interfacing systems. Exelon believes this meets NEI 00-04 as well as 10 CFR 50.69(c)(1)(v) for the reasons discussed below.

NEI 00-04, Section 4, discusses system selection and boundary definitions which "includes defining system boundaries where the system interfaces with other systems." Once a system is selected for categorization, the system boundary is defined. System boundaries can be identified in a number of ways; typically, a unique identifier in the station Component Record List is used. However, system boundaries can be defined in a number of ways, including system function, P&ID boundaries, maintenance rule functions, etc. Once the system boundary is defined, all components within that boundary that support the system safety functions or the design and licensing basis of that system are included, not just specific components within that system boundary. Some components associated with the system may not have the same system designator. For example, passive components, such as piping, that are in the pressure boundary of the system and are required to fulfill the system safety functions are included yet typically absent from the station Component Record List. Therefore, boundaries are defined to ensure that the interfacing SSCs are assigned to the right system to support a correct categorization outcome.

The initial steps for categorization include defining system boundaries and defining the list of functions for the system being categorized. If the system being categorized includes interfacing functions/SSCs, then "support functions" are created to 1) identify the interfacing system(s) and 2) to describe the support functions to the interfacing system(s).

Per NEI 00-04, Section 4, for interfacing systems, all impacts an SSC has on interfacing systems will be considered. In the event an SSC in a system being categorized supports multiple systems, there are two options possible (see below). The first option is to fully categorize the interfacing SSC. If categorized, its risk significance will be reflected in the System Categorization Document (SCD) for the system defined by its boundaries and its impact on all interfacing systems. Per Section 7.1 of NEI 00-04, an SSC being categorized will be assigned the highest risk significance of any portion of the process, whether it be from a function, "support function," or component level evaluation. Interface components can be categorized even if the interface component failure prevents the performance of the interface system(s) function (i.e. the components would be candidate safety-significant).

The second option is to choose to not categorize the interfacing system until the impact on all the interfacing systems is categorized in their entirety.

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 3 of 8 Docket No. 50-410 Exelons interpretation of NEI 00-04, Section 4, categorization process allows for two options when categorizing an SSC that interfaces with multiple systems:

Option 1: An interface SSC is categorized along with the system it is assigned to.

An SSCs impact on all systems it interfaces with will be considered in the SCD for the system the SSC belongs too. Consistent with NEI 00-04, Section 7.1, component and functional importance assessments would be performed and documented. This includes identification of all functions of the interface SSC in its assigned primary system as well as all functions of the interface SSC to all other systems outside this primary system.

The safety-significant SSCs from the component safety significance assessment (NEI 00-04, Section 5) are mapped to the appropriate function(s) for which they have safety significance. If any SSC is safety-significant, from either the PRA-based component safety significance assessment (NEI 00-04, Section 5) or from the defense-in-depth assessment (NEI 00-04, Section 6), then the associated system function is preliminarily safety-significant. All other functions/SSCs can be preliminarily assigned low safety significance. All preliminary categorizations assigned as candidate safety-significant or low safety-significant are then taken to the Integrated Decision-Making Panel for final review and approval. The overall process used in integrating the various categorization inputs is depicted in NEI 00-04 Figure 7-1.

Once a system function has been identified as safety-significant, then all components that support this system function are assigned a preliminary safety-significant categorization. All other components are assigned a preliminary LSS categorization.

Due to the overlap of functions and components, a significant number of components may support multiple functions. In this case, the SSC is assigned the highest risk significance for any function that the SSC supports. Consistent with NEI 00-04, Section 7, for safety-significant functions/SSCs, the critical attributes that make the function/SSC safety-significant will be identified.

An example of this approach is described below for a portion of the residual heat removal system (RHR) Loop A. (Note that this is an example only as no systems have yet been categorized at NMP2.)

Within the RHR Loop A is heat exchanger E1A which is part of the RHR system. The heat exchanger interfaces with (rejects heat to) the Service Water System (SWS).

When categorizing the RHR system in this first option, heat exchanger E1A cannot be categorized until the interfacing role with the SWS is considered. This would be accomplished by creating SWS "support functions" for heat exchanger EIA that fully describes the heat exchangers role to the SWS system.

All component and functional importance assessments would be performed and documented. The assessments include the impact of failure of the heat exchanger on both the RHR and SWS systems using all process steps of the NEI 00-04 categorization process (e.g., active and passive categorization, defense-in-depth, etc.). The outcome of each of the individual process steps' significance determination would be identified

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 4 of 8 Docket No. 50-410 and the highest significance from any process step is assigned to the heat exchanger.

Critical attributes for any safety-significant function/SSC will be identified. The safety significance of all components associated with the support function will have been assessed since all functional analysis steps will have been performed. Only the heat exchanger in the RHR system will get assigned a RISC result because other SWS components would not have been defined in the RHR system boundary. All remaining SSCs and functions of the SWS could later be categorized when the SWS system is categorized, and the shared interface components and piping categorized as part of the RHR categorization would retain the same significance level for both systems. Periodic reviews of the SSC categorization results per NEI 00-04 will be conducted no longer than every two refueling outages to ensure that the categorization process and results of the categorization are maintained valid.

Option 2: An interface SSC is left uncategorized until after all interfacing or supported systems are categorized in their entirety.

This option highlights the "example" listed in NEI 00-04, Section 4, concerning cooling water system piping on a ventilation system cooler. Existing treatments will continue to be applied to SSCs that are uncategorized.

In summary, there are two options available. For option 1, an SSC that interfaces with multiple systems will be categorized as HSS if it supports an HSS function. In option 1, once the SWS is subsequently categorized, the process will confirm the same Risk-Informed Safety Class (RISC) result for the interface SSC as was achieved when the RHR system was earlier categorized. As mentioned above, periodic reviews of the SSC categorization results for all systems categorized will be conducted no longer than every two refueling outages to ensure that the categorization process and results of categorized systems are maintained valid.

Exelon interprets either option to be consistent with 10 CFR 50.69, Section (c)(1)(v), and the guidance provided in NEI 00-04. The choice of which option to apply is a business decision based on which systems the station believes would benefit from categorization. There may be situations where an interface system would not benefit from categorization and current treatments for the interface system would remain in place. There may be other situations where the interface system may benefit from categorization. The choice of which systems to categorize and when is up to each individual licensee. From option 1, the initial categorization results of the interfacing SSC will not change during subsequent categorization of other interface systems because all steps of NEI 00-04 will have been performed.

Therefore, it is ensured that the sub-set of all the interfacing system functions associated with the interfacing component can and will be correctly and comprehensively identified without fully categorizing the interfacing system.

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 5 of 8 Docket No. 50-410

RAI 2

2) Section 5.3 of NUREG-1855, Revision 1, "Guidance on the Treatment of Uncertainties Associated with PRAs in Risk-Informed Decisionmaking, Final Report," dated March 2017 (ADAMS Accession No. ML17062A466), presents guidance on the process of identifying, characterizing, and qualitative screening of model uncertainties. Discussion of the PRA model uncertainty process is provided in License Amendment Request (LAR) Section 3.2.7.

The LAR states that the process for identifying key assumptions and sources of uncertainties for the internal events and the fire PRAs was performed using the guidance in NUREG-1855, Revision 1. It is further stated in the LAR that the internal events and the fire PRA models and notebooks were reviewed for plant-specific key assumptions and sources of uncertainty. Further, the LAR states that generic sources of uncertainty for the internal events PRA were identified from Electric Power Research Institute (EPRI) Technical Report (TR)-1016737, "Treatment of Parameter and Modeling uncertainty for Probabilistic Risk Assessments" and for the fire PRA from EPRI TR-1026511, "Practical Guidance of the Use of Probabilistic Risk Assessment in Risk-informed Applications with a Focus on the Treatment of Uncertainty." The LAR does not describe the process and the criteria used to identify, from the initial comprehensive list of assumptions and sources of uncertainty in the base PRA model(s) (including those associated with plant-specific features, modeling choices, and generic industry concerns), the specific key assumptions and sources of uncertainties presented in the LAR.

Describe, separately for the internal events, internal flooding and the fire PRAs, the process used to identify and evaluate key assumptions and sources of model uncertainty. Address the following in the response:

a. Discuss how a comprehensive list of plant-specific and generic industry key assumptions and sources of uncertainty were identified as a starting point for this evaluation.

b. Explain how the comprehensive list of key assumptions and sources of uncertainty sources was screened to a list of uncertainties that were specifically evaluated for their impact on the 50.69 application.

c. Explain what criteria, qualitative or quantitative, or what additional analysis were used to evaluate the impact of the key assumptions and sources of uncertainty on the 50.69 application.

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 6 of 8 Docket No. 50-410 Exelon Response to RAI 2

a.

This evaluation process is consistent with NUREG-1855, Revision 1, as referenced in Reg. Guide 1.200, Revision 2 guidance. In particular, the NUREG Stage E process, Assessing Model Uncertainty, was used as described below:

Step E-1.1 (Identification of Sources of Model Uncertainty and Related Assumptions): Tables A-1 and A-2 of EPRI 1016737 were used to identify potential sources of model uncertainty from the internal events and internal flooding PRA models. Appendix B and Appendix E of EPRI 1026511 were used to identify potential sources of model uncertainty in the fire PRA and Level 2 PRA model, respectively. Unique plant-specific issues were also considered in the identification process for the PRA models.

Step E-1.2 (Identification of Relevant Sources of Model Uncertainty and Related Assumptions): This step allows for screening of potential sources of model uncertainty based on the parts of the models used for the application. Since the 50.69 categorization uses the complete model, no specific potential sources of uncertainty were screened out for this application.

Step E-1.3 (Characterization of Sources of Model Uncertainty and Related Assumptions): Per the guidance in NUREG-1855 and the associated EPRI reports, the characterization process involves identifying: 1) the part of the PRA model affected, 2) the modeling approach or assumptions utilized in the model,

3) the impact on the PRA model, and 4) representation of conservative bias (if applicable). These considerations were included in the evaluation of the potential sources of model uncertainty.

Step E-1.4 (Qualitative Screening of Sources of Model Uncertainty and Related Assumptions): This step allows for screening out potential sources of model uncertainty by referencing consensus model approaches. The evaluation process included identifying the approach utilized (e.g., consensus approach or other applicable guidance) and using those considerations as the means to qualitatively screen potential impacts on the application.

Step E-1.5 (Identification and Characterization of Relevant Sources of Model Uncertainty and Related Assumptions Associated with Model Changes): The implementation of the 50.69 program utilizes the base PRA models. As such, no new sources of model uncertainty have been introduced for the application.

Therefore, using the approved process as described above, a comprehensive list of plant-specific and generic industry key assumptions and sources of uncertainty were identified as a starting point for this evaluation.

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 7 of 8 Docket No. 50-410

b.

The results of the base PRA evaluations were reviewed to determine which potential uncertainties could impact the 50.69 application. Assumptions or uncertainties determined not to be key are those that do not meet the definitions of key uncertainty or key assumption in RG 1.200 Revision 2, NUREG-1855 Revision 1, or related references.

Specifically, the following considerations were used to determine those assumptions and uncertainties that do not require further consideration as key to the application:

The uncertainty or assumption is implementing a consensus model as defined in NUREG 1855 Revision 1.

The uncertainty or assumption will have no impact on the PRA results and therefore no impact on the decision of the high safety-significance (HSS) or low safety-significance (LSS) for any SSC.

There is no different reasonable alternative to the assumption which would produce different results and/or there is no reasonable alternative that is at least as sound as the assumption being challenged.

The uncertainty or assumption implements a conservative bias in the PRA model, and that conservatism does not influence the results. These conservatisms are expected to be slight and only applied to minor contributors to the overall model.

Therefore, the steps above provide the screening process for the list of uncertainties that are specifically evaluated for their impact on the 50.69 application.

c.

As described in NUREG-1855, only the relevant sources of uncertainties and related assumptions with the potential to challenge the applications acceptance guidelines are considered key. For 10 CFR 50.69 categorization, the PRA acceptance guidelines are threshold values for Fussell-Vesely (FV) and Risk Achievement Worth (RAW) for each SSC being categorized, above which the SSC is categorized as candidate high safety-significant (HSS), and below which the SSC is categorized as candidate low safety-significant (LSS). For the 50.69 Program, the guidance in NEI 00-04 specifies sensitivity studies to be conducted for each PRA model to address key sources of uncertainty. The sensitivity studies are performed to ensure that assumptions and sources of uncertainty (e.g., human error, common cause failure, and maintenance probabilities) do not mask the SSC(s) importance.

As part of the required 50.69 PRA categorization sensitivity cases directed by NEI 00-04, internal events / internal flood and fire PRA models human error and common cause basic events are increased to their 95th percentile and also decreased to their 5th percentile values. In addition, maintenance unavailability terms are set to 0.0. For the fire PRA model only, a sensitivity case is required to allow no credit for manual suppression. These results are capable of driving a component and respective functions HSS and therefore the uncertainty of the PRA modeled HEPs and CCFs are accounted for in the 50.69 application. Also, per NUREG-1855, if any sources of uncertainty do

Response to Request for Additional Information Adopt 10 CFR 50.69 Page 8 of 8 Docket No. 50-410 challenge the acceptance guidelines, then appropriate compensatory measures or performance monitoring should be identified to help minimize the risk. In the case of 50.69, since no predictive model of the increase in unreliability following alternative treatment of LSS SSCs exists, this option is appropriate for 50.69. In fact, the example of a performance monitoring approach to address key uncertainties/assumptions given in NEI 00-04, Section 8.5, is the factor of increase sensitivity combined with the performance monitoring process described for 50.69 in NEI 00-04.