ML24102A242

NRR E-mail Capture - Draft Request for Additional Information Quad Cities TSTF-505 and 50.69 Amendments

ML24102A242
Person / Time
Site:	Quad Cities
Issue date:	04/10/2024
From:	Robert Kuntz NRC/NRR/DORL/LPL3
To:	Steinman R Constellation Energy Generation
References
Download: ML24102A242 (5)

Text

From: Robert Kuntz Sent: Wednesday, April 10, 2024 6:57 AM To: rebecca.steinman@constellation.com

Subject:

DRAFT Request for additional information RE: Quad Cities TSTF-505 and 50.69 amendments

Dear Rebecca Steinman,

By letters dated June 8, 2023 (Agencywide Documents Access and Management System (ADAMS) Accession Nos. ML23159A249 and ML23159A253, respectively), Constellation Energy Generation, LLC (Constellation, the licensee) submitted two license amendment requests (LARs) for Quad Cities Nuclear Power Station (Quad Cities), Units 1 and 2. The proposed amendments would modify Renewed License Nos. DPR-29 and DPR-30, and the Technical Specifications (TSs) to adopt Technical Specifications Task Force (TSTF) Traveler TSTF-505, Revision 2, Provide Risk-Informed Extended Completion Times, RITSTF [Risk-Informed Technical Specification Task Force] Initiative 4b (ML18183A493), and to allow for the implementation of the provisions of Title 10 of the Code of Federal Regulations, Part 50 (10 CFR 50), section 50.69, Risk-informed categorization and treatment of structures, systems, and components [SSCs] for nuclear power reactors. By letter dated March 19, 2024 (ML24079A122) Constellation supplemented the LARs and the supplement included a request to adopt TSTF-591 Provide Risk-Informed Completion Time (RICT) Program.

The NRC staff has determined that additional information is needed to support its review concerning the probability flood hazard analysis for the LARs, specifically Enclosure 4, Reference 52 to the TSTF-505 LAR titled Probabilistic Flood Hazard Assessment Report for the Mississippi River" (referred to as the PFHA report hereafter). Included is the NRC staffs draft request for additional information (RAI). If Constellation requires clarification on the included draft RAI contact me to schedule a clarification discussion. I no clarification is required then this draft RAI becomes a formal RAI and the staff anticipates a response within 30 days of this message. If you have any questions, contact me for clarification.

Robert Kuntz Senior Project Manager NRC/NRR/DORL/LPL3 (301) 415-3733

DRAFT REQUEST FOR ADDITIONAL INFORMATION

BY THE OFFICE OF NUCLEAR REACTOR REGULATION

QUAD CITIES NUCLEAR POWER STATION -

LICENSE AMENDMENT REQUESTS TO ADOPT

TSTF-505, RISK INFORMED COMPLETION TIMES, RITSTF INITIATIVE 4b

AND

10 CFR 50.69, RISK-INFORMED CATEGORIZATION AND TREATMENT OF STRUCTURES, SYSTEMS AND COMPONENTS FOR NUCLEAR POWER PLANTS

EXHB RAI 01

Regulatory Guide (RG) 1.200, titled "Acceptability of Probabilistic Risk Assessment Results for Risk-Informed Activities" (ML090410014), section C.1.1, explains that for external hazards such as floods, the risk to a facility can be evaluated qualitatively, quantitatively, or both, albeit in a simplified manner. Section C.1.2.8 (Technical Elements for External Flood, At-Power Probabilistic Risk Assessment) of this guide further details that external flood hazard analysis determines the frequency of external floods at a site through site-specific probabilistic hazard analysis. It ensures that uncertainties in models and parameters must be appropriately considered to derive a mean hazard curve from the family of hazard curves obtained.

Appendix A, titled "Quad Cities Multi-Site Stochastic Daily Weather Simulation," of the PFHA report, describes that 49 out of 250 weather gauging stations were used for stochastic modeling. The PFHA report states that this was simplified to 10 subbasins for hydrologic modeling, covering approximately 88,600 square miles. The PFHA assumed that for the hydrologic model precipitation depths within each subbasin fluctuate over time but stay constant across space. However, this simplification could result in underestimating uncertainties in flood hazard estimates, especially at low-frequency levels. Also, the staff lacks clarity on the handling of missing weather data, if any, prior to modeling.

To address these concerns:

a) Discuss the potential reduction in variability of daily precipitation and temperature due to weather field simplification. Elaborate on the uncertainties introduced by using simplified weather fields represented by 10 subbasins for hydrologic modeling.

b) Explain the methodology employed to handle missing weather data in stochastic modeling.

EXHB RAI 02

RG 1.200 delineates, in part, the quality of probabilistic risk assessments for external hazards relevant to plant safety analysis in terms of scope, detail level, and technical adequacy.

The licensee employed the RMAWGEN model for spatial multi-site stochastic generation of daily time series of temperature and precipitation. Cordano and Eccel (Cordano E. and E. Eccel E. (2016). Tools for Stochastic Weather Series Generation in R Environment, Italian Journal of Agrometeorology, doi:10.19199/2016.3.2038-5625.031), which is one of the key references to RMAWGEN, states that the vector auto-regressive model (VARs) implemented in the RMAWGEN works correctly for normally distributed variables. However, unlike minimum and maximum temperatures, precipitation data in the PFHA report, even after Gaussian transformation, does not exhibit normal distribution (see Figure 2.c). This may lead to underestimation of flood hazards and their uncertainty bounds especially on low frequency flood analysis. Moreover, the licensee states that they generated synthetic weather data using a general linear model in RMAWGEN following Wilks' approach for spatial correlation. However, the report lacks clarity regarding the implementation of Wilks' approach for spatial correlations in RMAWGEN.

The staff requests clarification on:

a) Whether RMAWGEN modeling adequately addresses non-normally distributed precipitation data and its impact on low frequency flood analysis.

b) The process and results of Wilks' approach implemented in RMAWGEN.

c) How well the generated data preserve statistics of measured weather data compared to generated data.

EXHB RAI 03

RG 1.200 defines the quality of probabilistic risk assessment in terms of scope, detail level, and technical adequacy.

Reviewing Appendix A to the PFHA report, the staff identified the following specific concerns related to setup and calibration of RMAWGEN:

• Lack of detailed information on the setup and its results of the stochastic models described in Section 3.2.

• Unclear variation of calibrated model parameters with different year data stated in Section 4.0.

• Weather models built using data only within the watershed basin, contradicting regulatory requirements of General Design Criteria 2 of Part 50.

To address these concerns:

a) Describe the process and results of VAR model calibration, including selection of the order (time-lag) of VAR models, utilization of exogenous variables, goodness-of-fit statistics, and variance of white noises.

b) Explain how calibrated parameters vary with different year calibration sets and their incorporation into final VAR model.

c) Discuss the potential impacts of using historical data both inside and outside the basin based on the concept of stochastic storm transposition on evaluating low frequency floods and their uncertainties.

EXHB RAI 04

Guidance C.1.2.8 in RG 1.200 recommends a probabilistic hazard analysis incorporating recent site-specific information and up-to-date databases.

Section 6 of Appendix A of the PFHA report discusses generating site-specific weather data for climate change projection scenarios based on the Intergovernmental Panel on Climate Change, fifth assessment report (IPCC, AR5). However, it's unclear to the staff why the climate change-based precipitation projection scenarios which could potentially increase the magnitude and frequency of future floods are not used and how weather data generation with climate change scenarios was performed.

To address these concerns:

a) Justify the exclusion of certain climate change-based precipitation projection scenarios from hydrologic simulation and flood frequency analysis, despite evidence from IPCC reports indicating increased heavy precipitation events.

b) Clarify the use of about 100-year monthly weather projections for IPCCs Representative Concentration Pathways (RCPs) 4.5 and 8.5 to generate 12 future 1000-year daily climate scenarios with RMAWGEN.

EXHB RAI 05

RG 1.200 sets standards for the quality of flood hazard analysis for PRA in terms of scope, detail level, and technical adequacy.

Section 4.3 of the PFHA report mentions recalibrating hydrologic model for continuous simulation. However, this section lacks sufficient detail for NRC staff to conclude the adequacy of model setup and recalibration.

To address these concerns:

a) Describe changes in key model parameters from recalibration and their impacts on estimating peak flow events, if any.

b) Discuss the adequacy of using 1991 calibration parameters for flood frequency analyses at the 1E-6 annual exceedance probability.

EXHB RAI 06

Guidance C.1.2.8 in RG 1.200 states that external flood hazard analysis estimates the frequency of external flood at the site using a site-specific probabilistic hazard analysis and that uncertainties in the models and parameter values are properly accounted for and fully propagated to allow the derivation of a mean hazard curve from the family of hazard curves obtained.

Section 7 of the PFHA report addresses uncertainties in flood frequency analysis considering the following:

• For aleatory variability, the licensee used a Monte Carlo sampling and hydrologic simulations, to cope with the variability of input data, assumptions, parameters, weather scenarios, and others.

• For epistemic uncertainty in flood frequency estimates, they fitted different probability distribution functions using the L-moment method, analyzed stage-discharge curves using different calibration years, and considering different combined flood events.

However, the licensee does not account for uncertainties associated with weather field simplification and different weather projection scenarios. To address these concerns:

a) Discuss mean flood frequency curve with appropriate uncertainty bounds, either quantitively, qualitatively, or mixed.

b) Discuss how uncertainties in flood frequency estimation impact suggested risk-informed flood protection or mitigation measures.

Hearing Identifier: NRR_DRMA Email Number: 2468

Mail Envelope Properties (SJ0PR09MB10897BEC4FB3692B175AE47D799062)

Subject:

DRAFT Request for additional information RE Quad Cities TSTF-505 and 50.69 amendments Sent Date: 4/10/2024 6:56:44 AM Received Date: 4/10/2024 6:56:00 AM From: Robert Kuntz

Created By: Robert.Kuntz@nrc.gov

Recipients:

"rebecca.steinman@constellation.com" <Rebecca.Steinman@constellation.com>

Tracking Status: None

Post Office: SJ0PR09MB10897.namprd09.prod.outlook.com

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