ML24145A106

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LLC Response to NRC Request for Additional Information No. 019 (RAI-10146-R1) on the NuScale Standard Design Approval Application
ML24145A106
Person / Time
Site: 05200050
Issue date: 05/24/2024
From: Shaver M
NuScale
To:
Office of Nuclear Reactor Regulation, Document Control Desk
References
RAIO-168416
Download: ML24145A106 (1)
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Text

RAIO-168416 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com May 24, 2024 Docket No.52-050 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738

SUBJECT:

NuScale Power, LLC Response to NRC Request for Additional Information No. 019 (RAI-10146-R1) on the NuScale Standard Design Approval Application

REFERENCE:

1. NRC Letter to NuScale, Request for Additional Information No. 019 (RAI-10146-R1), dated April 5, 2024 The purpose of this letter is to provide the NuScale Power, LLC (NuScale) response to the referenced NRC Request for Additional Information (RAI).

The enclosure to this letter contains the NuScale response to the following RAI question from NRC RAI-10146:

x 19.3-4 This letter makes no regulatory commitments and no revisions to any existing regulatory commitments.

If you have any questions, please contact Amanda Bode at 541-452-7971 or at abode@nuscalepower.com.

I declare under penalty of perjury that the foregoing is true and correct. Executed on May 24, 2024.

Sincerely, Mark W. Shaver Director, Regulatory Affairs NuScale Power, LLC Distribution:

Mahmoud Jardaneh, NRC Getachew Tesfaye, NRC Alina Schiller, NRC : NuScale Response to NRC Request for Additional Information RAI-10146, nonproprietary

RAIO-168416 NuScale Power, LLC 1100 NE Circle Blvd., Suite 200 Corvallis, Oregon 97330 Office 541.360.0500 Fax 541.207.3928 www.nuscalepower.com NuScale Response to NRC Request for Additional Information RAI-10146, nonproprietary

NuScale Nonproprietary NuScale Nonproprietary Response to Request for Additional Information Docket: 052000050 RAI No.: 10146 Date of RAI Issue:04/05/2024 NRC Question No.: 19.3-4 Regulatory Basis

  • The scope, criteria, and process used to determine Regulatory Treatment of Nonsafety Systems (RTNSS) for the advanced passive plant designs are established in:
1. SECY-94-084, Policy and Technical Issues Associated with the Regulatory Treatment of Nonsafety Systems in Passive Plant Designs, dated March 28, 1994 (Agencywide Documents Access and Management System Accession No. ML003708068) and associated Staff Requirements Memorandum (SRM), June 30, 1994 (ML003708098);
2. SECY-95-132, Policy and Technical Issues Associated with the Regulatory Treatment of Nonsafety Systems (RTNSS) in Passive Plant Designs, dated May 22, 1995 (ML003708005),

and associated SRM, June 28, 1995 (ML003708019); and

3. SECY-96-128, Policy and Key Technical Issues Pertaining to the Westinghouse AP600 Standardized Passive Reactor Design, June 12, 1996 (ML003708224), and associated SRM, January 15, 1997 (ML003755486).

Issue The NuScale SDAA design is an advanced passive light water reactor, which is covered by the Commission policy identified above. The NuScale SDAA internal events full power core damage frequency (CDF) is 6x10-9/yr. For comparison, the GEH Economic Simplified Boiling Water Reactor (ESBWR) certified design is also an advanced passive design covered by the Commission direction above. NUREG-1966, Final Safety Evaluation Report Related to the

NuScale Nonproprietary NuScale Nonproprietary Certification of the Economic Simplified Boiling-Water Reactor Standard Design, Chapter 19, page 19-18 (ML14100A187), provides the ESBWR internal event full power CDF as 1.65x108/yr. The staff considers this risk result to be comparable to the NuScale SDAA CDF given that these are design probabilistic risk assessments (PRAs) with associated assumptions and uncertainties. The staff notes that the RTNSS initiating event guidance, as described in Standard Review Plan (SRP) 19.3 does not have a minimum baseline threshold on CDF. The staff has concluded that the Commission direction on RTNSS applies in its entirety to the NuScale SDAA design as it did to the design certification application (DCA) design. SDAA FSAR Section 19.3, where NuScale discusses how it meets the Commission policy on RTNSS without any exceptions, confirms the staffs conclusion.

The staff notes that the core damage risk profile for the US460 (SDA) design is significantly different from the US600 (DCA) design. In the US600 design, drop of a module during refueling comprised over 95 percent of total CDF. Consequently, the staff did not identify any structures, systems, and components (SSCs) that needed to be included under RTNSS. In contrast, in the US460 design, incomplete emergency core cooling system (ECCS) actuation dominates the core damage risk profile. Design changes in the US460 design result in increased ECCS demand, which increases the potential for incomplete ECCS actuation thereby resulting in core damage. Loss of AC power for longer than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> results in ECCS actuation.

The staff uses guidance contained in SRP 19.3, Revision 0, dated June 2014, Regulatory Treatment of Non-Safety Systems for Passive Advanced Light Water Reactors, (ML14035A149) to conduct its review of an applicants RTNSS evaluation. NuScale uses this same guidance per Section 19.3 of the SDAA FSAR, which states, [t]he RTNSS scope, process, and criteria are consistent with the guidance of NUREG-0800 Section 19.3. In accordance with SRP 19.3, Revision 0 (page 19.3-6), staff responsible for the review of the applicants PRA verifies that the applicant has determined those nonsafety-related SSCs, if any, used to prevent the occurrence of initiating events and, based on their importance to risk as determined from the PRA, has included these nonsafety-related SSCs, as appropriate, in the scope of RTNSS.

SDAA FSAR Section 19.3.2.3 states, No nonsafety-related SSC are credited to meet NRC safety goals, to reduce the occurrence of initiating events, or to compensate for the uncertainties regarding passive systems in the PRA and in the modeling of severe accident phenomenology. Therefore, no nonsafety-related SSC meet the RTNSS C criteria.

NuScale Nonproprietary NuScale Nonproprietary Based on the staffs review of the SDAA FSAR and multiple documents available during the regulatory audit, the staff cannot verify that NuScale completely and satisfactorily addressed the following screening criteria, as stated on page 19.3-10 of SRP 19.3, Revision 0, in general, and for the two backup diesel generators (BDGs), in particular:

1. Does the calculation of the initiating event frequency consider the nonsafety-related SSCs?
2. Does the unavailability of the nonsafety-related SSCs significantly affect the calculation of the initiating event frequency?
3. Does the initiating event significantly affect the CDF and LRF (i.e., contribute to more than 10 percent of the at power or shutdown internal events CDF as stated in the footnote on page 19.3-10 of SRP 19.3, Revision 0)?

Based on its review of the event trees submitted in Chapter 19 of the SDAA FSAR, the BDGs are the only SSCs that completely avoid the need for ECCS actuation in the US460 design. The staffs review of the internal events CDF core damage sequences from the NuScale PRA, as reported in FSAR Table 19.1-17: Dominant Core Damage Sequences (Full Power, Internal Events, Single Module), identified that over 25 percent of the internal events CDF caused by losses of offsite power is mitigated by the two BDGs without the need to initiate ECCS.

Therefore, successful operation of the BDGs directly impacts the SDAA frequency of station blackout (SBO) and ECCS actuation for each NuScale Power Module (NPM). The staff also notes that the two BDGs support all six NPMs in the US460 design, compounding the impact of the reliability of the BDGs. Additionally, based on material available during the regulatory audit, the failure probabilities of the BDGs in the PRA are based on data for SBO DGs for operating plants. While the SBO DGs are scoped into the SBO rule for operating reactors, currently there is no regulatory program to ensure that the reliability and availability of the BDGs for the US460 design will be consistent with what is assumed in the PRA. Since loss of offsite power events are multi-unit events, the reliability of the BDGs impacts the defense-in-depth of the design.

From the staffs review of the material available during the regulatory audit, the staff also noted that the initiating event frequency for Loss of Offsite Power (LOOP) in the internal events PRA includes extratropical straight winds, F0 and F1 tornadoes, and Category 1 and 2 hurricanes.

The staffs review of material available during the regulatory audit identified that the two BDGs are Seismic Category III, which appear to be qualified for F0 and F1 tornadoes, and Category 1 and 2 hurricanes. These details are not included in the FSAR.

NuScale Nonproprietary NuScale Nonproprietary Information Requested:

To support the staffs finding against 10 CFR 52.137(a)(25) on the SDAAs conformance with the Commissions direction on RTNSS, NuScale is requested to:

Demonstrate how the RTNSS criteria related to the initiating event frequency stated on page 19.3-10 of SRP 19.3, Revision 0, dated 2014, are applied for the SDAA. Provide the results of NuScales evaluation against these criteria.

1. Since the successful operation of the two BDGs substantially decreases both the SBO and ECCS actuation frequency for all six NPMs in the US460 design and consequently, over 25 percent contribution to internal events CDF for a single NPM due to the potential for partial ECCS actuation, provide FSAR markups that (i) document the assessment of the two BDGs against the criteria on page 19.3-10 of SRP 19.3, Revision 0, dated 2014, (ii) identify the two BDGs as risk significant and nonsafety-related SSCs, (iii) identify the augmented quality and design details for the two BDGs, and (iv) identify the associated Inspections, Tests, Analysis, and Acceptance Criteria, or provide a means to ensure that the reliability of the two BDGs will be achieved in operation (e.g., a COL item to scope the BDGs within 10 CFR 50.65).
2. Provide FSAR markups for Chapter 19 that describe that the Seismic Category III building housing of the two BDGs is qualified to withstand F0 and F1 tornadoes, and Category 1 and 2 hurricanes consistent with the discussion in the PRA high winds notebook.

NuScale Response:

Standard Review Plan (SRP) 19.3, Revision 0, states The RTNSS process applies broadly to those nonsafety-related SSCs that perform risk significant functions and, therefore, are candidates for regulatory oversight. Per SRP 19.3, determination of structures, systems, and components (SSC) that satisfy Regulatory Treatment of Nonsafety Systems (RTNSS) scoping criterion C includes identifying SSC that are used to prevent the occurrence of initiating events.

NuScale does not use nonsafety-related SSC to prevent the occurrence of initiating events (i.e.,

lower initiating event frequency), and therefore there are no such SSC to include in the scope of RTNSS.

NuScale Nonproprietary NuScale Nonproprietary The Review Procedures section of SRP 19.3, under Area of Review - Identification of SSCs in the Scope of the RTNSS Program, states:

The staff responsible for review of the applicants PRA use information in Chapters 15 and 19 of the FSAR pertaining to initiators of transients and accidents affecting the nuclear steam supply system, and consult as necessary with reviewers of those sections, to verify that the applicant has correctly identified the SSCs that require evaluation of risk-significance based on their contribution to initiating event frequencies.

The staff then verifies that the applicant has completely addressed the following screening criteria for assessing risk significance of those SSCs with respect to initiating event frequency...

Prior to evaluating nonsafety-related SSC against the three RTNSS initiating event frequency criteria on page 19.3-10 of SRP 19.3, the applicant identifies SSC that support lowering initiating event frequency. Because NuScale does not use nonsafety-related SSC to prevent the occurrence of initiating events, no SSC require evaluation using the three screening criteria.

1. Consistent with the response above, the backup diesel generators (BDGs) are not used to prevent the occurrence of initiating events, and therefore do not require evaluation using the three screening criteria. The staff state that after a loss of offsite power (LOOP), successful operation of the BDGs directly impacts the SDAA frequency of station blackout (SBO) and ECCS actuation. Station blackout is not an initiating event; station blackout is a scenario without either offsite or onsite (provided by the BDGs) alternating current (AC) power sources following the initiating event Loss of Offsite Power, as described in Final Safety Analysis Report (FSAR) Section 19.1.4.1.1.4, Accident Sequence Determination. The LOOP initiating event frequency is based solely on industry operating experience without consideration of any equipment to lower the initiating event frequency. Availability of the BDGs does not prevent a LOOP; the BDGs mitigate the event, as the staff note. Similarly, an emergency core cooling system (ECCS) actuation following a loss of all AC power occurs from the low AC voltage 24-hour timer; this ECCS actuation is a design function, not an initiating event.
2. Table 8.3-2 of the FSAR, Classification of Structures, Systems, and Components, indicates that all components of the backup power supply system, which includes the BDG enclosures, are Seismic Category III.

NuScale Nonproprietary NuScale Nonproprietary Per the BDG data sheet, developed for the Standard Design Approval Application US460 design, the BDG enclosure is rated for wind speeds in excess of the weather-related events considered in the LOOP initiating event (EF0 and EF1 tornadoes and Category 1 and 2 hurricanes).

Impact on US460 SDAA:

There are no impacts to US460 SDAA as a result of this response.

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