ML24215A062
| ML24215A062 | |
| Person / Time | |
|---|---|
| Site: | 05200050 |
| Issue date: | 08/02/2024 |
| From: | NuScale |
| To: | Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML24215A000 | List:
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| References | |
| LO-169995 | |
| Download: ML24215A062 (1) | |
Text
Response to SDAA Audit Question Question Number: A-3.9.2-22 Receipt Date: 10/09/2023 Question:
"Since preliminary stress evaluations of the NuScale power module (NPM)-20 reactor vessel internals (RVI) and steam generator (SG) under seismic and transient loading conditions are not available for evaluation (current estimated availability date (( 2(a),(c),ECI the staff is evaluating NuScales planned RVI and SG stress analysis procedures based on: Preliminary NPM-160 American Society of Mechanical Engineers (ASME) Service Level D stress margins provided in RAIO-0819-66530 (NuScale response to NRC eRAI 8911 Question
- 18) for design certification The staff is also comparing NPM-160 and NPM-20 RVI resonance frequencies and mode shapes, where available, for the components with the highest stressed locations in the NPM-160 (Table A-10 ASME BPVC compliance - components of RAIO-0819-66530), including:
Hex head cap screw (LOCATION UNKNOWN) Control rod assembly (CRA) Guide tube support plate Upper control rod drive shaft (CRDS) support Upper riser section Core support block top plate Upper riser hanger ring Bellows (not highly stressed in the NPM-160 but a location of significant interest) As well as the highest stressed lower SG tube location (( }} 2(a),(c) in RAIO-0819-66530). Finally, the staff is evaluating the differences between the NPM-160 and NPM-20 dynamic loads induced by transient and seismic events (addressed in Audit Question 21). The staff requests the following missing information to evaluate the planned stress assessments (NPM-160 data should already exist and should not require additional NuScale analysis effort): 1. Details of the stress margin calculations for the most highly stressed RVI components in the NuScale Nonproprietary NuScale Nonproprietary
NPM-160 analysis, including: (( }} 2(a),(c),ECI The staff assumes this information already exists in a NuScale internal report. 2. An overview of NuScales planned approach for estimating the NPM-20 RVI and lower SG structural response and stresses due to transient events which include significant dynamic content. The staff notes that NuScale performed static evaluations of the stresses induced by transient loads for NPM-160 in RAIO-0819-66530. Based on the limited NPM-20 pressure time histories provided (( }} 2(a),(c),ECI The staff provides the following guidance from NUREG-0609, Asymmetric Blowdown Loads on PWR Primary System, traditionally followed for evaluations of dynamic transient analyses: Last paragraph on Page 4: o These loads provide the information needed to evaluate the dynamic response of the supports of the reactor pressure vessel, the reactor core structures, and the primary coolant loop and its loop components. From Section 3.1 Internal Subcooled Blowdown Loads: o The second step involves the conversion of these transient pressure and velocity data into equivalent transient forces throughout the primary system. The transient forces developed in these two steps are then included, along with the remaining LOCA loads, in a time-history structural analysis of the primary system.
- 3.
Any planned updates to the procedures to be used to evaluate NPM-20 RVI and SG tube stresses due to seismic loading. In particular explain the modes which will be used for the upper RVI stress analyses. Figures A-11 and A-17 of RAIO-0819-66530 indicate that the upper RVI dynamic stress analyses were based on ((
}}
2(a),(c),ECI However, the appropriate modes of the RVI are actually of the overall RVI assembly. The staff is concerned that the mode shapes will be non-conservative (due to restricted motion of the interface plane) and the modal frequencies will be too high (also NuScale Nonproprietary NuScale Nonproprietary
non-conservative) if the same procedure is used for NPM-20 stress analyses. 4. Address the potential impacts on stress margins due to apparent loading increases in the NPM-20 compared to those assumed in the NPM-160 including: (( }} 2(a),(c),ECI
Response
Item 1: NPM-20 hex head cap screw location and NPM-160 RVI stress margin calculations. For the NPM-20, which is the module reflected in the US460 standard design approval application (SDAA), cap screws exist at multiple locations, one of which includes a hex head cap screw (labeled as Socket Head Cap Screw in NPM-20) which attaches the core support mounting bracket top plate to the core support mounting bracket gusset. The NPM-20 RPV Lower SubAssembly drawing ED-122049, Revision 1, provides a level of detail beyond what is shown in the simplified US460 SDAA Figure 3.9-4. While excerpts from the drawing is provided within the response, drawing ED-122049 is uploaded to eRR. NuScale Nonproprietary NuScale Nonproprietary
RPV Lower SubAssembly (( }}2(a),(c),ECI Details of the stress margin calculations for the most highly stressed NPM-160 reactor vessel internals (RVI) components are addressed in the approved US600 Design Certification Application (DCA). NPM-160 details are not provided as part of this response as the US460 Standard Design Approval Application (SDAA) is a stand alone application and design and modeling techniques implemented for NPM-20 do not facilitate a direct comparison with NPM-160. SDAA Table 3.9.-7 defines the load combinations and stress limit for the RVI. Item 2: NPM-20 structural response and stresses due to transient events, including asymmetric pressurization (blowdown) loads. The NPM-20 structural response and stresses due to transient events are not estimated; they are calculated for the specified service loadings in accordance with the requirements of ASME BPVC Section III, Subsection NCA-2142.2 Service Loadings. NuScale Nonproprietary NuScale Nonproprietary
Asymmetric pressurization (blowdown) loads are included in the Load Combinations for Service Level C and D analyses. SDAA Section 3.9.1 describes acceptable analytical methods for Seismic Category I components and supports designated ASME BPVC, Section III, Division 1, Class CS. The plant and system operating transient conditions, including postulated seismic events and DBE, that provide the basis for the design of the RVI are provided in SDAA Section 3.9.3. The dynamic model used for the blowdown analysis includes the containment vessel (CNV), reactor pressure vessel (RPV), RVI, and control rod drive mechanisms (CRDMs). Appendix 3A has a representative diagram of the model and additional information regarding dynamic loading. The SG supports and SG tube supports are Seismic Category I components. The SG supports and SG tube supports are designated as internal structures in accordance with ASME Section III, Subsection NG. The load combinations and stress limit are consistent with those listed in SDAA Table 3.9-7. As identified in US460 SDAA Table 3.9-7, the following events are defined for RVI stress analysis:
The Design Basis Pipe Break (DBPB) includes the absolute sum of the internal pressure wave load from the pipe break, any applicable pipe whip load, and the asymmetric pressurization (blowdown) load (for pipe breaks inside containment).
ECCS includes the absolute sum of the jet impingement load from the valve discharge and the asymmetric pressurization (blowdown) load.
RSV includes the absolute sum of the jet impingement load from the valve discharge and the asymmetric pressurization (blowdown) load. Item 3: NPM-20 modal analysis. EC-107251, Revision 1, (NPM-20 Seismic Simulation) includes ((
}}2(a),(c),ECI Further modeling details are included in EC-107251, Revision 1, which is provided in the eRR.
NuScale Nonproprietary NuScale Nonproprietary
In DCA RAIO-0819-66530, the stresses were not calculated based on modal analysis; they were based on multi-point response spectrum analysis. The modal solution needs to be obtained before the spectrum solution. For a spectrum analysis, the excitation locations are (( }}2(a),(c),ECI In addition, since all the loads required for the upper RVI are included in the analysis, there is no need to model the entire RVI. Hand calculations, static analyses, and transient analyses are options for performing NPM-20 stress analyses. Item 4: Potential impacts on stress margins from ISRS loads. EC-107251, Revision 1, (NPM-20 Seismic Simulation) includes (( }}2(a),(c),ECI As stated above in item 3, it is not yet determined if the same approach (spectrum analysis) will be used in NPM-20 stress analyses, nor do we know the major modes for the complete set of NPM-20 components. Spectrum analysis is generally a conservative approach for stress analysis. If the ISRS loading amplitudes in a specific frequency range increase, but no major modes are observed in this frequency range, the impact of the structural response may be insignificant. No changes to the SDAA are necessary. NuScale Nonproprietary NuScale Nonproprietary}}