ML24215A217
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Response to SDAA Audit Question Question Number: A-19.1-41 Receipt Date: 09/18/2023 Question:
There are several locations in the NuScale SDA in Chapter 19 where editorial changes can provide clarification. Explain the following or clarify the SDA text in the following locations:
1.
SDAA Table 19.1-60 indicates that the External Flooding LRF mean value is 1.4D-13/yr.
This value is also found in r Section 5.3.1 of External Flood Notebook and is lower than the point estimate of 9.1E-13 /yr. Confirm that this value is a a typo and should read 1.4E-12, as shown in the Figure 5-2 of External Flood Notebook. If this is not the case, provide the updated External Flood Notebook with associated corrections.
2.
In Table 19.1-32, the HCLPF values for the Trip Valves for RRV and RVV trip valves appear to be flipped compared with Table 4-7 of SMA Notebook (ER-116292). Provide the correct HCLPF values in an SDAA markup and confirm that no additional changes are needed.
3.
On page 19.1-43 of the SDAA, a reference is made to EPRI NP-6041-NL. Confirm that this reference should be to EPRI NP-6041-SL and provide the associated SDAA markup.
4.
The last statement in FSAR Section 19.1.5.1.1.1 on Page 19.1-48, [r]esults of the fragility calculation for the NPM supports are shown in Table 19.1-32." appears to be out of place. It appears to belong to NuScale Power Module Supports subsection located on page 19.1-45.
Confirm that this is the correct interpretation for this statement and provide an FSAR markup.
Response
- 1. The external flooding large release frequency mean value of 1.4E-13/mcyr in standard design approval application (SDAA) Table 19.1-60, Summary of Results, is a typographical error, and NuScale has corrected this value to 1.4E-12/mcyr.
NuScale Nonproprietary NuScale Nonproprietary
- 2. The high confidence of low probability of failure values for the reactor recirculation valve trip valves and the reactor vent valve trip valves in SDAA Table 19.1-32, Seismic Margin Assessment Fragility, are swapped. NuScale has corrected this error.
- 3. The reference to EPRI NP-6041-NL on page 19.1-43 of the SDAA should read, EPRI NP-6041-SL. NuScale has corrected this error.
- 4. NuScale intended the referenced statement in SDAA Section 19.1.5.1.1.1, on page 19.1-48, to refer to the results of all fragility calculations, not just for the NuScale Power Module supports.
NuScale has corrected this error.
Markups of the affected changes, as described in the response, are provided below:
NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-43 Draft Revision 2 safe shutdown equipment list (SSEL). SSC that contribute to the seismic margin are determined by applying the MIN-MAX method described in Section 19.1.5.1.2.
Audit Issue A-19.1-41 The HCLPF ground motion for SSC that contribute to the seismic margin is obtained by performing fragility analysis using the separation of variables method, as endorsed by Section 19.0 of NUREG-0800, Revision 3.
Separation of variables, described in EPRI 103959 (Reference 19.1-23), is a best-estimate methodology to determine SSC fragility parameters (median capacity, randomness, and modeling uncertainty) as a combination of several independently determined factors (e.g., capacity, structure response, equipment response). The fragility parameters are then used to calculate the HCLPF. For SSC that don't contribute to the seismic margin, fragilities are described conservatively using the conservative deterministic failure margin method or by utilizing generic fragilities. The conservative deterministic failure margin method, described in EPRI NP-6041-SL (Reference 19.1-12) uses conservative input parameters (e.g., material strength, seismic demand) to calculate a conservative estimate of the HCLPF capacity directly. SSC with generically defined fragilities utilize conservative capacity values with a conservative application of design-specific seismic demands. In either case, a composite uncertainty is used to define the HCLPF capacity. As a result, the hybrid method, as described in EPRI NP-6041-SNL, is used to define parameter estimates for randomness and modeling uncertainty specific to different types of SSC.
The controlling failure mode of the structural events and their direct consequences are shown in Table 19.1-32. For components, seismic failures are either considered functional failures or mapped to specific equivalent random failures (such as a valve failing to open on demand).
Seismic Structural Events Fragilities for structural failures are modeled as basic events in the SMA model with median failure accelerations and uncertainty parameters. For each structural fragility, boundaries are defined such that relevant seismically-induced failure mechanisms are accounted for (e.g., failures to supporting sections, intersecting structures, nearby structures).
Seismically-induced structural failures are then assumed to lead directly to core damage and large release without opportunity for mitigation. This is a simplifying assumption for modeling catastrophic failure mechanisms.
Structural events differ from component failures in that they do not correspond to a random event in the internal events PRA. In all cases, the consequences of structural events are assumed to lead to both core damage and large release without opportunity for mitigation. This is a simplifying assumption for modeling catastrophic failure mechanisms.
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-48 Draft Revision 2 Conservative seismic demands are determined according to whether a component may be considered rigid (e.g., valves). If an SSC is rigid, indicating a high natural frequency, seismic demands are applied using a zero period acceleration. If an SSC is not rigid, the peak acceleration of the ISRS is used. For SSC located in the RXB, an enveloped floor ISRS for all locations on an elevation is used to describe the SSC seismic demand. For SSC located on or near the NPM, but do not contribute to the seismic margin (e.g., DHRS heat exchangers), broadened ISRS is used at the equipment anchorage location.
Audit Issue A-19.1-40 Each SSC fragility is calculated based on floor responses. Consequently, each fragility is multiplied by the PGA of the CSDRSRE (0.5g) to anchor the median capacity to the seismic input defined for design (i.e., the CSDRS). Each component fragility is then determined as a function of design loads, placement, and site response.
The HCLPF is then defined as the acceleration level where there is a 95 percent confidence of less than 5 percent failure probability. The HCLPF can also be approximated as the acceleration with a one percent probability of failure on the mean fragility curve.
Audit Issue A-19.1-41 Results of the fragility calculation for the NPM supports are shown in Table 19.1-32.Table 19.1-32 contains the results from the fragility calculations.
19.1.5.1.1.2 Systems and Accident Sequence Analysis Plant response analysis maps the consequences of seismic initiators combined with seismic and random failures. This analysis produces event trees with seismically induced initiating events, component and structural events, and non-seismic unavailability.
The SAPHIRE computer code is used for quantification of the logic models utilized in the NuScale SMA.
Seismically-Induced Initiators Plant response after a seismic event is mapped using seismically-induced initiating events, as illustrated in Figure 19.1-14. These events are modeled using similar logic to corresponding random internal events PRA initiating events. Plant response is modeled only for earthquakes with a non-negligible probability of causing a reactor trip.
Audit Issue A-19.1-20 The seismic hazard for the NuScale design SMA is partitioned into fourteen seismic event trees. The underlying logic for each event tree is identical; however, each event tree represents a different ground motion
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-148 Draft Revision 2 Audit Issue A-19.1-41 Table 19.1-32: Seismic Margin Assessment Fragility SSC HCLPF (g)
Controlling Failure Mode Assumed Consequence Reactor Building Crane Supports 0.92g Weld failure Core damage/Large Release Bioshield - normal operation (single stack) 0.93g Bolt shear failure Core damage/Large Release Bioshield - refueling of adjacent NPM (double stack) 0.93g Bolt shear failure Core damage/Large Release when configuration present Reactor Building 0.97g Roof in-plane shear failure Core damage/Large Release Reactor Building Crane 1.11g Plate bending failure Core damage/Large Release NPM Supports 1.14g Weld failure Core damage/Large Release Reactor Recirculation Valves 1.38g Valve body deformation Valve failure to open Reactor Vent Valves 2.69g Valve body deformation Valve failure to open Containment Isolation Valves 3.92g Valve body deformation Valve failure to open Reactor Safety Valves 6.00g Valve body deformation Valve failure to open Trip Valves for Reactor Recirculation Valves 8.447.14g Valve body deformation Valve failure to open Trip Valves for Reactor Vent Valve 7.148.44g Valve body deformation Valve failure to open Note:
- HCLPF = High-Confidence (95%) of a Low Probability (5%) of Failure (EPRI 103959)
NuScale Final Safety Analysis Report Probabilistic Risk Assessment NuScale US460 SDAA 19.1-217 Draft Revision 2 Audit Issue A-19.1-25 Audit Issue A-19.1-41 Table 19.1-60: Summary of Results Full Power (per mcyr)
Hazard CDF (mean values) 5th percentile 95th percentile LRF (mean values) 5th percentile 95th percentile Internal Events 6.0E-09 2.2E-10 2.1E-08 6.6E-13
<1E-15 1.5E-12 Internal Fires 4.6E-09 9.7E-11 1.6E-08 1.3E-11 3.9E-15 3.0E-11 Internal Floods 1.6E-10 1.8E-12 5.5E-10 3.4E-14
<1E-15 2.8E-14 External Floods 9.5E-09 1.4E-10 3.5E-08 1.4E-1312 5.9E-15 4.3E-12 High Winds (Tornado) 2.6E-09 2.6E-11 9.5E-09 1.6E-13
<1E-15 4.9E-13 High Winds (Hurricane) 1.9E-08 1.9E-10 7.0E-08 1.3E-12
<1E-15 4.3E-12 Seismic 1 (SMA) 0.92g Low Power and Shutdown (per year)
Hazard CDF (mean values) 5th percentile 95th percentile LRF (mean values) 5th percentile 95th percentile Internal Events 4.0E-11 9.8E-13 1.4E-10 3.5E-12 4.2E-14 1.2E-11 Module Drop 1.8E-08 2.5E-10 6.9E-08 NA 2 NA 2 NA 2 Internal Fires negligible5 negligible5 Internal Floods negligible5 negligible5 External Floods negligible5 negligible5 High Winds (Tornado) negligible5 negligible5 High Winds (Hurricane) negligible5 negligible5 Seismic 1 (SMA)
NA Multi-Module Hazard Conditional Probability of Core Damage Conditional Probability of Large Release Multi-Module 0.213 0.033 Composite CCFP < 0.14 Notes:
1.A seismic margins assessment is performed; results are presented in terms of the HCLPF (i.e., peak ground acceleration at which there is 95% confidence that the conditional failure probability is less than 5%).
2.A module drop does not result in a large release.
3.Results are presented in terms of a bounding estimate on the conditional probability that multiple modules would experience core damage (or large release) following core damage (or large release) in a single module.
4.Composite CCFP reflects contributions from all hazards.
5.Based on qualitative evaluation.