ML24215A214
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Response to SDAA Audit Question Question Number: A-19.1-30 Receipt Date: 06/19/2023 Question:
In response to A-19.1-8, Question 1, NuScale indicated that (( 2(a),(c)
- 1. Clarify the status of the storage location for the vertical bioshield (e.g., ((
}} 2(a)(c) is one potential option).
- 2. Explain how the options for the refueling storage location for the vertical bioshield are treated
(( }} 2(a),(c) Relevant supporting evaluations should be added to the eRR for staff review.
Response
- 1. NuScale is evaluating options for storage of the vertical bioshield section to satisfy design requirements. However, the storage location for vertical bioshield sections during a refueling outage does not impact seismic risk insights and is not evaluated as a risk contributor in ER-116292, Seismic Margins Assessment Notebook, which is available in the SDAA Audit Section 19.1-19.3 eRR, for the following reasons.
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Per the design specification for Reactor Building (RXB) components, the design of the bioshield shall accommodate removal and storage of bioshield parts during refueling activities to reduce the possibility of damage to other NuScale Power Modules (NPMs).
There are limited locations to temporarily store bioshield components while also maintaining the space needed to support refueling activity and Reactor Building crane NPM transport requirements.
(( }}2(a),(c) (as dispositioned in ER-115817, Fragility Analysis of the Bioshield, which is available in the SDAA Audit Section 19.1-19.3 eRR).
- 2. Seismic risk is not impacted by storage of the vertical bioshield section during refueling. The vertical section is evaluated in the seismic margin assessment (SMA) model ((
}}2(a),(c) Given the size of the vertical section (e.g., length, width) relative to an operating bay and the barrier protections in place for operating NPMs (e.g., bay walls, opposing bioshields), contact between the vertical section in its storage location and an operating NPM is not modeled as a risk contributor to an operating NPM in the NuScale SMA.
NuScale has added this information to FSAR Section 19.1.5.1.1.1, Seismic Analysis Methodology and Approach, under the header, Bioshield. With respect to loading of other structures (such as the end RXB bay wall), the RXB fragility considers ((
}}2(a),(c) as input to identify critical sections and associated failure modes to determine the governing failure for the RXB fragility. In the RXB structure configuration evaluated for the RXB fragility, six NPMs are operating normally in their respective operating bays, each with a single horizontal slab and vertical section secured in place.
NuScale performed analyses to evaluate the sensitivity of the RXB structural response to ((
}}2(a),(c) ER-105150, US460 RXB Sensitivity Analyses, has been uploaded to the SDAA Audit Section 19.1-19.3 eRR. With respect to (( }}2(a),(c) remains appropriate for use as input to the RXB fragility analysis.
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As is common practice for fragility development, NuScale reviews design evaluations (e.g., demand-to-capacity ratios, loads for critical sections) to determine critical RXB failure modes. In the case of the RXB bay wall, this review results in the determination that the governing failure mode is in-plane shear failure of bay walls ((
}}2(a),(c) Consistent with the results of sensitivity analyses documented in ER-105150, because the configuration involving the vertical bioshield storage location is not controlling, (( }}2(a),(c)
For the RXB fragility in general, critical sections are evaluated that correspond to several different substructures (e.g., roof, basemat, bay walls, pool walls, exterior walls). Initially, a conservative estimation of the structure high confidence of low probability of failure (HCLPF) capacity is performed for each substructure within the RXB using conservative deterministic failure margin. ((
}}2(a),(c) Therefore, the storage location for the vertical bioshield section is not identified as a key assumption in the evaluation of the RXB fragility or seismic risk in the SMA.
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 19.1-46 Draft Revision 2 failure mode evaluated in the fragility. The NPM support fragility is governed by failure of the base support; specifically, failure of the weld connection between the stiffener plate and a steel-plate composite wall. Given the loss of support to an NPM, failure is assumed to lead directly to core damage and large release. The designs of the NPM skirt restraint and the NPM lug restraint are discussed in Appendix 3B. Bioshield The bioshield fragility considers the major structural members (i.e., the horizontal slab and the vertical section) as well as the anchorages that provide support between the bioshield structures and the bay wall. The qualification calculations for the major members and the anchorages were reviewed to determine the controlling failure mode to evaluate with the separation of variables method. The bioshield fragility is considered in two configurations for an NPM operating at full power. Nominally, a single horizontal slab and vertical section are anchored to the bay walls of an operating NPM. When an NPM is in a refueling outage, the associated horizontal slab member is removed and placed on top of the bioshield of an operating NPM. Thus, a second configuration exists for an operating NPM with two stacked horizontal slabs, in addition to the vertical section. For an operating NPM, failure modes for each member relevant to both configurations are reviewed. For the configuration with a single horizontal slab, the controlling failure mode is shear failure of the bolts between the support plate and the bay wall uplift post. For the stacked horizontal slab configuration, each slab is anchored independently using the same design for providing seismic restraint, neither slab imparts a load on the other slab, and the controlling failure mode corresponds to shear failure of the bolts performing the same function for the stacked bioshield as the bolts that control the single slab configuration. Consequently, the fragility for the stacked configuration is the same as the single horizontal slab fragility for an operating NPM. Audit Question A-19.1-30 During refueling operations, the vertical bioshield section is moved to another location in the RXB; this storage location does not impact the seismic risk. Given the size of the vertical section (e.g., length, width) relative to an operating bay and the barrier protections in place for operating NPMs (e.g., bay walls, opposing bioshields), contact between the vertical section in its storage location and an operating NPM is not a risk contributor to an operating NPM. Components Similar to fragilities developed for structural failures, fragilities for component failures are modeled as basic events with median failure}}