ML24346A303
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Response to SDAA Audit Question Question Number: A-15.9-3 Receipt Date: 06/28/2024 Question:
FSAR Section 15.9.1 notes that, "The Reference 15.9-1 methodology and conclusions, as well as the conditions and limitations of the associated Safety Evaluation Report, have been reviewed and verified to be applicable to the NPM for the NuScale Power Plant US460 standard design."
Please describe and justify the rationale for that applicability. Provide markups to FSAR Section 15.9.1 to describe the basis for the applicability determination.
Response
TR-0516-49417-P-A, Revision 1, Evaluation Methodology for Stability Analysis of the NuScale Power Module, was previously reviewed and approved by the NRC during the Design Certification of the US600 design using the NuScale Power Module 160 (NPM-160). TR-0516-49417-P-A, Revision 1, is also used in support of the Standard Design Approval Application of the US460 design using the NuScale Power Module 20 (NPM-20). In determining the applicability of TR-0516-49417-P-A, Revision 1, to the NPM-20 design, the following steps were taken: ((2(a),(c) NuScale Nonproprietary NuScale Nonproprietary
(( }}2(a),(c) In summary, NuScale confirmed design and operational changes did not introduce new physics not captured in the existing PIRT or mathematical models. With the appropriateness of the mathematical representation confirmed, NuScale updated the inputs and re-performed the analysis suite consistent with Section 10.4 of the topical report and Section 4.0 of the NRC staff SER. Final Safety Analysis Report (FSAR) Section 15.9 is revised as shown in the attached markup to include clarifying details from the above response. Markups of the affected changes, as described in the response, are provided below: NuScale Nonproprietary NuScale Nonproprietary
NuScale Final Safety Analysis Report Stability NuScale US460 SDAA 15.9-1 Draft Revision 2 15.9 Stability Events that could result in thermal-hydraulic instability within the reactor pressure vessel are considered significant only for boiling water reactors (BWRs), where individual fuel assemblies with high power-to-flow ratios may undergo instabilities or the neutronic conditions may lead to power oscillation. Pressurized water reactors use pumps for forced circulation, which keeps core flow essentially constant with power level. The NuScale Power Module (NPM) employs natural circulation. With this design feature, flow through the core is not held constant by pumps providing forced circulation. Thus, variations in flow may result in changes in power level and vice versa. The identification and evaluation of the significance of these mechanisms is addressed in Section 4.4.7. Perturbations and bounding flow instability are evaluated in this section. The evaluation includes reactivity coefficients that span beginning of cycle (BOC) to end of cycle (EOC), and demonstrates that the NPM is protected from unstable flow oscillations provided that operation is limited by a defined pressure-temperature exclusion zone such that no boiling in the riser area above the core is allowed. The negative moderator reactivity coefficient may stabilize flow even if riser boiling occurs, but this is conservatively not credited in the stability methodology. 15.9.1 Consideration of Thermal-Hydraulic Stability The NuScale Stability Evaluation Methodology Topical Report (Reference 15.9-1) presents an analysis of the thermal-hydraulic stability of a representative NPM and demonstrates compliance with 10 CFR 50, Appendix A, General Design Criteria (GDC) 10 and GDC 12. The topical report considers potential power and hydraulic stability mechanisms during anticipated operational occurrences (AOOs) and normal operating conditions. Thermal-hydraulic instability during infrequent events or accidents is not considered because the acceptance criteria for such events allow for conditions beyond the specified acceptable fuel design limits imposed on AOO events experiencing instabilities. The topical report considers flow stability without making assumptions based on comparisons to other nuclear systems. The topical report describes computational methods developed for the analysis of the limiting instability modes for a representative NPM design during steady state, normal operation, and anticipated transients. Audit Question A-15.9-2 The region exclusion stability protection solution is shown in Reference 15.9-1 to be an acceptable approach for preventing the occurrence of instabilities in an NPM. The region exclusion is defined by specifying riser subcooling margin. The riser subcooling margin is maintained by the operational boundaries enforced automatically by the module protection system (MPS) as shown in Figure 4.4-2. The limiting instability mode is a natural circulation instability as described in Reference 15.9-1. Audit Question A-15.9-3 The Reference 15.9-1 methodology and conclusions, as well as the conditions and limitations of the associated Safety Evaluation Report, have been reviewed and verified to be applicable to the NPM for the NuScale Power Plant US460 standard
NuScale Final Safety Analysis Report Stability NuScale US460 SDAA 15.9-2 Draft Revision 2 design. Design and operational differences of the US460 standard design compared to the reference plant described in Reference 15.9-1 do not introduce new physics or phenomena requiring changes to the methodology. The conditions and limitations in the Safety Evaluation Report associated with Reference 15.9-1 have been met for application of the methodology to the US460 standard design. Updated stability analyses using inputs in a manner consistent with Reference 15.9-1, but with NPM parameters reflecting the US460 standard design, are presented in this section. Audit Question A-15-2, Audit Question A-15-4, Audit Question A-15-5 Although not described in the Reference 15.9-1 methodology, Section 15.9.5 presents evaluations of continuously-present, imposed oscillations on the secondary side through feedwater flow rate or main steam pressure. 15.9.2 Stability Analyses Several cases are analyzed over a wide range of power and primary system flow operating conditions and possible scenarios to demonstrate that stability is maintained during routine power operations in the NPM. 15.9.2.1 Stability Analysis for Power Operations This section analyzes power operations over a range of power level and flow conditions in the presence of a small perturbation in operating conditions as described in Section 8.1 of Reference 15.9-1. Only the limiting case is presented in detail in Section 15.9.2.1.1 through Section 15.9.2.1.4. The analyses show that the NPM is stable at power levels above 5 percent, with EOC conditions providing more damping (i.e. more stability) than BOC conditions. The flow stability condition that is the least stable occurs at 12.5 MW core power (5 percent of rated) with a BOC reactivity condition. The primary coolant flow response shows damped oscillations with a period of over a minute at this power level. The flow is less stable than the higher power cases, but with the low power level, there is no challenge to fuel limits. 15.9.2.1.1 Identification of Causes and Event Description The 12.5 MW power case at BOC is the least stable of the power operations considered. 15.9.2.1.2 Sequence of Events and Systems Operation No systems operations occur in response to the event.}}