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Federal Register / Vol. 86, No. 124 / Thursday, July 1, 2021 / Proposed Rules NuScale are the main steam line break outside containment, rod ejection accident, fuel handling accident, steam generator tube failure, and the failure of small lines carrying primary coolant outside containment.
To address the source term regulatory requirements, NuScale Power submitted topical report TR091517565, Revision 3, Accident Source Term Methodology, dated April 2019
ADAMS Accession No. ML19112A172.
The topical report proposes a methodology to develop a source term based on several severe accident scenarios that result in core damage, taken from the design probabilistic risk assessment. This source term is the surrogate radiological source term for a core damage event.
The topical report also provides methods for determining radiation sources not developed from core damage scenarios for use in the evaluation of environmental qualification of equipment under 50.49, Environmental qualification of electric equipment important to safety for nuclear power plants. Specifically, the report describes an iodine spike source term not involving core damage, which is a surrogate accident that bounds potential accidents with release of the reactor coolant into the containment vessel.
The staff submitted a related information paper to the Commission, SECY190079, Staff Approach to Evaluate Accident Source Terms for the NuScale Power Design Certification Application, dated August 16, 2019
ADAMS Accession No. ML19107A455, describing the regulatory and technical issues raised by unique aspects of NuScale Powers proposed methodology and the staffs approach to reviewing topical report TR091517565.
The NRCs review and findings of topical report TR091517565, Revision 3, are documented in the topical report final safety evaluation report issued on October 29, 2019 ADAMS Accession No. ML19297G520. The approved version TR091517565NPA, Revision 4 ADAMS Accession No.
ML20057G132 is discussed in the DCA
safety evaluation report Section 12.2, Radiation Sources, Section 12.3, Radiation Protection Design Features, Section 3.11 Environmental Qualification of Mechanical and Electrical Equipment, and Section 15.0.3, Radiological Consequences of Design Basis Accidents. The NRC
found the accident source terms acceptable for the purposes described in each of the above safety evaluation report sections.

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G. Boron Redistribution During Passive Cooling Modes The NRC evaluated the effects of boron volatility and redistribution during long term passive cooling.
During this mode of operation, boronfree steam will enter the downcomer and containment which can potentially challenge reactor core shutdown margin and could lead to a return to power. The NRC reviewed analyses provided by NuScale Power demonstrating that the reactor remains subcritical and that specified acceptable fuel design limits are not exceeded. The NRC evaluated the technical basis for NuScale Powers approach and conducted confirmatory calculations and independent assessments to determine its acceptability. The staffs review is primarily documented in Chapter 15, Section 15.0.5, Long Term Decay Heat and Residual Heat Removal, and Section 15.6.5, Loss of Coolant Accidents Resulting from Spectrum of Postulated Piping Breaks within the Reactor Coolant Pressure Boundary, of the safety evaluation report.
Specifically, the staff concluded that the top of active fuel remains covered with acceptably low cladding temperatures and that for beginning-of-cycle and middle-of-cycle conditions, with no operator actions, the core remains subcritical. The potential for an end-ofcycle return to power is discussed in Section IV.D, Exemption to General Design Criterion 27, Combined Reactivity Control Systems Capability,
of this document. In addition, Chapter 19, Section 19.1.4.6.4, Success Criteria, Accident Sequences, and Systems Analyses, of the safety evaluation report concludes that an operator error during recovery of the module from an uneven boron distribution scenario is unlikely to lead to core damage and is not a significant risk contributor.
H. Exemptions NuScale Power submitted a total of 17
requests for exemptions from the following regulations, including those discussed as part of the significant technical issues mentioned previously see Table 1.141, NuScale Design Certification Exemptions, in Chapter 1
of the final safety evaluation report ADAMS Accession No.
ML20204A986:
1. 50.46a and 50.34f2vi Reactor Coolant System Venting 2. 50.44 Combustible Gas Control 3. 50.62c1 Reduction of Risk from Anticipated Transients Without Scram 4. Appendix A to 10 CFR part 50, GDC
17, Electric Power Systems; GDC

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18, Inspection and Testing of Electric Power Systems; and related provisions of GDC 34, Residual Heat removal; GDC 35, Emergency Core Cooling; GDC
38, Containment Heat Removal;
GDC 41, Containment Atmosphere Cleanup; and GDC 44, Cooling Water Electric Power Systems GDCs 5. Appendix A to 10 CFR part 50, GDC
33, Reactor Coolant Makeup 6. 50.54m Control Room Staffing Alternative to meet the regulation 7. Appendix A to 10 CFR part 50, GDC
52, Capability for Containment Leakage Rate Testing 8. Appendix A to 10 CFR part 50, GDC
40, Testing of Containment Heat Removal System 9. Appendix A to 10 CFR part 50, GDC
55, Reactor Coolant Pressure Boundary Penetrating Containment, GDC 56, Primary Containment Isolation, and GDC
57, Closed Systems Isolation Valves Containment Isolation 10. Appendix K to 10 CFR part 50
Emergency Core Cooling System Evaluation Models 11. 50.34f2xx Power Supplies for Pressurizer Relief Valves, Block Valves, and Level Indicators 12. 50.34f2xiii Pressurizer Heater Power Supplies 13. 50.34f2xivE Containment Evacuation System Isolation 14. 50.46 Fuel Rod Cladding Material 15. Appendix A to 10 CFR part 50, GDC
27, Combined Reactivity Control Systems Capability 16. 50.34f2viii Post-Accident Sampling 17. Appendix A to 10 CFR part 50, GDC
19, Control Room NRCs safety evaluation report for Chapter 1, Introduction and General Discussion Section 1.14, Index of Exemptions, lists these exemption requests with the corresponding sections of the safety evaluation reports where these exemption requests have been evaluated. The NRC granted each exemption request.
V. Discussion Final Safety Evaluation Report NuScale Power submitted the final revision of the NuScale DCA, Revision 5, in July 2020 ADAMS Accession No.
ML20225A071. In August 2020, the NRC issued a final safety evaluation report ADAMS Accession No.
ML20023A318 after the Advisory Committee on Reactor Safeguards ACRS performed its final independent review and issued its letter to the Commission in July 2020 on its findings
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