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Federal Register / Vol. 86, No. 124 / Thursday, July 1, 2021 / Proposed Rules
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on the Use of Probabilistic Risk Assessment Methods in Nuclear Regulatory Activities and the White Paper on Risk-Informed and Performance-Based Regulation in SRM
SECY98144, White Paper on RiskInformed and Performance-Based Regulation, and Yellow Announcement 99019.
Based on the NRCs historic application of the single failure criterion and Commission direction on the subject, as described in SECY77439, Single Failure Criterion ADAMS
Accession No. ML060260236, SRM
SECY94084, Policy and Technical Issues associated with the Regulatory Treatment of Non-Safety Systems and Implementation of Design Certification and Light-Water Reactor Design Issues ADAMS Accession No. ML003708098, and SRMSECY190036, the NRC has retained discretion, in factor application-specific circumstances, to decide when to apply the single failure criterion. The Commissions decision in SRMSECY190036 provides direction regarding the appropriate application and interpretation of the regulatory requirements in 10 CFR part 50 to the NuScale IAB valves function to close.
This decision is similar to those in previous Commission documents that addressed the use of the single failure criterion and provided clarification on when to apply the single failure criterion in other specific instances.
D. Exemption to General Design Criterion 27, Combined Reactivity Control Systems Capability NuScale Power determined that, under certain end-of-cycle scenarios with one control rod stuck out, the NuScale reactivity control systems could not prevent re-criticality and return to power. This result does not meet GDC 27 of appendix A to 10 CFR
part 50, which covers reactivity control systems to reliably control reactivity changes under postulated accident conditions with margin for stuck control rods. Therefore, NuScale Power submitted an exemption request for GDC 27 refer to Section 15, 10 CFR 50, Appendix A, Criterion 27, Combined Reactivity Control Systems Capability, of DCA Part 7, Exemptions.
NuScale Power analyses determined that the specified acceptable fuel design limits would not be exceeded and that core cooling would be maintained during a return to power under these scenarios. The global core power level would be less than 10 percent and within capacity of the safety-related, passive decay heat removal system. The NRC independently verified NuScale Powers results and found that NuScale
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achieves the fundamental safety functions for nuclear reactor safety, which are to control heat generation, remove heat, and limit the release of radioactive materials. Chapter 15, Section 15.0.6.4.1, of the safety evaluation report contains details of the evaluation of this exemption request.
Additional information is provided in SECY180099, NuScale Power Exemption Request from 10 CFR part 50, Appendix A, General Design Criterion 27, Combined Reactivity Control Systems Capability ADAMS
Accession No. ML18065A431, dated October 9, 2018. The NRC granted the exemption request.
E. Safety-Related Class 1E AC or DC
Electrical Power NuScale does not contain safetyrelated Class 1E AC or DC electrical power systems. The purpose of appendix A to 10 CFR part 50, GDC 17, Electric Power Systems, is to ensure that sufficient electric power is available to accomplish plant functions important to safety. NuScale provides passive safety systems and features to accomplish plant safety-related functions without reliance on electrical power.
NuScale incorporates several innovative features that reduce the overall complexity of the design and lower the number of safety-related systems necessary to mitigate postulated accidents. NuScale has no safety-related functions that rely on electrical power.
For example, the emergency core cooling system performs its safety function without reliance on safetyrelated electrical power or external sources of coolant inventory makeup.
NuScale Power provided a methodology to substantiate its assertion that the safety-related systems do not rely on Class 1E electrical power in topical report TR081516497, Safety Classification of Passive Nuclear Power Plant Electrical Systems, dated February 23, 2018 ADAMS Accession No. ML18054B607. The NRC reviewed topical report TR081516497 and concluded that NuScale Power demonstrated that the safety-related systems do not rely on Class 1E
electrical power. The NRCs review and conclusions are documented in a safety evaluation report approving topical report TR081516497 ADAMS
Accession No. ML17048A459 issued December 13, 2017, as described in the final safety evaluation report for Chapter 1, Introduction and General Discussion, ADAMS Accession No.
ML20204A986.
Because no safety-related functions of NuScale rely on electrical power,
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NuScale does not need any safetyrelated electrical power systems.
Therefore, NuScale Power requested an exemption from GDC 17, which requires the provision of onsite and offsite power to provide sufficient capacity and capability to assure that 1 specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences and 2 the core is cooled and containment integrity and other vital functions are maintained in the event of postulated accidents. The NRC
determined that, subject to limitations and conditions stipulated in its safety evaluation report for TR081516497, the underlying purpose of GDC 17 to ensure sufficient electric power is available to accomplish the safety functions of the respective systems, is met without reliance on Class 1E
electric power. In other words, the onsite and offsite electric power systems are classified as non-Class 1E systems and electric power is not needed 1 to achieve or maintain safe shutdown, 2
to assure specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded as a result of anticipated operational occurrences, or 3 to maintain core cooling, containment integrity, and other vital functions during postulated accidents.
Further, the onsite and offsite power systems are not needed to permit functioning of structures, systems, and components important to safety.
Therefore, NuScale Power was granted an exemption from GDC 17. The NRCs evaluation of NuScale Powers exemption request from the requirements of GDC 17 is documented in Section 8.1.5, Technical Evaluation for Exemptions, of the final safety evaluation report for Chapter 8, Electric Power ADAMS Accession No. ML20023B614.
F. Accident Source Term Methodology The NRC reviewed NuScale Powers methods for developing accident source terms and performing accident radiological consequence analyses. As defined in 50.2, Definitions, a source term refers to the magnitude and mix of the radionuclides released from the fuel, expressed as fractions of the fission product inventory in the fuel, as well as their physical and chemical form, and the timing of their release.
NuScale Power developed source terms for deterministic accidents for NuScale that are similar to those which have been used in safety and siting assessments for large light water reactors. The design-basis accidents for
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