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Federal Register / Vol. 86, No. 124 / Thursday, July 1, 2021 / Proposed Rules
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decay heat removal system piping against acoustic resonance, and control rod assembly guide tube against turbulence buffeting. The steam generator tube margins against fluidelastic instability and vortex shedding will be validated in the TF3 testing facility as described in DCA Part 2, Tier 1, Section 2.1.1, Design Description.
In addition, the initial startup testing will confirm that flow-induced vibration will not cause adverse effects on the plant system components including the steam generator tubes. With the exception of the steam generator tube and inlet flow restrictor issue discussed previously, the NRC found the comprehensive vibration assessment program adequate to ensure the structural integrity of the NuScale power module components.
B. Containment Safety Analysis NuScale incorporates novel and unique features which result in transient thermal-hydraulic responses that are different from those of currently licensed reactors.
There are several peak containment pressure analysis technical issues unique to NuScale, including the associated thermal-hydraulic analyses.
In support of containment safety analysis, NuScale Power submitted technical report TR051649084P, Revision 3, Containment Response Analysis Methodology, May 2020
ADAMS Accession No. ML20141L808
that describes the conservative containment pressure and temperature safety analyses for several design-basis events related to the containment design margins. NuScale also submitted topical report TR051649422, Loss-ofCoolant Accident Evaluation Model, Revision 1, dated November 2019
ADAMS Accession No. ML19331B585.
This topical report describes the evaluation model used to analyze the power module response during a design-basis loss-of-coolant accident.
The NRC reviewed this topical report as part of the containment safety analysis.
The NRC also observed thermalhydraulic performance testing at NuScale Powers integrated system test facility, which validates the analytical model. Based on initial testing results and thermal-hydraulic analyses, NuScale Power made design changes to increase the initial reactor building pool level and the in-containment vessel design pressure to account for some uncertainties.
The NRC reviewed the details of the computer thermal-hydraulic evaluation model described in the DCA Part 2, Tier 2, Section 6.2.1.1 to determine whether any uncertainties were properly
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accounted for and found the containment design margins to be acceptable. The associated safety evaluation report approving topical report TR051649422 was issued on February 18, 2020 ADAMS Accession No. ML20044E199. The NRCs review and specific findings, including independent analyses and observation of NuScale testing, are documented in Chapter 6, Engineered Safety Features, Section 6.2.1.1, Containment Structure, of the safety evaluation report.
C. Emergency Core Cooling System Inadvertent Actuation Block Valve The NuScale emergency core cooling system relies on natural circulation cooling of the reactor core by releasing the heated reactor coolant steam from the top of the reactor pressure vessel through three reactor vent valves into the containment vessel and returning the cooled condensed reactor coolant water to the reactor pressure vessel through two reactor recirculation valves.
Each reactor vent valve and reactor recirculation valve consists of a first-ofa-kind arrangement of a main valve, an inadvertent actuation block IAB valve, a solenoid trip valve, and a solenoid reset valve. The IAB valve for each reactor vent valve and reactor recirculation valve is designed to close rapidly to prevent its corresponding emergency core cooling system main valve from opening when the reactor coolant system is at high pressure conditions. Premature opening of the emergency core cooling system main valves could result in fuel damage. The IAB valve then opens at reduced reactor coolant system pressure to allow the main valve to open and permit natural circulation cooling of the reactor core in response to a plant event. Although the valve assemblies are considered an active component, NuScale does not apply the single failure criterion to the IAB valve, including to the IAB valves function to close. Consistent with Commission safety goals and the practice of risk-informed decisionmaking, the NRC evaluated the NuScale emergency core cooling system valve system without assuming a single active failure of the IAB valve to close.
During design demonstration tests of the first-of-a-kind emergency core cooling system valve system performed under 50.43e, NuScale Power implemented design modifications to the main valve and IAB valve to demonstrate that the IAB valve will operate within a specific design pressure range. The DCD specifies that the emergency core cooling system valves including the IAB valves will be
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qualified under American Society of Mechanical Engineers Standard QME
12007, Qualification of Active Mechanical Equipment Used in Nuclear Power Plants, as endorsed by NRC
Regulatory Guide 1.100, Revision 3, Seismic Qualification of Electrical and Active Mechanical Equipment and Functional Qualification of Active Mechanical Equipment for Nuclear Power Plants, prior to installation in a NuScale nuclear power plant.
Additionally, the NRC regulations in 50.55a require that a NuScale nuclear power plant satisfy American Society of Mechanical Engineers Operation and Maintenance of Nuclear Power Plants, Division 1, OM Code: Section IST OM
Code as incorporated by reference in 50.55a for inservice testing of the emergency core cooling system valves, unless relief is granted or an alternative is authorized by the NRC. The NRCs review and findings related to the IAB
valve are documented in safety evaluation report Chapter 3, Design of Structures, Components, Equipment, and Systems, Section 3.9.6, Functional Design, Qualification, and Inservice Testing Programs for Pumps, Valves, and Dynamic Restraints. These findings show that the NRC regulatory requirements and DCD Part 2, Tier 2
provisions provide reasonable assurance that the emergency core system valve system will be capable of performing its design-basis functions in light of the safety significance of the required opening and closing pressures for the individual IAB valves.
Further, Chapter 15, Transient and Accident Analyses, Section 15.0.0.5, Limiting Single Failures, of the safety evaluation report states that the IAB
valve is a first-of-a-kind, safetysignificant, active component integral to the NuScale emergency core cooling system. NuScale does not apply the single failure criterion to the IAB valve, and the Commission directed the staff in SRMSECY190036, Staff RequirementsSECY190036
Application of the Single Failure Criterion to NuScale Power LLCs Inadvertent Actuation Block Valves, ADAMS Accession No. ML19183A408
to review Chapter 15 of the NuScale Design Certification Application without assuming a single active failure of the inadvertent actuation block valve to close. The Commission further stated that this approach is consistent with the Commissions safety goal policy and associated core damage and large release frequency goals and existing Commission direction on the use of risk-informed decision-making, as articulated in the 1995 Policy Statement
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