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NEI 18-04 - RIPB LB LTR Public Meeting 6-7-2021 Final
	ML21187A108
Person / Time
Site:	99902071
Issue date:	06/07/2021
From:	; Nuclear Energy Institute
To:	; Office of Nuclear Reactor Regulation
	Vechioli L
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ML21189A227	List: ML21187A108; ML21189A223;
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Risk-Informed Performance-Based Licensing Basis:

X-energys Approach to NEI 18-04 Implementation U.S. Nuclear Regulatory Commission (NRC) Public Meeting Travis Chapman and Steve Vaughn, Licensing Team, X-energy 07 June 2021

Agenda

Objectives

Overall Approach

Content

- Initial Set of Licensing Basis Events (LBEs)

- Preliminary Set of Safety Functions and Functional Design Criteria

- Process for Safety Related SSC Classification

- Framework for Defense-in-Depth (DID) Adequacy

Challenges

Path Forward

Questions and Closing Remarks

Objectives

Communicate the X-energy approach to implementation of NEI 18-04 and the development of a Risk-Informed Performance-based (RIPB) Licensing Basis (LB) approach

Receive feedback on the Xe-100 implementation of the NEI 18-04 approach to support design, analysis, and license application development

An approved RIPB LB topical report will provide:

- Stakeholder confidence in implementing a recently endorsed licensing process

- Specific feedback on the approach to implement NEI 18-04 to support licensing bases development

Overall Approach

Given that NEI 18-04 is an iterative and flexible process, the X-energy approach to NEI 18-04 implementation is to:

- not deviate from the guidance

- clarify cases where there is flexibility to ensure a common understanding

- optimize the organization of licensing and design bases results to maximize traceability and effective communication

Leverage the Integrated Decision-making Process (IDP) to allow multidisciplinary teams to identify and iterate a set of LBEs, safety functions/SSC classifications, and functional design criteria while the PRA continues to develop

Implement the Defense-in-Depth adequacy evaluation process early in design to allow effective consideration of changes as the design matures and allow for risk-informed decision-making opportunities within the framework

Licensing Basis Events (LBEs)

Based on the NEI 18-04 guidance, details from the Phase 0 PRA, and review by the Integrated Decision-making Process (IDP) team, an initial set of LBEs has been identified.

LBE totals by type:

- AOOs (11)

- DBEs (16)

- BDBEs (34)

Key internal initiating events (IEs) and associated LBEs initially determined to be the largest risk contributors:

- Steam Generator Tube Rupture (SGTR)

- Small/Medium/Large Helium Depressurizations

LBE list will be updated after Phase 1 PRA effort is complete (fall 2021) and a new F-C Target curve will be populated

DBEHLs are being evaluated for SR SSCs

Safety Functions (RSFs and PSFs)

Per NEI 18-04, a preliminary set of required safety functions (RSFs) and PRA safety functions(PSFs) have been identified.

RSFs for the Xe-100 are derived from IAEA-TECDOC-1570, 2007 Proposal for a Technology-Neutral Safety Approach for New Reactor Designs

PSFs are reflective of the Xe-100 technology-specific design, operational philosophy, and the requirements of ASME/ANS RA-1.4-2021 PRA Standard for Advanced Non-LWR NPPs

Preliminary Xe-100 Safety Functions (RSFs and PSFs)

Control Transport from Core Control Transport from Primary Boundary Control Transport from Reactor Building Control Transport from Site Retain Radionuclides in Fuel Particles Retain Radionuclides in Pebble Fuel Elements Control Heat Removal Control Reactivity Control Water/Steam Ingress PSFs RSFs Higher-level Functions

Preliminary Required Functional Design Criteria (RFDC)

Required Safety Function Preliminary Required Functional Design Criteria 1

Retain Radionuclides in Fuel Particles The reactors in the plant shall be designed, fabricated, and operated in such a manner that radionuclide releases from the fuel to the primary heat transport fluid will not exceed acceptable values.

1.1 Control Reactivity The reactors in the plant shall be designed, fabricated, and operated in such a manner that the inherent nuclear feedback characteristics and the reactivity control systems will ensure that the acceptable fuel performance limits are not exceeded.

1.2 Control Heat Removal The reactor characteristics including the geometry, materials, core power density, internals, and vessel, and the passive cooling pathways from the core to the environment shall be designed, fabricated, and operated in such a manner that the fuel performance limits are not exceeded.

1.3 Control Water/Steam Ingress The reactor systems and structures that prevent or mitigate the ingress of water and steam to the primary system shall be designed, fabricated, and operated in such a manner that core geometry is maintained.

Preliminary Required Safety Function Decomposition

1. Retain Radionuclides in Fuel Particles 1.1. Control Reactivity 1.2. Control Heat Removal 1.3. Control and Mitigate Water/Steam Ingress 1.1.1. Control with Passive Reactivity Feedback 1.1.3. Maintain Geometry for Insertion of Removable Poisons 1.1.2. Reactor Shutdown Capability 1.2.1. Transfer heat from Fuel to Vessel Wall 1.2.3. Transfer Heat from Vessel Wall to UHS 1.2.2. Radiate Heat from Vessel Wall 1.2.4. Maintain Geometry for Conduction and Radiation 1.3.1.

Control Water/Steam Ingress from SG 1.3.2.

Control primary system pressure

Based on the Phase 0 PRA, the initial list of DBEs, and review by the Integrated Decision-making Process (IDP) team.

For each DBE, the IDP team identified:

- The system(s) response to the IE

- The RSFs being satisfied by each system

- The systems engaged in each RSF from a prevention and mitigation perspective

For each RSF (Control Heat Removal, Reactivity, Water/Steam Ingress):

- Determined various sets of SSC Candidates that could provide the RSF across all DBEs

- If a set of SSCs could meet a particular RSF across all DBEs, then they are identified as a potential set of SR SSCs

Structured around Section 5.9.3 of NEI 18-04, IDP Actions to Establish DID Adequacy as informed by NEI 00-04.

- Plant capability DID is adequate Risk margins against F-C Target and cumulative risk targets Role of SSCs in prevention and mitigation SSC classification Independence among design features at each layer of defense

- Plant programmatic DID is adequate Performance targets for reliability and capability Uncertainties are identified with appropriate design margins:

- LBE frequency, plant response, and mechanistic source term Special treatments are sufficient

Managing the iterative workflows between design, safety analysis, and PRA to support NEI 18-04 outcomes and deliverables

DID Adequacy:

- Determining what the terms sufficient, appropriate, adequate, met signify in context

Alignment on what is (and is not) explicitly in the licensing basis (e.g., reliability targets, PRA model)

As the Xe-100 design continues to mature, safety analyses and PRA developments will allow refinements in LBE selection, SSC classification, and DID adequacy

Consider non-reactor sources of radionuclides:

- Fuel handling

- Spent fuel

- Release from other process systems

Stay engaged with TICAP and ARCAP activities

ML21187A108

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