ML25280A087

3 - NRC Review of LBE Identification and Scope

ML25280A087
Person / Time
Site:	Kemmerer File:TerraPower icon.png
Issue date:	10/08/2025
From:	Reed Anzalone, Hart M, Hanh Phan, Radel T NRC/NRR/DANU/UTB2
To:
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Download: ML25280A087 (1)

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NRC Review of Kemmerer Unit 1 Construction Permit Application LBE Identification and Scope Reed Anzalone, Hanh Phan, Michelle Hart, Tracy Radel NRR/DANU/UTB2 ACRS Subcommittee Meeting October 8-9, 2025

Topics

• LBE identification process

• Hazards in scope for KU1 LMP implementation

• Internal events PRA - Addressed in Chapter 3 safety evaluation and presented during Chapter 3 presentation

• Other hazards assessment - Internal, external, and coexistent hazards, screened and dispositioned consistent with staff guidance

• LBEs included in the PSAR

• Initiating event families

• LBE categorization

• Derivation of DBAs from DBEs 2

Context in LMP Framework

• This presentation covers tasks 1, 3, 4, and 6 from NEI 18-04 section 3

• Tasks 5 and 7 are discussed in the context of safety classification and safety analysis

• Task 2 was evaluated to the extent necessary to ensure adequate safety analysis and design information 3

NEI 18-04 Figure 3-2. Process for Selecting and Evaluating Licensing Basis Events

NEI 21-07 and PSAR Content NEI 21-07

• NEI 21-07: PSAR should identify deviations from the NEI 18-04 process, describe how the event sequence families were grouped to define the LBEs, and summarize the role of the PRA and risk insights in confirming completeness and classification of LBEs.

PSAR & Supporting Calculations

• PSAR section 3.4 states NEI 18-04 process was used without deviation

• PSAR sections 3.5-3.9 describe LBEs and categorization

• PSAR section 3.1 describes PRA and risk insights

• Supporting detail provided in NAT-4767, LBE Selection Report, Rev. 2, and NAT-7127, Natrium PRA Initiating Events Analysis, Rev A which staff audited 4

LBE Identification

• Candidate list of IEs identified from conceptual design of plant and literature review

• USO cast a wide net in identifying IEs from literature as applicable

• Grouped and refined through PRA and LMP - potentially several event sequence families for each LBE

• Noteworthy omissions (as identified elsewhere in literature):

• Events not included in CP-stage PRA

• Fire - addressed by DBHLs at CP stage

• Seismic - addressed by DBHLs at CP stage + seismic DBA

• Vessel leak - currently OQE, will be analyzed as BDBE or DBE at OL

• Core disruptive accidents 5

A Note on Core Disruptive Accidents

• Historically considered for SFRs; not initially included in PRISM PSID based on initial PRA but NRC requested it to be analyzed as a BDBE

• No reasonable events provide conditions necessary for CDA to occur

• Several reactivity events (control rod withdrawal, seismic DBA) and severe loss of flow/heat sink transients considered in CPA

• No LBEs or OQEs result in coolant boiling or fuel melt

• Some event sequences retained in PRA at very low frequency (all cutsets sum to ~1E-9) are assumed to result in local (not bulk) coolant boiling and melt fuel

• Fuel melt CDA

• TREAT testing of metallic fuel to significant overpower conditions (>4x normal power) demonstrated molten metallic fuel exits top of fuel assembly and travels upward with coolant rather than consolidating into a single mass

• Same test could not cause bulk melt of U-10Zr 6

Hazards Assessment - Overview A.

Hazards Identification Comprehensive list of potential hazards developed for KU1 Informed by PRISM analysis and supplemented with generic database B. Screening Analysis Qualitative: per ASME/ANS non-LWR PRA standard criteria Quantitative: risk-informed, frequency-based thresholds C. Hazards Considered for Evaluation Hazards carried forward to PRA Hazards addressed under DBHL framework D. Conclusion 7

A. Hazards Identification

• Documented in NAT-8294 Natrium PRA Screening of External Hazards

• 125 potential hazards initially identified for KU1

• Based on:

• PRISM Analysis

• Historical hazard data

• Site-specific characteristics and regional hazard databases

• Hazards list further reviewed for:

• Completeness - covering internal, external, and coexistent hazards

• Applicability - relevance to KU1 site and current design features 8

B. Hazards Screening (Qualitative)

• Screening criteria - ASME/ANS Non-LWR PRA Standard (SR HS-B5):

(a) Cannot physically impact plant or operations (b) No potential to cause trip/shutdown (manual or automatic)

(c) Covered within another hazard definition (d) Less severe than another hazard with higher frequency (e) Slow-developing; sufficient time for response or mitigation (f) Maximum consequences 10% of background radiation dose

• Each hazard evaluated against criteria using engineering judgment, operating experience, and available site data

• 98 hazards screened out qualitatively

• Remaining hazards subject to further quantitative screening 9

B. Hazards Screening (Quantitative)

• Absolute risk contribution evaluated against risk significance criteria

• Event sequence families subject to screening must:

Have mean occurrence frequency < 1E-7/plant-year (conservative analysis)

Ensure aggregate risk contribution < 1% of cumulative PRA risk targets

• Eight hazards screened out quantitatively:

1.

Small volcano 2.

Truck explosion (onsite/offsite transport risk) 3.

Meteorite impact 4.

Orbital debris 5.

Excavation work onsite 6.

Heavy transportation accidents 7.

Vehicle accident (onsite traffic) 8.

Vehicle explosion 10

C. Hazards Considered for Further Evaluation

• Hazards not screened out progress to PRA modeling and evaluation

• Per RG 1.253, these hazards (excluding internal events) can be deferred until OL-stage PRA

• During CP stage, these hazards addressed under DBHL framework and presented in Chapter 6 of KU1 PSAR and SE

• Screened-in hazards (currently addressed under DBHL framework):

• Internal hazards:

Internal fire (including sodium fire, cask transporter fire, electrical faults)

Explosions (fuel, hydrogen)

Internal floods, pipe rupture effects (pipe whip, spray, jets)

• External hazards:

Extreme winds, tornadoes, cyclones, and high/low pressure events Seismic hazards (including soil liquefaction)

• 27 coexistent hazards evaluated and dispositioned 11

D. Conclusion

• KU1 hazards assessment is reasonable for CP application and consistent with LMP guidance

• Staff expectations for USO:

• Validate all assumptions during OL stage

• Reperform screening using final plant design & site-specific data

• Develop all-hazards PRA at OL stage, integrating applicable:

• Internal events

• External events

• Coexistent hazards 12

PSAR LBEs 13 7 Types

• Loss of primary flow

• Increase or decrease in heat removal

• Reactivity

• Release from ex-vessel systems

• Fuel handling

• Local fuel faults

• Other 33 IE families

• Loss of primary sodium pump (DHP-L1PP)

• Loss of all primary sodium pumps (DHP-LAPP)

• Loss of offsite power (DHP-LOOP)

• Loss of offsite power while at low power (SUD-LOOP)

• Loss of a single medium voltage AC bus (OTH-LMAC) 85 non-DBA LBEs

• DHP-L1PP-BL

• DHP-L1PP-1

• DHP-L1PP-2

• DHP-L1PP-3

• DHP-L1PP-4 21 DBAs

• DHP-L1PP-CN

LBE Categorization

• NEI 18-04

• AOOs: Fmean > 1E-02

• DBEs: 1E-02 > Fmean > 1E-04

• BDBEs: 1E-04 > Fmean, F95 > 5E-07

• USO followed NEI 18-04 except for identifying BDBEs based on mean instead of 95th percentile

• Would result in two additional BDBEs, both of which were classified as OQEs 14

Derivation of DBAs from DBEs

• Based on NEI 18-04, need to derive DBAs from DBEs and AOOs/BDBEs whose uncertainty bands go into the DBE region

• USO did not do second step, but committed to performing for OL

• 17 AOOs and BDBEs extend into DBE region

• 4 have bounding DBAs in IE family, 1 is bounded by other DBAs

• 5 do not have relevant DBAs but are bounded by other events crediting only SR controls that meet FC target

• Rest dispositioned by staff in SE

• 2 may result in design changes for OL 15

Acronyms 16 ACRS - Advisory Committee on Reactor Safeguards ANL - Argonne National Lab ANS - American Nuclear Society AOO - Anticipated Operational Occurrence ASME - American Society of Mechanical Engineers ARCAP - advanced reactor content of application BDBE - Beyond Design Basis Event CDA - Core Disruptive Accident CFR - Code of Federal Regulations CP - Construction Permit CPA - Construction Permit Application DBA - Design Basis Accident DBE - Design Basis Event DBHL - Design Basis Hazard Level DID - Defense In Depth EAB - Exclusion Area Boundary F-C - Frequency-Consequence PSAR - Preliminary Safety Analysis Report PSID - Preliminary Safety Information Document PSF - PRA Safety Functions QHO - Quantitative Health Objective RG - Regulatory Guide RSF - Required Safety Functions SE - Safety Evaluation SFR - Sodium Fast Reactor SR - Safety Related SRDC - Safety Related Design Criteria SSC - Structures, Systems, and Components TICAP - technology inclusive content of application TREAT - Transient Reactor Test Facility USO - US SFR Owner IDP - Integrated Decision-making Process IDPP - Integrated Decision-making Process Panel IE - Initiating Event KU1 - Kemmerer Unit 1 LBE - Licensing Basis Event LMP - Licensing Modernization Project LPZ - Low Population Zone LWR - Light Water Reactor NEI - Nuclear Energy Institute NRC - Nuclear Regulatory Commission NRR - Office of Nuclear Reactor Regulation NSRST - Non-safety-related with Special Treatment NST - No Special Treatment OL - Operating License OQE - Other Quantified Events PRA - Probabilistic Risk Assessment PRISM - Power Reactor Innovative Small Module