Text

6 - DID & Integrated Risk Evaluation
	ML25280A090
Person / Time
Site:	Kemmerer File:TerraPower icon.png
Issue date:	10/08/2025
From:	Reed Anzalone, Hart M, Hanh Phan; NRC/NRR/DANU/UTB2
To:	;
References
	Download: ML25280A090 (1)
	v • d • e

NRC Review of Kemmerer Unit 1 Construction Permit Application Integrated Risk Evaluation and Defense-in-Depth Hanh Phan, Michelle Hart, Reed Anzalone NRR/DANU/UTB2 ACRS Subcommittee Meeting October 8-9, 2025

Topics

Integrated risk evaluation

Defense-in-depth (DID) adequacy evaluation

Plant capability DID

Programmatic DID

Integrated decision process (IDP) and integrated decision process panel (IDPP) 2

Integrated Risk Evaluation Hanh Phan, Senior Risk and Reliability Analyst, NRR/DANU/UTB2 Michelle Hart, Senior Reactor Engineer, NRR/DANU/UTB2 3

LMP Cumulative Risk Metrics The total mean frequency of exceeding a site boundary dose of 100 mrem from all licensing basis events (LBEs) should not exceed one per plant-year The average individual risk of early fatality within one mile of the exclusion area boundary (EAB) from LBEs, based on mean estimates of frequencies and consequences, should not exceed 5x10-7 per plant-year The average individual risk of latent cancer fatalities within 10 miles of the EAB from all LBEs, based on mean estimates of frequencies and consequences, should not exceed 2x10-6 per plant-year 4

Integrated Plant Performance Assessment

PSAR section 4.1 describes the preliminary integrated risk assessment and comparison to the NEI 18-04 LMP cumulative risk metric targets

Integrated risk assessment included the LMP non-DBA LBEs (AOOs, DBEs, and BDBEs) consistent with guidance in NEI 21-07, as endorsed by RG 1.253

Uses the KU1 PRA which is consistent with the overall sufficiency for use in the LMP process for the PSAR

Departed from LMP guidance by also including PRA other quantified events (OQEs) beyond the LBE frequency cutoff

Provides additional information than the LMP integrated risk assessment envisioned 5

Consequence Analyses for Risk Assessment

Implemented NAT-9391-A TR methodology LBE evaluation model

Event-specific source terms as input

Used consequence analysis computer code to calculate for each source term

Individual total effective dose equivalent (TEDE) at site boundary

Average individual risk of early fatality within 1 mile of the EAB

Average individual risk of latent cancer fatality within 10 miles of the EAB

Site data

Uniform population

No credit for emergency response actions

Generic meteorological data assumed to be conservatively representative of site

Uncertainty including weather trials to determine mean consequences 6

Integrated Risk Assessment Results Total mean frequency, without uncertainties, of exceeding a site boundary dose of 100 mrem (1 mSv) from all AOOs, DBEs, BDBEs, and OQEs 3.12x10-3 per plant-year LMP Target

< 1 per plant-year Mean frequency, without uncertainties, of the cumulative average individual risk of early fatality within 1 mile of the EAB, summed over all AOOs, DBEs, BDBEs, and OQEs 0 per plant-year LMP Target

< 5x10-7 per plant-year Mean frequency, without uncertainties, of the cumulative average individual risk of latent cancer within 10 miles of the EAB, summed over all AOO, DBEs, BDBEs, and OQEs 3.86x10-9 per plant-year LMP Target

< 2x10-6 per plant-year 7

Integrated Risk Assessment Conclusions

The integrated risk results reported in PSAR section 4.1 demonstrate a large margin to each of the LMP cumulative risk metrics

Staff determined that the integrated risk assessment

Included the range of potential radiological release events for the facility, based on internal events, as consistent with the CP stage of review

Used acceptable methods to quantify the consequences

Provided mean risk results to compare to the LMP cumulative risk metrics

PSAR provides a complete, consistent, and integrated summary of the plant risk to public health and safety, given the preliminary design information 8

Defense-in-Depth Adequacy Evaluation Reed Anzalone, Senior Nuclear Engineer, NRR/DANU/UTB2 Hanh Phan, Senior PRA Analyst, NRR/DANU/UTB2 9

NEI 21-07 Considerations

Plant capability DID

Margin to the F-C target curve is maintained for all individual LBEs

Identify functions needed to keep frequency of risk-significant DBEs and BDBEs below 1E-02 and 1E-04 respectively and individual risks below the QHOs (previous discussion)

Sufficient independence between DLs

Assurance against over-reliance on a single feature across DLs

Balance between prevention and mitigation in the DLs

Programmatic DID 10

LBE Margin

PSAR figure 3.5-1 shows margin to F-C target curve visually and tables 4.2-1 and 4.2-2 show margin for risk significant LBEs as identified by both mean and 95th percentile

Staff concluded adequate margin was demonstrated consistent with NEI 21-07 11 PSAR Figure 3.5-1. F-C Chart for LBEs with Uncertainty Bands

Layers of Defense

5 DLs consistent with NEI 18-04 table 5-2

DLs used to ensure multiple independent & diverse functions available to perform FSFs for LBEs by evaluating DLs across initiating event family 12 NEI 18-04 Table 5-2. Guidelines for Establishing the Adequacy of Overall Plant Capability Defense-in-Depth

DL Strategy 13 From TerraPower presentation on I&C DID and diversity presentation 4/2023 (ML23100A225)

DLs - Quantitative Layer Guideline

DL2 functions for risk-significant AOOs identified as necessary to keep DBEs below 1E-02/year

No risk-significant AOOs

DL3 functions for risk-significant DBEs identified as necessary to keep BDBEs below 1E-04/year

DL3-RR1 (primary functional containment boundary), DL3-RR3a (EVHM to reactor head barrier function)

DL4 and 5 functions identified to maintain risks below QHOs

DL4-RR7 (fuel handling building barrier) for RFH-ESWR-2 14 NEI 18-04 Table 5-2. Guidelines for Establishing the Adequacy of Overall Plant Capability Defense-in-Depth

DLs - Independence, Single Feature Reliance,

& Prevention Mitigation Balance

IE families evaluated across DLs to ensure independence, single-feature reliance, and prevention-mitigation balance

Independence established through DL strategy

One LBE has single feature reliance: RRS-ISPL-CN, DBA break of SPS-I that releases tritium accumulated in cold trap

Dose does not exceed DBA criteria even when considering 40 years of accumulated tritium with no containment credited

Prevention-mitigation balance assured by ensuring preventative features (DL1) exist for each IE family 15

Example DL Evaluation for DHP-LOOP Family 16 LBE ID LBE Cat.

Reactivity Control Heat Removal Radionuclide Retention DHP-LOOP-BL AOO Gravity scram (DL3-RC1), control rod driveline scram follow (DL4-RC3), alternative shunt trip (DL4-RC6)

Non-passive IAC (DL2),

passive IAC (DL4-HR1), RAC (DL3-HR4)