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Kairos Power, LLC, Presentation Slides for the March 17, 2026 Pre-Submittal Meeting on Hermes 2 Structural Design Criteria
	ML26070A199
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Site:	Hermes File:Kairos Power icon.png
Issue date:	03/11/2026
From:	Kosbab B, Peebles D, Schumacher N; Kairos Power
To:	; Office of Nuclear Reactor Regulation
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	KP-NRC-2603-002
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KP-NRC-2603-002 Presentation Slides for the March 17, 2026 Pre-Submittal Meeting on Hermes 2 Structural Design Criteria (Non-Proprietary)

Hermes 2 Structural Design Criteria Drew Peebles - Director, License Applications Ben Kosbab - Senior Principal (SGH)

Nick Schumacher - Senior Consulting Engineer (SGH) 17 MARCH 2026

Our mission is to enable the worlds transition to clean energy, while improving peoples quality of life and protecting the environment.

To achieve this mission, we are focused on delivering a technology that is both safe and affordable.

3 HERMES 2 STRUCTURAL DESIGN CRITERIA Agenda

Introductions

Public Session Precast Concrete for Safety-Related Structures Extreme Wind Design Criteria

Closed Session (as needed)

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Precast Concrete for Safety-Related Structures HERMES 2 4

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5 HERMES 2 STRUCTURAL DESIGN CRITERIA Precast Concrete in Hermes 2

As presented in PSAR, the safety-related portion of reactor building is:

3.5.1: A hybrid of cast in place and precast concrete structural elements.

3.5.3: A reinforced concrete structure designed to meet ACI 349-13.

3.5.3: Designed consistent with ASCE 43-19.

Additional details about Hermes 2 design for today's discussion:

Will use emulative precast concrete for safety-related structures per ACI 349-13 and ASCE 43-19.

Will use a structural system with a combination of CIP and precast concrete with emulative detailing.

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6 PRECAST CONCRETE IN HERMES 2 ACI 349-13 Provisions

ACI 349-13 adopts and amends ACI 318-08 provisions.

Specific sections relevant for precast:

Chapter 16 of ACI 349-13 and ACI 318-08 contains provisions for precast concrete.

ACI 349-13 does not modify Chapter 16.

Chapter 21 of ACI 349-13 and 318-08 contains provisions for seismic design of concrete structures.

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7 PRECAST CONCRETE IN HERMES 2 ACI 349-13 Chapter 21

ACI 349-13 Section 21.1.1 prohibits precast concrete structural systems, as defined in ACI 318, for use in the design of lateral-force-resisting systems unless it is demonstrated by experimental evidence and analysis that the proposed system will have strength and ductility equal to or exceeding that provided by permitted systems.

ACI 349-13 R21.1.1 states: The proportioning and detailing requirements in Chapter 21 are based predominantly on field and laboratory experience with monolithic reinforced concrete building structures and precast concrete building structures designed and detailed to behave like monolithic building structures.

Per the NEHRP Recommended Seismic Provisions, precast seismic systems can be defined as either emulative or jointed.

Emulative systems behave like CIP equivalents with the same material properties and dimensions.

ACI 349-13 implicitly permits appropriately detailed emulative precast concrete structural systems because they behave like monolithic reinforced concrete building structures, unlike jointed designs.

Jointed systems behave uniquely and are not considered for Hermes 2.

Defined precast concrete structural systems in ACI 318-08 were jointed only.

The NRC has considered this concept in NUREG/CR-6486, which studied the application of modular construction, including precast concrete, for nuclear SR structures:

"ACI Code 349 can be used as the design/analysis construction basis [for precast construction], provided the precast module meets the requirements which would apply to the corresponding [CIP] concrete structure.

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8 PRECAST CONCRETE IN HERMES 2 ASCE 43-19 Provisions

ASCE 43-19 does not prohibit precast structural walls in the seismic force-resisting system.

The commentary states that the criteria presented in this standard does not discourage the use of any structural system with reliable seismic performance.

Nonductile seismic force-resisting systems are prohibited.

ASCE 43-19 Chapter 6 describes some attributes of a system that can achieve ductile response under seismic loads, including:

a) Connections are designed to be stronger than the weakest connected component to force inelastic deformation into appropriately detailed cross sections.

b) A framing system that is regular in stiffness and strength, with minimal or no plan and/or vertical irregularities, and c) Components supporting gravity loads that are not part of the seismic load path are detailed to retain their strength for seismic displacements expected in design basis and beyond design basis loadings.

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9 PRECAST CONCRETE IN HERMES 2 ASCE 43-19 Provisions

ASCE 43-19 uses limit states to control the expected deformation of SSCs.

Limit State D SSCs are intended to remain elastic.

Ability to impose Limit State D on precast concrete connection components to force inelastic deformation away from connections.

See precedent in ACI 349-13 for diaphragms and foundations, where certain portions (whether CIP or precast) are designed to remain elastic in order to force inelastic behavior first into desirable areas of the structure.

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10 PRECAST CONCRETE IN HERMES 2 Intended Hybrid Concrete Structural System

ACI 550.1 is a published ACI document that provides guidance for detailing precast concrete structures to meet building code requirements for seismic demands by emulating CIP reinforced concrete design.

The NEHRP Recommended Seismic Provisions and ACI 550.1 suggest that emulative detailing can be achieved by one of two methods:

1) Using ductile connections that permit nonlinear action at the connection site.

2) Using strong connections that force nonlinear action away from the connection site.

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11 PRECAST CONCRETE IN HERMES 2 Summary

Consistent with Hermes 2 PSAR, precast concrete will be used for safety-related structures

In implementing this strategy, this presentation summarized how the reactor building design can include pre-cast concrete consistent with industry codes/standards:

ASCE 43-19 includes provisions for the use of emulative precast concrete as a ductile structural lateral force resisting system.

ACI 349-13 includes provisions for emulative precast concrete to be designed and detailed to behave like CIP concrete.

Seeking feedback from the NRC staff if there is any misalignment on these conclusions - that this design approach is consistent with codes and standards.

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13 HERMES 2 STRUCTURAL DESIGN CRITERIA Extreme Wind Design Criteria

Hermes 2 PSAR referenced RG 1.76 and RG 1.221 for tornado and hurricane protection.

Updated design basis for FSAR will use criteria from FEMA P-361 (2024) for tornado and hurricane design of the safety-related superstructure.

KP-FHR relies on functional containment: TRISO fuel and Flibe coolant.

The safety-related superstructure of the building thus is not designed as a physical containment for fission product retention like other reactors, but still provides protection to safety-related SSCs from the effects of natural phenomena and external event hazards.

NUREG-1537 Part 1, Section 3.2 states:

Design criteria for protection from meteorological damage should be identified based on factors such as historical data on maximum wind velocity, vertical velocity profiles, gust factors, applied loads, recurrence intervals, and tornado loadings.

The applicant may refer to local building codes, standards, or other criteria to ensure that significant meteorological damage to the facility is very unlikely.

Further, the design criteria should provide reasonable assurance that potential meteorological damage would not significantly affect designed SSCs (i.e., they would continue to perform necessary operational and safety functions).

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14 EXTREME WIND DESIGN CRITERIA FEMA P-361: Introduction

FEMA P-361 presents design and construction criteria for safe rooms that provide near-absolute protection from wind and wind-borne debris from extreme events, including tornadoes and hurricanes.

Since the original development of FEMA (Federal Emergency Management Agency) guidelines in 1998, with tens of thousands of safe rooms built, there has not been a single reported failure of a safe room constructed to FEMA criteria.

FEMA P-361 builds on knowledge of the Mitigation Assessment Team (MAT) field investigations, research, and technical reports prepared by FEMA and other national and state agencies.

FEMA P-361 uses the International Code Council (ICC) standard, ICC-500, as a reference standard. ICC-500 is referenced by the International Building Code (IBC) where storm shelters are required.

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15 EXTREME WIND DESIGN CRITERIA History of FEMA P-361 and ICC-500

Building code development of tornado design parameters largely started in the 1970s after an EF-5 hit Lubbock, Texas.

Defense Civil Preparedness Agency (DCPA) was a US DoD agency between 1972 and 1979 that oversaw limited research and publications on tornado preparedness and protection.

DCPA functions were absorbed by FEMA in 1979. FEMA developed the 1st edition of P-361, Design and Construction Guidance for Community Shelters in 2000.

In the early 2000s, the ICC partnered with FEMA to form a national committee (ICC-500) to develop a consensus standard to codify the design and construction requirements of extreme wind storm shelters.

ICC-500 was first published in 2008, with updates in 2014 and 2020. In 2009, the IBC first referenced ICC-500. ICC 500-compliant storm shelters are currently required for critical facilities and K-12 schools in certain parts of the country.

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16 EXTREME WIND DESIGN CRITERIA FEMA P-361 Methodology

FEMA P-361 and ICC-500 are similar with respect to design parameters for community safe rooms, with the former referencing the latter.

ICC 500 is a minimum consensus standard adopted by building codes. FEMA P-361 adopts more conservative requirements for federally-funded safe rooms.

FEMA P-361 Part A is for owners/operators to address project planning, economic considerations, and operations and maintenance.

FEMA P-361 Part B parallels ICC-500.

FEMA P-361 safe rooms, and ICC-500 storm shelters, are designed for greater wind speeds (and corresponding wind pressures), and larger and more energetic wind-borne missiles than conventional buildings governed by model building codes.

Hermes 2 will adopt applicable structural design and testing provisions from FEMA P-361 Part B as described in the following slides.

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17 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.3 Load Combinations:

Safe room shall be designed to resist the load combinations specified in ICC 500 Section 302.2 or 302.3 that are adapted from ASCE 7 requirements.

The effects of tornado and hurricane loads will be combined in accordance with ICC 500 Section 302.2 (LRFD) or 302.3 (ASD), and supplemented with normal loads not specified in ICC 500 (e.g. pipe reactions Ro, and thermal effects and loads during normal operations To).

NUREG-1537 does not prescribe load combination requirements for determining demands on SSCs.

The ICC 500 load combinations are storm shelter-specific load combinations that were adapted from ASCE 7. The load combinations account for loading conditions anticipated during the extreme wind incident and include unmodified dead and live loads concurrent with the extreme wind loads. The ICC 500 load combinations bound the extreme environmental load combinations in PSAR (Table 3.5-1).

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18 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.5.1 Design Wind Speeds:

Tornado and hurricane design wind speeds (VT and VH) shall be per Figure B3-1 and B3-2, respectively.

The Hermes 2 design tornado wind speed, VT, is 250 mph; the design hurricane wind speed, VH, is 160 mph.

NUREG 1537 Section 3.2 states that design criteria for protection from meteorological conditions should be based on site characteristics from historical data including tornado loadings.

The proposed tornado design wind speed (250 mph) is ~21% greater than the highest magnitude wind speed recorded within 10 miles of the site (206 mph).

The proposed hurricane wind speed (160 mph) is based on FEMA P-361 Figure B3-2, a 10,000 year mean recurrence interval (MRI) map.

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19 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.5.2 Calculating Wind Loads:

Wind loads for main wind force resisting systems (MWFRS) and components and cladding (C&C) shall be calculated using the application of the ASCE 7-16 with modifications in FEMA P-361 to the site exposure factor, directionality factor, and topographic factor.

The Hermes 2 MWFRS and C&C wind pressures for protective structures will be calculated using ASCE 7-22 (record code for the City of Oak Ridge) with modifications specified in FEMA P-361.

A comparative analysis will be performed to demonstrate that wind pressures from tornadoes will control over wind pressures from hurricanes.

NUREG 1537 does not prescribe a method for calculating wind loads. The guidance states that severe wind loads, e.g., tornado strength including translational speed, rotational speed, and the maximum pressure differential should be considered.

ASCE 7-22 is the governing code for the City of Oak Ridge. ASCE 7 methodology for the computation of extreme wind pressures was described in PSAR.

Modifications in FEMA P-361 include i) the requirement to conservatively use a directionality factor (Kd) of 1.0, and ii) provisions that lead most safe rooms to using an internal pressure coefficient (GCpi) of +/-0.55. Design pressures determined using wind-induced internal and external pressure coefficients are comparable to the pressures determined using a combination of wind-induced external pressure coefficients and atmospheric pressure change (APC)-

induced pressures.

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20 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.6.1 and B3.2.6.2 Debris Hazards - Test Missile Criteria:

Elements of the safe room structure and its components (including windows, doors, and other impact-protective systems) should resist failure from wind pressures and debris impacts. Tornado missile impact criteria are specified in Table B3-3, and hurricane missile impact criteria are specified in Table B3-4.

The Hermes 2 safety-related superstructure will consider tornado and hurricane missile impacts. The design test missile for tornadoes will be a 15-pound 2x4 sawn lumber member traveling 100 mph at vertical surfaces and 67 mph at horizontal surfaces. The effects of the tornado missile will control over the effects of the hurricane missile (9-lb 2x4 traveling 80 mph at vertical surfaces and 16 mph at horizontal surfaces).

NUREG 1537 Section 3.2 that the applicant may refer to local building codes, standards, or other criteria to ensure that significant meteorological damage to the facility is very unlikely. Further, the design criteria should provide reasonable assurance that potential meteorological damage would not significantly affect designed structures, systems, and components.

FEMA P-361 selected a 15 lb 2x4 sawn lumber as the representative missile for the 1st Edition (2000), which was subsequently adopted by ICC-500, a consensus standard regulated by the International Code Council. The missile material, size, and weight are based on observational research at Texas Tech in the 1970s based on the missile to cause the most damage in actual tornadoes, and which are most likely to get lifted and thrown long distances at high speeds. The impact speeds were subsequently validated by computer simulations of tornado missile trajectories at Texas Tech (McDonald, 1999). FEMA P-361 and ICC-500 criteria have evolved since the initial issuances in the early 2000s; however, the reconnaissance damage assessments (FEMA MATs) have substantiated that the 2x4 missile spectrum continues to be representative of damaging tornado-borne debris hazards. The FEMA P-361 missile also bounds other rod-and-plate-type missiles that have aerodynamics conducive to lift. Building system performance when subjected to projectile impact is confirmed via testing to provide confidence in their protection capability (FEMA P-361 Section B3.2.7 and B8).

FEMA P-361 addresses other extreme wind-induced physical impact hazards (besides the class represented by 2x4 projectiles) by laydown, falling debris, and rollover hazard criteria in Section B3.2.6.5 (see section below).

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21 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.6.5 Laydown, Falling Debris, and Rollover Hazards:

Design professionals are required to quantify the loading from laydown and falling debris hazards where the safe room is within the radius of the hazard. Quantifying the impact load of either must include a minimum impact factor of 2.0 times the weight of the identified hazard.

Where multiple impact loads have been determined, they may be considered one at a time, but each should be applied to the roof of the safe room simultaneously with all other applicable uniform live loads.

The Hermes 2 safety-related superstructure will consider laydown and falling debris hazards in accordance with FEMA P-361 Section B3.2.6.5.

NUREG 1537 Section 3.2 states that the applicant may refer to local building codes, standards, or other criteria to ensure that significant meteorological damage to the facility is very unlikely. Further, the design criteria should provide reasonable assurance that potential meteorological damage would not significantly affect designed structures, systems, and components.

The FEMA P-361 laydown, falling debris, and rollover hazard provisions require considering project-specific extreme wind-induced hazards (which are present at the site and could credibly affect the components requiring their function be maintained) other than the representative missile in Section B3.2.6.1 and B3.2.6.2. These include laydown and falling of adjacent, taller (or elevated) building elements, and rollover of vehicles and small buildings.

For Hermes 2, the impact of elements of the high-bay non-safety-related roof enclosure (i.e., host building) on the safety-related maintenance deck is one example of a laydown hazard that will be considered. Rollover hazards would include tumbling vehicles along grade.

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22 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B3.2.7 Envelope Component Testing and Design and B8 Test Methods for Impact and Pressure Testing:

All safe room envelope components (e.g., impact protective systems and wall, roof, and door assemblies) should successfully pass the component-specific testing requirements set forth in ICC 500 Section 306.

When applicable, impact and pressure testing should be conducted in accordance with the provisions of ICC 500 Chapter 8.

The Hermes 2 safety-related superstructure will comply with FEMA P-361 Section B3.2.7 and B8.

Published missile impact tests (i.e.,

pre-qualified building assemblies) can be used to demonstrate adequacy of the protection boundary, rather than Kairos-performed local and global design analysis.

NUREG 1537 states the applicant may refer to local building codes, standards, or other criteria to ensure that significant meteorological damage to the facility is very unlikely.

FEMA P-361 requires all components of the envelope to be tested for impact following the test procedures in Section B8. The test assemblies must accurately replicate the proposed assembly. The impact testing follows the provisions of Section 11.1 through 11.3 of ASTM E1886. The test protocol considers criteria pertaining to the impact test apparatus, calibration, impact procedure, test missile properties and speed, test temperature, impact angle, impact locations (e.g., center, edge, or corner of specimen),

number of impacts, and pass/fail criteria. The pass/fail criteria considers perforation, dislodgement and disengagement, spalling, permanent deformation, and maximum deflection. Successful performance testing provides reasonable assurance that damage from such wind-induced loading is very unlikely.

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23 EXTREME WIND DESIGN CRITERIA Hermes 2 Extreme Wind and Missile Criteria FEMA P-361 Section Application to Hermes 2 Technical Justification B7.2.1 Protection of Critical Support Systems:

Critical support systems should resist the same design wind pressures and wind-borne debris as the safe room they serve and should remain functional for at least as long as the minimum period of safe room occupancy for the designated safe room storm type.

Hermes 2 structures that protect SSCs performing a safety function will be designed for the same wind pressures and wind-borne debris specified in FEMA P-361 B3.

FEMA P-361 Section B7.2.1 includes design considerations to ensure continuous operation of essential equipment and other support systems during and after an extreme wind event. Applying the FEMA P-361 specified extreme wind pressures, wind-borne debris, and functionality requirements provides reasonable assurance that SSCs performing a safety function will be adequately protected.

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24 EXTREME WIND DESIGN CRITERIA Summary

The Hermes 2 reactor building will still be designed to the principle design criteria described in the PSAR. However, the industry code being used for the detailed extreme wind design criteria is being updated to FEMA-P-361.

This update is consistent with the guidance in NUREG 1537 that applicants should use criteria to ensure that significant meteorological damage to the facility is very unlikely.

The technical justification for the sufficiency of the FEMA-P-361 standard being used for this application is provided in this presentation.

Seeking feedback from the NRC staff if there is any misalignment on these conclusions - that this design approach is consistent with the guidance in NUREG 1537.

No Reproduction or Distribution Without Express Written Permission of Kairos Power LLC.

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