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Kairos Power LLC, Presentation Slides for the April 18, 2023 ACRS Kairos Power Subcommittee Meeting
	ML23102A240
Person / Time
Site:	99902069, Hermes File:Kairos Power icon.png
Issue date:	04/18/2023
From:	; Kairos Power
To:	; Office of Nuclear Reactor Regulation
Shared Package
ML23102A238	List: ML23102A239; ML23102A240; ML23102A241;
References
	KP-NRC-2304-004
	Download: ML23102A240 (1)
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KP-NRC-2304-004 Presentation Slides for the April 18, 2023 ACRS Kairos Power Subcommittee Meeting (Non-Proprietary)

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

JORDAN HAGAMAN - DIRECTOR OF RELIABILITY ENGINEERING AND QUALITY ASSURANCE ACRS KAIROS POWER SUBCOMMITTEE MEETING APRIL 18, 2023 Hermes PSAR 12.9 Quality Assurance 1

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12.9 Quality Assurance

10 CFR 50.34 (a)(7) A description of the quality assurance program to be applied to the design, fabrication, construction, and testing of the structures, systems, and components of the facility.

The Quality Assurance Program Description (QAPD) for the design, construction, and operation of the Hermes reactor is based on ANSI/ANS 15.8-1995 (R2005), Quality Assurance Program Requirements for Research Reactors Endorsed by NRC Regulatory Guide 2.5, Quality Assurance Program Requirements for Research and Test Reactors (RG 2.5) 2

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Quality Assurance Program Description

The Hermes QAPD applies to design-phase, construction-phase, and operations-phase activities affecting the quality and performance of safety-related structures, systems, and components (SSCs).

Safety-related SSCs within the scope of the Hermes QAPD are identified by design documents.

Technical aspects are considered when determining program applicability including, as appropriate, the SSCs design safety function.

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Quality Assurance Program Description

The Hermes QAPD includes discussion of eighteen design, construction, and modifications program elements:

4 Organization Quality Assurance Program Design Control Procurement Document Control Procedures, Instructions, and Drawings Document Control Control of Purchased Items and Services Identification and Control of Items Control of Special Processes Inspections Test Control Control of Measuring and Test Equipment Handling, Storage, and Shipping Inspection, Test, and Operating Status Control of Non-Conforming Items and Services Corrective Actions Quality Records Assessments

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DR. MATTHEW DENMAN - DISTINGUISHED ENGINEER, RELIABILITY ACRS KAIROS POWER SUBCOMMITTEE MEETING APRIL 18, 2023 Hermes PSAR Chapter 13 Accident Analysis 1

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Safety Case Summary

10 CFR 50.34(a)(4) requires a preliminary safety analysis to assess the risk to public health and safety from operation of the facility, including determination of the margins of safety

To demonstrate compliance with 10 CFR 100.11 dose reference values, a Maximum Hypothetical Accident (MHA) that bounds the postulated events is analyzed for dose consequences by challenging the performance of functional containment The Hermes MHA approach is consistent with guidance in NUREG-1537 The Hermes MHA is not physical The Hermes MHA includes conservatisms that maximize source term The Hermes MHA includes a postulated release of radionuclides

To ensure that the postulated events are bounded by the MHA:

The list of postulated events is comprehensive to ensure that any event with the potential for significant radiological consequences has been considered Initiating events and scenarios are grouped, so that a limiting case for each group can be qualitatively described in CPA (quantitative results will be provided with OLA)

Acceptance criteria are provided for the important figures of merit in each postulated event group to ensure that the potential consequences of that event group remain bounded by the MHA as the design progresses Prevention of an event initiator is justified in the PSAR 2

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Relationship between Dose Limits, the Maximum Hypothetical Accident, and Postulated Events

The Maximum Hypothetical Accident (MHA) is constructed to:

Be conservatively non-physical to overestimate potential off-site dose consequences Provide confidence that sufficient safety margin exists Ensure that reasonable design constraints will result in bounded postulated event doses

At the PSAR stage, only the MHA dose is:

Quantitatively evaluated Needed to ensure that sufficient margin exists to 10 CFR 100.11 dose reference values 3

100.11(a)(1-2) Reference Values EAB/LPZ Dose (relative)

MHA Dose Potential Postulated Event Doses Margin To Dose Ref.

Values Hypothetical Conservatism Design Basis Conservatism

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Maximum Hypothetical Accident: Narrative (1 of 2)

The shutdown and heat removal systems are assumed to perform their safety functions but are not modeled. Instead, hypothetical temperature curves are used to conservatively drive radionuclide movement through the functional containment. Individual release pathways are discussed on the next slide.

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Maximum Hypothetical Accident: Narrative (2 of 2)

Radionuclides are postulated to diffuse from TRISO particles The distribution of TRISO particles account for both manufacturing defects and in-service failures Pre-transient diffusion of radionuclides from the fuel kernels are hypothetically and conservatively not modeled to maximize fuel inventory for release

Radionuclides are postulated to evaporate and degas from the Flibe driven by conservative natural convection boundary conditions. No holdup of gases in Flibe is credited.

Tritium is conservatively assessed to maximize both its inventory and release The initial inventory of tritium is conservatively assessed The release of tritium is conservatively postulated to:

desorb from in-vessel graphite as a function of temperature instantaneously release from both steel and Flibe

The Ar-41 inventory that is held up by closed graphite pores is instantaneously released 5

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MHA: Methodology (1 of 3)

The Hermes MHA uses the methodology from the approved KP-FHR Mechanistic Source Term Methodology Topical (KP-TR-012-P-A). The following concepts directly leverage the topical report:

Radionuclide grouping and transport approaches for the TRISO Fuel and Flibe coolant

Mass transfer correlations for tritium into graphite reflectors and pebbles

The gas space is not credited for confinement of the radionuclides that release from the Flibe free-surface

Two-hour holdup assumptions for radionuclides transporting through the reactor building

Conservative, unfiltered, ground level releases are modeled to maximize offsite doses 6

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MHA: Methodology (2 of 3)

The following non-physical conditions provide additional hypothetical challenges to the functional containment (beyond what is described in KP-TR-012-P-A):

Prescribed hypothetical temperature histories are applied to the transient. This ensures that the MHA will bound the system temperatures from the postulated event groups.

Pre-transient diffusion of radionuclides from the fuel in the reactor core is neglected.

This ensures that the maximum inventory is available for release at the initiation of the transient.

A bounding vessel void fraction is assumed to facilitate the release of low volatility species in the vessel via bubble burst.

Additional conservatism in tritium modeling to address limitations associated with tritium modeling in graphite is described in KP-TR-012-P-A.

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MHA: Methodology (3 of 3) 1.

Identify and account for the sources of material at risk (MAR) and the barriers to release 2.

Evaluate release fractions for every combination of barrier, radionuclide group, and time interval 3.

RADTRAD and ARCON evaluate dose consequences at the exclusion area boundary (EAB) and the low population zone (LPZ)

Fuel kernels Circulating activity Structural MAR Tritium Argon-41 Flibe Gas space Graphite grains for non-Flibe tritium TRISO layers Graphite pores Sources Barriers Legend:

8 Three sources of MAR and associated release barriers

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Maximum Hypothetical Accident:

Sources of MAR (1 of 2)

1. TRISO Fuel

Serpent 2 evaluation provides fuel inventory

Pre-transient depletion of radionuclides from the fuel is neglected to maximize inventory available for release

2. Circulating Activity

Bounding value of circulating activity is assumed in the analysis

Expected to be a variable controlled by technical specification 9

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Maximum Hypothetical Accident:

Sources of MAR (2 of 2) 3.

Structural (steel, reflector, pebbles)

Tritium

The inventory conservatively bounds the operating lifetime at full capacity factor with margin while accounting for differential uptake rates for pebbles and reflector

Transfer from Flibe to structures

Born in the Flibe but transferred to and sorbed in structures (primarily graphite)

Transport speciation is conservatively assigned as tritium fluoride to maximize tritium sorption

Transfer from Flibe to structures determined by mass transfer coefficients from Flibe flow characteristics

Sorption within structures

Sorbed solely as a function of mass transfer from the Flibe to structures (i.e., no diffusion resistance)

Retained without modeling steady state release mechanisms (i.e., perfect absorber)

Argon-41

Produced via neutron activation of Ar-40 to Ar-41

The inventory available for release consists of Ar-41 contained within the graphites closed porosity 10

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Maximum Hypothetical Accident:

Release models for the TRISO Fuel

Transport through TRISO fuel layers is modeled using Ficks laws of diffusion The CORSOR model is used for kernel diffusion IAEA correlations are used for layer diffusion

Diffusion is driven by the fuels hypothetical temperature curve

Transient diffusion of fission products:

Is negligible if even a single PyC layer remains intact Total releases are thus dominated by releases from exposed kernels 11 Fuel Fuel Fuel Particle

< 1 mm diameter

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Maximum Hypothetical Accident:

Release models for the Flibe Coolant

Flibe provides a secondary functional containment barrier to:

Bounding circulating activity In-transient release of fission products from TRISO

Two release mechanisms are modeled for Flibe Bubble burst Evaporation Impurities

Certain radionuclide groups bypass the Flibes functional containment No credit for gas retention High volatility noble metals evaporate near instantaneously 12

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Maximum Hypothetical Accident:

Release models for the Structural MAR

Tritium in graphite grains No-holdup of tritium in the Flibe instantly drops the concentration of tritium outside of graphite grains drops to zero Tritium rapidly diffuses out of the graphite grain due to the non-physical concentration gradient

MAR outside of graphite grains (e.g., steel, pores) are instantly released at the start of the transient Reflectors Pebbles 13

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Maximum Hypothetical Accident:

Release models for the Gas/Atmospheric Transport

RADTRAD:

Input: Mobilized material-at-risk activities Depletion mechanisms Radioactive decay Aerosol settling (i.e., Henry correlation)

Leakage rates (two-hour holdup)

ARCON96:

Inputs Hourly Meteorological Data Distance from the reactor building to the following areas:

Exclusion area boundary Low population zone Approved values from KP-TR-012 Outputs Time averaged dispersion values 14 Conservative and Prescriptive Values ARCON96 Source Reactor Building Reactor Building to Environment Environment RADTRAD ARCON96

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Maximum Hypothetical Accident:

Dose Consequences Location and Duration Whole Body Dose (rem)

Thyroid Dose (rem) 10 CFR 100 MHA Result 10 CFR 100 MHA Result Exclusion Area Boundary (First 2 hrs at 250m) 25 0.227 300 0.235 Low Population Zone (30 days at 800m) 25 0.059 300 0.081 Dose results meet 10 CFR 100.11 reference values at the EAB and LPZ with significant margin 15

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Summary

The MHA dose consequence results meet the site dose reference values in 10 CFR 100.11(a)(1-2) at the EAB and LPZ with significant margin

The MHA dose is bounding because it employs non-physical conditions that are beyond the design basis 16

ML23102A240

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