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X-Energy, LLC - Graphite Qualification Topical Report - Presubmittal Slides
	ML22270A227
Person / Time
Site:	99902071
Issue date:	09/27/2022
From:	Baylis S, Vechioli L; NRC/NRR/DANU/UAL1, X-Energy
To:	;
	Vechioli L
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	Download: ML22270A227 (17)
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Xe-100 graphite qualification: open session S Baylis xx/xx/2022

Objectives, scope and outline of presentation Objective: introduce X-Energys proposed approach to qualifying graphite for use in the Xe-100 reactor Scope of presentation:

Will discuss graphite material for top and bottom reflectors, inner and outer side reflector. Will not be discussing pebble material.

Will focus on work to qualify the material under ASME III-5. Will not discuss detailed component design, structural performance (e.g., seismic response) or safety analysis.

Outline:

Introduction to graphite - historical context and material structure Graphite response to irradiation Effects of oxidation ASME III-5 code requirements

Introduction to Graphite - Operating Experience

Graphite Led the way into the Nuclear Age!

Graphite was first used in CP-1

Higher temperature gas reactors were designed and built, leading to a great deal of operating experience in the temperature range of interest for the Xe-100

Xe-100 is fortunate to have a team with members who worked on several previous HTGRs.

Introduction to Graphite Graphite has a layered structure where the carbon atoms in each layer are bonded in hexagonal arrays with covalent bonds (effectively graphene)

The layers are bonded to each other by weaker Van der Waals interactions The weak bonds among the layers determine weak shear strength Graphite crystals are anisotropic, as properties depend on the direction of force application Bulk graphite is very porous from the formation process Bulk anisotropy depends on average orientation of crystals IG-11 H-451

Graphite Aging with Irradiation - Crystal Dimensional Change Neutron irradiation displaces atoms and creates vacancies (Frenkel Defects)

Most displaced atoms return to lattice positions, but they may lodge at interstitial sites, vacancies or at grain boundaries At low temperature, when mobility is also low, the distance between the displaced atom and its resulting vacancy is small. Consequently, Wigner energy builds up.

At elevated temperature (>250°C), mobility is high enough The preferential interstitial site is in the weaker Van Der Waals inter-planer area rather than in the covalent-bonded, graphene planes.

Standard model: mobile vacancies aggregate to form line defects and heal, shrinking basal planes. Interstitials aggregate to form new graphene sheets. Swelling in C-axis and shrinkage in A-axis.

Buckle, ruck and tuck model: at low temperature (<250°C),

pinning of dislocations by defects leads to buckling of planes. At higher temperatures, dislocations interact and pile up, leading to folding of planes.

Graphite Aging with Irradiation T

As graphite is irradiated at temperature it initially becomes denser. This is a result of C-axis expansion filling porous voids while A-axis contraction leads to net shrinkage.

Simultaneously, crystal strain leads to new pore generation. When the rates of densification and pore generation are equal the process undergoes Turn-around.

Beyond this point, the graphite swells at an increasing rate A key point, known as nil-swelling, nullity or unity, is reached where the graphite has the same size as when it started.

Code stipulates that up to a 10%-

dimensional change is acceptable.

The curve shifts up and to the left with temperature.

Irradiated Material Properties The processes behind dimensional change drive changes in material properties. Shape of curves depend on irradiation temperature Low dose (~1dpa): rapid increase in elastic modulus, strength and CTE, rapid decrease in thermal conductivity (pinning effects)

Moderate dose: continued increase in elastic modulus and strength, CTE and thermal conductivity fall to plateau High dose (beyond turnaround): modulus and strength begin to fall, further reduction in thermal conductivity Irradiation creep Rate of dimensional change depends on applied load. Difference in strain between stressed and unstressed material referred to as irradiation creep Primary creep: rapid initial strain on similar dose scale to pinning, magnitude roughly equal to elastic strain. Fully recoverable - strain anneals out with further irradiation if load is removed.

Secondary creep: slower, permanent strain Some evidence for greater creep recovery than suggested by simple model, tertiary creep at very high dose, flux dependence etc.

Strong evidence for significant influence of creep strain on CTE. Weaker evidence for small influence of creep strain on elastic modulus.

Graphite oxidation - context In addition to neutron irradiation, the other major physical phenomenon affecting graphite material properties is oxidation The majority of current and historic gas-cooled reactors use(d) CO2 coolant (Magnox, UNGG, AGR): significant radiolytic oxidation in all reactors, maximum graphite temperatures limited by potential for thermal oxidation.

Modern HTGR designs use helium coolant: much lower rates of oxidation in normal operating conditions, allows operation with graphite at higher temperatures.

Potential for limited chronic oxidation by coolant impurities (water, hydrogen, CO, CO2 etc.): will be evaluated but not expected to challenge component integrity.

Acute oxidation: steam and/or air ingress in postulated accident conditions could cause rapid oxidation of graphite.

ASME code recommends simplified, conservative treatment of oxidation (HHA-3141):

Account for strength reduction at >1% weight loss Scope of code does not include simultaneous (chronic) oxidation >1% and irradiation

>0.25dpa (but does include acute oxidation of irradiated material)

Any region with strength reduction >50% is not credited in stress evaluation Any region with weight loss >30% is regarded as removed from geometry

Changes with Oxidation - Chronic vs Acute Uniform Bulk oxidation at lower temperature causes more strength loss than surface-localized oxidation at high temperature considering the same amount of Oxygen.

Oxidation is much faster at higher temperatures, causing more to occur on the surface than the bulk Acute Oxidation at high temperatures leads to faster dimensional change Barring specific data, generally, 10% weight reduction is around 50% strength reduction.

© 2020 X Energy, LLC, all rights reserved 10 Oxidation-from Models to Strength Reduction Estimates 50 % Strength Loss 75 % Strength Loss 30 % Strength Loss Virgin 10 % Mass Loss 20 % Mass Loss 5 % Mass Loss Virgin Calculating an expected oxygen partial pressure, time and temperature allows for an estimate of weight reduction This weight reduction translates to a strength reduction Strength reduction can be considered with structural analysis and PoF calculations.

© 2020 X Energy, LLC, all rights reserved 11 The Code Requirements The main governing code of Concern is the ASME B&PV Code,Section III, Division 5. The code provides standards and guidance on the Qualification of the Material:

Irradiated Material Properties (HHA-2220 and HHA-III-3300)

Qualification Envelope states maximum of 200°C Temperature increments for damage dose and any properties that are temperature dependent.

Properties of interest are:

o Dimensional Change o Strength o Elastic Modulus [Dynamic Youngs Modulus]

o Creep Coefficient [Use appropriately validated and calibrated model for relevant range of dose, temperature and stress]

o Coefficient of Thermal Expansion [in relevant temperature range, including effect of creep strain, HHA 3142.3]

o Thermal Conductivity [in relevant temperature range]

Material being tested for qualification use must be representative (HHA-III-4200)

Historical Data must be shown to be applicable if used (HHA-III-5000)

Reporting Requirements for Material Data Sheets (HHA-II-4000)

Material Specification and use of ASTM standards (HHA-III-1000 and HHA-I-1110)

Etc.

© 2020 X Energy, LLC, all rights reserved 12 The Code Requirements The main governing code of Concern is the ASME B&PV Code,Section III, Division 5. The code provides standards and guidance on the Irradiation Limits:

Irradiation Fluence Limits (HHA-3142.1) o < 0.001 dpa the effects of neutron irradiation are negligible and may be ignored o >0.001 dpa the effect of neutron irradiation on thermal conductivity shall be taken into account o > 0.25 dpa all effects of neutron irradiation (described in HHA-2200) shall be considered, and a viscoelastic analysis applied Stored (Wigner) Energy (HHA-3142.2) for graphite irradiated:

o To significant fluence [> 0.25 dpa]

o And at a temperature < 200°C, Then the effect of stored (Wigner) energy buildup shall be accounted for during thermal transient evaluation.

© 2020 X Energy, LLC, all rights reserved 13 The Code Requirements - Design of Components The main governing code of Concern is the ASME B&PV Code,Section III, Division 5. The code provides standards and guidance on the Design of Graphite Components and Assemblies Graphite Core Components are to be designed such that (HHA-3212):

All mechanical loads that occur shall be transferred to the adjacent load-carrying or supporting structures within allowable limits.

Displacement or deformation of adjacent Graphite Core Component in opposing directions do not cause constraint and thus hinder expansion or shrinkage due to temperature or irradiation.

Changes in the shape of a Graphite Core Component due to irradiation do not adversely affect the stability or functionality of the core assembly.

The compensation of the differential strains inside the Graphite Core Assembly and in the surrounding structures does not lead to stresses exceeding the HHA-3211 limits in the Graphite Core Components.

Movement of blocks and the accumulation of gaps inside the Graphite Core Assembly are within allowable limits.

Changes in shape of the Graphite Core Component due to radiation and temperature effects are within allowable limits and do not affect the function and stability of the core assembly.

Design channels for the gas flow through Graphite Core Components are such that the shielding effect of the graphite internals is within allowable limits.

Grooves, keyways, dowel holes, and other recesses in the blocks are to be blended. The minimum fillet radius shall be five times the maximum grain size as documented in the Mandatory Appendix HHA-II Material Data Sheet.

External edges of Graphite Core Components shall be chamfered.

© 2020 X Energy, LLC, all rights reserved 14 The Code Requirements - Certification The main governing code of Concern is the ASME B&PV Code,Section III, Division 5. The code provides standards and guidance on the Designer and Supplier Certification:

G - certificate (HAB-330):

Designer of Graphite Core Components must have a G-Certificate (HAB-3320). The Designer is responsible for designing the components and assemblies, preparing the Design Specification, Design Report, Design Drawings and Construction Specification GC - certificate (HAB-3430):

Holder is responsible for construction and assembly of components, prepares the Construction Report, tests components (HAB 3420)

G Quality System - certificate (HAB-3800):

Holder is a Graphite Material Organization (GMO), and the certification is provided by ASME or a GC-certificate holder, based on the GMOs quality system program. The holder is responsible for material manufacture, graphite core component machining and installation (HAB-3830)

All machining (HHA-4220), examination (HHA-4230, HHA-5220, HHA-5400, HHA-5500), testing (HHA-4240), packaging (HHA-4250) and installation (HHA-5300) must be done by a responsible party (G-certificate, GC-certificate or G Quality System-Certificate holder)

Graphiteblocksaremerelystackedoneontopoftheotherandcannotbecementedorbondedtogetherinanywaydueto hightemperaturesandneutronirradiation.

Allblockstacksmustconformtothesinglecolumnprinciple.Thismeansthatblocksintheverticalcanonlybesupported byblockdirectlybeneath.Theremaythusbenoareaswherethepotentialcandevelopforbridgingtooccurasthiswill placetheblocksunderatensilestressconditionwhichcouldresultinblockcrackingandultimatelyfailure.

Rockville, MD 20852 x-energy.com @xenergynuclear X Energy, LLC

ML22270A227

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