development of an asme code for graphite core components · pdf fileconstruction of graphite...

Development of ASME Rules for the

design and construction of Graphite

Core Components

Technical Meeting on High-Temperature Qualification of

High-Temperature Gas Cooled Reactor Materials

IAEA,Vienna

Jun 10-13, 2014

Tim Burchell, BPC-III Working

Group Graphite & Composite Design

Contents

• Present some information on the rules for the design and construction of graphite core components of a High Temperature Gas Cooled Reactor.

• Contents:

– HTRs and their Graphite Core Components (GCC)

– Progress and structure of the code

– proposed criteria for the design of Graphite Core Components

• Introduction (GCC In Safety Case)

• Modes of failure, Stress categories and Stress Limits

• Design Margin Comparison

• Verification of methods

• Graphite data mandated by the code

• Conclusions

Graphite Core Components – Prism Type

HTR (HTTR)

Graphite Core Components – Pebble Type

HTR (PBMR)

Overview of the code documents • Subpart HAB: General

Requirements for Graphite Core Components

– HAB-1000 Introduction

– HAB-2000 Classification of Graphite Core Components

– HAB-3000 Responsibilities and Duties

– HAB-4000 Quality Assurance

– HAB-5000 Authorized Inspection

– HAB-7000 Reference Standards

– HAB-8000 Certificates and Data Reports

• Subpart HHA: Graphite Materials – HHA-1000 Introduction

– HHA-2000 Materials

– HHA-3000 Design

– HHA-4000 Machining

– HHA-5000 Installation and Examination

– HHA-8000 Nameplates, Stamping and Reports

– Appendix HHA-I Graphite Material Specifications

– Appendix HHA-II Requirements for creation of a material datasheet

• HHA-II-2000 Materials data sheet forms

• HHA-II 3000 Detailed Requirements for the derivation of the materials data sheet - as-manufactured properties

• HHA-II-4000 Detailed Requirements for the derivation of the materials data sheet – irradiated materials properties

– Appendix HHA-III Requirements for Generation of Design Data for Graphite Grades

ARTICLE 2000: MATERIAL DATA NEEDA

7

Application of the materials Data to the ASME B&PV Code, Division III, Sect 5:HTRs, Rules

for Construction of Nuclear Facility Components

Sub Section HH, Class A Nonmetallic Core Support Structures

Subpart A, GRAPHITE

HHA-2220 IRRADIATED MATERIALS PROPERTIES

The Materials Data Sheet shall include the properties specified in Mandatory Appendix HHA-

II. Fast neutron irradiation effects on the following properties shall be required for

compliance with this Subpart:

1) Dimensional change

2) Creep coefficient

3) Coefficient of thermal expansion

4) Strength

5) Thermal conductivity

6) Elastic modulus

(b) The magnitude of the material property change depends on the damage dose and

irradiation temperature. The damage dose and temperature range for the measurements shall

cover the qualification envelope range of HHA-2131(a), or as required in the application of the

graphite grade in the Graphite Core Assembly

Graphite irradiation limits

8

HHA-3142 Irradiation Effects

Neutron Fast Fluence

Limit

(DPA)

Condition Design Analysis Requirement

<0.001 Unirradiated Simplified

>0.001 Irradiated Effect on thermal

conductivity shall be

considered (thermal

stress and stress

gradients)

>0.25 Irradiated Full viscoelastic

analysis.

Full effects of neutron

irradiation (HHA-2220)

Graphite analysis and life limits

9

HHA 3142.3 Internal Stresses Due to Irradiation

– The internal stresses in a graphite core component [that exceed the dose limits described in

HHA-3142.1(c)] shall be calculated. This calculation shall be completed by viscoelastic

modeling of the material behavior.

– (a) Irradiation induced property changes creep and changes in properties (elastic modulus,

CTE, thermal conductivity) shall be accounted for in this analysis. The interaction between

irradiation creep and the CTE shall be included in this assessment.

– (b) The analysis shall account for stress concentrations resulting from Graphite Core

Component geometry.

– (c) The stress analysis shall account for superposition of stresses resulting from all of the

loads that a Graphite Core Component is exposed to simultaneously.

HHA-3142.4 Graphite Cohesive Life Limit

– A temperature-dependent cohesive life limit is to be defined for the graphite grade used for

the Graphite Core Components. Material that exceed this life limit is considered to provide

no contribution to the structural performance (stiffness and strength) of the Graphite Core

Component. This fluence limit shall be set to the fluence at which the material experiences a

+10% linear dimensional change in the with-grain direction. For full assessment (HHA-

3230) this material shall not be included in the volume of the Graphite Core Component

assessed.

HHA-3000: Design

• Article HHA-3000: BPV Section III Div 5

HTRs, HHA Code on Graphite Core

Components (BPV III)

• Status:

– On the.

– Articles 4000, 5000 and 6000 being revise to

notmalize the code.

Major Design Code Issues

• Selected design methodology – Probabilistic:

– Design margin related to material uncertainty.

– Defined in Appendix 3 Material Qualification, not prescribed in the code

• Core Component vs. assembly design, catering for damage tolerance assessment.

– Designer selection and classification of parts for structural reliability.

• Design for effect of environmental effects over operating life (Irradiation, Oxidation)

HHA-3000: TABLE OF CONTENTS HHA-3100 GENERAL DESIGN

HHA-3110 PART CLASSIFICATION

HHA-3120 Loading Criteria (Design & Service loadings)

HHA-3130 SPECIAL CONSIDERATIONS HHA-3131 Oxidation

HHA-3132 Irradiation Effects

HHA-3133 Abrasion and Erosion

HHA-3134 Fatigue

HHA-3135 Compressive Load

HHA-3200 DESIGN BY ANALYSIS HHA-3210 DESIGN CRITERIA

HHA-3211 Requirements for acceptability

HHA-3212 Basis for Determining Stresses

HHA-3214 Stress Analysis

HHA-3220 Stress Limits for GCC – Simplified Assessment (Stress intensity based limits. These are related to the failure probability limits in the next section.)

HHA-3230 Probability of Failure Limits for GCC – Full Assessment (Failure probability limits and how to assess to them.)

HHA-3240 Experimental limits - Design by test (For both static and fatigue strength.)

HHA-3300 GRAPHITE CORE ASSEMBLY DESIGN

Design Criteria for GCC

• Brief overview of the criteria for the design of the

GCC. Supporting Article HHA-3000.

– Key concepts: Role of GCC in A HTR Safety Case.

– Modes of Failure addressed

– Determination of Limits

– Material Reliability Curve

– Probabilistic Assessment – Simplified Assessment

– Probabilistic Method – Full Assessment

– Comparison of Margins

– Verification

GCC In safety Case • Graphite is quasi-brittle.

• Graphite Strength shows a high variability.

• It is not necessarily possible to ensure against cracking of graphite components.

• A Graphite Core Assembly design shall ensure that the Failure (cracking) of a GCC does not result in loss of functional integrity of the Graphite Core Assembly.

• As opposed to a pressure vessel, damage tolerance in GCA in ensured by limiting the consequence of failure of a single GCC, thus damage tolerance is ensured by assemblies of many components where no single component is critical to the functional integrity of the component.

Modes of failure

• The identified modes of failure for graphite are:

– Brittle fracture

• Based on small numbers of parts cracking. Related to loss of function. Material dependent.

– Fatigue

– Buckling (Elastic Instability)

– Environmental effects.

• Oxidation

• Water vapour

• Irradiation

Determination of limits

• Key Claims:

– It is possible to design parts by comparing calculated

stresses to strength limits based on specimen test results

and incorporating adequate Design Margin.

– For graphite, fixed Design Margins do not ensure uniform

reliability, variability in the graphite grade must be

accounted for.

– It is possible to characterise the material variability

statistically, and from this determine the design margin.

Determination of limits

• Probabilistic approach selected.

• Design Margins to be provided by means of reliability targets, allocated for stress categories based on part classification. Table HHA-3000-1

Material Reliability Curve

• The variability in material strength is characterised by the material reliability curve.

– Proposed. Use a Weibull Distribution to characterise the material strength (Ho, Schmidt, Nemeth & Bratton)

– Conservatism introduced using 95% confidence limits.

Simplified Assessment

• Simplified assessment: – Compare the highest stress

calculated in the part to a design stress value, calculated from the Material reliability curve and the target POF for the part for this service level.

• Using Weibull:

StSm

Material Dependent,

Based on POF Required

Design Margin

Conservative 95%

CI based on data availability

1

mallowS Sc ln 1 POF

Note: The Allowable stress is now a function of material quality.

Simplified Assessment (Contd.)

• The Design margin can be calculated as a function of

required POF and material variability (m)

• Note: For a typical grade, 5 < m < 15 are typical.

Simplified Assessment

• Schematic of simplified assessment.

Full Assessment:

• Note that the simplified assessment assumes that the entire part is at the same stress (or at least in a simple stress distribution in the case of bending).

• Full assessment takes account of the actual distribution of stress in the part.

– Smaller volumes of material at the same stress level will result in a lower probability of failure of the part.

Full Assessment • Full Assessment (CDF)

Full Assessment

• Full Assessment (pdf)

How is this achieved?

• Typically be means of some integral such as

Weibull’s weakest link.

• Or, a modified Weibull approach

d ),,(

exp 1

V

m

ofV V

zyxP

VL

VS

j

mn

j c

j

1

exp

Comparison to other margins • Incorporation of design margin guards

against failure by backing off from the load at which failure is anticipated.

• The design margin can be compared to design margin values that are in use internationally today.

• Sources – ASME CE Draft

– JAEA Design Methodology

– RDMCI Methodology (South Africa)

– UK Methodology

• Assessed for Both Core Blocks and Core Supports for materials with different levels of variability (Weibull moduli from 5 to 15).

• Converted to the same units (distance from the mean tensile strength (St) to the Design Stress (Sm)

Comparison on margins

Verification

• Verification of Methods

– Work completed by volunteers to integrate the

verification case into a criteria document.

• Test of the methods over a range of problems.

– Demonstrate accuracy or conservatism.

Verification - Acceptance Criteria

• What is a suitable basis for acceptance?

• Material variability and experimental bias:

– Material data for all material of the same grade

– Typical part tests from few billets

• Analysis of typical (NBG-18) billet data provides the following: (Billet mean tensile strength values of 24 billets)

– 50% of billets fall within +/- 6% of the material mean

– 95% of billets fall within +/-18% of the material mean

• No additional uncertainties included in the acceptance criteria.

– Analysis accuracy / convergence

– Experimental accuracy (confidence in mean prediction)

0

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Problem ID

Denninghof Method with:

Sc = 21.488 [MPa]

S0 = 10.930 [MPa]

m = 4.484

R = 0.266

nu = 0.210

Rel

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Verification - Typical Results

From Hindley e.a., 2009

HHA-3000: DESIGN-Ongoing

Activities

• Fatigue Rules

• Guidance: Oxidation

• Guidance on Stresses due to Irradiation

Graphite and the ASME Code

32

MANDATORY APPENDIX HHA-III

REQUIREMENTS FOR GENERATION OF DESIGN DATA FOR GRAPHITE GRADES

HHA-III-3300 IRRADIATED GRAPHITE

For irradiated graphite [HHA-3132.1(b) and (c)] the following properties shall be determined:

(a) thermal conductivity – temperature dependent

(b) dimensional change [HHA-3132.1(c) only]

(c) creep coefficients [HHA-3132.1(c) only]

(d) CTE –temperature dependent [HHA-3132.1(c) only]

(e) strength [HHA-3132.1(c) only]

(f) elastic modulus [HHA-3132.1(c) only]

Test data shall represent and envelope the irradiation conditions in service, i.e., they shall mimic reactor

neutron fluence and temperature ranges. Data shall be reported in accordance with ASTM C625.

FORM MDS-1 MATERIALS DATA SHEET (SI UNITS)

This form provides a template for the required graphite properties.

Conclusions

• The sub-group graphite core components has

established code requirements for the design and

construction of Graphite Core Components.

– Code is largely complete but is being edited & revised.

– Design requirements provided an acceptable basis for

design of GCC.

• Material data (including irradiation data) are being

developed and will be introduced into the code

• Data as a code case which is then adopted by the

ASMA Code

Thank You!

Feel free to contact:

•Christian Sanna of ASME by email

at [email protected] or by phone at +1-

212-591-8513.

•Tim Burchell, ORNL by email

[email protected] or by phone at

+1-865-576-8595.

•Mark Mitchell, EON Consulting by

email Mark Mitchell

[email protected] or by phone

at +27-329-3337

mailto:[email protected]



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