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Slide 2
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
Slide 3
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
Slide 4
Graphite Core Components – Prism Type
HTR (HTTR)
Slide 5
Graphite Core Components – Pebble Type
HTR (PBMR)
Slide 6
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.
Slide 10
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.
Slide 11
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)
Slide 12
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
Slide 13
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
Slide 14
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.
Slide 15
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
Slide 16
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.
Slide 17
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
Slide 18
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.
Slide 19
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.
Slide 20
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.
Slide 21
Simplified Assessment
• Schematic of simplified assessment.
Slide 22
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.
Slide 23
Full Assessment • Full Assessment (CDF)
Slide 24
Full Assessment
• Full Assessment (pdf)
Slide 25
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
Slide 26
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)
Slide 27
Comparison on margins
Slide 28
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.
Slide 29
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
20
40
60
80
100
120
140
VP
-12
.00
VP
-12
.12
VP
-12
.01
VP
-12
.04
VP
-12
.02
VP
-12
.13
VP
-12
.03
VP
-12
.05
VP
-12
.14
VP
-12
.06
VP
-12
.15
VP
-00
.0c2
VP
-12
.07
VP
-00
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VP
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f
VP
-17
.0c2
VP
-17
.0c
VP
-17
.0f
VP
-12
.08
VP
-12
.11
VP
-12
.09
VP
-12
.10
VP
-19
.0
VP
-05
.1
VP
-05
.2
VP
-05
.3
Problem ID
Denninghof Method with:
Sc = 21.488 [MPa]
S0 = 10.930 [MPa]
m = 4.484
R = 0.266
nu = 0.210
Rel
ativ
e E
rro
r (A
nal
ysi
s/E
xp
erim
ent)
[%
]
Slide 30
Verification - Typical Results
From Hindley e.a., 2009
Slide 31
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.
Slide 33
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
Slide 34
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