A S M E

N T B - 4 - 2 0 2 1

Backgrou nd Information fo r Addre ssing Ade qu acy o r

Optimizatio n of ASME BPVC

Se ctio n III, Division 5 Ru le s fo r No nmetallic Core Co mpo ne nts

ASME NTB-4-2021

BACKGROUND INFORMATION FOR ADDRESSING ADEQUACY OR OPTIMIZATION OF ASME BPVC SECTION III, DIVISION 5 RULES FOR NONMETALLIC CORE COMPONENTS

Prepared by:

Josina W. Geringer, Oak Ridge National Laboratory Timothy D. Burchell, Oak Ridge National Laboratory Mark Mitchell, Ultra Safe Nuclear Corporation

Date of Issuance: June 30, 2021

This manuscript was sponsored by the US Department of Energy, Office of Nuclear Energy, under contract DE- AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT Battelle LLC. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The United States Government retains, and by accepting the work for publication, the publisher acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or allow others to do so, for United States Government purposes.

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Copyright © 2021

THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS

All Rights Reserved

ASME NTB-4-2021: BACKGROUND INFORMATION FOR ADDRESSING ADEQUACY OR OPTIMIZATION OF ASME BPVC SEC. III, DIV. 5 RULES FOR NONMETALLIC CORE COMPONENTS

TABLE OF CONTENTS

Table of Contents iii

Foreword iv

Abbreviations and Acronyms v

1. INTRODUCTION 1

2. CODE APPROACH 4

3. GRAPHITE STRENGTH 14

4. CODE VERIFICATION 16

5. SUMMARY 22

References 23

Appendix A: Graphite Core Components Code Development Progress Presented to ASME Section III (2005) 26

LIST OF TABLES

Table 1: Comparison of the Behavior of Steels and Nuclear Graphite 2

Table 2: Safety Margins for Core Support Structures (In Tensile Loading) 8

Table 3: Design Allowable Probability of Failure 9

Table 4: Set of Verification Problems (VP) 18

LIST OF FIGURES

Figure 1: JAEA Methodology: (a) Design Stress Limit for Core Support Graphite and (b) Fracture Probability Density Function 6

Figure 2: Proposed ASME CE-3550-1 Service Categories and Stress Intensity Levels 7

Figure 3: Illustration of Design Margin 8

Figure 4: Design Allowable Stresses Flowchart for SRC-1 9

Figure 5: Schematic of Simplified Assessment Methodology (CDF) 10

Figure 6: Schematic of Full Assessment Methodology (CDF) 11

Figure 7: Weibull Probability Failure Prediction with the Full Assessment Method (PDF) 11

Figure 8: Typical Graphite Core Component Design Sequence 12

Figure 9: Typical Graphite Core Component Design Sequence (Continued) 13

Figure 10: Comparison of Predicted and Experimental Mean Failure Load for Verification Problems20 Figure 11: Comparison of Experimental Mean Failure Load and Code-Allowable Loads for Verification Problems 21

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ASME NTB-4-2021: BACKGROUND INFORMATION FOR ADDRESSING ADEQUACY OR OPTIMIZATION OF ASME BPVC SEC. III, DIV. 5 RULES FOR NONMETALLIC CORE COMPONENTS

FOREWORD

The purpose of this document is to provide background information on the scope, development, and verification of elevated-temperature design and construction rules as defined in the ASME Boiler and Pressure Vessel Code (“BPVC”), Section III Rules for Construction of Nuclear Facility Components, Division 5 High Temperature Reactors Subsection HH, Class A Nonmetallic Core Support Structures, Subpart A Graphite Materials, 2017 edition. The general requirements applicable to nonmetallic core components are discussed in BPVC Subsection HA General Requirements, Subpart B, Graphite Materials.

Acknowledgements

The authors extend their appreciation to Dr. W.E Windes and Ms. A.L. Mack from Idaho National Laboratory (managed and operated by Battelle Energy Alliance) for their input and technical review. Stellenbosch University granted permission to use the work of the late Dr. M.P Hindley. His research was conducted as part of his studies undertaken at Stellenbosch University.

Established in 1880, ASME is a professional not-for-profit organization with more than 100,000 members promoting the art, science, and practice of mechanical and multidisciplinary engineering and allied sciences. ASME develops codes and standards that enhance public safety, and ASME provides lifelong learning and technical exchange opportunities benefiting the engineering and technology community. Visit www.asme.org for more information.

STLLC is a not-for-profit limited liability company, with ASME as the sole member, formed in 2004 to carry out work related to new and developing technologies. STLLC’s mission includes meeting the needs of industry and government by providing new standards-related products and services, which advance the application of emerging and newly commercialized science and technology and provides the research and technology development needed to establish and maintain the technical relevance of codes and standards. Visit https://asmestllc.org/ for more information.

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ASME NTB-4-2021: BACKGROUND INFORMATION FOR ADDRESSING ADEQUACY OR OPTIMIZATION OF ASME BPVC SEC. III, DIV. 5 RULES FOR NONMETALLIC CORE COMPONENTS

ABBREVIATIONS AND ACRONYMS

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ASME NTB-4-2021: BACKGROUND INFORMATION FOR ADDRESSING ADEQUACY OR OPTIMIZATION OF ASME BPVC SEC. III, DIV. 5 RULES FOR NONMETALLIC CORE COMPONENTS

1 INTRODUCTION

Before its appearance in the 2011 edition of ASME BPVC, there was no internationally recognized graphite core design code. Although the need for such a code was recognized by stakeholders such as the US Nuclear Regulatory Commission (NRC) and several reactor designer/constructers, it was not until 2002 that ASME formed a project team to initiate a graphite design code. Graphite is used extensively for reactor internal components in high-temperature gas reactor concepts, as it is needed to establish core geometries allowing coolant flow, reactivity control, and shutdown element insertion; serve as a moderator while supporting the nuclear heat generation process; and provide a passive heat removal flow path in certain licensing basis events [1].

Those functions are key to the operation and safety of the reactor system. One critical difference between graphite core components and the pressure vessel is that the graphite core assembly is a structure comprising many hundreds of components. The designers take measures to ensure that the failure of a single component does not compromise the function of the assembly.

The prior approach to graphite design was deterministic, similar to the approach applied for metallic components today. Graphite, with no strength in the plastic regime, was treated as a linear Hookean material. Any component that suffered cracking was considered a “failed component” and was removed. Additionally, only nonirradiated graphite use was addressed. This approach has been found to be inadequate for design and regulatory licensing. Graphite, because of its nature, is inherently cracked; and the absence of cracking cannot be ensured nor used as an indicator of absolute reliability, as it can be for metals.

After assessment, a new probabilistic approach was adopted. It concluded that the designer can allow for cracks in the component but must demonstrate through analysis and testing that the component can maintain the assigned safety function. Moreover, the design should account for the effects of irradiation on the thermal and mechanical properties of the graphite in the design of the graphite core and consider statistical strength variations within a billet, as well as variation from billet to billet due to different production runs. The new approach also does not follow the standard ASME practice of defining primary and secondary stresses but instead uses a combined stress approach that incorporates the largest stress contributors — irradiation-induced stresses and mechanical stress concentrations — as well as lesser stress contributors like combined membrane, bending, and peak stresses.

It was initially envisioned that the design code would be applied for helium-cooled high-temperature reactors, as that was the leading technology at the time.

The purpose of this document is to provide background information on the scope, development, and verification of elevated-temperature design and construction rules as defined in ASME BPVC Section III, Division 5 Subsection HH, Class A Nonmetallic Core Support Structures, Subpart A Graphite Materials, 2017 edition. The general requirements applicable to nonmetallic core components are discussed in BPVC Subsection HA General Requirements, Subpart B, Graphite Materials.

Similar to the rules for metallic components, BPVC Section III, Division 5, High Temperature Reactors is structured to provide a central location for all aspects of construction for high-temperature reactors, including nonmetallic components or, more specifically, graphite components, in the 2017 edition. For nonmetallic components, according to ASME code terminology, “construction” includes all aspects of Materials (HHA-2000), Design (HHA-3000), Machining, Examination and Testing (HHA-4000), Installation and Examination (HHA-5000), and Nameplates (HHA-8000). The rules stipulate details for material specifications (HHA-I), the requirements for preparing a material data sheet (HHA-II), and the requirements for generating design data for different graphite grades (HHA-III). It also gives reference guidance for consideration of factors such as graphite as a structural material (HHA-A) and the environmental and oxidation effects in graphite (HHA-B).

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