ML24341A135

1851-0001-RPT-002, Revision 0, Degradation Management Program for the Abilene Christian University Molten Salt Reactor (Public)

ML24341A135
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Site:	Abilene Christian University
Issue date:	12/05/2024
From:	Angelici V Abilene Christian University, Nuclear Energy eXperimental Testing Lab
To:	Office of Nuclear Reactor Regulation
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R24014 1851-0001-RPT-002, Rev 0
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Proprietary Proprietary Abilene Christian University Molten Salt Research Reactor Degradation Management Program Revision 0 November 2024

© 2024 Abilene Christian University NEXT Lab

ENGINEERING STUDY R24014 1851-0001-RPT-002 DEGRADATION MANAGEMENT PROGRAM FOR THE ABILENE CHRISTIAN UNIVERSITY MOLTEN SALT REACTOR REVISION 0 QA CLASSIFICATION: NON-SAFETY RELATED Prepared by:

11/25/2024 Valentina Angelici, MPR & Associates Date Reviewed by 11/25/2024 Robert Keating, MPR & Associates Date Approved by:

12/1/2024 Suzanne McKillop, MPR & Associates Date Accepted by:

12/2/2024 Ramin M Bahai Date Zachry Nuclear Engineering, Inc.

14 Lords Hill Road Stonington, CT 06378 Client 322GEN/Natura Zachry Project No. 116011 Page 1 of 52 Total number of pages including Attachments - 52

PROPRIETARY ii 1851-0001-RPT-002, Rev 0 Degradation Management Program for the Abilene Christian University Molten Salt Research Reactor RECORD OF REVISIONS Revision Number Pages /Sections Revised Revision Description 0

All Contributors Valentina Angelici Robert Keating Suzanne McKillop Lara Ojha R24014 Page 3 of 52

PROPRIETARY iii 1851-0001-RPT-002, Rev 0 Acronyms and Abbreviations ACU Abilene Christian University API American Petroleum Institute ASME American Society of Mechanical Engineers DAS Distributed acoustic sensing DM Degradation mechanism DMP Degradation management program DTS Distributed temperature sensing EAF Environmentally assisted fatigue FID Flow-induced degradation FIV Flow induced vibration FP Fission Products GWT Ultrasonic guided wave testing HALEU High Assay Low Enriched Uranium HAZ Heat affected zone HCF High cycle fatigue HF Hydrogen Fluoride IASCC Irradiation assisted stress corrosion cracking IGSCC Intergranular stress corrosion cracking ISI In-Service Inspection ITPR Independent third-party reviews LCF Low cycle fatigue MSRR Molten Salt Research Reactor NDE Non-destructive examination NRRA Natura Resources Research Alliance PIE Post-irradiation examination PT Surface penetrant testing PWHT Post weld heat treatment QA Quality Assurance RAI Requests for additional information R24014 Page 4 of 52

PROPRIETARY iv 1851-0001-RPT-002, Rev 0 RCI Request for Confirmation of Information RIM Reliability and Integrity Management SCC Stress corrosion cracking SERC Science and Engineering Research Center SME Subject matter experts SRC Stress relaxation cracking SSCs Systems, structures, and components UT Ultrasonic testing UV Ultraviolet R24014 Page 5 of 52

PROPRIETARY v

1851-0001-RPT-002, Rev 0 Table of Contents Acronyms and Abbreviations..................................................................................... iii 1

Introduction...................................................................................................... 1-1 1.1 Purpose................................................................................................................. 1-1 1.2 Background.......................................................................................................... 1-1 1.3 Scope.................................................................................................................... 1-2 1.4 DMP Basis........................................................................................................... 1-2 2

Program Implementation Requirements and Management......................... 2-1 2.1 General Process.................................................................................................... 2-1 2.2 Selection of Components in the scope................................................................. 2-3 2.3 Degradation Assessment and Mitigation Strategies............................................ 2-4 2.4 Programmatic Requirements.............................................................................. 2-10 2.5 Transition to Plant Operation............................................................................. 2-14 3

Assessment of Degradation Mechanisms..................................................... 3-1 3.1 Gross Structural Deformation.............................................................................. 3-1 3.2 Fatigue.................................................................................................................. 3-2 3.3 High Temperature Cracking................................................................................ 3-4 3.4 Embrittlement...................................................................................................... 3-6 3.5 Thermal Aging..................................................................................................... 3-8 3.6 Interference from Differential Thermal Expansion............................................. 3-8 3.7 Corrosion.............................................................................................................. 3-8 3.8 Chemical Reactions........................................................................................... 3-11 3.9 Fretting............................................................................................................... 3-11 3.10 Galling................................................................................................................ 3-11 3.11 Seal-ring Leakage.............................................................................................. 3-12 3.12 Flow-Induced Degradation................................................................................ 3-12 4

Mitigation Strategies....................................................................................... 4-1 4.1 Design.................................................................................................................. 4-1 4.2 Fabrication........................................................................................................... 4-4 4.3 Operation.............................................................................................................. 4-6 5

References....................................................................................................... 5-1 R24014 Page 6 of 52

PROPRIETARY Table of Contents (contd.)

vi 1851-0001-RPT-002, Rev 0 A

Flow Charts..................................................................................................... A-1 R24014 Page 7 of 52

PROPRIETARY vii 1851-0001-RPT-002, Rev 0 Figures Figure A-1.

Program Description........................................................................................... A-1 Figure A-2.

General Process. A, C, and E are addressed in the following figures................ A-2 Figure A-3.

A: Design Procedure........................................................................................... A-3 Figure A-4.

C: Fabrication...................................................................................................... A-4 Figure A-5.

E: Operation........................................................................................................ A-5 R24014 Page 8 of 52

PROPRIETARY 1-1 1851-0001-RPT-002, Rev 0 1

Introduction 1.1 PURPOSE The purpose of this report is to establish a comprehensive and structured program to manage the degradation of metallic safety-related components and other supportive systems that pose a potential nuclear, personnel, or investment risk at the Abilene Christian University (ACU)

Molten Salt Research Reactor (MSRR).

1.2 BACKGROUND

ACU is designing and constructing the 1 MWt Molten Salt Research Reactor (MSRR) in their existing Science and Engineering Research Center (SERC). The reactor will not produce electricity and will be fueled with High Assay Low Enriched Uranium (HALEU) in the form of UF4 dissolved in lithium fluoride and beryllium fluoride (FLiBe) salt. To support the design effort, ACU is working with Zachry Nuclear, Inc., and Teledyne Brown Engineering. ACU submitted a construction permit application for the MSRR on August 12, 2022, that the NRC docketed on November 18, 2022. The NRC approved the application and issued a construction permit for the MSRR on September 16, 2024.

After docketing, the NRC issued a number of audit questions and requests for additional information (RAIs). Several of these questions requested information regarding how ACU will address the effects of expected environmental conditions on metallic components. In particular, how ACU is either precluding the possibility that degradation will occur or monitoring in-service the progression of degradation when its potential occurrence cannot be excluded by design.

To address the NRCs concerns, MPR issued a report to evaluate the effects of embrittlement, creep, and fatigue on the Reactor Vessel (Reference 1). While addressing the NRCs RAIs, MPR and ACU identified the need for a structured program that could systematically manage degradation in the MSRR components and provide the technical bases for the mitigation strategies adopted for the reactors metallic components.

The concept of a program to address degradation was included in the RAI answers, verbally discussed with the NRC, and documented in a Request for Confirmation of Information (RCI) that was issued by the NRC and accepted by ACU (Reference 2). The NRC accepted the RAI answers, provided that a DMP will be developed and will be maintained up to date throughout the life of the reactor (i.e., design, fabrication, and operation).

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PROPRIETARY 1-2 1851-0001-RPT-002, Rev 0 1.3 SCOPE This document defines the ACU degradation management program (DMP). The document is structured as follows:

Section 2: Defines the program requirements for implementation, documentation, independent review, and all the other programmatic requirements. The program will be implemented, controlled, documented, and maintained by a DMP Database. Executing the program and establishing a controlled DMP Database requires several key inputs, such as:

SSCs in the scope of the program Potential degradation mechanisms Mitigation strategies for applicable degradation mechanisms Section 3: Identifies degradation mechanisms that are associated with the MSRR environment, materials, and SSC. The program will, as a minimum, address these mechanisms, as appropriate. The DMP Database is the controlled documentation of the results of these assessments. Section 3 does not preclude consideration of other degradation mechanisms that are identified at any state of the program.

Section 4: Provides a list of available mitigation strategies that can be applied during design, fabrication, or operation. The DMP Database is the controlled documentation of the selected mitigation strategies for each component and each degradation mechanism.

Section 4 does not preclude consideration of other mitigation strategies or the use of multiple mitigation strategies, as the program determines to be necessary.

1.4 DMP BASIS The MSRR safety related systems, structures, and components will be constructed to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code),Section III, Rules for the Construction of Nuclear Facility Components, Division 5, High Temperature Reactors (Section III-5). Depending on the relative safety significance, components are constructed to various Code Classes (A, B, SM). Some components are constructed to ASME Code Section VIII when the required materials of construction are not available in Section III-5. Lastly, the reactor enclosure is constructed to ASME Code Section III, Subsection NE (Class MC Containments). Safety related piping is constructed according to ASME B31.3.

1.4.1 Degradation in Service Section III-5 provides the rules for construction, design allowable values, and provides mechanical properties to be used for design of components in high-temperature reactors.

However, as stated in Section III-5, HBB-1110(g) the Code rules do not provide methods to evaluate deterioration that may occur in service as a result of corrosion, mass transfer phenomena, radiation effects, or other material instabilities. Therefore, addressing deterioration in service is the responsibility of the plant owner.

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PROPRIETARY 1-3 1851-0001-RPT-002, Rev 0 1.4.2 Owners Responsibility Specifically, Section III-5, HBB-2160 Deterioration of Material in Service, states that consideration of deterioration of material caused by service is generally outside the scope of the Code and it is the responsibility of the Owner (ACU) to select material suitable for the conditions stated in the Design Specifications with specific attention being given to the effects of service conditions upon the properties of the material.

The ASME Code states that the Owner is responsible for addressing deterioration in service, due to service and environmental conditions; however, the Code does not provide any specific requirements or recommendations on how the Owner is to meet this responsibility. For the MSRR, the DMP Database is the means by which the Owner manages degradation in service and meets this responsibility.

1.4.3 Basis for the DMP In the operating LWR fleet, degradation is generally managed by the ASME Code Section XI, Division 1, along with selected other industry and regulatory programs. When advanced reactor designs began in earnest, it became clear that the deterministic ASME Code,Section XI, Division 1 In-Service Inspection (ISI) Program approach is not suited to advanced, non-LWR technologies. ASME Code,Section XI, Division 1 requirements may be non-conservative, overly burdensome, or based on different technical assumptions and safety bases than are applicable to or required for advanced reactors.

To respond to this need, the ASME Code,Section XI community along with its international members chartered a special committee to develop the Reliability and Integrity Management (RIM) Program for nuclear power plants. RIM is a non-deterministic, technology neutral, approach to plant lifecycle management that follows a system-based Code approach. The RIM Program requirements are in ASME Code Section XI, Division 2 (Reference 3). The NRC has endorsed the RIM Program approach in Regulatory Guide 1.246 for non-LWR reactors.

Since the MSRR is a small research reactor, a full scope RIM program is neither necessary nor required by regulation. However, the process by which RIM evaluates degradation mechanisms and determines appropriate degradation management represents the state-of-the-art consensus process and thus provides a useful strategy to manage the degradation concerns for the MSRR.

The DMP is inspired by the RIM approach and applies a similar structured approach to design, fabrication, and operation. The program is appropriate in that it assesses and guides how credible degradation mechanisms are to be addressed during design, fabrication, or operation to ensure the safety of the plant is maintained.

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PROPRIETARY 2-1 1851-0001-RPT-002, Rev 0 2

Program Implementation Requirements and Management The process developed for the MSRR DMP assesses and manages credible degradation mechanisms during design, fabrication, and subsequent operation to ensure the safety of the plant is maintained. The development of the program entails the development of the following steps:

1.

Scope Definition: Identify the SSCs that are included in the scope of the DMP.

2.

Assessment Process: Identify the active degradation mechanisms for the SSCs within the scope of the DMP that are credible to the MSRR.

3.

Performance Requirements: Based on the plant safety objective and basis, assess the desired performance requirements of each component and consequences of failure due to degradation.

4.

Mitigation Strategies: Select all appropriate mitigation strategies and ensure that each applicable degradation mechanism is addressed, and the safety objectives are met.

5.

Performance Monitoring: Determine any performance monitoring or examinations required during the life of the plant.

6.

Documentation: Document the results of the previous steps in a DMP Database.

7.

Updates during operation: The DMP Database is a living document that is updated throughout the operational life of the reactor.

2.1 GENERAL PROCESS The degradation management program involves a structured approach to identify, assess, mitigate, and monitor challenges to the performance of key components due to in-service degradation mechanisms.

Defining the components included in the program is the first key step. For this effort, all safety-related metallic components are in the scope of the program. In addition, ACU may choose to include any non-safety-related components that potentially challenge the MSRR research mission or investment.

((

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PROPRIETARY 2-2 1851-0001-RPT-002, Rev 0

.))

Appendix A provides a set of flowcharts that explain the process and the interconnection between the different pieces that constitute the program. The diagram in Figure A-1 describes the process used to structure the program. The following diagrams (Figure A-2 through Figure A-5) describe the sequence of steps and activities that the owner will carry out when the program is applied. Although the flowcharts depict the steps in a sequential order, the actual sequence of steps may vary depending on the specific component being analyzed.

The flowcharts clearly indicate the decision points, where the owner makes choices and evaluates conditions, clarifying dependencies and relationships between different steps in the program. The flowcharts define the boundaries of the process, clearly indicating the start and the end, and include feedback loops and iterative processes to highlight where steps are repeated or revised based on certain conditions or outcome. The owner of the program will leverage the flowcharts to create the DMP Database and maintain it throughout the lifetime of the reactor.

The next paragraphs in Section 2 describe the DMP process and constitute the explanation of how the flowcharts in Appendix A should be interpreted.

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PROPRIETARY 2-3 1851-0001-RPT-002, Rev 0 Due to the iterative nature of the program, the program owner will be required to execute and reassess the DMP results multiple times throughout the reactor's lifecycle. This continuous process is essential to ensure the program remains aligned with evolving conditions and requirements. There are several key reasons that may necessitate revisiting and refining the general process:

((

))

2.2 SELECTION OF COMPONENTS IN THE SCOPE Selecting components for the program requires a systematic approach. The criteria identified by this program are used to select components included in the DMP. These criteria are listed below:

[

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PROPRIETARY 2-4 1851-0001-RPT-002, Rev 0

.))

While all the metallic safety-related components are included in the DMP, key assets and mission critical components are included when the owner and stakeholders deem it beneficial.

The classification, functions and consequences of components will change over time as the design evolves. Therefore, the program, including the selection of the components in the DMP, will be reassessed periodically, as appropriate.

Components not meeting the selection criteria are not included in the DMP, and normal industrial practices for managing degradation are applicable.

2.3 DEGRADATION ASSESSMENT AND MITIGATION STRATEGIES For each component included in the DMP, all applicable or potential degradation mechanisms are identified though the Assessment Process. The assessment and bases for assessing degradation mechanisms is documented and independently reviewed by subject matter experts (SME).

2.3.1 Assessment Process The evaluation of applicable degradation mechanisms for each component requires a systematic approach to identify potential causes of deterioration or failure over time considering the specific operating conditions for each component. According to the specifics of the component and the operating conditions, the following general steps are included:

((

.))

Environmental factors, such as temperature, pressure, exposure to salt and gases, redox potential, and thermal cycling, can significantly affect the degradation and are to be clearly identified prior to assessing the applicable degradation mechanisms. This process is expected to meet ANSI 15.8.

Potential degradation mechanisms that are most likely applicable to the MSRR are identified in Section 3. As a minimum, all of these mechanisms are to be considered. Further, while each is generally listed separately, as applicable, the cumulative effects of multiple mechanisms are to be considered. Section 3 may not address all potential degradation and the program must include any potential degradation that is determined to be credible, even if not listed in Section 3.

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PROPRIETARY 2-5 1851-0001-RPT-002, Rev 0

((

.))

Following this structured approach, the program will systematically identify and select degradation mechanisms, or the combination of degradation mechanisms when applicable, that have the potential to affect MSRR components.

2.3.2 Mitigation Strategies Selecting mitigation strategies is the process of determining how to best mitigate (manage or eliminate) each identified credible degradation mechanism. Mitigation strategies are selected following a structured process that involves the following:

((

.))

A preliminary list of mitigation strategies and guidance specifically applicable to the MSRR is provided in Section 4. These are not intended to limit mitigation to only those options provided in Section 4 and multiple strategies, providing defense in depth, are preferred. Mitigation strategies may be applicable to multiple phases of the life cycle, such as design, fabrication, and operation, as described below.

Design Mitigation strategies during the design phase proactively address degradation mechanisms.

[

.))

Design strategies to address degradation mechanisms, with the technical basis, shall be documented, referenced, and maintained by the program. All applicable requirements must be provided to the responsible designers to be implemented and controlled during the design. If R24014 Page 16 of 52

PROPRIETARY 2-6 1851-0001-RPT-002, Rev 0 during detailed design, designers cannot fully implement a requirement, the program must be notified and the DMP process reevaluated, as needed.

Possible mitigation strategies during design can be divided into the following categories:

((

))

Requirements to support mitigating design strategies such as ((

)) are documented in the DMP Database, to provide continuity throughout the lifecycle of the reactor. Comprehensive documentation may include system descriptions, design specifications, or other reports that document the rationale ((

)), and specific mitigation strategies employed to address identified degradation mechanisms.

The documentation outlines how these changes impact component performance, and how the degradation mechanism is addressed. Additionally, documentation must address the final design simulations, calculations, or testing results to validate the effectiveness of the selected configuration or design margin against the degradation mechanism. Clear records of revisions, updates, and maintenance procedures ensure that a complete understanding of the design evolution is maintained to support future operations, repair and replacement (modification) activities and maintenance. Figure A-3 in Appendix A describes the process of mitigation during design.

Fabrication Mitigation strategies during fabrication proactively address degradation mechanisms.

Fabrication strategies focus on incorporating fabrication requirements that minimize, or R24014 Page 17 of 52

PROPRIETARY 2-7 1851-0001-RPT-002, Rev 0 eliminate, the potential impact of the applicable degradation mechanisms. Addressing these during fabrication involves implementing preventive measures such as:

((

.))

The program maintains (in the DMP Database) detailed records of requirements to be implemented during fabrication, including ((

.)) These mitigation strategies are incorporated into the fabrication and design specifications.

The DMP Database maintains the bases of the selection and the rationale behind it to establish traceability. Figure A-4 in Appendix A describes the process of mitigation during fabrication.

Periodic Replacement Limiting the service life of a component is the planned replacement of the component before degradation challenges its safety or functionality. Replacement is planned before the component has reached the end of its operational lifespan and prior to a degradation mechanism challenging the performance.

This strategy involves removing the degraded component and installing a new one to ensure continued functionality. Many factors can affect the choice of a planned component replacement, including ((

.))

Replacement activities are carefully planned to minimize disruption to operations. The system design must account for planned component replacements, and comprehensive documentation records the rationale for replacement decisions and the technical specifications of the new components. ((

.))

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PROPRIETARY 2-8 1851-0001-RPT-002, Rev 0 Operation Mitigation of a degradation mechanism during operation requires determination of appropriate techniques to evaluate, monitor, or determine the status of degradation and ensure the functionality of the component throughout the operational life. Mitigation during operation may include the following options:

((

.))

Designing components for effective in-service mitigation requires the integration of features and consideration to facilitate ((

.)) These may include:

((

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PROPRIETARY 2-9 1851-0001-RPT-002, Rev 0

.))

Documentation of a mitigation strategy during operation is crucial for ensuring that the strategy is implemented effectively, accounted for during design, monitored consistently, and adjusted as necessary to maintain its effectiveness. Thus, when this mitigation strategy is selected, the program includes changes of the technical specifications, as well as operating procedures.

Figure A-5 in Appendix A describes the process of mitigation during operation.

2.3.3 Assessing the Impact of Failure The mitigation strategies employed during the different stages of the life of the reactor may not fully mitigate a degradation mechanism. Once the possibility of mitigating a degradation mechanism through design, fabrication, and operation is assessed and strategies selected, the program reconsiders the consequences of the failure of the component. This is consistent with the defense in depth principle, as any mitigation strategy is applied independently of the consequences of a failure.

Considering the consequences of failure due to the degradation mechanism, together with the mitigation strategies prior to in-service operation (i.e., design and fabrication), provides four possible outcomes:

[

.))

((

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PROPRIETARY 2-10 1851-0001-RPT-002, Rev 0

))

2.4 PROGRAMMATIC REQUIREMENTS The program includes requirements to ensure its effectiveness in maintaining the performance of the components. The key programmatic requirements include ((

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PROPRIETARY 2-11 1851-0001-RPT-002, Rev 0 R24014 Page 22 of 52

PROPRIETARY 2-12 1851-0001-RPT-002, Rev 0 R24014 Page 23 of 52

PROPRIETARY 2-13 1851-0001-RPT-002, Rev 0 R24014 Page 24 of 52

PROPRIETARY 2-14 1851-0001-RPT-002, Rev 0

.))

2.5 TRANSITION TO PLANT OPERATION The DMP is developed during the design phase of the reactor to inform the MSRR design and manage strategies to mitigate the expected degradation mechanisms systematically. The transition from design and fabrication to operation marks a critical phase where theoretical plans and strategies are put into practical effect. As the program transitions into operation, implementation of the strategies identified during design begins. Continuous adaptation and refinement based on real-time feedback and monitoring results are key to ensuring the program remains responsive and evolves with the environmental conditions and stakeholder needs.

During operation, the MSRR will follow operating procedures that incorporate the results of the DMP.

The DMP Database structure remains valid during operation. Technical and scientific justification are modified if new scientific results are uncovered and need to be incorporated into the documents. The owner updates the decision records to adapt to the changes that will inevitably occur during operation. During operation, monitoring reports are produced, and their feedback will feed the decision records. The decision records will be used to update the operating procedure to reflect the changes.

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PROPRIETARY 3-1 1851-0001-RPT-002, Rev 0 3

Assessment of Degradation Mechanisms After selecting the SSCs included in the DMP, the Assessment Process is the next key step to define the DMP. This step selects the potential active degradation mechanisms that are credible in the MSRR environment and might affect the reliability of MSRR components.

This section provides a preliminary list of potential active degradation mechanisms and initial considerations on the components that are expected to be affected by each degradation mechanism. Degradation mechanisms other than those presented in this report may need to be considered. A full assessment of the applicability of each degradation mechanism to all the components identified will be performed and documented in the DMP Database.

3.1 GROSS STRUCTURAL DEFORMATION Gross structural deformation refers to a significant change in the configuration of a structure under load. The deformation experienced is generally in the plastic or permanent regime. Two different structural deformations are accounted herein: plastic collapse/rupture and excessive deformation.

3.1.1 Plastic Collapse/Rupture Plastic collapse is a type of failure where a material undergoes excessive plastic deformation under a constant load. The effect of load on material deformation is dependent on material properties such as yield strength and toughness. Excessive plastic deformation can develop and eventually lead to structural instability and loss of load-bearing capacity, which can lead to rupture. Structural collapse can result from excessive through-wall stresses to ductile components. Affected components include all vessels, pipes or pressure boundary components that operate at pressures higher or lower than ambient pressure, especially those that are part of the fuel-salt boundary.

Designers must design pressure boundaries and supports in accordance with ASME Code,Section III requirements. In general, the Code limits are intended to provide an appropriate margin against plastic collapse/rupture, provided all applicable loads are considered in the design.

3.1.2 Excessive Deformation Excessive through-wall strain or creep strain can result in excessive deformation. Strain beyond the yield strength of a material will result in permanent, plastic deformation. All vessels in service at an operating pressure less than or greater than ambient are susceptible. Excessive deformation can be a challenge especially at closures and flanges which must maintain geometric integrity for proper function.

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PROPRIETARY 3-2 1851-0001-RPT-002, Rev 0 Designers must design pressure boundaries and supports in accordance with ASME Code,Section III requirements. In general, the Code limits are intended to provide an appropriate margin against excessive deformation, provided all applicable loads are considered in the design.

It should be noted that austenitic steels typically have two allowable stresses listed, one at 90%

yield and one at 66% yield. The lower allowable stress is intended for locations where distortion is critical, such as at flanges.

3.2 FATIGUE Fatigue damage is the result of repeated cyclic stress loading. The loading cycles of concern in nuclear plant pressure components are either from vibration (mechanical loading) or repeated cycles of heating and cooling, especially when the material's free thermal expansion is constrained. The constraint characterization of thermal growth can be either free expansion (e.g.,

linear expansion of a hot structure) or self-constraint (e.g., cylinder with a differential thermal gradient through thickness. Fatigue is common in materials and structures subjected to cyclic temperature changes.

3.2.1 High Cycle Fatigue High cycle fatigue (HCF) is a type of fatigue failure that occurs when the material is subjected to a high number of stress cycles, typically above 100,000 cycles. The stress levels in HCF are well below the yield strength; therefore, deformation is elastic and not plastic. HCF can initiate and propagate a new crack or propagate an existing crack.

High cycle thermal fatigue is generally independent of the structural geometry and is the result of excessive surface stress from rapid thermal stratification or mixing events. Pressure components that are subjected to thermal mixing, cycling, or rapid fluctuations in temperature are all susceptible.

High cycle vibration can also result in fatigue damage. Typically, this is the result of an excitation of a vibrational input, such as a motor, or flow induced vibrations (FIV). Excitation and vibration are dependent on the geometry, stress concentrations and the excitation input.

Pressure components that have potential excitation loads and generally low natural frequencies (less than around 100Hz) are susceptible. Tubes in heat exchangers and small diameter drain lines are typical examples of susceptible geometries.

Generally, high cycle fatigue is addressed by keeping the alternating stresses well below the material dependent endurance limit. One strategy to prevent this degradation mechanism is by limiting or eliminating thermal mixing events. If there is a sufficient margin against fatigue, inspection and monitoring is generally not needed. Inspections would require Ultrasonic Testing (UT) or a surface examination, as cracking will typically manifest as a surface phenomenon.

Accelerometers and thermocouple monitoring can be used as mitigation if the instrumentation is located in an area of concern and could detect the high cycle events.

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PROPRIETARY 3-3 1851-0001-RPT-002, Rev 0 3.2.2 Low Cycle Fatigue Low cycle fatigue (LCF) is a type of fatigue failure that occurs in materials subjected to cyclic stresses of higher amplitude and lower frequency, typically below 100,000 cycles. The stress levels in LCF are often high enough to cause plastic deformation in the material. Fatigue cracking can be induced by the structural deformations associated with normal operating thermal transients. Any component that experiences changes in temperature (thermal growth) during operating life is susceptible. For example, heat-up, cool-down, pressurization and depressurization, or changes in temperature during events can create an environment conducive to LCF. LCF can also be an issue for elements that are structurally repeatedly preloaded and unloaded (i.e., bolted closures). Components with materials of different thermal expansion such as dissimilar metal welds can also be affected.

Designers must design pressure boundaries to the ASME Code,Section III requirements. The ASME Code addresses LCF by keeping the alternating stresses low and meeting the ASME Code usage limit (1.0). However, the ASME fatigue curves are best estimate curves and based on fatigue testing to loss of load, not crack initiation. Therefore, meeting the ASME Code usage limit, alone, will not ensure that fatigue cracking cannot occur.

The rate of thermal transients can be limited to keep cyclic stresses lower; however, the limiting stress is often the end state condition; therefore, slowing transients can have a limited usefulness.

In general, the lower the calculated fatigue usage, the lower the possibility of fatigue degradation.

Should monitoring be required, volumetric UT can detect and size cracking in service.

Monitoring via instrumentation in relevant areas is typically used to ensure that heat up rates are maintained, and the number of cycles are counted and tracked. This can also provide assurance against fatigue damage.

3.2.3 Environmentally Assisted Fatigue Accelerated crack propagation and failure can result from cyclic stresses in a corrosive environment via environmentally assisted fatigue (EAF). Environmental conditions, such as contained fluids, can result in lower fatigue life than would be otherwise predicted by the ASME Code. Exacerbation of fatigue damage due to the molten salt environmental conditions relative to in-air ASME Code fatigue life can occur. Generally, EAF is most significant on the slowest transients and the effect can be quite high. Any component that is subjected to low cycle thermal fatigue and is wetted by a fluid may be susceptible to EAF.

Designers must design pressure boundaries to the ASME Code,Section III requirements; however, the Code does not explicitly account for environmental effects. The effects of salt environments on fatigue are not well understood and were not considered during the development of the ASME code. EAF is typically addressed by use of a multiplier (Fen factor) on the ASME Code calculated usage, which was based on in-air testing. Common environmental multipliers range from 10 to 200.

As EAF is a multiplier on fatigue usage, the designer should keep ASME Code fatigue to as low as practical, unless there is data or confidence in the EAF multipliers for the salt environment.

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PROPRIETARY 3-4 1851-0001-RPT-002, Rev 0 Since the effect of the environment is not well understood, an extremely low (or zero) fatigue usage is a desirable target.

If there is a very large margin against fatigue, EAF could be considered not credible, and inspection and monitoring may not be required. Monitoring is typically used to ensure that heat up rates are maintained, and the number of cycles are counted and tracked. Inspections could utilize UT to detect and size cracking in service, especially around welds.

3.3 HIGH TEMPERATURE CRACKING High temperature cracking of components can occur due to several degradation mechanisms.

Typically, these include stress relaxation cracking, creep, and creep-fatigue.

3.3.1 Stress Relaxation Cracking (SRC)

Stress relaxation cracking is a high temperature, intergranular failure mode associated with intergranular crack propagation. SRC is known to occur in austenitic stainless steels and nickel alloys at moderate to high temperatures, specifically in the coarse grain heat affected zones of welds adjacent to fusion boundaries. Initiation generally occurs in areas of high residual stresses and high hardness (generally >200Hv) (Reference 5-11). This includes areas that have been cold worked and thick welded sections.

SRC is an early failure phenomenon, typically occurring early in service (within one year of life) or during post weld heat treatment. Heating during service promotes stress relaxation via plastic deformation while simultaneously creating precipitants that serve as dislocation obstacles, decreasing ductility. The inhibition of dislocation motion prevents ductile stress relief and leads to stress relief via cracking and failure. Any austenitic materials or Ni-alloys that operate between 550-750 are susceptible (Reference 5). Thinner welds below 12 mm are not generally a concern (Reference 1), as there is less residual stress build up. While according to the American Petroleum Institute (API), Type 316H stainless steel is the least susceptible of austenitic materials to display SRC, failures have still been known to occur (Reference 1).

It is difficult to assess vulnerability to SRC using traditional mechanical testing or NDE. To avoid SRC, minimizing or eliminating welds in the design, or strategically locating them in lower temperature locations, would be very effective (see Section 4.1.3 for more details).

Residual stresses can be minimized via optimized joint geometry such that they are not highly constrained or heavily loaded. A mockup of weld configurations to quantify the residual stresses through hardness testing will highlight susceptible welds. Optimized welding procedure might include use of a high deposition-rate low heat-input process such as arc pulsation or oscillation methods to produce a finer more equiaxed weld microstructures, a reduced HAZ width, and reduced residual stress.

Lastly, the API recommends a post weld heat treatment (PWHT) for all butt welds thicker than 12 mm, including any weld repairs. PWHT should be done in a carefully controlled manner, avoiding fast heating above the precipitant solvus line, to prevent reheat cracking.

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PROPRIETARY 3-5 1851-0001-RPT-002, Rev 0 Because SRC failure occurs early in life, in service inspection is unlikely to detect it. A longer period of hot functional testing (without fuel) for at least several months could help with detection of failure before fueled operation. Monitoring via surveillance coupon exposure tests under operating conditions and accident conditions can provide information about intergranular carbide precipitation but will likely not provide information early enough for sufficient mitigation.

3.3.2 Creep Cracking Creep cracking results from long-term, high temperature, load controlled, constant stresses. It can result in crack propagation and ultimately rupture due to excessive accumulated strain. This form of degradation can affect base material, welds, or bolts. Any austenitic component or structure that has service temperatures above 425 is susceptible. Residual stresses around welds may be present and affect the creep parameters.

Designers must design pressure boundaries to the ASME Code,Section III requirements. Creep failures are addressed by keeping load-controlled stresses low and meeting the ASME Code creep usage limit (1.0). Creep life is generally dependent on the structural geometry, pressure, loading and time at temperature. These variables can be controlled in design to keep the creep usage below the Code limits. For example, residual stresses near welds can be annealed at operating temperature.

Creep can also be affected by the environment (see Section 3.3.4) and relatively large margins are desirable. Creep damage is generally detectable by metallurgical examinations such as surface replica or boat samples. Advanced creep cracking can be detected by surface examination or UT. Monitoring is another option; however, surveillance specimens will need to be under load and located in a comparable environment and removed periodically for testing.

3.3.3 Creep-Fatigue Cracking Creep-fatigue cracking results from long-term, high temperature, combined with cyclic stresses.

The combination of creep and fatigue can be significantly worse than either creep or fatigue considered alone. Creep can greatly increase accumulated strains and lead to fatigue failures at fewer cycles than would otherwise be expected without creep. Any austenitic component or structure that has service temperatures about 425°C and cyclic loading is susceptible.

Both creep usage (see Creep Cracking) and fatigue usage (see Thermal Fatigue) should be minimized. The ASME Code has interaction limits that account for creep-fatigue. Creep-fatigue can be largely precluded by design and construction. Similar inspection techniques as are employed in fatigue and creep mitigation can be employed in this case. Monitoring is likely not an option as there is no means of load cycling surveillance coupons.

3.3.4 Environmentally Assisted Creep Creep damage can accumulate faster when combined with time-dependent corrosion mechanisms. Intergranular cracking occurs in conjunction with surface attack and material depletion due to the environment. Austenitic components with service temperatures above R24014 Page 30 of 52

PROPRIETARY 3-6 1851-0001-RPT-002, Rev 0 425°C that are exposed to corrosive environments are susceptible. Residual stresses around welds may be present and affect the creep parameters.

Strategies to avoid creep (Section 3.3.2) can also be employed to mitigate environmentally assisted creep. Regarding the environment, redox chemistry in salt can be actively controlled.

Surveillance coupons subject to stress and exposed to fuel salt can be studied periodically.

Further corrosion mitigation strategies (discussed in Section 3.7) can also be employed here.

3.4 EMBRITTLEMENT Type 316H is a very ductile material with excellent fracture toughness. ASME Code,Section III, Division 5, HBB-3241(b) exempts Type 316H from rules controlling nonductile fracture, unless fabrication effects alter the material, such that a nonductile fraction becomes a plausible failure mode. By extension, provided radiation and other embrittling mechanisms do not result in a plausible brittle failure, the Code does not require a nonductile failure evaluation. Therefore, provided embrittlement potential is low and stress margins are high, embrittlement is not a credible failure.

Embrittlement can largely be precluded by design and operations that maintain very low stress levels and manageably low neutron and gamma dose. Also, the operating temperature of the MSRR is such that embrittlement will likely be countered by the annealing occurring at high temperatures.

A high flaw tolerance may be useful in demonstrating margin against nonductile failure. In-service, cracking can be detected by surface penetrant testing (PT) or UT, especially around welds. Environmental conditions can be maintained using redox control in the case of tellurium embrittlement (see below) and by tuning the amount of beryllium or UF4 in salt. If deemed necessary. surveillance coupons in similar temperature, radiation, and chemical conditions during operation could be sampled periodically for characterization via XRD and metallography to observe any phase or microstructural changes as well as intergranular attack.

The following sections enumerate specific types of embrittlement that may be a factor for the MSRR.

3.4.1 Neutron and Radiation Embrittlement The irradiation of steels (i.e., Type 316H) is known to produce helium in the bulk of the material, which can cause material swelling. Irradiation can also change material microstructure, resulting in embrittlement and reduction of ductility and fracture toughness. Components made of Type 316H that are exposed to neutron and gamma radiation are affected, including welds. A dose of over 1 dpa is generally required for embrittlement to be a significant concern (Reference 1).

3.4.2 Fission Product Embrittlement - Tellurium Tellurium is produced as a fission product in molten salt reactors and can embrittle nickel-based alloys by diffusing into the alloy and forming intermetallics. These intermetallics form at grain boundaries near the surface of the metal exposed to molten fuel and include nickel telluride R24014 Page 31 of 52

PROPRIETARY 3-7 1851-0001-RPT-002, Rev 0 (Ni3Te2), molybdenum telluride (MoTe2), and chromium telluride (Cr-Te) intermetallics. These intermetallics weaken interfaces and decrease fracture toughness and ductility of an alloy. The rate and depth of diffusion, and thus reduction of fracture toughness and ductility, is dependent on temperature. Nickel-based alloy components exposed to molten fuel salt are especially susceptible. Type 316H is less susceptible to tellurium attack, as the amount of Ni is lower and the formation of Ni-Te compounds is less likely, but it is still a possible degradation mechanism.

Control over the redox potential can mitigate tellurium embrittlement. Chemical analysis of salt samples for fission products can also be useful in providing information about the environment and the likelihood of this degradation mechanism to occur.

3.4.3 Hydrogen Embrittlement Hydrogen, which could be produced by the reaction of HF with Type 316H or nickel, can diffuse into the alloy and react with the alloy to form hydride phases. These phases embrittle high strength steels, resulting in loss of tensile ductility and transition from ductile to brittle failure modes. At elevated temperatures, sensitization1 can also occur, which enhances the effects of hydrogen embrittlement. Components made of Type 316H that are exposed to hydrogen would be susceptible, including piping.

The effects of hydride in the metals reach a maximum at room temperature and decrease as temperatures increase. Increasing the temperatures, the mobility and solubility of hydrogen increases beyond the materials ability to trap it, reducing the resulting embrittlement.

3.4.4 Embrittlement due to Cold Work Work hardening (cold work) due to bending or straining of components induces dislocations in the bulk of the material which reduce ductility. This degradation mechanism mainly occurs during fabrication. If a component is work hardened excessively, embrittlement can occur.

Flexible steel tubing or other components that are repeatedly bent during fabrication could be susceptible.

3.4.5 Phase Formation Embrittlement (Beryllium)

Excess beryllium in the salt in contact with structural alloys can form Ni-Be or Fe-Be phases in Type 316H, especially towards the exposed surface of the steel. These phases can embrittle the material. Components that are in contact with beryllium as well as all components that contain fuel salt which contains beryllium may be susceptible. Careful testing to determine and establish the limits of beryllium amount on the introduction of beryllium and subsequent control of its concentration should be specified to prevent an excess or accumulation in fuel salt and adverse reaction with Type 316H.

1 When alloys that contain chromium and nickel are exposed to temperatures ranging from approximately 530 to 820ºC, the carbon can form intergranular carbides, resulting in depletion of chromium along the grain boundaries, a process called sensitization.

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PROPRIETARY 3-8 1851-0001-RPT-002, Rev 0 3.5 THERMAL AGING For long-term exposure at high temperatures, precipitation of embrittling carbides and intermetallics including sigma, chi, and eta (,, and ) is expected. These microstructural changes cause thermal aging embrittlement, affecting yield strength, tensile strength, elongation, toughness, and creep response of the materials and welds over service life. Reductions in tensile strength can affect time-independent allowable stress. Thermal aging can result in embrittlement (see Section 3.4) and overstress situations late in service. All components at high temperatures would be susceptible to this degradation mechanism. Valve seats or closures, or pressure boundary seals could be affected by a reduction in yield strength leading to leakage.

Type 316H is expected to behave better than other materials due to its austenitic phase, high carbon content, and common use in other industry. The ASME Code has requirements that designers must account for the reduction of strength over time. The designer can include them in the design and implement sufficient margins. In service, valves can be monitored for seal leakage and closure flanges can be monitored by use of redundant seals.

3.6 INTERFERENCE FROM DIFFERENTIAL THERMAL EXPANSION Different materials have differing thermal expansion coefficients and can operate at different temperatures, resulting in changes in gaps and clearances. This difference can cause gaps or stresses at interfaces between dissimilar materials at high temperatures. For example, the Type 316H core support has a large thermal expansion coefficient relative to graphite. This must be considered to prevent loose or damaged graphite blocks. Additionally, instances of dissimilar valves (Type 316H/Ni) could be affected, leading to leaks.

Design considerations such as the use of margins and fasteners to account for expansion can prevent buildup of stress or warping of structures. For example, mechanical interconnection can prevent separation of components during heat up processes.

3.7 CORROSION General corrosion occurs due to a chemical interaction between a metal or metal alloy and its environment. Aggressive environments such as chlorides or acidic solutions can increase the rate and severity of corrosion. During corrosion, there is often removal of metal mass which can degrade mechanical properties. Oxide layers and corrosion products that form can also lead to surface roughness and decreased thermal emissivity. Salt-wetted steel surfaces, interfaces of dissimilar metals, and components where there is potential for corrosive attack by hydrofluoric acid are susceptible. This includes welds with different filler material, copper tubing near compressed air systems, and valves.

Data can be collected from Type 316H corrosion tests in fuel salt at operating and maximum temperatures during accident scenarios to produce kinetics curves that can predict corrosion over time. These data can inform corrosion allowances used to define material thicknesses. Careful choice of materials such as Type 316H which has excellent corrosion resistance, Ni 201 which is resistant to corrosive attack by HF, and Ni-alloy valves to resist NF3 corrosion, can mitigate the risk of severe corrosion. Additionally, designers can avoid pairing dissimilar metals that could R24014 Page 33 of 52

PROPRIETARY 3-9 1851-0001-RPT-002, Rev 0 form a galvanic couple, including in piping and dissimilar welds. Several strategies could also be implemented to foster a less corrosive environment. For example, beryllium additions to fuel salt will lower redox potential to within a controlled window.

The purification of fuel salt (i.e., via hydrofluorination-hydrogen process and mechanical filters) and the removal of oxidant impurities can help control the impurities in the salt. Tanks can be inerted to replace oxygen and prevent oxidation. Sampling of fuel salt to consistently monitor concentrations of chromium or other alloying elements or oxides, as well as to monitor redox potential, would be advantageous. Eddy current tests can be performed during inspections for tube thickness loss (except in the case of Selective Dissolution of Active Alloys, see below).

Surveillance coupons immersed in fuel salt may be removed at intervals for characterization of chemical attack and possibly mass loss.

In the following sections, specific relevant types of corrosion for the MSRR facility are discussed.

3.7.1 Selective Dissolution/Dealloying The main corrosion mechanism expected in the MSRR is selective dissolution. Selective dissolution, or dealloying occurs when one or more components of a solid solution alloy (generally the most active ones) are selectively removed from a solid alloy. This phenomenon is characterized by the formation of a porous microstructure on the exposed surface and a reduction in density. As a result, the material may lose strength, depending on how deep from the surface the porous structure is formed. In the case of Type 316H, active elements including chromium, manganese, and sometimes iron may leech out and get into the fuel salt. Selective dissolution is not expected to occur for Ni vessels as long as oxidation potential is kept low.

Selective dissolution does not change the dimensions of the component, and thus will not be able to be monitored or detected using traditional eddy current testing methods. This degradation mechanism can be mitigated by controlling the impurities in the salt via purification and by controlling redox potential via beryllium addition. A corrosion allowance should also be factored in based on the estimated dealloying rate.

3.7.2 Effects of Fission Products on Corrosion Fission products can cause corrosion of structural alloys. Any salt wetted components may be susceptible. The presence of fission product can affect the redox potential of the fuel salt. The measurement and monitoring of redox potential and salt composition (i.e., adding beryllium) can help manage FP attack.

3.7.3 Stress Corrosion Cracking & Intergranular Cracking Stress corrosion cracking (SCC) occurs in a material when tensile stress is applied in a corrosive environment. SCC requires the presence of three factors to occur simultaneously: a susceptible material (e.g., Type 316H), tensile stress, and a corrosive environment. Intergranular cracks propagate through the material in conjunction with sustained environmental attack within the crack tip. Intergranular stress corrosion cracking (IGSCC) can occur in response to directly applied stresses or residual stresses such as those found around welds or cold worked materials.

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PROPRIETARY 3-10 1851-0001-RPT-002, Rev 0 Crack propagation may initiate at flaws or pores (see Dealloying) that serve as stress concentrators. All salt wetted components that experience stress could be susceptible, particularly the primary heat exchanger.

IGSCC has not been studied in an MSRR equivalent salt system, so there is little existing data available. One example of testing that could be done to monitor SCC in MSRR conditions would be the use of U-bend samples in FLiBe salt solution. Exterior inspections can be done to detect cracks, especially near welds, using UT methods. If selected, testing intervals should be selected such that the smallest detectable flaw will not have propagated substantially or catastrophically before the components next inspection.

3.7.4 Irradiation Assisted Stress Corrosion Cracking Irradiation assisted stress corrosion cracking (IASCC) is an age-related degradation mechanism where materials exposed to neutron radiation become more susceptible to SCC. IASCC is a combined effect of elevated corrosion potential of reactor coolant due to fission products and embrittlement processes such as neutron embrittlement. Austenitic stainless steels are susceptible to SCC. Therefore, components made of Type 316H that are subjected to tensile stress and exposed to neutron and gamma radiation, including welds, might experience IASCC.

Design should limit radiation dosage to minimize neutron embrittlement in reactor components (see Section 3.4.1).

3.7.5 Galvanic Corrosion Galvanic corrosion occurs when two dissimilar metals with different electrode potentials (different nobilities) are in contact during service in an electrolyte solution. When this configuration exists, the more noble material becomes the cathode, and the more active material becomes the anode subject to mass loss. Instances of two different metals interfacing in salt wetted environments are susceptible, including places where Ni and Type 316H pipes and valves connect. There may also be a galvanic effect between graphite and Type 316H.

Designing to avoid as many instances as possible of dissimilar metal interactions can prevent galvanic corrosion. When two different metals must connect, designers should avoid this being in a salt wetting area. A higher corrosion allowance can be used to accommodate accelerated corrosion due to graphite or other noble metals, including isolating the core from the reactor vessel and adding an allowance on the grid plates that come into contact with the core.

3.7.6 Corrosion due to Thermal Gradient Corrosion of a metal in molten fluoride will stop if the solubility limit of the metal fluoride is reached. The equilibrium ion concentration as well as the ion solubility limit of ions of active alloying elements in molten salts increase with temperature. If there is a temperature gradient, as the temperature of salt decreases in cold areas, the equilibrium constants of the corrosion reactions and solubility limit of dissolved metal will decrease, causing the dissolved alloying element to precipitate out of the salt. The precipitates can deposit on the metal surface, diffuse into the alloy, or remain suspended within the salt as a solid metal particulate. The thermal gradient may prevent the system from reaching chemical equilibrium, potentially causing higher R24014 Page 35 of 52

PROPRIETARY 3-11 1851-0001-RPT-002, Rev 0 overall corrosion in the hot areas of the reactor. The higher the thermal gradient, the greater the effect on corrosion in the hot area.

Thermal gradients can be minimized through design considerations and the implementation of appropriate operating procedures.

3.8 CHEMICAL REACTIONS 3.8.1 Reaction with Hydrogen Fluoride HF can etch glass and react with concrete, rubber, and many metals. HF reaction with metals can produce hydrogen gas, which can pose an explosion hazard. Within the MSRR facility, fluorides may react with water to produce HF, so any components containing fluoride salts and water may be affected.

The HF absorbent canister is designed to filter out HF from the system. Mass spectrometry can be employed to detect the presence of HF gas (see Section 4.3.2).

3.8.2 Carburization Carburization is a thermo-chemical process that can occur in steels. At high temperatures, surrounding carbon-bearing material can diffuse into the steel (Reference 7). Increased carbon content hardens steel, but also embrittles it by impeding dislocation movement. The graphite core will be the primary source of carbon within the MSRR facility.

3.9 FRETTING Fretting is a phenomenon that occurs at the interface between two contacting surfaces subjected to slight relative motion, typically induced by vibration or thermal expansion and contraction.

This micro-motion leads to repeated cycles of frictional rubbing, which can result in several detrimental effects, which include wear, abrasion, or material loss. This phenomenon is often encountered in heat exchangers.

Design considerations can help prevent areas of induced vibration or thermal expansion. This includes selecting materials with similar hardness and compatible surface finishes. Monitoring including UT and eddy current testing (see Section 4.3.2) can be used to monitor surface effects of frictional rubbing, such as material loss.

3.10 GALLING Galling is a form of wear that occurs when two metal surfaces slide against each other under pressure, causing material from one surface to transfer to the other. This phenomenon typically happens between metals with similar hardness and properties, where the friction and heat generated during sliding cause the surfaces to weld together. As the movement continues, the material can be torn away from one surface and deposited on the other, leading to rough, uneven surfaces, and ultimately causing the components to seize or lock together.

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PROPRIETARY 3-12 1851-0001-RPT-002, Rev 0 Design considerations can help mitigate galling in components, such as material selection, coatings, surface finish, and design clearances.

3.11 SEAL-RING LEAKAGE Leaking of a seal-ring is generally due to incorrect dimensioning of the seal-ring, excessive stretching or compression of the seal-ring, or differential thermal expansion between the seal-ring and surrounding components, and damage or debris on the sealing surfaces. Seal-rings in the reactor system exposed to high temperatures are particularly susceptible.

Design considerations regarding seal-ring dimensions and material can help prevent large differences due to extreme environments. The use of double seals permits leak testing which provides for monitoring and alert operators of a potential seal-ring failure.

3.12 FLOW-INDUCED DEGRADATION Flow-induced degradation (FID) refers to the degradation of materials or components due to fluid flow within the system. Three different phenomena are discussed in the next section, which are abrasion and erosion, cavitation, and flow-induced vibration.

Monitoring of salt flow conditions can be performed to ensure that abrasion and erosion rates are minimal. Allowances can be designed into pipe and vessel thicknesses. UT testing can be done periodically to identify any flaws introduced and eddy current testing can show loss of thickness in thin tubes.

3.12.1 Abrasion and Erosion Continuous flow of molten fuel and coolant salts through piping and other components such as heat exchangers may lead to abrasion, or erosion over time. Abrasion is the gradual loss of material due to the passage of hard particles over a surface. Erosion can occur due to particle impact on a surface.

3.12.2 Cavitation Cavitation is a phenomenon characterized by the formation and collapse of vapor bubbles (cavities) within a flowing liquid, and it occurs when the local pressure in the liquid drops below the vapor pressure of the liquid at a given temperature. These are generally generated due to changes in pressure inside pumps which may bring static pressure momentarily below the liquids vapor pressure. These tiny bubbles can create repeated shock waves that can erode components (see above).

3.12.3 Flow-Induced Vibrations (FIVs)

Flow induced vibrations result from turbulence in a flow of fluid due to discontinuities such as bends, partially closed valves, and defect. These vibrations can vary in frequency and amplitude and can cause fatigue damage (see high-cycle fatigue). Piping as well as components that have fuel or coolant salt flowing within them, including heat exchangers, may be susceptible.

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PROPRIETARY 4-1 1851-0001-RPT-002, Rev 0 4

Mitigation Strategies This section provides a preliminary list of mitigation strategies that can be employed in the MSRR to mitigate the identified degradation mechanisms. Mitigation strategies are divided into three main groups based on the timing of their implementation 1) Design, 2) Fabrication, and

3) Operation.

4.1 DESIGN In many cases, degradation mechanisms can be precluded during the design of the components.

Design codes, materials selection, geometries, component configurations, or margins can all have a major impact on the susceptibility of components to degradation. This discussion provides guidance on the use of design to minimize or preclude relevant degradation mechanisms.

4.1.1 Material Selection The material selection involves a systematic approach that considers both the operational environment and the specific degradation mechanisms expected to affect the component. Key factors include assessing the temperature, pressure, chemical exposure, radiation fields, and mechanical stresses the material will endure, and whether the material is approved for the conditions of interest by the ASME code.

The selection process involves evaluating the material's mechanical properties, such as strength, toughness, and fatigue resistance, to ensure longevity under operational stresses. Additionally, factors like cost-effectiveness, availability, and compatibility with fabrication processes are considered to optimize performance and minimize the risk of degradation over the plant's lifecycle.

4.1.2 Code Design Margins The MSRR is designed to meet the ASME Code Section III, Division 5,Section VIII, or B31.3, depending on the component function and environmental condition. The ASME Code is intended to address some, but not all failure mechanisms. Although the ASME Code does not typically account for degradation mechanisms and the potential effect on failure mechanisms, following the Code promotes practices aimed at ensuring the integrity and robustness of the component. Possible uses of the ASME code directions to prevent degradation mechanisms are provided below.

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PROPRIETARY 4-2 1851-0001-RPT-002, Rev 0 Plastic Collapse Plastic collapse is a degradation mechanism already defined in Section 3.1.1. The ASME Code provides well established margins against overload failures, such as plastic collapse. Designing in accordance with these Codes and meeting the allowable stresses, properly defining all loading conditions, and providing adequate corrosion allowances will generally preclude credible failures due to over loading.

Fatigue The ASME Code provides rules to address cyclic loading, in particular protection against fatigue. However, there are limitations that need to be considered. These are summarized below:

Fatigue curves are based on testing of specimens, generally to load drop (i.e., failure) and not to crack initiation. Meeting the Code fatigue rules does not preclude fatigue cracking.

Fatigue testing was conducted in air at room temperatures. While the design margins are intended to account for in-service conditions, effects of environment and other enhancing degradation mechanisms may not be fully addressed.

Proper fatigue design requires fully defining the loading events. Fatigue failures in-service are most commonly associated with loading events that were not considered in design.

When designing against fatigue, using the ASME Code rules is an excellent mitigation strategy, but uncertainties may dictate larger design margins. This can be accomplished by design in selecting geometries and in limiting loading events. In general, the lower the fatigue usage, the greater the likelihood that fatigue may be precluded by design.

Creep Creep is a high temperature degradation that results from a combination of high stresses held at high temperature for a long period of time. The ASME Code rules address creep; although some creep phenomena can be exacerbated by environmental conditions that are not accounted for by the ASME Code rules.

When designing against creep, using the ASME Code rules is an excellent mitigation strategy, but uncertainties and environmental effects may dictate larger design margins. This is generally accomplished through conservative design (i.e., thicker walls) or limiting the load-controlled stress to very low values. Temperature and time are typically a given, leaving stress as the most important variable. Welds are also known to be much more susceptible to creep than wrought material; therefore, welds should be minimized and located in the lowest temperature and stress regions practicable. In general, the lower the creep usage, the greater the likelihood that creep may be precluded by design.

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PROPRIETARY 4-3 1851-0001-RPT-002, Rev 0 Thermal Aging Thermal aging is a degradation mechanism that is addressed by the ASME Code rules.

Designing in accordance with these Codes and meeting the allowable stresses, properly defining all loading conditions, will generally preclude credible failures due to thermal aging.

Nonductile Failures The ASME Code addresses nonductile failures either by requiring the use of ductile materials (e.g., impact testing limitations) or by evaluation against non-ductile failures. The Code does not require any special evaluations of Type 316H, as this material is very ductile with a high toughness. However, the Code does not address embrittlement due to service conditions.

Embrittlement due to service conditions is best addressed by limiting conditions that could significantly embrittle Type 316H, maintaining high structural margins (i.e., low stress) and locating welds in less susceptible locations. These strategies would likely mitigate embrittlement by design.

4.1.3 Weld Geometry and Locations Welds are often a weak point in component construction due to the interface between dissimilar metals, energy input leading to residual stresses, and microstructural changes in the heat affected zone (HAZ). The most effective design strategy is to limit or avoid welds or locate them in the least susceptible locations. Examples are:

Use of pipe that is bent rather than constructed of welded fittings.

Use of ring forgings rather than rolled and welded plate.

Machining components from larger forgings, rather than welding plate or smaller fittings.

Locate welds in areas of lowest temperature, stress, and radiation.

Residual stresses can be minimized via optimized joint geometry such that components are not highly constrained or heavily loaded.

Nozzle welds can be located in heads using full penetration butt welds rather than larger constrained corner welds.

4.1.4 Thermal Transients and Mixing Fatigue degradation mechanisms such as HCF, LCF, and EAF are all the result of cyclic thermal events. Thermal stratification, high cycle mixing events, rapid transients, and frequency heat up and cooldown cycles all induce cyclic stresses. Designing systems to limit the severity or frequency of thermal cycling can preclude fatigue damage.

4.1.5 Environmental Controls Many degradation mechanisms (e.g., Corrosion, Flow Induced Degradation, Embrittlement) are environmentally dependent. Designing to ensure controlled flow of fuel and coolant salt throughout the system can help preclude these DMs. For example, speed of flow can be R24014 Page 40 of 52

PROPRIETARY 4-4 1851-0001-RPT-002, Rev 0 designed to be low enough to preclude abrasion or erosion. The geometry of the flow path can be optimized to avoid disturbances that could lead to cavitation. While neutron embrittlement is more effectively mitigated through material selection and fabrication methods, it is conceivable to protect susceptible components with neutron shields. Redox potential of fuel salts can be controlled through purification, achieved via the hydrofluorination-hydrogen process and mechanical filtration, and through beryllium or UF4 additions, which can eliminate conditions required for corrosion mechanisms.

4.1.6 Corrosion Allowances Corrosion allowance refers to an additional thickness of material intentionally added to a component to compensate for expected corrosion over its intended service life. The allowance is specified during the design phase. Building a corrosion allowance into the design guards the component from failure so it can remain in accordance with ASME Code and functional requirements throughout the lifetime of the plant, despite corrosion over time. For the MSRR, corrosion is mainly due to the presence of salt with impurities, as discussed in Section 3.7.

4.2 FABRICATION Mitigation during fabrication encompasses the use of strategies that are applied to materials to adjust properties based on intended use. A list of preliminary mitigation strategies for fabrication is provided below.

4.2.1 Material Specifications Once the material is selected during design, additional specifications can be added to the material to optimize its behavior. For example, grain size may be adjusted, as finer grain sizes enhance mechanical properties such as strength and toughness, while coarser grains provide better resistance to creep deformation. Consultation with material suppliers can help obtain guidance for the specific material and application requirements.

4.2.2 Cold Work Cold working, also known as cold deformation or cold forming, is a process used to improve the mechanical properties and performance of metals and alloys by applying plastic deformation at temperatures below their recrystallization point. Cold work can effectively mitigate degradation mechanisms through several mechanisms, which include:

Increased Strength and Hardness: Cold work introduces dislocations and deformation within the crystal lattice of the material, which increases its strength and hardness. This makes the material more resistant to deformation under load and improves its ability to withstand wear and abrasion.

Improved Fatigue Resistance: By refining the grain structure and introducing compressive stresses, cold work enhances the material's resistance to fatigue failure.

Enhanced Yield and Tensile Strength: Cold work typically increases both the yield strength and tensile strength. This improvement makes the material more capable of withstanding higher loads without permanent plastic deformation or failure.

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PROPRIETARY 4-5 1851-0001-RPT-002, Rev 0 Improved Corrosion Resistance: Certain forms of cold work, such as peening, can impart a work-hardened surface layer that improves the material's resistance to corrosion and environmental degradation. This is achieved by densifying the surface and reducing the availability of sites for corrosive attack.

While cold work can be used to improve properties such as strength and hardness, the introduction of dislocations causes a sacrifice of ductility. In cases where ductility is required, limiting the cold work could be an option to mitigate degradation mechanisms.

4.2.3 Heat Treatment Heat treatments are used on metals to improve their mechanical properties, enhance their performance, and mitigate potential DMs when they are incorporated into components. The main purposes are:

Strengthening: to increase the strength and hardness of the metal by modifying the microstructure through controlled heating and cooling processes.

Stress Relief: to relieve internal stresses induced during fabrication processes such as welding, machining, or forming to help prevent deformation and cracking.

There are many different heat treatments, and the effect of their application depends on the material. Available heat treatments include:

Annealing: used to soften a material, relieve internal stresses, and refine grain structure.

Hardening: consists of rapid cooling (quenching) used to increase hardness.

Tempering: reduces embrittlement caused by hardening, while retaining some strength and hardness.

Normalizing: refines material grain structure and improves toughness and machinability.

Stress Relieving: reduces internal stresses caused by processes like welding or machining.

Precipitation Hardening: consists of controlled precipitation of particles within the solid solution matrix to increase strength and hardness.

Solution Treatment: improves material properties by dissolving precipitates in an alloy.

Aging: consists of a heat treatment at a lower temperature after solution treatment to precipitate fine particles and improve strength.

The appropriate heat treatment depends on the degradation mechanism to be mitigated. The selection of the appropriate heat treatment should be performed with the material vendor and manufacturer.

4.2.4 Coatings and Claddings Coatings and claddings provide protective layers that help prevent or minimize degradation processes such as corrosion, wear, galling, erosion, and thermal fatigue. Coatings are thin layers R24014 Page 42 of 52

PROPRIETARY 4-6 1851-0001-RPT-002, Rev 0 of material applied to the surface of a substrate, while claddings involve the application of a thicker layer of material onto a substrate to provide more substantial protection or to change the surface properties significantly.

For corrosion protection, a coating or cladding can act as a barrier between the metal substrate and the corrosive environment, preventing direct contact and inhibiting the penetration of corrosive agents. Both can also provide chemical resistance, cathodic protection, and mitigate wear and abrasion by increasing surface hardness of a component.

The employment of a coating for a specific material or component should be carefully evaluated and tailored for the intended use of the component and scope of the coating.

4.3 OPERATION During operation, the DMP identifies techniques to assess, monitor, and manage degradation to ensure the component's functionality throughout its operational life. As discussed in Section 2, mitigation during operation has different objectives depending on whether [

))

4.3.1 Monitoring on-Line ((

))

If the degradation mechanism is fully mitigated during design and fabrication, monitoring instrumentation can be installed ((

)) are discussed below:

Operating Parameters Monitoring Monitoring operating parameters in a research reactor is critical to ensuring failure of a component can be predicted and is identified when it occurs.

Continuous monitoring of temperature is essential to prevent overheating, which could lead to thermal stresses and potential equipment failures. Advanced temperature sensors placed strategically can provide real-time data to operators, enabling prompt adjustments to cooling systems or reactor power levels as needed.

Additional integrated monitoring systems utilize pressure sensors, flow meters, fluid level sensors, vibration sensors, and radiation detectors to detect and localize out of range parameters.

Gas Detection Tubes Gas detection tubes are portable devices that use chemical reagents to detect specific gases.

These tubes provide a quick, qualitative assessment of gas presence and can be useful for preliminary leak detection in confined spaces or remote areas.

Leak Testing Defects that may cause leaks can be detected by applying pressure to generate fluid flow toward lower pressure - where the leak is. Inspections should look for cracks, holes, weak seals, and R24014 Page 43 of 52

PROPRIETARY 4-7 1851-0001-RPT-002, Rev 0 other flaws that could allow fluid to leak out of a system or component. Several different leak testing methods exist and can be chosen from, including vacuum decay, pressure decay, and pressure crack methods.

Visual Inspection Visual inspections can be used to identify failure of components that are visually accessible. For components that are exposed and visible during operation, visual inspections with naked eye, or using a camera and a robot, can quickly and efficiently reveal issues without the need for disassembly.

Other In-service Monitoring There are several other in-service monitoring techniques available for use throughout the MSRR.

Possible instruments that could supply useful information about the conditions of operation include accelerometers, proximity probes, thermal detectors, radiation detectors, and strain gauges. Some examples of other available monitoring techniques include seal leak monitoring and pressure tests.

4.3.2 Monitoring ((

))

If the degradation mechanism is not fully mitigated during design and operation, [

] These are especially useful when there is uncertainty regarding the remaining life of a component, as in-service monitoring can provide updated information on the degradation mechanism trend. Inspections should be periodic. The decision on frequency will depend on the predicted rate of the degradation mechanism. In some cases, NDE inspections may require the reactor to shut down. Preferably, these inspections can be scheduled during a service period.

Visual Inspection Visual examination can identify visual anomalies (e.g., pitting, coating failures, build-up) on its own or characterize anomalies identified with the other inspection methods. Visual inspections can be used to identify issues to components that are visually accessible. The simplest method for visual examination is with the naked eye and is easily conducted for piping that is excavated or exposed. Visual examination can also be conducted internally using borescopes or in-line robotic crawlers equipped with video. Remote visual inspection using cameras and robotic systems equipped with video capabilities could be used to inspect areas that are difficult to access manually. The inspector can assess the components physical conditions and confirm that the component meets the design specifications and performance standards. Proper alignment, clearances, and absence of deformation can also be confirmed. Visual inspection can be accompanied by field measurements of the components distortion at regular intervals. Early signs of degradation that can be detected with a visual inspection, such as cracks, discoloration, or surface irregularities, are indicators that can reveal the onset of various forms of damage, including corrosion, wear, and leaks.

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PROPRIETARY 4-8 1851-0001-RPT-002, Rev 0 Ultrasonic Testing UT is a non-destructive testing method useful for detecting small defects such as porosity, cracking, and thickness variation. UT can penetrate deep into materials, depending on the frequency of the ultrasound waves and the material properties of the surface. This allows inspection of thick components including reactor vessel, piping, and welds. As a limitation, UT requires a smooth and accessible surface for the transmission and reception of the ultrasonic waves. Rough or contaminated surfaces may hinder effective inspection or produce unreliable results. Additionally, the effectiveness of UT can be limited in highly curved or irregularly shaped components where sound waves may be reflected or scattered. API 579 provides directions on how to establish UT use.

Ultrasonic Guided Wave Testing Ultrasonic guided wave testing (GWT) provides a volume inspection of a large length of pipe from a single external inspection location. Unlike UT transducers which act perpendicular to the pipe wall, GWT transducers send sound waves down the length of the pipe, most often in the form of a collar and can detect anomalies based on the nature of the reflected signal. GWT can indicate areas with wall loss, pitting, and embedded discontinuities as well as give a general indication of the remaining wall thickness.

Surface Penetrant Testing Surface Penetrant Testing (PT) is a non-destructive testing method that uses a low-viscosity dye to detect surface-breaking defects on non-porous and relatively smooth surfaces. This technique can detect leaks, hairline cracks, fatigue cracks, surface porosity, and other flaws. Specifically, it can be used around welded areas to check for cracks due to differential cooling or residual stress.

Eddy Current Testing Eddy current testing is a non-destructive testing method that uses electromagnetic induction to detect flaws along piping and tubes. Moving an electronic probe along a conductive material induces an eddy current (a current in the opposite direction) which is then measured. It can be used to detect material loss within pipes due to corrosion, abrasion, or erosion. However, it is worth noting that it will not be able to detect selective dissolution (see Section 3.7.1), as no measurable dimensional change would result.

Dye Penetration Dye penetration methods involve injecting a colored dye into the system. This dye can be visually observed under ultraviolet (UV) light and can point to leaks and cracks.

Mass Spectrometry Mass spectrometers can detect minute quantities of trace gases that may leak from a system.

This highly sensitive method is useful for identifying leaks in vacuum systems, refrigeration systems, and other critical applications where even small leaks can be significant.

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PROPRIETARY 4-9 1851-0001-RPT-002, Rev 0 Fiber Optic Sensing Fiber optic cables can be used to detect changes in temperature or strain that could indicate the presence of leaks. Distributed sensing techniques, such as distributed temperature sensing (DTS) or distributed acoustic sensing (DAS), provide continuous monitoring along the length of the fiber optic cable. Fiber optics are affected by radiation, which can cause attenuation of the signal and damage of the material over time.

Surveillance Coupons Coupons help understand how reactor materials degrade over time due to environment exposure and operational stresses. Surveillance coupons are installed during reactor construction or refueling outages using specialized handling and deployment tools designed for radiation environments. Retrieval can also be done during service periods. Retrieval procedures involve careful handling and transport to designated laboratories for post-irradiation examination (PIE),

where detailed analysis and testing are conducted to evaluate material properties and degradation mechanisms.

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PROPRIETARY 5-1 1851-0001-RPT-002, Rev 0 5

References

1.

MPR Associates, Inc., MSRR Management of Reactor Vessel Degradation, Embrittlement, Creep, and Fatigue of Type 316H Stainless Steel. Report No. 1851-0001-RPT-001, Revision 0, 2024.

2.

Nuclear Regulatory Commission, Construction Permit Application Documents for the MSRR - Abilene Christian University, June 27, 2024, https://www.nrc.gov/reactors/non-power/new-facility-licensing/msrr-acu/documents.html, accessed on July 2nd, 2024.

3.

2023 ASME Boiler and Pressure Vessel Code, American Society of Mechanical Engineers, 2023,Section XI, Division 2.

4.

Balazik, Michael, Nuclear Regulatory Commission, Materials Degradation RCIs, email message, addressed to Benjamin Beasley and Lester Towell, Abilene Christian University, May 30, 2024, https://www.nrc.gov/docs/ML2415/ML24158A230.pdf.

5.

Al-Buraiki, Iyad, Al-Ismail, Sadiq, and Mohammed Abu Alsaud, "Stress Relaxation Cracking of Thin Alloy 800/800H Electric Heater Tubular Heating Elements," AMPP Annual Conference + Expo, San Antonio, Texas, USA, March 2022.

6.

Material, Fabrication, and Repair Considerations for Austenitic Alloys Subject to Embrittlement and Cracking in High Temperatures 565 °C to 760 °C (1050 °F to 1400

°F) Refinery Services, API Technical Report 942-B, API, May 2017.

7.

Hamidreza Torbati-Sarraf, Amir Poursaee, 9 - Influence of the microstructure of the carbon steel reinforcing bar on its corrosion in concrete, Poursaee, Amir (ed.),

Corrosion of Steel in Concrete Structures (Second Edition), Woodhead Publishing, 2023.

8.

Preliminary Safety Analysis Report, Abilene Christian University Molten Salt Research Reactor Preliminary Safety Analysis Report, Rev 2, July 2024.

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PROPRIETARY A-1 1851-0001-RPT-002, Rev 0 A

Flow Charts

((

))

Figure A-1.

Program Description R24014 Page 48 of 52

PROPRIETARY A-2 1851-0001-RPT-002, Rev 0

((

))

Figure A-2.

General Process. A, C, and E are addressed in the following figures.

R24014 Page 49 of 52

PROPRIETARY A-3 1851-0001-RPT-002, Rev 0

((

))

Figure A-3.

A: Design Procedure R24014 Page 50 of 52

PROPRIETARY A-4 1851-0001-RPT-002, Rev 0

((

))

Figure A-4.

C: Fabrication R24014 Page 51 of 52

PROPRIETARY A-5 1851-0001-RPT-002, Rev 0

((

))

Figure A-5.

E: Operation R24014 Page 52 of 52