ML24081A122
| ML24081A122 | |
| Person / Time | |
|---|---|
| Issue date: | 03/20/2024 |
| From: | Boyce T, Colon-Gonzalez F, Reed A NRC/RES/DE |
| To: | |
| Wendy Reed 3014157213 | |
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W. REED et al.
1 NRC RESEARCH ACTIVITIES IN SPENT FUEL STORAGE AND MANAGEMENT OF ADVANCED FUELS FOR ADVANCED REACTORS W. REED U.S. Nuclear Regulatory Commission Rockville/MD, United States of America Email: wendy.reed@nrc.gov F. COLN-GONZLEZ U.S. Nuclear Regulatory Commission Rockville/MD, United States of America T. BOYCE U.S. Nuclear Regulatory Commission Rockville/MD, United States of America Abstract With the increased interest and activities in development of commercial advanced reactors and their fuels, the U.S.
Nuclear Regulatory Commission (NRC) is assessing various advanced non-light water reactor (ANLWR) fuel types to ensure its readiness to license the storage and transportation of these fuels within its flexible regulatory framework. These advanced fuel types include metal fuel, TRISO pebbles, and molten salt fuels. To aid this assessment, the NRC staff is engaged in several research activities to expand their understanding of the key technical and regulatory considerations related to these ANLWR fuel types. These considerations may include various waste forms and waste streams, radiochemistry of constituents, storage canister performance and degradation, actinide separation, tritium management strategies, and other attributes. The outcome of these activities is intended to ensure staff readiness to review near-term licensing/certification actions, support pre-application engagements, and inform guidance development, as needed. This document will provide an overview of the various research activities and a summary of the recently completed reports.
- 1.
INTRODUCTION In the rapidly evolving landscape of nuclear technology, ANLWRs have emerged as a promising solution to revolutionize the energy sector. Despite their potential, these innovative technologies also pose unique challenges. These challenges encompass a wide range of areas across the fuel cycle. In addition to preparing for licensing and regulating new technologies in reactor design, fuel qualifications, and plant controls, the staff is assessing challenges in the management of waste, particularly storage and transportation of spent fuel from ANLWRs.
In December 2016, the NRC issued its Vision and Strategy: Safely Achieving Effective and Efficient Non-Light Water Reactor Mission Readiness, outlining the objectives, strategies, and activities necessary to ensure readiness for potential ANLWR license applications. The vision and strategy encompass three key objectives: enhancing technical readiness, optimizing regulatory readiness, and optimizing communication. To achieve these objectives, the NRC developed an implementation action plan with six strategies and associated near-and mid-term goals.
The NRC is exploring potential challenges regarding the storage of advanced reactor fuels (ARFs) as part of its action plans. A key challenge is characterizing the spent fuel and waste from the new ARF types so that it is clear what performance characteristics are needed for the canisters and casks to support long-term storage.
The NRC is particularly interested in understanding whether the current approaches and technology for storing spent fuel can be compatible with ARF types such as metal fuel, tristructural isotropic (TRISO) fuel, and molten salt reactor (MSR) fuel. For instance, the NRC is examining whether the chemical and physical characteristics of the waste forms can create new degradation processes for the containers used to store and transport the ARF types and potential waste forms.
This document provides an overview of the approach that NRC is using to guide its research activities regarding ANLWR spent fuel, as well as a summary of recently completed reports. These reports include an
IAEA-CN-142 assessment of the current state of knowledge of storage and transportation of these ARF types and potential challenges for future storage and transportation and systems.
- 2.
NRC PLANNING FOR NEW FUELS LICENSING, OVERSIGHT AND RESEARCH The environment in new fuels is evolving quickly as new reactor vendors emerge with a variety of reactor designs and fuel characteristics. The NRC has developed an approach to prepare for its reviews of new fuels across the fuel cycle, including reviews of fuel facilities on the front end of the fuel cycle and transportation and storage of spent fuel on the back end of the fuel cycle. The below graphic (Fig. 1) depicts at a high-level the approach that the NRC is using to help plan its work. It includes the consideration of licensing timelines, preparations for oversight activities, and research activities. It also includes consideration of rulemaking, coordination with international partners, and communications with public stakeholders. More detailed information can be found on the NRCs New Fuels website at: https://www.nrc.gov/materials/new-fuels.html.
FIG 1. New Fuels Infographic - Phases of the front and back end of the fuel cycle The NRCs approach to research activities is depicted below (Fig. 2). The top row shows the various stages of the fuel cycle. The vertical portion on the left side lists the various technical areas that the NRC reviews as described in its Standard Review Plans for fuel facilities and storage and transportation systems.
The figure is intended to illustrate the planning process rather than depict definitive NRC plans, recognizing the dynamic nature of the ANLWR environment. We identify some activities, note if its not applicable or not expected, or if its not really ripe to assess. Where available, references are also cited. The NRC has a similar research planner table for each of the three ARF fuel types.
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3 FIG 2. Research Planner
- 3.
CHARACTERISTICS OF ADVANCED REACTOR SPENT FUEL The NRC has conducted assessments of the state of knowledge regarding ARF types to inform the NRCs new fuels regulatory research planner. An overview of these assessments is provided in Section 5 and an overview of the fuel types is provided below.
3.1.
TRISO Fuel Based on decades of coated particle fuel testing simulating in-reactor conditions, a number of potential coated particle failure mechanisms have been identified [1-2]. These mechanisms include migration of the fission product palladium from the fuel kernel during irradiation, which could chemically attack the silicon carbide (SiC) layer of TRISO-coated particles by forming palladium silicides at localized reaction sites and compromise the structural integrity of the SiC layer. However, these failure mechanisms require high stressors (e.g., temperature, radiation rates, and forces) that are not expected to arise in storage settings of the TRISO-coated particle fuel.
High burnup and enriched TRISO-coated particle fuel poses potential challenges to safe storage beyond those associated with light water reactor spent fuel storage. Storage of TRISO fuel discharged from HTGRs with higher burnup and higher enrichment combinations will likely require new criticality benchmark data and also burnup credit and depletion analysis for storage container designs and fuel burnup loading curve evaluations.
Additionally, TRISO fuel discharged from a fluoride salt-cooled high temperature reactor may contain some solid fluoride salts, which can undergo radiolysis at low temperatures and generate fluorine gas that is toxic and potentially corrosive. Degradation mechanisms involving the interaction of solid fluorine salt and TRISO fuel did not appear to be well documented in existing research. Therefore, materials evaluations for dry storage of spent TRISO fuel with residual salt material would need to ensure that a dry storage system provides adequate materials performance for safely storing spent solid coated particle fuel.
3.2.
Metallic Fuel Metal fuel has characteristics important to storage and transportation that differ from light water reactor spent fuel, and which may require detailed analysis to fully understand performance for storage of these spent
IAEA-CN-142 fuel types. These differences include factors unique to the composition of the fuel, thermal limits, criticality, and material performance.
Water ingress into spent metal Na-bonded fuel could cause an exothermic reaction. Given the possible pre-existence of fuel cladding degradation from reactor operation, and the difficulty with monitoring the storage environment, factors affecting storage of sodium-bonded metal fuel have not been fully addressed through past experience.
Some of the stored fuel has been treated chemically to deactivate the sodium, generating metallic and ceramic waste forms suitable for disposal in a permanent repository. As a result, the storage of spent nuclear metal fuel comprises both original waste forms and converted forms.
However, if the cladding has defects that allow moisture to contact the metal fuel during wet and dry storage, moisture and O2 are expected to react quickly with sodium, producing Na2O, NaOH, and H2, which can cause secondary reactions.
3.3.
Salt Fuel Salt waste management from fluid-fueled molten salt reactors, in particular, represents a fundamental challenge to the established infrastructure regarding managing radioactive waste, including spent fuel. Salts are highly hygroscopic, therefore will require active management after exiting the reactor, or immobilization for long term storage. Additionally, some proposed salts for use in these reactors contain beryllium (Be), which is a known toxic element and will need to be carefully managed. Salts may need to be processed to separate out the various constituents depending on the waste management strategy; for example, the salt may undergo a dehalogenation process to allow better encapsulation of the radioisotopes in a vitrified waste form [3].
With regard to molten salt reactors, there is no direct experience with converting the used salt into waste forms. In the case of the molten salt reactor experiment (MSRE), at shutdown, the fuel salt (LiF-BeF2-ZrF4-UF4) was drained into three metal drain tanks in the late 1960s and has been actively managed since then.
Management of MSR waste potentially represents a fundamental challenge to the established infrastructure for LWRs. When considering putting the waste into a form that is suitable to be used, several characteristics of the waste need to be considered:
decay heat; concentrations of total halogen, alkali, alkaline earths, rare earths, transition metals, U, Pu, and other actinides; ratios of different halogens (e.g., Cl:F:I) and alkalis (e.g., Li:Na:K:Cs) and, to a lesser extent, alkaline earths, rare earths, and transition metals; concentration of long-lived isotopes (e.g., I-129, Cl-36, Se-79, Tc-99, Pu-242, Np-237); and fissile content (e.g., U-233, U-235, Pu-239, Pu-241).
- 4. NRC RESEARCH ON ADVANCED REACTOR SPENT FUEL WASTE FORMS AND STORAGE The NRC is using the approach in the Research Planner to conduct additional research to enhance understanding of the potential challenges associated with the storage and transportation of advanced reactor fuel types and their wastes, and to determine any appropriate regulatory actions that might be needed (e.g., revisions to guidance documents and regulations).
In general, the NRC is assessing potential challenges that must be met by canisters and casks to be used for both short-term and long-term storage and transportation of the waste from the new fuel types such as metal fuel, TRISO pebbles, and molten salt. As discussed in the previous section, each of advanced reactor spent fuel type has its own technical challenges. MSR salt fuels are in liquid form during reactor operation, which is very different from the solid clad fuel in most other Light Water Reactor (LWR) and ANLWR reactor concepts.
MSRs will require several types of fuel salt processing operations including noble gas (e.g., Kr, Xe) removal, removal of fission products not soluble in the fuel salt (e.g., noble metals), and recovery of fissile or fertile material. Additionally, the management of waste streams may require novel waste forms from post-reactor operations to limit the volumes of generated spent fuel or high-level waste, which will need to be evaluated in the context of the regulatory framework for independent storage (Title 10 Code of Federal Regulations (10 CFR) Part 72) and transportation (10 CFR Part 71).
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5 The NRC is currently conducting a technical assessment of continued storage options for MSR salt waste. This work will provide NRC staff insights into characteristics of various waste forms and loading with a view to informing future safety and licensing reviews related to storage and transportation of ANLWR waste and waste forms. In addition, the work looks to form the basis of regulatory guidance to support licensing and certification of spent fuel storage and transportation options.
With regard to spent TRISO fuel, NRC is looking to understand the performance of TRISO coating layers under storage and transportation conditions. For metallic fuel, NRC is planning on assessing the applicability of codes and standards to better understand some of the safety implications of storing sodium-containing metal fuels, and to assess the performance of stainless-steel cladding and sodium-bonded spent metal fuel under storage environments.
- 5.
SUMMARY
OF RECENTLY COMPLETED REPORTS This section provides a summary of some recently completed reports from the NRC regarding spent fuel storage and management of advanced reactor fuels. The reports below have informed the NRCs regulatory research planner by providing characteristics of the fuel types as discussed in Section 3 and identifying challenges to be assessed for storage and transportation.
5.1.
Assessment of the Current State of Knowledge on Storage and Transportation of Molten Salt Reactor Waste [4]
The NRC report provides an assessment of the current state of knowledge of storage and transportation of MSR waste. The public documentation for many MSR designs does not provide information on the extent to which molten fuel salt would be processed after discharge from a reactor. This is because reactor vendors are not emphasizing the waste management aspects of the designs. However, for some MSRs, actinides can be recovered and reused as fuel, which is a significant concept as radioelements such as plutonium dominate the waste fuel salt activity after about 300 years. Recovering actinides would result in a salt waste stream consisting of shorter-lived fission products. Given the hygroscopic nature of salts, they will likely require long-term immobilization. Therefore, processing the molten salt could reduce the total volume of waste that may need to be stored and could also reduce the amount of waste remaining during long term storage or disposal timeframes, ultimately allowing for more efficient waste form production. It is feasible that some or all potential waste processing operations, including waste form development, could be carried out at the reactor site or at another site, such as a centralized processing facility which would service more than one MSR.
5.2.
Potential Challenges with Storage of Spent (Irradiated) ARF Types [5]
The NRC report discusses potential challenges with the storage of spent ARF, focusing on both canister performance and the configuration of the spent fuel. It highlights issues with two types of fuel: TRISO and nuclear metal fuel. High burnup and enriched TRISO fuel pose unique challenges. In the case of TRISO fuel used in molten salt cooled reactors, there is the potential generation of toxic and corrosive fluorine gas due to radiolysis of solid fluoride salts. The report suggests that further research is needed to ensure safe storage of spent TRISO fuel with residual salt material. For sodium-bonded fuel with stainless-steel cladding, the report notes that degradation can occur in a dry storage environment due to moisture penetration, emphasizing the importance of maintaining canister integrity. The report concludes that factors affecting the storage of sodium-bonded metal fuel have not been fully addressed and that differences in thermal and radiological characteristics between spent metal fuel LWR and Spent Nuclear Fuel (SNF) may impact the structural characteristics of Dry Storage Systems (DSSs). Cladding degradation could occur at elevated storage temperatures, typically greater than 400 °C (752 °F), primarily induced by internal stress.
The NRC report also discusses additional thermal-induced degradation mechanisms under dry storage conditions that can challenge material performance. Sensitization, uranium oxidation, and pyrophoric reactions involving uranium hydrides can occur at temperatures greater than 200 °C (392 °F). The degrees to which these degradation mechanisms affect material performance vary and are affected by the presence of moisture.
Degradation from embrittlement is of concern, depending on the amount, morphology, and distribution of the
IAEA-CN-142 ferrite phase in the weld, the composition of the stainless-steel cladding, and the time spent in the necessary temperature region. An assessment of physical and chemical properties of spent metal fuels and how they interact with proposed storage environments would facilitate a better understanding of the possible degradation mechanisms applicable to these waste forms. This assessment, including an initial characterization of the cladding integrity and of the quantity and thermal characteristics of spent fuel to be loaded in DSSs, would be performed as part of the certification process. The current state of materials evaluations for spent metal fuel and TRISO DSSs and DSFs does not completely characterize specific canister and non-LWR fuel performance under all storage conditions and storage environments that could be expected for ARF. Additional characterization of material performance and the influence of the different possible physical and chemical ARF waste forms, possible ARF configurations, radiation effects, thermal loading, and long-term canister material performance under a variety of storage environments will clarify the fuel and material performance of ARF and address challenges observed from past storage experience.
5.3.
Storage Experience with Spent (Irradiated) ARF Types [6]
The NRC report reviews the storage of spent non-LWR fuels, including TRISO-coated particle fuel and nuclear metal fuel. The study aims to identify factors contributing to fuel degradation during storage. The report discusses the physical and chemical changes in irradiated non-LWR fuel, and reviews literature on storage experiences for various types of reactors. For TRISO-coated particle fuel, failure mechanisms have been identified such as the migration of fission product palladium, which can attack the SiC layer of the particles.
However, these mechanisms are not expected to occur during storage due to lower stresses, temperatures, and radiation fields.
Nuclear metal fuel, characterized by its porosity and anisotropic swelling, undergoes changes during reactor operation, including redistribution of fuel constituents and sodium fusion. The cladding housing the fuel can degrade, leading to thinning and cracking. The report also reviews storage conditions and fuel performance during storage for spent non-LWR fuel from various reactors. TRISO-coated particle fuel elements are stored at different facilities, with no records of fuel failure under storage conditions at one facility, but moisture compromised the integrity of canisters at another, causing a release of gaseous radionuclides.
Spent nuclear metal fuel has been stored in both wet and dry conditions. Issues arise from the potential interaction of sodium with the storage environment, leading to the production of hydrogen and sodium hydroxide. Some fuel has been chemically treated to deactivate the sodium, resulting in waste forms suitable for disposal. The remaining fuel will continue to be treated. The report concludes that the degradation issues experienced during storage and the changing inventory of waste forms pose challenges for regulating long-term storage.
- 6. CONCLUSION/KEY REMARKS The NRC is currently engaged in a variety of research activities to better understand the technical considerations regarding the storage and transportation of advanced reactor spent fuel types. As part of its efforts, the NRC continues to expand engagement with international groups and regulatory counterparts to promote the exchange of technical and regulatory experience while establishing pathways for collaboration and research. The desired outcome of these activities is to have sufficient analytical capabilities within the needed timeframes to support regulatory interactions and reviews of applications for licenses, certifications, and approvals. The NRCs efforts demonstrate a comprehensive and proactive approach to addressing the complex challenges associated with ANLWR spent fuel management.
ACKNOWLEDGEMENTS The authors would like to thank members of the Nuclear Regulatory Commission for their technical review and contributions. Special thanks are extended to Dr. Raj Iyengar, Branch Chief of the NRCs Reactor Engineering Branch, and Mr. Jesse Carlson, a Materials Engineer from the NRCs Materials and Structural Engineering Branch.
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