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Author: James Rubenstone
The Nuclear Regulatory Commission must balance its preparations for policy changes with the safe, secure operation of existing facilities, the availability of resources, and the constraints of current law.
Under the current structure for the management of commercial spent nuclear fuel (SNF)1 in the United States, the licensee is responsible for its safe and secure storage and transportation. The U.S. Nuclear Regulatory Commission (NRC) ensures safety and security through licensing and regulatory oversight. NRC also has regulatory authority over the disposal of SNF and other types of high-level radioactive waste (HLW)2 in a geologic repository.
NRC’s authority to regulate SNF and HLW derives from federal law, including the Atomic Energy Act of 1954, the Energy Reorganization Act of 1974, and the Nuclear Waste Policy Act of 1982, as amended in 1987. NRC implements its regulatory authority through regulation, guidance, inspection, orders, enforcement, and other oversight actions.
Because of recent policy decisions, the U.S. program for future management of SNF and other HLW is undergoing a transition. Until a new national policy emerges, NRC is continuing to carry out its primary mission of ensuring the “safe use of radioactive materials for beneficial civilian purposes while protecting people and the environment.” At the same time, NRC staff is working to prepare itself to fulfill its role in the new national policy for managing SNF and other HLWs, specifically in the areas of licensing and regulation of the back end of the nuclear fuel cycle.
Current Regulatory Framework
Most commercial SNF in the United States is owned by the utility that produced it and is stored—either underwater in spent fuel pools or in dry-cask systems—on the sites of operating and decommissioned nuclear power plants. NRC regulation of wet storage at nuclear power plant sites falls within the operating license for each reactor issued under the regulations in Title 10 of the U.S. Code of Federal Regulations (CFR) Part 50.
Most reactor facilities were not designed to store the full amount of SNF that the reactor generates over its operational life. Therefore, as pools reach their storage capacity, the power plants generally move SNF into dry storage, usually on site. Dry storage facilities, on sites of active and decommissioned nuclear power plants, are referred to as independent spent fuel storage installations (ISFSIs) and are licensed under 10 CFR Part 72.
Regulating Storage Systems
The regulations in 10 CFR Part 72 apply to both facilities where SNF is stored and the certification of the cask systems used for dry storage. Licenses and certificates are issued for fixed terms and can be renewed for additional fixed terms. Most existing ISFSIs were licensed for an initial period of 20 years. NRC has recently revised 10 CFR Part 72 so that the initial and renewal licenses may now be issued for terms of up to 40 years.
The regulations in 10 CFR Part 72 do not limit the number of renewals of an ISFSI license or storage system certificates. Aging management3 for SNF must include potential degradation processes and other effects of aging, as well as maintenance, inspection, and monitoring, all of which are important factors in license-renewal decisions.
Dry storage systems consist basically of stainless steel inner canisters with concrete and steel outer structures. Intact SNF assemblies are loaded directly into inner canisters, under water in the spent fuel pool. Assemblies identified as damaged (a small fraction of the total) are loaded in specially designed damaged-fuel cans that provide additional containment. Canisters are subsequently dried, backfilled with inert gas, and sealed, either by welding or with bolted metal-seal lids. Licensed designs include both integrated and modular systems with horizontal or vertical orientations.
About 20 different dry storage system designs (including variations) from three principal vendors have been certified by NRC and are in use in the United States (NRC, 2011a). Dry storage systems are designed and built by vendors following design criteria in 10 CFR Part 72 to perform specific safety functions, including confinement of radioactive material, radiation shielding, control of criticality, removal of decay heat, and maintenance of structural integrity. The design criteria also address maintaining the retrievability capability of SNF in storage (10 CFR 72.122(l)). Staff guidance describes retrievability as the ability to handle individual or canned4 spent fuel assemblies by normal means (NRC, 2010a).
NRC’s principal role in regulating transportation of SNF and HLW is through certification of transportation packages, as provided in 10 CFR Part 71. Shipments of radioactive and other hazardous materials are also subject to regulation by the U.S. Department of Transportation.
Criteria for SNF transportation casks include similar safety functions as for storage. Transportation casks are further evaluated for performance under severe accident conditions (such as impacts, fires, and full immersion in water). Both truck and rail casks for transporting SNF have been certified by NRC, including some dual-certified storage-transportation designs. NRC has published a number of technical reports and public information documents on the safety of SNF transportation (e.g., NRC, 1977, 2003a, 2012a; Sprung et al., 2000).
Security requirements for SNF storage and transportation include both physical protection (10 CFR Part 73) and material control and accounting (10 CFR Part 74). NRC is currently revising 10 CFR Part 73 requirements that pertain to storage of SNF (NRC, 2009). Any additional revisions to security regulations that may be needed to address extended storage and transportation will be considered when the current rulemaking for 10 CFR Part 73 is complete, or as new needs for storage and disposal are identified.
NRC has regulatory authority over the disposal of commercial SNF and HLW. Currently two sets of NRC regulations apply to this area. Disposal of SNF and HLW in geologic repositories, in a generic sense, is governed by 10 CFR Part 60. Regulations specific to geologic disposal in a repository at Yucca Mountain, Nevada, are in 10 CFR Part 63. In each case, the NRC regulation implements standards set by the Environmental Protection Agency (EPA). Although these two sets of NRC regulations and EPA standards have some common features, they also differ in several respects.
The generic HLW disposal regulations in 10 CFR Part 60, first issued in 1983, implement EPA standards in 40 CFR Part 191. These regulations and standards provide for the evaluation of the long-term performance of a repository, after permanent closure of the facility, against a cumulative release standard, an individual protection dose standard, and a groundwater protection standard. The regulations in Part 60 implement the EPA release standard, in part through criteria for release by various subsystem components of the barrier system (e.g., waste package, groundwater path). NRC has not applied 10 CFR Part 60 to any disposal facility, as national policy has focused on a single site for a repository.
The regulations in 10 CFR Part 63 involve a more risk-informed, performance-based approach to evaluating a geologic repository. These regulations specify an explicit role for performance assessment models in demonstrating compliance and include requirements for such models. The regulations in Part 63 implement EPA standards in 40 CFR Part 197 for post-closure performance, which address individual protection, human intrusion, and groundwater protection, but do not include a cumulative release standard for post-closure.
NRC staff’s review plan for applying Part 63 (NRC, 2003b) provides further guidance on the regulatory requirements. NRC staff experience in using this regulation and guidance is captured in one volume of a safety evaluation report for the Yucca Mountain license application (NRC, 2010b) and three technical evaluation reports (NRC, 2011b,c,d).
In issuing 10 CFR Part 63, NRC acknowledged that this more risk-informed, performance-based approach provides a better regulatory framework for geologic disposal of HLW and SNF than the approach in 10 CFR Part 60. At that time, NRC stated that the “generic Part 60 requirements will need updating if applied to sites other than Yucca Mountain” (NRC, 2001). NRC has not yet begun rulemaking to effect this update.
State of the U.S. Program
Current U.S. inventories of commercial SNF are on the order of 65,000 metric tons (heavy metal equivalent), and are increasing by ~2,000 metric tons per year from an operating fleet of 104 light-water reactors. Nearly one-third of this material is now in dry storage at 63 licensed sites (NRC, 2011a), and almost all of it is owned by the power utilities that produced it. The Nuclear Waste Policy Act requires that the federal government, through the U.S. Department of Energy (DOE), take possession and permanently dispose of SNF used for commercial power generation in the United States.
The United States does not currently have an active program for reprocessing commercial SNF to produce new reactor fuel, although some interest has been expressed in developing commercial reprocessing. DOE has possession of the HLW generated from prior U.S. reprocessing of commercial fuel, in addition to a larger inventory of HLW from its environmental management activities at defense sites. The proposed repository at Yucca Mountain was designated to dispose of 7,000 metric tons (heavy metal equivalent) of DOE-owned HLW and spent fuel along with 63,000 metric tons of commercial SNF.
In 2009, President Obama announced that the proposed repository site at Yucca Mountain, Nevada, was “no longer considered a workable option” for disposal of SNF and HLW. The following year, DOE sought to withdraw from consideration its application to construct a repository at Yucca Mountain, which had been under review by the NRC since 2008. NRC suspended its review and licensing process in 2011, when no further funds were appropriated for this purpose. DOE and NRC actions on the Yucca Mountain proceedings have been challenged in federal court, but a decision is still pending.
In 2010, the Secretary of Energy established an advisory body to DOE, the Blue Ribbon Commission on America’s Nuclear Future (BRC), to conduct a comprehensive review of policies for managing the back end of the nuclear fuel cycle in the United States. In its final report, the BRC reaffirmed the need for geologic disposal of HLW, while acknowledging that one outcome of current U.S. policy is the expectation that SNF will have to be stored for extended periods of time (BRC, 2012).
Several of the BRC’s formal recommendations could directly affect NRC’s regulatory role. The next section focuses on how these recommendations align with NRC’s current activities and potential future plans.
The Changing Policy Environment
NRC has directed its staff to prepare for potential changes in national policy on the back end of the nuclear fuel cycle and the management of commercial SNF and HLW. It is important to note that, as an independent regulator, NRC does not develop national policy for the nuclear fuel cycle. That role clearly belongs to Congress and the Executive Branch.
Nevertheless, NRC can be prepared to respond to policy changes by adjusting its regulatory framework, in keeping with its mission to ensure the safe use of radioactive materials for beneficial civilian purposes. Of course, NRC must also balance its preparation for timely response with competing priorities for continued safe and secure operation of existing facilities, the availability of resources, and the constraints of current law.
NRC recognized the possibility of extended storage of SNF in the recent update of its Waste Confidence Decision (NRC, 2010c). The term “waste confidence” refers to NRC’s finding of “reasonable assurance” that sufficient disposal capacity will be available for commercial SNF and HLW and that storage can be managed safely and securely, under NRC regulation, in the interim. The current Waste Confidence Decision, which includes five separate findings that support these conclusions, is embodied in NRC regulation 10 CFR 51.23 as a generic determination that temporary storage of SNF after cessation of reactor operation has no significant environmental impact. The current update (2010) of the Waste Confidence Decision made this determination for at least 60 years beyond the licensed life of reactor operation.
NRC staff’s efforts related to the back end of the fuel cycle are focused on extended storage and subsequent transportation of commercial SNF, geologic disposal alternatives, and assessment of potential environmental impacts of an extended Waste Confidence Decision.
Extended Storage and Transportation of Spent Nuclear Fuel
NRC staff has been looking into the possibility that the current regulatory framework will need revisions to accommodate extended periods of SNF storage and the subsequent transportation of older spent fuel. As previously noted, current storage regulations allow for multiple renewals of ISFSI licenses and cask certificates. Such renewals include reviews of aging management plans, with a focus on the degradation of system components over time. Initial staff efforts have focused on identifying necessary technical information related to degradation processes of various components of dry storage systems.
The NRC staff is considering both the existing level of knowledge for degradation processes in storage applications and how degradation may affect the safety of both storage and subsequent transport. This work draws on previous technical evaluations of extended storage (EPRI, 2011; Hanson et al., 2012; NWTRB, 2010; Sindelar et al., 2011) and is informed by staff experience with current licensing and understanding of risk (NRC, 2007).
The NRC staff draft report (NRC, 2012b) prioritizes the technical needs and identifies those that should be addressed first. The highest priority areas include stress corrosion cracking of stainless steel canister bodies and welds, degradation of cask bolts, and swelling or pressurization of fuel pellets and rods over time. Areas of slightly lower priority include effects of aging on fuel cladding, assembly hardware, and neutron absorbers; microbiologically influenced corrosion of canister and seal materials; and degradation of concrete structures. Depending on the initiation time and rate of progression of degradation, many of these age-related processes may not be significant until far into an extended storage period.
The NRC staff draft report also identified three high-priority areas as cross-cutting topics that affect a number of components and safety functions: more realistic thermal calculation models; the effects of residual moisture after drying; and in-service methods of monitoring storage systems and components.
Thermal evaluations for current SNF storage applications, for example, focus on the maximum temperature the fuel cladding may reach, based on models with conservative assumptions that provide upper temperature bounds. During extended storage, as decay heat decreases, these models may over-predict temperatures both inside and on the exterior of the storage canister.
The model bias noted above may be problematic, because other degradation processes will potentially come into effect at the lower temperatures expected during extended storage. These processes include the susceptibility of stainless steel canisters to stress corrosion cracking in the presence of chloride salts and atmospheric moisture and possible low-temperature ductile-to-brittle transitions in fuel cladding. More realistic thermal models could help determine the potential impacts of these processes. NRC has begun technical work on this and several other high-priority areas to provide bases for potential changes in storage and transportation regulations and guidance over extended periods of time.
NRC staff is also monitoring work by other groups, both in the United States and worldwide, that are examining technical issues related to extended storage of SNF. For example, the Electric Power Research Institute has established the Extended Storage Collaboration Program (ESCP), which coordinates work by different organizations in the United States (including DOE and industry) and other countries on technical issues related to extended storage (EPRI, 2011).
Among other topics, ESCP has been active in early planning for a possible cask demonstration project, in which pre-characterized spent fuel assemblies would be stored for some period (10 to 15 or more years) in a well-instrumented, monitored cask. Fuel assemblies would then be removed for post-storage characterization to benchmark and verify models and expectations of fuel behavior over longer periods of time. There is particular interest in monitoring a cask containing high-burnup5 fuel as a complement to an earlier examination of dry-stored, low-burnup fuel (e.g., Kimball and Billone, 2002).
NRC activities related to extended storage are consistent with conclusions and recommendations of the BRC report on the likely need for SNF storage over a longer time as efforts proceed to site and develop facilities for geologic disposal. The BRC report also recommends siting and establishment of consolidated storage facilities as an interim step. Current NRC regulations allow for the licensing of privately operated, away-from-reactor storage facilities for SNF from multiple power plants (for example, the Private Fuel Storage Facility licensed in 2006).
Geologic Disposal of Spent Nuclear Fuel and High-Level Waste
NRC has not begun formal rulemaking proceedings for revising any regulations for geologic disposal of SNF and HLW. However, NRC staff is examining technical areas potentially relevant to alternative disposal options in mined geologic repositories. To this end, the staff has developed a scoping-level assessment model to investigate how different aspects of the geologic environment and waste characteristics may affect potential repository performance.
The model, referred to as SOAR (for Scoping of Options and Analysis of Risks), is based on relatively simple or generic representations of features, events, and processes (FEPS) for a conceptual repository system. In the model, FEPS are parameterized to consider the characteristics of a variety of alternative waste forms; engineered barrier materials; and geologic, hydrologic, and geochemical settings. Uncertainties in parameters that could potentially affect radionuclide release and receptor dose can be evaluated through the stochastic sampling of parameter values.
SOAR is implemented in a visual-based software environment that allows flexibility and modular model design (GoldSim Technology Group, 2010). A full description of SOAR can be found in the User Guide for version 1.0 (Markley et al., 2011).
The general structure of the SOAR model includes five principal component modules (Figure 1). The main components are Waste Form, Waste Package, Near Field environment (engineered and disturbed zones), Far Field environment (natural system), and Biosphere. A secondary component, Disruptive Events, complements the Waste Package component (which focuses mostly on failure caused by corrosion) for modeling other processes that could cause waste packages to fail (such as earthquakes).
SOAR provides insights into comparative risks for different potential repository systems. Even though its process models are relatively abstracted (i.e., necessarily simplified representations of complex processes), these insights help to focus technical work on relevant performance aspects of alternative repository designs and waste inventories. Technical investigations are designed to lay the groundwork for potential revisions of regulations for geologic disposal, as may be required by future changes in national policy.
In the BRC report, the study commission explicitly recommends that NRC begin revising 10 CFR Part 60 and engage with EPA to develop a new disposal standard to support those revisions. As previously noted, NRC is focusing on developing technical information to support potential rulemaking but has not begun the formal rulemaking process.
The BRC report also recommends that work begin on a regulatory framework for deep borehole disposal (e.g., Arnold et al., 2011), which is beyond the scope of existing law and NRC regulations for HLW disposal and would require the development of an appropriate technical basis and rulemaking. NRC staff has done only limited work in this area thus far but is monitoring technical investigations by other groups on potential deep borehole disposal.
Potential Environmental Impacts of Future Fuel Cycle Scenarios
In conjunction with the most recent update of the Waste Confidence Decision and rule (NRC, 2010c), NRC directed its staff to conduct a comprehensive analysis of potential future impacts on the environment of extended storage and transportation in a formal Environmental Impact Statement (EIS). The staff was directed to assess the potential impacts of longer term storage of SNF to inform a possible extension by NRC of the time specified in the Waste Confidence rule.
To this end, NRC staff developed preliminary assumptions and proposed a scenario-based approach to assessing potential environmental impacts beyond the time period in the current Waste Confidence rule (NRC, 2011e). The scenarios are designed to capture credible variations in how the fuel cycle might develop and are not intended to endorse a particular position or approach. The four proposed scenarios include extended storage at different locations (on site and at one or more consolidated sites), along with possible commercial SNF reprocessing. Each scenario involves some transport of SNF and HLW as well as the hand-ling and repackaging of SNF. All four scenarios end with disposal in a geologic repository. NRC staff is now developing relatively high-level system models for the back end of the fuel cycle to help explore the impacts of the four scenarios in the EIS.
Although the BRC report does not specifically recommend an environmental impact analysis, NRC staff efforts related to the EIS touch on several specific elements in the proposed BRC approach, including consolidated (interim) storage, preparations for large-scale transport, and potential reprocessing of SNF. Because an EIS covers a broad range of environmental impacts, NRC staff considers its efforts part of a comprehensive approach to understanding the implications of future directions in the fuel cycle.
In preparation for possible changes in U.S. national policy affecting the back end of the nuclear fuel cycle in response to the BRC recommendations, NRC staff is examining the regulatory basis for extended storage, subsequent transportation, and geologic disposal of SNF and HLW. Technical investigations by NRC staff and other groups will contribute to a basis for any necessary revision to existing regulations, whether driven by new information or changes in national policy. NRC’s efforts are generally consistent with the recommendations of the Blue Ribbon Commission on America’s Nuclear Future.
The author thanks T. McCartin, B. Hill, K. Compton, R. Einziger, C. Markley, C. Pineda, D. Dunn, A. Mohseni, and L. Kokajko of the NRC staff for thoughtful insights, discussion, and support. The views expressed herein are those of the author and do not constitute a final judgment or determination of the matters addressed or of the acceptability of any licensing action that may be under consideration by the U.S. Nuclear Regulatory Commission. Use of commercial products or trade names does not constitute endorsement by the U.S. Nuclear Regulatory Commission.
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1 NRC uses the term “spent nuclear fuel” (SNF) to refer to irradiated reactor fuel that has been removed from service and has not been chemically separated. Some other groups use the term “used fuel” when referring to the same material.
2 The term “high-level radioactive waste” (HLW), as used by NRC, covers a broader category than SNF. HLW includes highly radioactive material that results from spent fuel reprocessing, as well as other materials with enough long-lived radioactivity to require permanent isolation. Distinctions among the details and history of radioactive waste classification are not addressed in this article.
3 “Aging management” refers to all actions that address the effects of aging, including prevention, mitigation, and monitoring.
4 “Canned” assemblies are those that have been placed in damaged-fuel cans prior to dry storage.
5 The term “high-burnup fuel” generally applies to spent fuel that has been irradiated in a reactor to a power output of greater than 45 gigawatt-days per metric ton of uranium.