ML23264A073
| ML23264A073 | |
| Person / Time | |
|---|---|
| Issue date: | 11/06/2023 |
| From: | David Esh, Priya Yadav NRC/NMSS/DDUWP/LLWPB, NRC/NMSS/DDUWP/RTAB |
| To: | |
| References | |
| Download: ML23264A073 (1) | |
Text
DAVID ESH AND PRIYA YADAV 1
U. S. NUCLEAR REGULATORY COMMISSION:
ENSURING SAFETY AND FOSTERING SUSTAINABILITY THROUGH RISK-INFORMED COMPLIANCE PERIOD FRAMEWORK FOR ANALYSES OF RADIOACTIVE WASTE DISPOSAL D.W. Esh US Nuclear Regulatory Commission Washington DC, USA Email: david.esh@nrc.gov P. Yadav US Nuclear Regulatory Commission Washington DC, USA Abstract The United States Nuclear Regulatory Commission (NRC) Low-Level Radioactive Waste Management (LLRWM)
Program continues to make progress with allowing for the safe use of nuclear technology and nuclear materials for industrial and medical uses while sustainably increasing disposal options for higher activity wastes. The NRC is embarking on a rulemaking to modify its low-level waste disposal regulations to allow more use of site-specific analyses. This will likely allow for near-surface disposal of higher activity waste (e.g., Greater-than-Class-C (GTCC) waste) and large volumes of depleted uranium, both of which can be similar to wastes described to be intermediate level wastes in many countries. The NRCs approach to maintaining safety and sustainability involves requiring licensees to perform technical analyses of the wastes proposed for disposal in a risk-informed, graded manner. Use of site-specific technical analyses balances the need to increase waste disposal options while protecting future generations from potential releases of radioactivity from long-lived radionuclides disposed in the near surface. The NRC plans to implement a graded compliance period framework of shorter duration of analyses required for typical low-level waste and a longer duration of analyses for waste streams containing significant quantities of long-lived radionuclides. The NRC staff has compiled research from the international community to inform staffs determination of the appropriate durations for each type of waste. Implementing this risk-informed compliance period framework will ensure that licensees evaluate protection of the member of the public as well as an inadvertent intruder in timeframes commensurate with the characteristics of the waste streams being disposed, thereby fostering sustainability for future generations.
- 1.
INTRODUCTION Radioactive waste can take many different forms with substantially different concentrations of radionuclides and radioactive decay and ingrowth characteristics. The hazards from some types of radioactive waste may be minimal and short-lived, whereas others may be substantial and persist for thousands of years and longer. Different approaches have been used to ensure that members of the public who may reside or engage in normal activities in and around a closed radioactive waste disposal facility are protected from disposed radioactivity. The approaches used rely on some combination of policy, design, and technical analyses. Policy is used to define what types of waste may be disposed and where it may be disposed. For example, some programs require all radioactive wastes to be disposed in a deep geologic facility. Most programs will define different types of radioactive waste and then establish appropriate design and technical analyses requirements for those different types of radioactive waste. Waste classification is used to broadly separate wastes into categories to facilitate management. Types of radioactive waste are usually established by either a source-or analyses-based approach.
In the US, the source-based approach was used in legislation and regulation. For example, low-level waste (LLW) is defined by what it is not (e.g., not spent nuclear fuel). LLW is then further subdivided into different classes: A, B, C, and Greater-Than-Class-C (GTCC) wastes. Fig. 1 is a representation of the International Atomic Energy Agency (IAEA) waste classification scheme compared with select US LLW classes [1]. Though the US approach is similar, there are some important differences. The upper bound (high-level waste (HLW)) is not based strictly on concentrations but rather under a source-based approach; it is based on how the radioactive waste was generated. The US system does not define waste classes below LLW. The approach used in some programs is to further subdivide the waste into classes based on the half-lives of radionuclides present such as short-lived LLW
IAEA-CN-318//351 FIG. 1. The IAEA waste classification system and approximations of the US LLW classifications (LLW-SL) and long-lived LLW (LLW-LL). This approach has some distinct advantages when it comes to establishing design and technical analyses requirements. The LLW Classes shown (A and C) are distinct lines in the US system as they were derived by the regulatory body and placed in regulation. The GTCC dividing line is shown as being thick to communicate that GTCC waste, and the suitability for disposal, are to be determined by site-specific technical analyses.
Prior to the early 1980s, LLW disposal facilities in the United States were developed and operated using design-based approaches with limited technical analyses. Many of these early facilities had operational difficulties that led to early closure and the need for remediation. Much of the operational challenges resulted from surface instability and engineered and natural systems not performing as anticipated. NRCs LLW disposal regulations were developed in the early 1980s in response to these challenges [2]. NRCs LLW disposal regulations are found in Part 61 of Title 10 of the United States Code of Federal Regulations (10 CFR Part 61).
Requirements associated with stability and technical analyses are specified. Waste classes are specified based on NRCs technical analyses of potential inadvertent intruders [3]. Wastes with concentrations of radionuclides above Class C limits are currently not generally acceptable for near-surface disposal unless specific proposals for the disposal are approved by the NRC. NRC made assumptions about the concentrations and quantities of radionuclides that were likely to be present in LLW when deriving the waste classes. NRC also allowed for alternate classification and waste characteristics to be authorized if it could be demonstrated if safety provisions could be achieved.
In the US nuclear industry, the types and quantities of radioactive wastes generated have, in some cases, deviated from what was previously assumed. NRCs waste classification limits specified in regulation (Table 1 and 2 of 61.55) did not provide values for uranium. NRC also did not specify what analyses should be performed for waste with concentrations of radionuclides above Class C limits. NRCs waste classification limits were derived assuming that the waste would be buried shallowly (less than 5 m from the land surface) such that an inadvertent intruder could excavate into the waste and disperse radioactivity at the land surface. Therefore, an operator could not take advantage of disposal practices if waste was placed more deeply in the near-surface (upper 30 m below the land surface). In the US, operators have requested to dispose of large quantities of depleted uranium, which is a concentrated, long-lived, alpha-emitting waste type, as well as GTCC wastes. These wastes have characteristics similar to some types of Intermediate Level Waste (ILW). To ensure safety and sustainability
DAVID ESH AND PRIYA YADAV 3
for future generations from the near-surface disposal of these wastes, NRC is conducting a rulemaking to revise and add requirements suitable for these types of wastes.
- 2.
ROLE OF SITE-SPECIFIC TECHNICAL ANALYSES Site-specific technical analyses have a critical role in risk-informed, performance-based regulation of LLW disposal. Site-specific technical analyses are the most efficient mechanism to identify necessary engineered and natural system performance characteristics on absolute and relative bases. In the US, there is a diversity of different wastes, engineered barrier systems, and especially geologic and natural system characteristics for LLW disposal facilities. Some locations may have more than 100 cm/yr of precipitation and a water table less than 5 m deep, whereas others may have less than 20 cm/yr precipitation and a water table that is hundreds of meters deep. Expected future land uses may be widely different. Generic regulatory analyses to establish limiting concentrations will, to achieve safety objectives, needlessly restrict sites with expected high performance because of the expected low performance of other sites. However, site-specific technical analyses may account for these differences. Use of generic regulatory analyses may be more useful for programs that may only have a single disposal facility.
NRCs LLW disposal regulations have many different requirements (e.g., siting, waste characteristics, performance objectives, control of the site, monitoring) to achieve safety and many of the requirements collectively are synonymous with IAEAs safety case methodology. The performance objectives provide for:
Protection of offsite individuals from releases of radioactivity after closure; Protection of an onsite individual who unknowingly uses the land after closure; Protection of workers and members of the public during operations; Stability of the disposal site after closure.
For protection of an onsite individual who unknowingly uses the land of the disposal facility (called an inadvertent intruder), NRC performed radiological dose calculations. These calculations assumed that the LLW was disposed shallowly (with 1 m of cover) and that a person excavated into the waste to a depth of 3 m to build a foundation for a home 100 years post-closure (i.e., at the end of the assumed institutional control period). NRC also made assumptions on what radionuclides may be present. The result was the establishment of generic waste concentrations that were protective of inadvertent intruders. However, these concentrations are overly restrictive if wastes are disposed more deeply, with additional barriers, or in a form that is less dispersible compared to that used in NRCs analyses. In addition, NRCs analyses also do not address wastes that have different radionuclides present that may result from advanced reactor designs, depleted uranium, or other advances in the use of nuclear materials.
- 3.
NRCS PROPOSED POST-CLOSURE SAFETY APPROACH New or different waste streams than originally considered are or have been proposed for commercial disposal in the US (e.g., large quantities of depleted uranium that can be up to 80 percent uranium by weight and GTCC waste). These waste streams may have characteristics that are different than wastes that considered in the original regulatory framework, and the safety of future generations must be examined. Site-specific technical analyses are required under current regulations (10 CFR Part 61) for evaluation of impacts to offsite individuals, however they are not currently required for an inadvertent intruder. Current NRC regulations also do not specify a time period for analysis of post-closure safety from LLW disposal. NRC is conducting a rulemaking to add relevant technical analyses requirements to address these new wastes and to modernize the regulations. For post-closure safety these requirements include the scope of the analyses, consideration of uncertainties, model support, and the time period of the assessment.
A challenging aspect of the disposal of long-lived materials, either radioactive waste or other materials, is what analyses should be performed and how far into the future should be assessed. Starting in approximately 2009, NRC sought to examine this issue and develop an approach appropriate for the different wastes and different disposal sites and designs that have been implemented in the past and may be implemented in the future in the United States. NRC performed an evaluation of the key considerations [4]. After careful deliberation and
IAEA-CN-318//351 discussion with stakeholders, the NRC staff are considering a risk-informed, graded approach to analyses timeframes1. The main components of the approach are:
A compliance period of 1,000 years if significant quantities of long-lived radionuclides are not present otherwise a 10,000-year compliance period followed by a performance period; Dose limits are prescribed for the compliance period and optimization-like criteria for the performance period; Long-lived radionuclides are when 1) More than 10 percent of the initial activity of the radionuclide remains after 1,000 years, 2) The peak activity from progeny occurs after 1,000 years, or 3) More than 10 percent of the peak activity of the radionuclide (including progeny) within 1,000 years remains after 1,000 years.
The process for determining what are significant quantities of different waste streams is site-specific and described in guidance.
The proposed approach is risk-informed because it will allow a disposal facility operator who either does not wish to dispose of significant quantities of long-lived waste or is prohibited for other reasons such as by policy to complete a shorter compliance analysis of post-closure safety. For many sites, especially those that have been selected and sited in a location with suitable and stable long-term characteristics, there is not a measurable difference in resources required to complete a 1000-year compared to a 10,000-year post closure safety assessment. Most of the effort is associated with compiling the necessary information and building, testing, and describing the models used to perform the evaluation. However, for some sites the features, events, and processes relevant for longer timeframes that are included in the modelling may differ from those for shorter timeframes and thus there would be additional resource demands for the assessment. The criterion to apply after the 10,000-year compliance period is that releases of radioactivity from the disposal site must be reduced to the extent reasonably achievable during the performance period, which has no ending timeframe. This criterion is similar to an optimization criterion. The goal of the criterion is to ensure transparency with stakeholders. Given current understanding, the operator should communicate what they expect will happen and what measures have been taken by site selection, inventory selection, design, and operation to reduce the impacts to the extent reasonably achievable. Post-closure safety assessments do not produce the doses a member of the public will receive in the future, rather they produce estimates given a wide range of uncertainties that can be one important source of information used in regulatory decisionmaking.
Many programs will use a peak dose approach for post-closure safety assessment. This approach does have merit especially if only one or a limited number of disposal facilities will be licensed. However, if numerous facilities may be licensed in very different environmental and geologic settings, then the peak dose approach could lead to the situation where poorer sites would be analysed for shorter periods of time and very favourable sites would be analysed for much longer times. In other words, there could be a disincentive to the operator to select the best sites. In addition, some facilities, in combination with the types of waste proposed for disposal, may pose very low risks. Analyses for these facilities should not be overly burdensome or complex.
- 4.
TECHNICAL AND POLICY CONSIDERATIONS In developing the proposed approach, the staff reviewed current practices within the US for commercial LLW disposal, within the US for other wastes, and internationally for near-surface disposal of radioactive wastes.
In the US there are four operating LLW disposal facilities in the States of Texas, Washington, Utah, and South Carolina. The regulation of these facilities is done by the States (termed Agreement States) under compatible regulations developed by the States. The Agreement States licensed the disposal facilities in part based on technical analyses. The assessments were completed for different periods of time, ranging from 2,000 to 1,000,000 years. The facility in Utah was originally licensed using a 500-year assessment because the facility was limited to accepting Class A waste and the groundwater is not potable. However, a proposal was made by the operator to accept large quantities of depleted uranium and for that assessment the regulator stipulated a 1 These criteria will be proposed to the NRC Commission in May 2024. The Commission may adopt, alter, or reject the staff recommendations.
DAVID ESH AND PRIYA YADAV 5
ACRONYMS: DOE - Department of Energy, ILW - Intermediate Level Waste, WIR - Waste Incidental to Reprocessing FIG. 2. The timeframes evaluated in post-closure safety assessments for operating and closed radioactive waste disposal facilities 10,000-year compliance period followed by an assessment of deep time extending well beyond the compliance period [5].
Consistent with the diversity of wastes, engineered designs, and disposal sites, the approaches used internationally are diverse. Fig. 2 is a compilation of the compliance periods or periods of time evaluated in the post-closure safety assessments for numerous operating and closed disposal facilities located throughout the world. The information plotted for the US facilities are compliance periods. The information plotted for the international facilities are better expressed as time evaluated as the requirements for compliance werent always clearly expressed. The y-axis is the concentration of long-lived alpha emitting radionuclides present in the waste expressed as a fraction of NRCs Class A limits. NRCs Class A limit for long-lived alpha emitting radionuclides is 370 Bq/g. In the analyses, the plutonium isotopes and Am-241 were used as a proxy for the total long-lived alpha emitting radionuclides present as many programs do not report a full list of radionuclides in the waste inventory.
A variety of different insights can be developed from the information presented in Fig. 2. It is important to understand the context of the information provided in the figure. The information was derived from available reports where usually the inventory information was contained in a single source document. Therefore, normal verification of the data could not be performed. However, the data are generally consistent which provides some confidence. In addition, there were no obvious outliers, and enough data have been obtained that even with changes to individual points the insights that can be derived would not change significantly. The data generally trend from lower left to upper right on the log-log scaled plot. That is, longer analyses are performed for higher concentrations of long-lived radionuclides. The closed facilities requiring remediation in the United States had little to no technical analyses when licensed. The licensing of these facilities were design-based approaches and the challenges experienced with surface water management and stability were not anticipated. The data are plotted at 500 years but could be plotted on the left-hand axis.
Most operating facilities were licensed using information from technical analyses completed for 10,000 years and longer in the post-closure safety assessment. One of the arguments expressed by stakeholders in the United States was use of a compliance period longer than 1,000 years would prevent licensing of disposal facilities and that the information developed in such analyses was so uncertain as to make the information not useful. The fact that information from greater than 1,000-year analyses was successfully used throughout the world over multiple decades strongly suggests uncertainty can be addressed with long-term safety assessments. The green area provided in Fig. 2 reflects the region encompassed by the NRCs proposed approach to analyses timeframes.
IAEA-CN-318//351 NRCs proposed approach is compatible with past regulatory experience. Besides closed facilities undergoing remediation and the few facilities in the world managing waste with characteristics similar to ILW, NRCs proposed approach is consistent with international practice. For wastes that contain limited quantities of long-lived radionuclides, it is appropriate to use a compliance period of 1,000 years. In guidance, the staff recommend it is advisable to perform technical analyses for longer periods if the peak has not been captured in the 1,000-year analysis and if the doses are increasing [6].
Most LLW disposed outside the United States has long-lived concentrations that are below NRCs Class A limits on a facility-averaged basis. Nonetheless, longer assessments are performed outside the United States than within. Part of this has to do with the land use around the facilities and the expectation of future land use.
As shown in Fig. 3, facilities outside the United States are generally located closer to potential receptors in the present day. The information in Fig. 3 was developed by using published facility location information in combination with Graphical Information Systems (GIS) software to estimate the nearest human receptors from the disposal facilities. As shown, present day receptors are generally located at greater distances from the disposal facilities in the United States as compared to elsewhere. Larger distances and depths are likely strongly correlated with reduced likelihood of future interaction with the disposed waste. Population density was not considered in developing Fig. 3.
A concern expressed by some stakeholders with the use of longer-term technical analyses, such as 10,000 years and longer, is uncertainty. NRC staff embraces the consideration of uncertainty in licensing decisions and has reflected it in the criteria provided for the technical analyses. The argument provided by some is that the calculated impacts have unrepresented uncertainties that makes the information not suitable for use in decisionmaking, therefore a shorter compliance period should be used (e.g., 1,000 years). It is true that some types of uncertainties are not reflected in most analyses, such as societal and technology development. It is also true that the technical analyses are to be developed using a features, events, and processes framework or other approach such that the scope of the analysis is representative of performance and includes key uncertainties. There is an obligation to provide safety and sustainability for future generations. Excess uncertainty is not a proper argument to reduce regulatory requirements, rather it is a reason to identify proper solutions to mitigate the impacts of significant uncertainties. Most larger programs have established different disposal concepts for different types of waste.
High-level waste is targeted for deep geologic disposal because of the uncertainties in long-term near-surface disposal. Uncertainties associated with near-surface disposal are generally higher than associated with deep geologic disposal because there is a higher likelihood of human interaction and disturbance, and near-surface geomorphology is complex and dynamic. Excess uncertainty is an argument against near-surface disposal not for near-surface disposal with criteria that can be reasonably demonstrated.
FIG. 3. Receptor distances and disposal depth for operating disposal facilities
DAVID ESH AND PRIYA YADAV 7
Stakeholders have offered differing views and interpretation of guidance provided by the IAEA. In IAEAs safety standards SSR-5, it is stated that isolation of radioactive waste shall be achieved for several hundred years for shorter-lived waste and several thousand years for intermediate and high-level waste [7]. This guidance was interpreted as the timeframes that are appropriate for use in post-closure safety assessment. In the safety guide for the safety case and safety assessment for the disposal of radioactive waste (SSG-23), the guidance on analyses timeframes states safety assessment calculations should cover a time period that is long enough to determine the maximum, or peak, dose or risk but acknowledges that due to a variety of factors that this is not always possible
[8]. In view of the complexity and variability of these factors, the guidance states it is not possible to establish a universal timescale over which meaningful quantitative results from modelling can be obtained. The context under which this guidance is provided is important. IAEA provides for, and member states implement, classification systems to establish what waste can be disposed and where it can be disposed. The near-surface environment, with its uncertainties, is not appropriate for all types of wastes. If wastes with considerable long-term hazards are to be disposed in the near surface, then they must be appropriately analysed including the uncertainties.
Stakeholders also stated that most LLW is short-lived (>98%) by activity, therefore the compliance period should be limited to 1,000 years or less consistent with the waste. While the statement is correct that in the US most of the LLW radioactivity is comprised of radionuclides with half-lives of less than 30 years such that by 500 years that activity has decayed, it represents a misunderstanding of the existing regulatory framework and purpose for that framework. Post-closure safety assessment has always been about ensuring that after the containment and isolation functions have been achieved that the remaining radioactivity will not, if released, result in impacts that exceed established safety standards. For releases to an offsite receptor, the radionuclides Tc-99, I-129, and C-14 are usually the most significant. These long-lived radionuclides are mixed with the short-lived radionuclides in most commercial LLW disposed in the US. If there were waste streams that could be quantified as containing only short-lived wastes then different criteria could be applied for those wastes, but that is not a practical solution in the current operating environment. The goal of post-closure safety assessment is ensuring the long-term safety from what may be released from the disposal facility. Only through this goal may we achieve sustainability.
As previously mentioned, some new waste streams have been proposed for disposal that have higher concentrations and quantities of long-lived radionuclides than traditional LLW. These wastes, combined with the different disposal sites and designs where they could be placed, create challenges associated with the analyses.
Fig. 4 is the result of probabilistic analyses of hypothetical sites with different thicknesses of the vadose or unsaturated zone and different amounts of recharge of precipitation to an underlying aquifer. Whereas for a thin, wetter site the expectation may be that the peak dose is realized within the first 1,000 years, for thicker and drier sites this becomes less to very unlikely. Even for a thin, wet site the peak dose may not be realized within the first 1,000 years resulting from other sources of uncertainty and variability. Some results increase and decrease reflecting the contribution from radionuclides with different mobility at different times. The primary consideration, and most accurately known, is the characteristics of the waste that will be disposed and how much long-lived radionuclides are contained in the waste.
- 5. CONCLUSIONS The NRCs approach to maintaining safety and sustainability involves requiring licensees to perform site-specific technical analyses of the LLW proposed for disposal. Use of site-specific technical analyses balances the need to increase waste disposal options while protecting future generations from potential releases of radioactivity from long-lived radionuclides disposed in the near surface accounting for differences in wastes, designs, and sites. The NRC is considering proposing to implement a graded compliance period framework of shorter duration of analyses required for typical low-level waste and a longer duration of analyses for waste streams containing significant quantities of long-lived radionuclides. The NRC staff compiled research from the international community to determine the appropriate durations for each type of waste. Implementing this risk-informed compliance period framework will ensure that licensees evaluate protection of the member of the public as well as an inadvertent intruder, thereby fostering sustainability for future generations.
IAEA-CN-318//351 FIG. 4. Hypothetical dose vs. time responses for disposal sites with different thicknesses of the unsaturated zone and different amounts of recharge ACKNOWLEDGEMENTS The authors would like to acknowledge A. Gross who completed the GIS analyses for the receptor locations. T. McCartin and H. Arlt contributed significantly throughout the rulemaking process and provided many valuable comments and insights. C. McKenney and S. Koenick provided helpful and insightful review comments.
REFERENCES
[1] IAEA, Classification of Radioactive Waste, General Safety Guide No. GSG-1, International Atomic Energy Agency, Vienna Austria, 2009.
[2] NRC, Licensing Requirements for Land Disposal of Radioactive Waste: Final Rule, Federal Register, Vol. 47, No. 248, pp. 57446-57482, December 27, 1982, U.S. Nuclear Regulatory Commission, Washington DC United States, 1982.
[3] Ridge, A., Esh, D., Gross, A., TableCalculator: a transparent public tool to replicate US NRC LLRW classification table calculations, WM Symposia 2019, Phoenix, AZ, March 3-7, 2019.
[4] NRC, Technical analysis supporting definition of period of performance for low-level waste disposal, U.S. Nuclear Regulatory Commission, Rockville MD, 2011, ADAMS Accession No. ML111030586.
[5] Neptune and Company, Inc., Deep time assessment, report for EnergySolutions Clive DU PA model, version 1, Appendix 13, Neptune and Company, Inc., Denver CO, 2011.
[6] NRC, Guidance for conducting technical analyses for 10 CFR Part 61, draft final report, U.S. Nuclear Regulatory Commission, Rockville MD, 2016, ADAMS Accession No. ML14357A072 (NOTE: This report is being revised consistent with the proposed regulatory requirements. It will be made public after Commission decision).
[7] IAEA, Disposal of Radioactive Waste, Specific Safety Requirements No. SSR-5, International Atomic Energy Agency, Vienna Austria, 2011.
[8] IAEA, The Safety Case and Safety Assessment for the Disposal of Radioactive Waste, Specific Safety Guide No.
SSG-23, International Atomic Energy Agency, Vienna Austria, 2012.