ML20138J638

Disposal of HIGH-LEVEL Radioactive Wastes in Unsaturated Z0NE:TECHNICAL Considerations and Response to Comments

ML20138J638
Person / Time
Issue date:	10/31/1985
From:	Evans D, Hackbarth C, Thomas Nicholson NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:
References
NUREG-1046, NUDOCS 8510290415
Download: ML20138J638 (49)

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NUREG-1046 i-i J

! Disposal of High-Level l Radioactive Wastes in the Unsaturated Zone:

Technical Considerations and Response to Comments i Final Report i

2 U.S. Nuclear Regulatory Commission Office of Nuclear Regulatory Research

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i C. J. Hackbarth, T. J. Nicholson, D. D. Evans I

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t NUREG-1046 Disposal of High-Level Radioactive _W _ astes in the Unsaturated Zone:

Technical Considerations and Response to Comments Final Report Manuscript Completed: September 1985 Date Published: October 1985 C. J. Hackbarth, T. J. Nicholson, D. D. Evans *

' Consultant Division of Radiation Programs and Earth Sciences i

Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Wcshington, D.C. 20555

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ABSTRACT On July 22, 1985, the U. S. Nuclear Regulatory Commission 'NRC) promulgated amendments to 10 CFR Part 60 concerning disposal of high-level radioactive waste (HLW) 'in geologic repositories in the unsaturated zone (50 FR 29641).

This report contains a discussion of the principal technical issues considered by the NRC. staff during the development of these amendments. It expands or revises certain technical discussions originally presented in draft NUREG-1046 (February 1984) based on public comment letters and an increasing understanding of the physical, geochemical, and hydrogeologic processes operative in unsatur-ated geologic media.

The following issues related to disposal of HLW within the unsaturated zone are discussed: hydrogeologic properties and conditions, heat dissipation and temper-ature, geochemistry, retrievability, potential for exhumation of the radioactive waste by natural causes and by human intrusion, the effects of future climatic changes on the level of the regional water table, and transport of radionuclides in the gaseous state.

The changes to 10 CFR Part 60 in definitions, siting criteria, and design criteria for the geologic repository operations area are discussed. Other criteria examined by the NRC staff but which were not changed in rule are the minimum 300-meter depth for waste emplacement, limitations on exploratory boreholes, backfill requirements, waste package design criteria, and provisions for ventilation.

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TABLE GF CONTENTS Page i

I. INTRODUCTION .............. ....................................... 1

a. Background on 10 CFR Part 60 and discussion of public comments related to HLW disposal in unsaturated zone........ 1

b. Changes in NUREG-1046......................................... 3

c. General discussion of the saturated and unsaturated zones..... 4 II. DISPOSAL OF HLW WITHIN THE UNSATURATED ZONE ....................... 7 l a. Background.................................................... 7

b. -Issues addressed by NRC in considering the disposal of HLW within the unsaturated zone................................. 12

1. Hydrologic properties of the unsaturated. zone............ 12

2. Heat dissipation and temperature......................... 13

3. Geochemistry............................................. 15

4. Retrievability........................................... 17

5. Potential for exhumation of the radioactive waste by natural processes...................................... 18

6. Potential for exhumation of the radioactive waste by

. human intrusion........................................ 18 7

7. Effects of future climatic changes on the level of the regional water table...... ............................ 19

8. Radionuclide transport in the gaseous state.............. 20 III. REVIEt! 0F NRC REGULATIONS ......................................... 23

a. Background.................................................... 23

b. Amendments to 10 CFR Part 60 and review of public comments.... 23

1. Definitions............................................... 24

2. Siting criteria. ......................................... 25

-3. Design criteria fc- the geologic repository

. operations area......................................... 29

c. Other Issues Reviewed by the NRC staff........................ 29

1. Minimum 300-meter depth for waste emplacement............ 29

2. Exploratory boreholes.............................. ..... 30

3. Backfill requirements.................................... 30

4. Waste package design criteria............................ 31 Provisions for

5. ventilation............................... 31 l IV. CONCLUSIONS ....................................................... 32 REFERENCES ............................................................. 33 APPENDIX - DEFINITIONS AND CONCEPTS .................................... A-1 v

List of Figures

_P,a g e

1. Schematic vertical section of a hypothetical ground-water flow system.......................................................... 5

2. Diagram showing distribution of water in an unsaturated medium... 6

3. Simplified view of the hydrologic cycle.......................... 8

4. Types of openings in selected geologic media..................... 9

5. Pressure head versus water content for three geologic media...... 10

6. Effective hydraulic conductivity versus water con'.ent for three geologic media.......................... ................ 11 vi

ACKNOWLEDGEMENTS Several additional people contributed to the production and internal review of this document, most notably C.N. Ostrowski, the original task leader of this project.

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I. INTRODUCTION I.a Background The Nuclear Regulatory Commission (NRC) has established a regulatory framework for licensing the disposal of high-level radioactive wastes (HLW) in geologic repositories. On February 25, 1981, NRC promulgated final licensing procedures in 10 CFR Part 60, " Disposal of High-Level Radioactive Wastes in Geologic Repositories" (46 FR 13971). Technical criteria against which license applica-tions will be reviewed under 10 CFR Part 60 were promulgated on June 21, 19t3 (48 FR 28194).

Prior to issuance in final form, the 10 CFR Part 60 technical criteria were proposed for public comment in 1981 (46 FR 35280). Ninety-three comment letters were received on these proposed technical criteria. Several commenters on the proposed rule, including the U.S. Department of Energy, the U.S. Department of the Interior, and, separately, the U.S. Geological Survey (USGS), took issue with a statement made by the Commission at 46 FR 35281 that explained that the proposed technical criteria were developed specifically for dispost! in satur-ated geologic media because DOE plans at that time called for HLW disposal at sufficient depth to be situated in the hydrogeologic regime termed the satur-ated zone. These commenters considered disposal in the unsaturated zone to be a viable alternative to disposal in saturated geologic media and noted that, since the technical criteria were generally applicable without regard to the possibility of saturation, their scope and applicability should not be unduly

. restricted. NRC found this criticism'to be well founded and stated its intent to promulgate for public comment specific criteria that apply to the unsatur-ated zone. To that end, NRC published specific techn'ical criteria for HLW disposal within the unsaturated zone as proposed amendments on February 16, 1984 (49 FR 5934).

In addition to these proposed amendments, NRC specifically requested public input on two questions related to ground-water travel time calculations within the unsaturated zone (49 FR 5937). In conjunction with the proposed amend-ments, NRC published for public comment a draft version of this report which contained a discussion of the principal technical issues considered by the NRC staff during the development of the proposed amendments.

A total of fourteen groups and individuals commented on the proposed amendments and draft version of this report. There was general acceptance of NRC's view that disposal of HLW within the unsaturated zone is a viable alternative to disposal within the saturated zone. The commenters addressed the specific questions on ground-water travel time within the unsaturated zone and provided additional comments suggesting word changes to improve the technical accuracy and to clarify the intent of the proposed amendments. The NRC staff considered these comments during the development of the following final amendments, which were published on July 22, 1985 (50 FR 29641):

1. New definitions of the terms " ground water" and " unsaturated zone" were added to S 60.2 and a conforming technical change was also made to the existing definition of the term " saturated zone" to read as follows:

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" Ground water" means all water which occurs belov the land surface.

" Saturated zone" means that part of the earth's crust beneath the regional water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric.

" Unsaturated zone" means the zone between the land surface and the regional water table. Generally, water pressure in this zone is less than atmos-pheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the fluid pressure locally may be greater than atmospheric.

2. Section 60,122 was amended by revising paragraphs (b)(2)(iii) and (c)(9) redesignating paragraph (b)(2)(iv) as (b)(7), and adding new paragraphs (b)(8), (c)(22), (23) and (24) to read as follows:

S 60.122 Siting criteria.

(b) ***

(2)***(111) Low vertical permeability and low hydraulic gradient between the host rock and the surrounding hydrogeologic units.

(7) Pre-waste-emplacement ground-water travel time along the fastest path of likely radionuclide travel from the disturbed zone to the access-ible environment that substantially exceeds 1,000 years.

(8) For disposal in the unsaturated zone, hydrogeologic conditions that provide--

(i) Low moisture flux in the host rock and in the overlying and underlying hydrogeologic units; (ii) A water table sufficiently below the underground facility such that fully saturated voids continuous with the water table do not encounter the underground facility; (iii) A laterally extensive low permeability hydrogeologic unit above the host rock that would inhibit the downward movement of water or divert downward moving water to a location beyond the limits of the underground facility; (iv) A host rock that provides for free drainage; or (v) A climatic regime in which the average annual historic precipitation is a small percentage of the average annual potential evapotranspiration.

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(9) Ground-water conditions in the host rock that are not reducing.

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(22) Potential for the' water table to rise sufficiently so as to cause saturation of an underground facility located in the unsaturated l

zone.

(23) Potential for existing or future perched water bodies that may saturate portions of the underground facility or provide a faster flow path from an underground facility located in the unsaturated zone to the accessible environment.

(24) Potential for the movement of radionuclides in a gaseous state through air-filled pore spaces of an unsaturated geologic medium to the accessible environment.

3. Although no changes to the design criteria were included in the proposed

, amendments, these provisions were addressed by commenters on the proposed

, amendments. Following consideration of discussions presented in these i public' comments, NRC revised S 60.133(f) and S 60.134(b)(1) to read as i follows:

S 60.133 Additional design criteria for the underground facility.

l (f) Rock excavation. The design of the underground facility shall incorporate excavation methods that will limit the potential for creating l

a preferential pathway for ground water to contact the waste packages or radionuclide migration to the accessible environment.

S 60.134 Design of seals for shafts and boreholes.

l" (b)(1) The potential for creating a preferential pathway for ground water to contact the waste packages or radionuclide migration through existing pathways.

I.b Changes in NUREG-1046 This report has been expanded and revised since its initial publication as a draft in February 1984. Six figures have been added to more clearly illustrate technical concepts. Appendix B, " Governing Unsaturated Flow and Transport Mechanisms and Equations," has been deleted with the expectation I i that it will form the core of a separate publication in the future. Public 1 comments received on the proposed unsaturated zone amendments are discussed in Section III.b, " Amendments to 10 CFR Part 60 and review of public l

comments." These comments are also incorporated into the revised technical discussions throughout this document, most extensively in Section II.b.8, "Radionuclide transport in the gaseous state."Section II.b.3, " Geochemistry,"

has been rewritten. Finally, the entire document has been edited and updated to improve clarity and to reflect an increasing understanding of the physical, geochemical, and hydrogeologic processes operative in unsaturated geologic media.

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I.c General discussion of the saturated and unsaturated zones The term " ground water" as defined in the final amendments to 10 CFR Part 60 refers to all water that occurs below the land surface, i.e., in both saturated and unsaturated zones. Todd (1980) noted that, generally, a single saturated zone is overlain by a single unsaturated zone that extends upward to the ground surface. These two sul' surface hydrogeologic zones are separated by an irregular surface called the water table (Linsley et al., 1982). The water table as defined in 10 CFR Part 60 (48 FR 28219) is that surface in a ground-water body at which water pressure is. atmospheric. Often nowever, locally saturated conditions may exist in portions of the unsaturated zone creating a perched water table. The existence and position of the perched water table varies with the magnitude of the initiating water recharge events, the infiltration properties of the soil, and the occurrence of less permeable hvdrogeologic units (e.g., aquitards) at depth. A schematic view of these hydrogeologic concepts is presented in Figure 1.

The saturated zone is defined in the final amendments as that part of the earth's crust beneath the regional water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric (i.e., positive gauge pressure). The saturated zone may extend to considerable depth, but as depth increases, the weight of overlying rock and soil materials also increases, usually causing less pore space to exist. This results in little ground water existing at depths greater than 3 kilometers (Linsley et al., 1982).

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As defined in the final amendments to 10 CFR Part 60, the unsaturated zone is the zone between the land surface and the regional water table. Generally, fluid pressure in this zone is less than atmospheric pressure, and some of the voids ccntain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies, the water pressure locally may be greater than atmospheric. '

Roseboom (1983) stated that placing radioactive wastes in the unsaturated zone will add a new and very effective natural barrier--an environment that should remain relatively dry during the desired period of containment. He further noted that because soil moisture in the unsaturated zone would ultimately percolate downward to the saturated zone, many, if not all, of the additional natural barriers that exist below in the saturated zone also apply to the isolation capabilities of the unsaturated zone.

Water flow and hence contaminant transport through the unsaturated zone will probably differ significantly from that through the saturated zone. The air contained within certain pores or fractures reduces the cross-sectional area for liquid water flow and provides channels for the movement of gases. As the water pressure becomes progressively more negative, smeller and smaller pores are drained, and water flow is confined to the remaining smaller water-filled pores as wedges of water at points of contact of soiid particles and a_s water films on solid particles (Figure 2). The reduction in cross-sectional area and the confinement of flow to the smaller pores or to film flow within the larger voids drastically reduce the hydraulic conductivity of the unsaturated zone.

The hydraulic conductivity of the unsaturated zone (termed ef fective hydraulic conductivity or unsaturated hydraulic conductivity) can be several orders of magnitude smaller than the saturated hydraulic conductivity of the same geologic material and is a function of water content a1d negative pressure of the water.

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Water flow through a thick unsaturated zone is normally controlled by gravity and is therefore vertically downward unless there are discontinuities that could cause a horizontal flow component. Thus one credible pathway for the migration of water-soluble contaminants from a geologic repository located in an unsaturated zone to the accessible environment is vertically downward to the underlying regional water table and subsequently through the saturated zone to the accessible environment. If the unsaturated zone beneath the geologic repository is hundreds of meters thick, travel times for contaminants in solu-tion could be exceedingly long, offering additional assurance of waste isolation.

The depth to the regional water table varies throughout the United States; consequently, the thickness of the unsaturated zone also varies. Temporal and spatial fluctuations of the regional water table are dependent upon a number of factors (see Figure 3), including infiltration, percolation to the water table, air entrapment during ground-water recharge, evapotranspiration and plant uptake, bank-storage effects near streams, ground-water pumping and injection, artifi-cial recharge from surface-water impoundments and surface depressions (e.g.,

conds,'iagoons, and landfills), agricultural irrigation and drainage, regional topography, and geotechnical drainage systems (Freeze and Cherry, 1979). The annual water budget of an arid climate (i.e., precipitation minus potential evapotranspiration) leads to large moisture deficits and little or no recharge to the regional water table except perhaps along stream channels or mountain front: where recharge is concentrated. Commonly, the recharge areas for underlying ground-water units are many kilometers from a given arid site.

Unsaturated geologic materials range from highly porous granular material such as alluvial fill to extremely low permeability consolidated rock with low fracture density such as granites and some densely welded tuffs. Included within this range are porous unconsolidated and consolidated materials with varying degrees of fracturing (for example, low permeability soils, slightly welded tuff, and certain types of sandstones).

The behavior of any water eventually entering the subsurface will be influenced by the water conductance and retention properties (e.g., effective hydraulic conductivity and pore size distribution) of the geologic medium (see Figure 4).

For example, in the case of unsaturated highly fractured rocks, the downward movement of water at certain times can be quite rapid (Roseboom, 1983), but the

quantity of flow may be extremely small. In contrast, in a highly porous medium ~with extremely low permeability (e.g., tight clays), the vertical move-ment of water may be very slow (Roseboom,1983). Unsaturated zone properties of interest in determining ground-water flow rates are water content, negative pressure heads, and the functional relationships of effective hydraulic con-ductivity with water content and with negative pressure head (see Figures 5 and 6). The water content can vary from full saturation (which is equal to the effective porosity) to virtually zero. Pore water pressures in the unsaturated rone range from above atmospheric (e.g., in perched water bodies) to highly l ne0ative pressures.

l II. DISPOSAL OF HLW WITHIN THE UNSATURATED ZONE II.a Background The unsaturated zone has been viewed as a potential location for the disposal of HLW for approximately 20 years. In recent years, one of the major advocates 7

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of the concept of HLW disposal within the unsaturated. zone has been the United States Geological Survey (Winograd, 1981). Many positive aspects and accompanying concerns as'sociated with HLW disposal in the unsaturated zone have been identified (Winograd, 1981; National Academy of Sciences, 1983; Roseboom, 1983; Wollenberg et al., 1983). The positive aspects and potential concerns associated with this concept will be discussed in the NRC evaluation of principal issues presented in the following sections of this chapter.

The NRC staff recognizes that there is little likelihood that a "oerfect" site will be identified within the unsaturated zone. As in the case of potential geologic repository sites within the saturated zone, the staff believes that a site should exhibit an appropriate combination of favorable conditions so as to encourage the selection of a site that is among the best that can reasonably be found. Meaningful comparisons of performance can only be made for specific sites, and will require consideration of non geologic factors as well as geo-logic and hydrologic factors. Further, any potentially adverse conditions identified at a particular site (whether in the saturated or unsaturated zone) should be assessed to ensure that they will not compromise the ability of the geologic repository to meet the performance objectives of 10 CFR Part 60. The staff reiterates the Commission's view set forth in 48 FR 28201 that a site would not be disqualified simply as a result of the absence of a favorable condition or the presence of a potentially adverse condition.

One of the most important issues that must be addressed in evaluating any potential geologic repository site is how effectively the site will isolate radioactive waste from the accessible environment for the necessary period of time. It is generally recognized that ground-water migration constitutes the most probable mechanism for radionuclide transport from a geologic repository to the accessible environment (Bredehoeft and Maini, 1981; Witherspoon et al.,

1981; Roseboom, 1983; Kocher et al., 1983). It is the importance of ground-water flow and transport mechanisms and the identification of credible pathways to the accessible environment that has led to serious consideration of the unsaturated zone in the siting of potential geologic repositories.

II.b Issues addressed by NRC in considering the disposal of HLW within the unsaturated zone The concept of HLW disposal within the unsaturated zone clearly has a number of important issues associated with it. The NRC staff believes it may be premature to divide these issues into definite advantages and disadvantages at this time because of the current paucity of scientific data in the area of ground-water flow through unsaturated geologic media other than porous granular materials. The purpose of the following sections is'to provide a generalized discussion of the principal issues that may be involved in disposal of HLW within the unsaturated zone. In some cases, the issues (e.g., human intrusion) are generally applicable to disposal within either the saturated zone or the unsaturated zone.

II.b.1 Hydrologic properties of the unsaturated zone: The final amendments to 10 CFR Part 60 identified " low moisture flux in the host rock, and in the overlying and underlying hydrogeologic units" as one of five favorable hydrogeologic conditions for disposal in the unsaturated zone (S 60.122(b)(8)).

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i The NRC staff considers the relatively low moisture flux of certain unsaturated geologic media to be a favorable condition in the siting of a geologic repository because the generally low moisture flux could minimize leaching of the waste packages and thus reduce significantly the rate of radionuclide transport from the underground facility by ground-water migration.

It is generally recognized that, in the unsaturated zone, vertical ground-water flux can be very small. The flow path to the saturated zone and its regional discharge points would be extremely long in arid regions where the unsaturated zone could extend to depths of up to 600 meters (Winograd, 1981; Wollenberg et al.,

1983). A fundamental principle of soil physics is that, as the water content decreases, larger pores drain before smaller pores and the effective hydraulic conductivity is drastically reduced (Klute, 1965) (see Figures 5 and 6).

Conversely, as the water content approaches saturation, the large pores are the last to fill. This principle explains " capillary barriers" (also termed the

" wick effect") where large contrasts in pore size distributions between distinct layers act to impede unsaturated flow. Although this concept is generally asso-ciated with geotechnical design criteria for creating restrictive flow boundaries, existence of natural geologic " capillary barriers" could enhance the low ground-water flux attributes for siting geologic repositories in the unsaturated zone.

Hence, "a water table sufficiently below the underground facility such that fully saturated voids continuous with the water table do not encounter the underground facility" is another favorable hydrogeologic condition for the unsaturated zone identified in S 60.122(b)(8).

One major concern raised with respect to disposal of HLW in unsaturated geologic media is the current lack of scientific data on ground-water flow in deep, unsaturated, fractured, geologic media. The principal hydrogeologic parameters which need to be characterized are (1) water storage properties, (2) changes in water content and pressure, (3) rate of ground-water movement (i.e., flux),

and (4) distribution and transport of contaminants. Techniques for measuring and monitoring these parameters have already been developed for unsaturated soils (e.g., Black et al., 1965; Bouma et al., 1974) and porous geologic media (e.g., Wilson, 1980). Therefore, the staff does not currently consider the present low level of scientific information on ground-water movement in deep unsaturated zones to be an insurmountable concern at this time.

II.b.2 Heat dissipation and temperature: The ability of the host rock to dissipate heat generated by the waste packages is a primary concern in designinn an underground facility (Wollenberg et al. 1983). Changes in the temperature of the host rock will have some effect on all processes involved in isolating HLW (Kocher et al., 1983). Rapid heating of the host rock due to HLW emplace-ment could induce fractures and hence alter ground-water flow patterns (Donath, 1980). This could have a positive effect of enhancing free drainage. Slow heating could have the opposite effect of causing rock expansion and reduction.

of fracture apertures (Kocher et al., 1983). Therefore, to evaluate the thermo:nechanical response of the host rock to HLW emplacement, it will be neces-sary to determine the thermomechanical properties of a host rock (e.g., heat capacity, thermal conductivity, and thermal expansion) as functions of temper-ature and degree of saturation (Wollenberg et al., 1983) and to determine the effects of thermally induced processes on the ability of the geologic repository to offectively contain and isolate the HLW for the required periods of time.

13

The thermal conductivity of a host rock is dependent upon physical parameters such as porosity, mineral composition, density, and degree of saturation (water content). The relative ratio of water to air present in the pore spaces affects the thermal conductivity of the host rock since air is a relatively poor thermol conductor as compared to water (Wollenberg et al., 1983). The relatively low thermal conductivities of unsaturated materials compared to their saturated equivalents have been raised as a principal point of concern (National Academy of Sciences, 1983; Wollenberg et al., 1983). The bulk thermal conductivity will control the maximum temperature rise for a given waste form and thermal loading (Wollenberg et al., 1983). Low thermal conductivity values will result in a higher temperature rise in response to a given thermal loading. In contrast, liquid convection induced by heat is less likely to be a potential mechanism for transport of radionuclides from an underground facility within the unsaturated zone to the accessible environment (Wollenberg et al., 1983).

Wollenberg et al. (1983) point out that data on thermal expansion of unsaturated media are currently scarce and that the increase in vapor pressure in response to heating and its ramification for radionuclide transport are presently not well understood. However, these authors also noted that the transport of latent heat by water vapor that is driven by temperature gradients can effectively increase heat transport through unsaturated geologic media, thereby reducing the temperature of the underground facility.

Mineralogical changes in the host rock may occur as a result of changes in temperature. For example, Lappin (1980) noted that welded tuff may experience contraction upon heating if the discontinuities (e.g., fractures) in the rock mass contain significant amounts of expandable clays or zeolites. Conversely, welded tuff containing cristobalite may undergo volumetric expansion due to phase inversion on heating to temperatures of 150-200 C (Wollenberg et al.,

1983). Further, in some instances the permissible thermal loading of an under-ground facility may be seriously limited since clays and zeolites dewater at temperatures of 100 C under atmospheric pore pressure (National Academy of Sciences, 1983). If a shallower depth of waste emplacement and equal regional thermal gradients are assumed for disposal in the unsaturated zone compared to that in the saturated zone, lower ambient temperatures should exist earlier in the life of an underground facility within the unsaturated zone (Wollenberg et al., 1983). Further, temperature rises in the underground facility also can be reduced by lowering thermal loading. This could be accomplished in several ways including extended surface storage of the waste (Donath, 1980; Wang et al., 1981) and increased canister spacing within the underground facility (Donath, 1980; Wollenberg et al., 1983). Also, heat may be dissipated more readily from a geologic repository operations area by an appropriate choice of backfill material (Donath, 1980) and by an efficient ventilation system.

However, tne relative benefits of maintaining a low temperature (approximately 100 C) in the unsaturated zone are currently being reconsidered by the NRC staff in light of continuing research on vapor phase transport. The possibility that a dry envelope could form around the waste packages and could effectively retard the migration of radionuclides by diverting the movement of ground water around the waste packages is currently under consideration by the staff.

The NRC staff expects that these and related issues would be addressed during site exploration and characterization of a candidate area containing a potential site within the unsaturated zone.

14

II.b.3 Geochemistry: After permanent closure of a geologic repository, the immediate vicinity of the underground facility will be partially drained and will have an air pressure of approximately one atmosphere. The presence of air in pores and fractures of the host rock and any packing and backfill that might be used could promote non-reducing conditions in the underground facility.

Unlike an underground facility in the saturated zone, an underground facility within the unsaturated zone could maintain non-reducing conditions and an air pressure of approximately one atmosphere throughout its lifetime because l surface air can be connected with water and air at the horizon of the under-ground facility through open pores and fracture channels within the host rock.

As the temperature in the underground facility rises, it is probable that any ground water present in pores and fractures of the host rock increasingly will be converted to water vapor. Dissolved solids in the ground water can then be precipitated. Expansion of heated air plus water vapor probably will cause air and water vapor to move away from the waste packages. Calculations by some authors (e.g., Preuss and Wang, 1984) suggest that, over time, gaseous oxygen originally present in the underground facility might be purged entirely from the immediate vicinity of the waste packages. If emplaced HLW heats the surrounding environment above about 100 C, then liquid water will boil. At some distance from the waste packages, lower temperatures should allow con-densation of escaping water vapor back into the liquid phase. The liquid water will tend to flow toward the waste packages owing to capillary action (Preuss and Wang, 1984; Travis et al., 1984). This liquid water movement can cause redistribution of chemical elements in any packing and backfill materials pre-sent as well as in the host rock. Depending on such factors as thermal loading, thermal conductivity, heat capacity, porosity, and the initial water content of the host rock and surrounding strata, a " dry zone" might develop around the waste packages.

Within the dry zone, hot vapor-laden air could cause accelerated corrosion of the waste packages, forming metal oxides and hydroxides (Evans, 1937). Host rock, backfill, and packing materials might also be altered. For example, volcanic glasses can undergo devitrification and dehydration to form clays, zeolites, oxides, hydroxides, and secondary silicates (Levy, 1984), and hydrated mineral phases in the host rock, backfill, and packing (e.g., clays) might dehydrate and collapse (Singh, 1981; Allen et al., 1984). If a waste package is breached while surrounded by a dry zone, glass waste forms can alter princi-pally via devitrification, surface hydration, and radiation damage (Bates and

, Steindler, 1983). Spent fuel might be subject to surface hydration and radia-I tion damage. However, the only radionuclides that might be expected to migrate to the zone of higher water content at the edge of the dry zone are those either released as a gas or those that readily form gaseous compounds (Roseboom, 1983)

(see also Section II.b.8 for further discussion).

The temperature within the geologic repository is expected to peak within a few hundred years of waste emplacement; as the temperature decreases, the volume of any dry zone will also decrease (Preuss and Wang, 1984; Travis et al., 1984).

Returning liquid ground water can react with previously altered minerals and redistributed chemical elements in the host rock, backfill, and packing, thereby changing ground water chemistry. Solids that were precipitated during the formation of a dry zone could be redissolved by returning ground water, thus increasing the ionic strength of the solutions. Reversibly dehydrated minerals 15

along the path of returning ground water might be rehydrated, so the time elapsed before liquid water can reach the canisters could, in part, be delayed by mass balance considerations. However, experiments on mixtures of bentonite and sand have shown that even brief exposures to water vapor at temperatures above about 150 C can cause bentonite to lose swelling ability with consequent increases in porosity and permeability (Couture, 1985).

Eventually, liquid water might reach the waste packages. Some calculations suggest that, during the final disappearance of a dry zone around the waste pack-ages, the ground-water flux through the underground facility might temporarily increase (Travis et al., 1984). If metallic components have been armored by earlier corrosion products, the ability of waste package materials to reduce the Eh of solutions might be compromised, and non-reducing conditions could exist even adjacent to the waste packages. High ionic strength solutions, especially under non-reducing conditions, could increase local corrosion rates (Uhlig, 1948) and add to the uncertainty in predicting waste package performance.

If liquid water reaches the waste form, radionuclides probably will be leached into the ground water. Thermal, chemical, and electrical potential gradients will be the driving forces for release of radionuclides from the waste packages (Carnahan, 1984). Radionuclide colloids could form during dissolution of the waste form, or by interactions between the waste package and released radio-auclides, or by sorbing onto preexisting colloids in the ground water (Avogadro and de Marsily, 1984).

Ground water containing dissolved radionuclide species and radionuclide colloids might be forced to flow along mineral surface films in the unsaturated zone.

l This low water to rock ratio could provide increased opportunity for reaction i between radionuclides and rock materials compared to radionuclide migration in the saturated zone. However, radionuclide solution species that form under non-reducing conditions might be more soluble and less retarded by interactions with packing, backfill, and host rock components, than are species that form under reducing conditions (Burney and Harbour, 1974; Palmer and Meyer, 1981; Salter and Jacobs, 1982, Serne and Relyea, 1982; Torstenfelt and Allard, 1984). The concentration of a given radionuclide should be lowered along a flow path by hydrody1amic dispersion, filtration, precipitation and coprecipita-tion, ion exchange, chemical substitution, isotopic exchange, diffusion into pores (matrix diffusion), diffusion into solids (volume diffusion), and electro-static attraction by mineral surfaces. As fluids migrate away from the geologic repository, cooler temperatures will generally cause reaction rates to be slower.

In sum, the NRC staff recognizes several positive geochemical aspects and possible concerns for high-level waste disposal in the unsaturated zone. Positive aspects include the overall low water content of the rocks, which could increase stability of the waste package and reduce corrosion. A dry zone may develop around the waste packages, possibly enhancing the expected performance of the waste pack-age materials. If the waste packages are breached, the lack of liquid water to transport radionuclides could hinder radionuclide movement from the underground facility. If the waste form contacts liquid water, a low water flux through the repository would limit the release rate of the radionuclides. Contaminated ground water flowing through unsaturated rock would have a low water-to-rock ratio, thus maximizing interactions between radionuclides and rock components and enhancing the performance of geologic barriers. Matrix diffusion could be an especially important retardation process for fracture flow through porous unsaturated rocks (0 gard et al., 1984).

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One probable concern related to HLW disposal in the unsaturated zone is ground-water conditions in the host rock that are not reducing. As with disposal in the saturated zone, corrosion rates of waste package materials may increase in a non-reducing environment. After a waste package is breached, radionuclides would tend to be more soluble and more mobile under non-reducing chemical condi-tions. Thus NRC has modified S 60.122(c)(9) to identify ground-water conditions l that are not reducing as a potentially adverse condition in both the saturated and the unsaturated zones. Another possible concern might be a temporary l increase in the ground-water flux through the underground facility during the final disappearance of a dry zone. Alteration of bentonite packing material by water vapor at high temperatures may degrade swelling properties. Finally, irreversible dehydration of minerals such as clays and zeolites during the early high temperature stage of the evolution of the geologic repository could cause collapse of these mineral structures and subsequent degradation of the engineering properties as well as the sorptive capacities of the geologic barriers.

l II.b.4 Retrievability: 10 CFR Part 60 specifies that the design of a geologic repository must maintain the option of waste retrieval throughout the period during which the wastes are being emplaced and thereafter until the completion of a performance confirmation program and Commission review of the information obtained from such a program. Retrieval of the HLW could occur for any reason pertaining to the public health and safety or the environment. The retrievabil-ity requirement does not imply ready or easy access to the emplaced waste at all times prior to permanent closure. NRC recognizes that a retrieval opera-tion would be an unusual event and could be a complicated and costly operation.

A geologic repository located in the unsaturated zone could be more accessible than a geologic repository located at the same depth in the saturated zone.

The relatively dry nature of the rock in the unsaturated hydrogeologic zone could minimize or eliminate the need for dewatering operations. Further, if DOE selects a backfill plan similar to that discussed by Roseboom (1983) it could be easier to gain access to the waste packages in an underground facility within the unsaturated zone. Therefore, retrieval of waste packages from a geologic repository operations area in the unsaturated zone could be less complicated and expensive than in the saturated zone. However, additional evaluations of the complications and expenses involved in such retrieval operations must be evaluated on a case-by-case basis.

In. connection with the issue of retrievability, the stability of underground openings must also be considered. Provisions of S 60.133(e) require that underground openings (1) permit operations to be carried out safely, (2) main-tain the retrievability option, and (3) reduce the potential for deleterious rock movement or fracturing of oves lying or surrounding rock (48 FR 28227).

( These provisions will apply to geologic repositories located within either i hydrogeologic zone. Stability of underground openings depends primarily on the in situ stress and strength of the rock in the excavation areas. Stress magnitude and orientation in relation to the geometry of the excavation and rock mass properties are the major factors that dictate the stability of an underground opening. Underground excavations can be located in either rock or unconsolidated alluvium.

17 l

Wollenberg et al. (1983) present a detailed discussion of the mechanical properties of alluvium and the relationship of these properties to repository construction. Two major factors influence the stability of underground open-ings in alluvium: cementation (i.e. , degree of induration) and the depth of the underground openings. These authors recognize that even relatively small amounts of cohesion can eliminate the need for immediate support provided by a shield and significantly reduce permanent support pressures.

Wollenberg et al. (1983) further state that the depth at which stability problems will require the use of supports or reinforcements can be estimated from the compressive strength of the alluvium. Korbin and Brekke (1975) have found that significant stability problems do not occur until the overburden pressure exceeds twice the compressive strength. This has been estimated as $210 m for alluvium by Wollenberg et al. (1983).

Based primarily upon results provided by Wollenberg et al. (1983) the NRC staff has concluded that it should be possible to maintain stable openings in both unsaturated alluvium and hard rock. Therefore, it should be possible to maintain the retrievability option for a geologic repository located in the unsaturated zone.

II.b.5 Potential for exhumation of the radioactive waste by natural processes:

Continuous geologic processes and discrete geologic events acting at a given site will progressively change the present geologic environment over time (Kocher et al., 1983). Such geologic processes that may either singularly or in combination affect a geologic repository site include tectonism, seismic activity, erosion, glaciation, flooding, meteorite impact, intrusive magmatism and volcanism. Changes in the geologic environment could affect a site in two major ways--by affecting ground-water transport rates and pathways through the subsurfaEe and by possible exhumation of a geologic repository. Some of these processes (e.g., glaciation) might act to remove some of the surface covering from the geologic repository, while other processes (e.g., tectonism and seismic activity) might create discontinuities (i.e., fracture zones) that could create new pathways between the geologic repository and the accessible environment with shorter travel times or larger release rates. The concern of exhumation by natural processes will be site dependent and thus must be evaluated on a site-by-site basis.

II.b.6 Potential for exhumation of the radioactive waste by human intrusion:

In addition to the uncertainties associated with the performance of a geologic repository, there has been repeated concern over the uncertainty of future human behavior during the lifetime of a. geologic repository. The question of human intrusion (whether inadvertent or deliberate) into a geologic repository has received considerable attention from NRC during the development of 10 CFR Part 60. NRC views on this issue are discussed in detail at 46 FR 35282-35283 and 48 FR 28199.

Human intrusion into a geologic reposito.'y within the unsaturated zone may be more likely than into one within the saturated zone if it is located at a significantly shallower depth than a geologic repository within the saturated zone. This may not necessarily be the case at potential sites where a thick unsaturated zone exists.

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With respect to a possible influx of ground water that may occur if a geologic repository were reopened, Roseboom (1983) believes there would be virtually no effect on a geologic repository in the unsaturated zone because of an assumed arid to semi-arid climate with very small net recharge as well as the negative water pressure and the low ground-water flux through the unsaturated zone.

Throughout the development of the technical criteria for 10 CFR Part 60, the question of future development of potential resources at the site of a geologic repository has been closely linked to the issue of human intrusion (48 FR 28199).

NEC has expressed the view that selection of a site with no foreseeably valuable resources could so reduce the likelihood of human intrusion as to reduce or eliminate any further need for it to be considered. Potential future develop-ment of resources at the site of a geologic repository would therefore be of concern regardless of whether the site was situated in the saturated or the unsaturated zone. Further, it is possible that the unsaturated zone could be penetrated if underlying rock units contain a valuable natural resource (e.g., minerals, petroleum, or even potable water in arid regions). The poten-tial for future development of resources is clearly a site-dependent issue that necessarily must be evaluated on a case-by-case basis regardless of whether the potential site is in the unsaturated zone or the saturated zone.

II.b.7 Effects of future climatic changes on the level of the regional water table: The impacts of climatic changes upon the earth's hydrologic cycle have been well documented in the geologic record. Major global climatic j changes have resulted in the waning and waxing of glaciers, fluctuations in sea level, and the expansion and drying of pluvial lakes (Flint, 1971). Short-term (101-102 years) climatic changes although important, might not constitute a major concern to the siting of a geologic repository. However, longer period (103-104 years) climatic changes as well as their potential driving mechanisms encompass time spans over which the components of the geologic repository must function effectively to isolate HLW from the accessible environment. For 4

example, the present Holocene Epoch is one in a series of relatively brief (7 x 103 to 2 x 104 years) interglacial periods within much longer periods of

glaciation (Wright, 1972; Broecker and Van Donk, 1970), while possible driving mechanisms such as the obliquity of the earth's axis and the precession of the equinoxes have cycles of s4.1 x 104 and s2.1 x 104 years, respectively (Broecker, 1968; Hays et al., 1976). Therefore, the long-term environmental stability of the site and the impact of future climatic changes need to be considered in designing the geologic repository to ensure long-term integrity of the geologic repository system.

The effects of a major climatic change on the hydrology of the earth's surface could be profound (Kocher et al., 1983). For example, global sea level dropped over 100 m during a previous glaciatian (Imbrie and Imbrie, 1979). Kocher et al.

(1983) noted that a worldwide heating (e.g., via the~ greenhouse effect) also could have important climatic consequences. While climatic change has long been recognized as a factor to be considered in siting a geologic repository within the saturated zone, it may assume even greater importance when applied to a geologic repository within the unsaturated zone.

l 19

Any climatic change that results in increased precipitation with subsequent higher net infiltration and ground-water recharge can cause a rise in the regional water table. A rise in the regional water table might reduce the ground-water flow path from the geologic repository to the accessible environ-ment. A corresponding increase in recharge could result in increased ground-water velocity and hence shorter residence times for dissolved radionuclides.

During a pluvial period, increased ground-water recharge could allow more i frequent water / waste contact, thereby increasing the probability of canister l corrosion and leaching of the waste (Wollenberg et'al., 1983). Increased precipitation could produce local perched water tables with increased lateral water movement above the more impermeable zones. These conditions could create locally saturated conditions and hence large recharge (moisture) fluxes. Finally, gross changes in sea levels related to climatic changes that affect the cryosphere could produce a new drainage stream base level

} with commensurate changes to both regional water tables and discharge points.

1 l It is possible for scientists to postulate future climatic conditions based d

upon paleoclimatic reconstructions for a specific area using geologic and bio-logic evidence. Techniques used in such reconstructions include, but are not limited to, the study of strand lines that mark the stillstands of former lakes

(e.g., Russell, 1885; Meinzer, 1922; Malde, 1968), records of the 180/180 ratio e

in continental ice sheets (Dansgaard et al. ,1971), analysis of paleodistribu- '

tion of floral and faunal species, investigations of fossil seeds and spores in packrat middens (Van Devender and Spaulding, 1979); and tree ring analyses (Smith and Stockton, 1981). Kocher et al. (1983) noted that, although effects of previous glaciations on surface land forms are readily observable, evidence of the effects of glaciation at depth is much more difficult to obtain. These authors believed that ground-water age dating might allow estimates to be made of changes in ground-water flow rates or flow directions that coincided with previous glacial /interglacial cycles. If it can be shown that such changes are not significant, the effects of future glaciation on ground-water flow at depth reasonably may be ignored (Kocher et al., 1983).

The NRC staff believes that a carefully designed geologic repository would not necessarily be affected adversely by cyclical climatic changes. As Wollenberg et al. (1983) noted, certain regions prone to development of local perched water tables'(e.g., past locations of playa lakes) obviously should be avoided. The effects of climatic changes on arid and semi-arid regions might not contribute sufficiently to the net amount of ground-water flux to compromise the isolation capability of the geologic repository. The potential thickness of the unsatur-ated zone in arid regions could allow designers to compensate for any predict-able rise in the regional water table during pluvial periods by locating the geologic repository well above the predicted maximum water table level and well below any periodic perched water tables. This consideration along with the consideration discussed in Section II.b.1 supports the NRC decision to identify "a water table sufficiently below the underground facility such that fully saturated voids continuous with the water table do not encounter the under-ground facility" as a favorable condition in 60.122(b)(8).

II.b.8 Radionuclide transport in the gaseous state: As stated in Sec-tion II.b.3., the unsaturated geologic media surrounding the underground facil-ity will be partially drained and will possess connected air-filled pores that s can form potential pathways for gaseous movement. Any " dry zone" formed by 20

the elevated temperatures in the immediate vicinity of the underground facility might increase the capacity of that zone to transmit gases. Radionuclides in the gaseous state could move from the underground facility through air-filled pore spaces in the surrounding host rock.

The issue of radionuclide transport in the gaseous state is a relatively new waste management issue that has developed from recent scientific investigations of the feasibility of HLW disposal in unsaturated geologic media. Because most of the investigations were only recently initiated, the issue of radionuclide transport in,the gaseous state from a HLW underground facility has not as yet been examined in any great detail by the scientific community.

Radionuclides in waste with significant vapor pressures are tritium, krypton-85, carbon-14.(as carbon dioxide, methane, or other gaseous compounds), and iodine-129 (Hamilton et al. ,1976) with half-lives of 12.26 years,10.76 years, 5730 years, and 1.7 X 107 years, respectively. Of these radionuclides, iodine-129 is recognized by Roseboom (1983) as being of primary concern because of its high

! vapor pressure and long half-life. Other radionuclides, including those with very low vapor pressures, may move within aerosol-size particles of ground water through pn unsaturated geologic medium (Green and Evans, 1985), but the signifi-cance of this transport mechanism has not yet been established.

Transport of radionuclides in the gaseous state can occur primarily by two processes: convection and molecular diffusion. Convection may occur because of the thermal cells developed in the non-isothermal field around the under-ground facility. Radionuclide convective flux and spatial distribution will depend upon Le specific radionuclide concentration and release rate at the source, the transient temperature distributions, the air permeability of the geologic medium, the viscosity of the air, the sorptive capacity of the geologic medium, and a geometric design factor. Molecular diffusion of gaseous radio-nuclides can be a combination of ordinary diffusion (concentration gradient driven), forced diffusion due to an electric field established around the waste, and thermal diffusion resulting from non-isothermal conditions (Green and Evans, 1985). The three types of diffusive flux can be additive and each component will depend primarily upon the same factors as indicated above for convection, with the exception that air permeability is replaced by effective diffusion coefficients. Additionally, forced diffusion flux will depend upon the electrical field strength surrounding the waste. All of these factor are specific to the site and the engineering design.

Ordinary diffusion is likely to be the most significant type of gaseous diffu-sion in an unsaturated geologic setting. It is a process in which a molecular constituent in a gaseous mixture moves under the influence of a concentration gradient. Under isothermal conditions, the diffusive flux is proportional to the concentration gradient. The proportionality parameter (diffusion coeffi-cient) is a function of the properties of the gases.and the prevailing temper-ature and pressure. Slattery and Bird (1958) presented an expression for the calculation of the diffusion coefficient of a dilute nonpolar gas at low pressure in a binary system. The expression was based on kinetic theory and included the critical temperature and pressure of the two gases as well as two constants that must be obtained experimentally. Green and Evans (1985) showed that, for hydrogen, methane, carbon dioxide, krypton, and iodine. the diffusion coefficients increase monotonically by a factor of 2 to 3 as temperature is increased from 30 to 250 C and that the coefficients are inversely related to pressure. ,

21 I

I l

1 The diffusion coefficients discussed above apply to diffusion through open spaces such as drained fractures, boreholes and shafts in consolidated rock.

Diffusion through a rock matrix or granular material makes use of an effective diffusion coefficient that depends upon the percentage of he rock volume that is air-filled pore space and the tortuosity of the pathways as well as the partitioning coefficients between gas and liquid and between liquid and solid.

A generalized equation for ordinary diffusion through a partially saturated porous medium is given by Weeks et al. (1982), from which an effective diffusion coefficient can be defined mathematically.

Diffusive flux by forced diffusion could be insignificant when compared to ordinary diffusion. However, it should be recognized that gamma radiation emitted during the decay of HLW can create an electric current by Compton scat-tered electrons. The magnitude of the developed electric field is related to the current density and the resistivity of the medium. Electrically charged molecules and atoms in the gaseous phase and charged aerosol particles will respond to the electric field. The average range of a gamma ray in air is at most a few meters and is much less in liquids and solids. Therefore, forced diffusion may be important only in the immediate vicinity of the waste package.

Thermal diffusion of gaseous radionuclides could also be important close to the waste package. This type.of diffusion can take place without a concentration gradient or a force field in the presence of a tem erature gradient. The thermai diffusive flux is proportional to the gra.ient of the logarithm of the i temperature. As noted previously, tempe uture gradients are expected to exist in the vicinity of an underground facility for very long time periods.

An analysis of radionuclide transport in the gaseous phase should include con-sideration -of the several related processes adjacent to the waste package as well as on a larger scale. Radionuclide transport in the gaseous phase may be effective in bridging a " dry zone" surrounding the emplaced waste packages, thus providing a mechanism whereby radionuclides can migrate to the zone of condensation (wetter region). Within the wetter zone, subsequent solution and l downward transport of radionuclides with moving ground water may be more rapid j than it would be if the geologic medium were at its initial water content.

This could result in a shorter travel time for the movement of some radionuclidas.

A larger scale situation may involve radionuclide transport in the gaseous state j from the underground facility to the accessible environment, probably upward

! toward the land surface. Diffusive and convective flux of radionuclides could

be through open natural or man-made channels and also through toe unsaturated

! rock matrix.

l l Radionuclide transport in the gaseous state may not necessarily be a potentially l adverse condition but, given the fact that gaseous-state transport could provide a realistic mechanism for radionuclide transport from an underground facility within an unsaturated zone, the NRC staff prefers to retain the opportunity to evaluate whether or not radionuclide transport in the gaseous state could adversely affect a geologic repository system. To that end, the potential for i

gaseous state transport of radionuclides from the underground facility to the accessible environment is identified as a potentially adverse condition in the amendments to 10 CFR Part 60 (S 60.122(c)(24)). NRC has not yet reached any conclusiens on the issue of radionuclide transport in the gaseous state as a significant mechanism for radionuclide migration from the underground facility 22

but rather is currently , sponsoring research on processes in unsaturated rock in an effort to better understand the subject.

For an underground facility located in the saturated' zone, a temporary, localized, unsaturated region could form around the facility as a result of activities related to construction and operation of a geologic repository operations area (e.g.,-dewatering of shafts and drifts). To date, the issue of gaseous state transport of radionuclides has not been raised for a geologic repository system within the saturated zone primarily because in this case the localized unsaturated region could be expected to be encompassed within a much larger saturated region.

Further, it is anticipated that the time required for waste package integrity (300 to 1000 years) will generally exceed the post-closure time required for resaturation of the underground facility (assumed by the NRC staff to occur within a few hundred' years following permanent closure). Therefore, the NRC staff does not consider it necessary at this time to identify gaseous-state transport as a potentially adverse' condition for high-level waste disposal within the saturated zone. However, if future research in the area of gaseous r transport challenges these current assumptions, the staff may recommend that l NRC broaden the provision of S 60.122(c)(24) to include both the saturated and the unsaturated zones.

III. REVIEW OF NRC REGULATIONS III.a Background Prior to developing amendments to 10 CFR Part 60 specifically related to HLW disposal in unsaturated geologic media, the NRC staff reviewed the final tech-nical criteria published in June 1983 to determine which provisions of the existing rule were equally applicable to HLW disposal within either the satu-rated or the unsaturated zone. The final technical criteria were generic in nature, and most provisions applied regardless of the hydrogeologic zone in t5ich a geologic repository might be located. Many of the issues discussed in Chapter II already are addressed in the final technical criteria since they pertain to the saturated zone as well as to the unsaturated zone. For example, the siting criteria of S 60.122 (48 FR 28225) identify favorable and potentially adverse natural conditions. These provisions address, among other items, the nature and rates of tectonic, hydrogeologic, geochemical and geomorphic proces-ses; structural defosmation; igneous activity; and seismicity. These provisions, as currently written, can be applied equally to sites within the unsaturated zone. Similarly, human activity, a possible concern regardless of hydrogeologic zone, is addressed in the provisions of 6 60.122.

III.b Amendments to 10 CFR Part 60 and review of public comments The issue of whether NRC regulations should adoress considerations relevant to 4 the disposal of HLW in all geologic repositories regardless of whether the sit-was situated in the unsaturated zone or the saturated zone was initially raised in 1981.' In the statement of considerations to the proposed technical criteria published for public comment on July 8, 1981, NRC explained that the proposed criteria were developed for disposal in saturated geologic media since the then-current DOE plans called for HLW disposal at sufficient depth to lie within that portion of the hydrogeologic regime termed the saturated zone (46 FR 35281). NRC further noted that additional or alternative criteria may need to be developed for regulating HLW disposal in the unsaturated zone.

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4 Several commenters on the proposed technical criteria criticized this approach on the basis that disposal in the unsaturated zone was a viable alternative and, since the technical criteria were generally applicable without regard to the possibility of saturation, their scope and applicability should not be unduly restricted. NRC reviewed the technical criteria in light of these pub-l'ic comments and found this criticism to be well founded. In the statement of considerations to the final technical criteria promulgated in June 1983, NRC recognized that although the final technical criteria are generally appropriate

+ to disposal in both the saturated and the unsaturated zones, certain distinc-tions were needed (48 FR 28203). Rather than promulgating the specific tech-1 nical criteria that would apply to the unsaturated zone at the time the final technical criteria were promulgated in June 1983, NRC preferred to issue such criteria in proposed form to afford further opportunity for public comment.

NRC published proposed amendments to 10 CFR Part 60 specifically.related to HLW disposal within the unsaturated zone on February 16, 1984 (49 FR 5934). As mentioned in Chapter I, NRC received fourteen comment letters on these proposed amendments. These comment letters were considered by the NRC staff during the i

development of the final amendments and resulted both in the addition of new amendments and in the modification of existing amendments for the sake of techn-ical accuracy. These changes, with differences between the proposed and final amendments shown in comparative text, are discussed below.

III.b.1 Definitions: The terms " ground water" and " unsaturated zone" have

! been added to S 60.2 Definitions as follows:

" Ground water" means all water which occurs below the land [Earthis ]

4 surface.

" Unsaturated zone" means the zone between the land surface and the regional [ deepest] water table. Generally, fluid pressure [ water] in this zone is [under] less than atmospheric pressure, and some of the 4

voids may contain air or other gases at atmospheric pressure.

Beneath flooded areas or in perched water bodies the [ water] fluid pressure locally may be greater than atmospheric.

t-

In the proposed amendments to 10 CFR Part 60, NRC defined the term " ground water" as "all water which occurs below the Earth's surface." Although some commenters on the proposed amendments commended NRC for proposing this definition other i commenters expressed the view that NRC should restrict the use of the term

" ground water" to the saturated zone. Although limiting the term " ground water" to water within the saturated zone currently might be a more widely accepted practice than applying it to all water below the Earth's surface, NRC notes that at present no unique definition of " ground water" appears to be universally accepted in the technical community. The NRC staff notes that no definition of this term was provided in U.S. Geological Survey Water Supply Paper 1988 (Lohman et al., 1972). However, in the Glossary of U.S. Geological Survey Wate'r - Supply Paper 2250 (U.S. Geological Survey, 1983) the following definition of " Ground water" is included:

"In the broadest sense, all subsurface water, as distinct from surface water; as more commonly used, that part of the subsurface water in the saturated zone." (p. 240).

1 4

24 1

Therefore, NRC has not actually redefined the term as one commenter suggested but rather has adopted one of the more commonly used doiinitions of " ground water" to effectively apply the provi: ions of 10 CFR Part 60 to the regulation of HLW disposal within unsaturated as well as saturaced geologic media.

Similarly, the terms, "hydrogeologic unit," "hydrogeologic properties," and "hydrogeologic conditions" refer to units, properties and conditions in either the saturated or unsaturated zore. One minor changt: has been made to the word-ing of the definition of ground water -- the phrase " Earth's surface" has been replaced by " land surface" for the sake of technical clarity and for consistency -

with the wording of the definition of "unsaturatad zone."

The proposed definition of " unsaturated zone" was derived from USGS Water-Supply Paper 1988 (Lohman et al., 1972). Two commenters on the proposed amendments stated that the phrase " deepest water table" introduced confusion into the definition of the term " unsaturated zone" (S 60.2). NRC had inferred that the phrase " deepest water table" as used by the USGS referred to the regional water table and hence adopted this same phraseology in its proposed definition. In light of the confusion expressed by co.nmenters, possibly due in part to an

ncorrect inference by some that the phrase " deepest water table" referred to local rather than regional water tables, the definition of the term " unsaturated zone-' was modified in the final amendments. For the sake of technical clarity the phrase " deepest water table" was replaced by " regional water table," and the phrase " water in this zone is under less than atmospheric pressure" was rewritten as " fluid pressure in this zone is less than atmospheric pressure."

. III.b.2 Siting criteria: The siting criteria of S 60.122 contain provisions for favorable and potentially adverse conditions that may exist at a potential site for a geologic repository. The majority of the provisions set forth at S 60.122 are equally applicable to both saturated and unsaturated zones.

However, given the natural differences between these two hydrogeologic regimes, it is necessary to differentiate between certain favorable and potentially adverse hydrogeologic conditions for each zone. The final technical criteria promulgated in June 1983 contained favorable and potentially adverse condi; ions that are generally applicable without regard to the hydrogeologic zone under consideration as well as specific hydrogeologic conditions for HLW disposal within the saturated zone. In reviewing the favorable hydrogeologic conditions for the saturated zone contained in the final rule, the NRC staff recognized that paragraph 60.122(b)(2)(iv) which was related to pre-waste-emplacement ground-water travel time, could apply equally to the unsaturated zone. There-fore, this provision was removed from S 60.122(b)(2), which contained hydro-geologic conditions specifically related to the saturated zone, and renumbered as new paragraph 60.122(b)(7). It should be noted that the provisions of S 60.122(b)(2)(iii) specifically relate to hydrogeologic conditions of the saturated zone, not the unsaturated zone. This paragraph was included in the proposed amendments because it was necessary tc delete the disjunctive between S 60.122(b)(2)(iii) and (iv) when, as discussed above, S 60.122(b)(2)(iv) was renumbered as S 60.122(b)(7). Further, NRC has modified the wording of S 60.122(b)(2)(iii) for the sake of technical clarity as suggested by a commenter on the proposed amendments. These provisions now read as follows:

S 60.122(b)(2)(iii) Low vertical permeability and low hydraulic

[petential] gradient between the host rock and the surrounding hydrogeologic units.

25

S 60.122(b)(7) Pre-waste-emplacement ground-water travel time along the fastest path of likely radionuclide travel from the disturbed zone to the accessible environment that substantially exceeds 1,000 years.

A new paragraph 60.122('s)(8) was added to the provisions of 10 CFR Part 60 to identify favorabic hydrogeologic conditions that may exist at a site in the unsaturated zone. The provisions of this new paragraph as set forth in the final amendments read as follows:

S 60.122(b)(8) "For disposal in the unsaturated zone, hydrogeologic conditions that provide--

(i) Low [and-nearly-constant] moisture flux in the host rock and in the overlying and underlying hydrogeologic units; (ii) A water table sufficiently below the underground facility such that fully saturated voids continuous with the water table do not encounter the underground facility; (iii) A laterally extensive low permeability hydrogeologic unit above the host rock that would inhibit the downward movement of water or divert dUwnward moving water to a location beyond the limits of the underground facility; (iv) A host rock that provides for free drainage; or (v) A climatic regime in which the average annual historic precipitation is a small percentage of the average annual potential evapotranspiration.

The NRC staff originally identified the favorable hydrogeologic conditions presented in S 60.122(b)(8)(i)-(v) for the following reasons:

(i) low moisture flux in the host rock and surrounding hydrogeologic units resulting from very low ground-water fluxes (i.e., little or virtually no outflow to deeper systems) would probably result in little solute transp?rt because (a) there would be little water contact with and hence little leaching of the waste packages and (b) the moisture flux would provide for a small water gradient to induce movement of what little water content was available, except by gravity drainage (see Sec-tion II.b.1 for further discussion).

(ii) in the event of both short- and long-term fluctuations of water table levels, fully saturated pores continuous with the water table would not encounter the underground facility, providing additional assurance that the rate of leaching of radionuclides from the waste packages would not be altered significantly (see sections II.b.1 and II.b.7 for further discussion).

(iii) inhibition or diversion of downward movement of water would minimize the potential for significant leaching of radionuclides from failed waste packages (see Sections II.b.3 and II.b.7 for further discussion).

'. 26 9

(iv) a freely drained host rock would promote low moisture content around the waste packages thus decreasing the amount of water available to corrode the waste packages, leach radionuclides from the waste and transport radionuclides away from the underground facility (see Sec-tion II.b.1 for further discussion).

(v) in arid zone environments, low amounts of precipitation will result in low recharge' flux (see Sections II.b.1 and II.b.7 for further i discussion).

In addition to the favorable conditions discussed above, the NRC staff identi-fied three potentially adverse conditions that could exist at a site in the unsaturated zone in the proposed amendments. These new paragraphs 60.122(c)(22),

(23), and (24) are additions to the existing provisions of 9 60.122(c). Further, NRC has modified S 60.122(c)(9) as suggested by one commenter on the proposed amendments so that the provision is now equally applicable to both the saturated and the unsaturated zone. These potentially adverse conditions read as follows:

60.122(c)(9) [For-disposal-in-the-saturated-zone-] Ground-water conditions in the host rock that are not reducing.

60.122(c)(22) Potential for the water table to rise sufficiently so as to cause saturation of an underground facility located in the unsaturated zone.

60.122(c)(23) Potential for existing or future perched water bodies that may [have-the-effect-of-saturating] saturate portions of the underground facility or [previding] provide a faster flow path [for-radiennelide-movement]

from an underground facility located in the unsaturated zone to the accessible environment.

60.122(c)(24) Potential for [ vapor-transport-of-radionuclides-from the-underground-facility-iecated-in-the-unsaturated-zone] the movement of radionuclides in a gaseous state through air-filled pore spaces of an unsaturated aeologic medium to the accessible environment.

The presence of ground-water conditions in the host rock that are not reducing has been identified in the final amendments as a potentially adverse condition for HLW disposal in either the saturated or the unsaturated zone. Previously, this provision (S 60.122(c)(9)) was applicable solely to disposal in the saturated zone (48 FR 28225). Following consideration of comments on this pro-vision, the NRC staff suggested that S 60.122(c)(9) be modified as discussed above. The staff's rationale for suggesting this amendment is based on the fact that corrosion rates of waste package materials may increase in a non-reducing environment. Further, if a waste package is breached, radionuclides would tend to be more soluble and more mobile under non-reducing geochemical conditions.

With respect to the potentially adverse condition set forth in S 60.122(c)(22),

the NRC staff assumes that the design for an underground facility located in the unsaturated zone can be quite different from one in the saturated zone, given the variation in hydrogeologic, geochemical, and thermal properties between the two hydrogeologic zones. Therefore, any condition that could cause a rise in the level of tne water table, and hence possibly result in the saturation 27

)

l of an underground facility within the unsaturated zone could potentially affect the integrity of a geologic repository that was not originally designed for saturated conditions.

The NRC staff identified the presence of existing or future perched water bodies as a potentially adverse condition in the unsaturated zone (S 60.122(c)(23))

because it may be possible for contaminated gas or ground water to diffuse into the rock surrounding the underground facility. If this contaminated gas or ground water encountered a perched water body, it could be transported horizon-tally. If this horizontal movement shortened the flow path to the accessible environment, the caologic repository system might not meet the performance objectives of 10 CFR Part 60. Minor word changes were made to clarify the intent of titis provision.

During the development of the proposed amendments, the staff identified vapor transport of contaminants as a potential concern associated with HLW disposal in the unsaturated zone. NRC noted at 47 FR 5935 that, in unsaturated geologic media, water is transported in both liquid and vapor phases. The relative con-tribution of transport via both these phases and their direction of movement with respect to a geologic repository was deemM to directly influence the containment of contaminants. The comments received on the discussion of vapor transport at 49 FR 5935 and on the wording of the proposed amendment,-S 60.122(c)(24), indi-cated a need for the NRC staff to clarify its intent with respect to vapor transport.

Vapor formation may not necessarily be a potentially adverse condition but, given the fact that vapor transport could provide a realistic mechanism for radionuclide transport within the unsaturated cone, NRC wanted the opportunity to evaluate whether or not vapor transport could adversely affect a geologic repository system. To that end the potential for vapor transport ' radio-nuclides from an underground facility located in the unsaturated zo..e to the accessible environment was identified as a potentially adverse condition in the proposed amendments (S 60.122(c)(24)). NRC has not reached any conclusions on vapor transport as one commenter on the proposed amendments incorrectly inferred, but rather is currently sponsoring research on vapor phase transport in unsaturated fractured rock in an effort to better understand this subject.

Some confusion was expressed by the commenters with respect to the use of the term " vapor transport" in the proposed amendments and draft NUREG-1046. In particular, one commenter stated that S 60.122(c)(24), as written, was ambiguous and meaningless and could, under ceit.ain conditions, be applied te any geologic repository site. The term " vapor transport" as used in the proposed amendments referred to both water vapor and the gaseous state of some constituent contaminants.

A second commenter on 7.his issue suggested that quantitative clarifications be added to this provision since the proposed wording allowed no potential vapor transport of radionuclides by molecular diffusion (i.e., the process whereby solutes are transported at the microscopic level because of variations in tracer concentrations within the liquid phase) or convective transport (i.e., heat transported along with flowing ground water). The same commenter noted that, while the flux values associated with these two transport processes might be miniscule, they would not be zero at any unsaturated site. NRC did not consider it appropriate to add quantitative clarifications to the final provisions of S 60.122(c)(24) as suggested by a commenter because the movement of radionuclides in the gaseous state is, to a large extent, dependent on site- and design-28

specific parameters. However, to alleviate the confusion surrounding proposed S 60.122(c)(24), the wording of this provision was modified extensively in the final amendments. Reference to vapor transport was deleted, and t'N nrovision now identifies the potential for the movement of radionuclides in a weous state through air-filled pore spaces of an unsaturated geclogic mediw.; to the accessible environment as a potentially adverse condition. The NRC staff believes the revised wording will more accurately convey its original intent and should remove any ambiguity associated with the previous wording, Jor example, one' commenter's query of where the vapor transport is occurring and when it is important.

III.b.3 Design Criteria for the Geologic Repository Operations Area: Sec-tions CO.131 through 60.134 specify minimum criteria for the design of the geologic repository operations area (48 FR 28225-28227). During the develop-ment of the proposed amendments to 10 CFR Part 60 specifically related to the unsaturated zone, the NRC staff reviewed the provisions of the design criteria set forth by the final rule and concluded that no changes were necessary to ensure that the design criteria were equally applicable to both the saturated and the unsaturated zone. The bases for this conclusion were provided at 49 FR 5935-5936. One commenter on the proposed amendments pointed out that the wording of S 60.133(f) and S 60.134(b) served to limit the potential for creating a preferential pathway. However, as this commenter noted, a prefer-ential ground-water pathway may be preferred to allow for a freely draining host rock. Asaan example, this commenter stated that it might be desirable to have a preferential pathway for ground water from an overlying unit where the l

ground water may tend to perch naturally to an underlying unit or completely through the underground facility horizon to an underlying permeable zone.

III.c Other issues reviewed by the NRC staff In addition to those issues relating to disposal of HLW within the unsaturated zone that.provided the bases for the development of the proposed amendments presented in the previous section, several other issues were reviewed by the staff. Some of these issues were identified by public commenters on the proposed 10 CFR Part 60 technical criteria. A more detailed discussion of most of these issues was presented by Roseboom (1983), and it is primarily his paper that forms the basis of the following discussion. The staff believes that it is useful to review these issues, although they resulted in no changes to the amendments.

III.c.1 Minimum 300-meter depth for waste emplacement: One commenter on the proposed 10 CFR Part 60 technical criteria (July 1981) advocated applying the rule equally to both the saturated and the unsaturated zones because he con-sidered it necessary to change the siting criterion that set a minimum depth of 300 meters for waste emplacement. However, the commenter incorrectly identified this provision (see S 60.122(b)) as a requirement rather than as a favorable condition. Hence, a minimum depth of 300 meters for waste emplacement is con-sidered a favorable condition because the deeper the HLW is emplaced below the land surface the less likely it is to be disturbed. In that light, such depth is a favorable condition irrespective of hydrogeologic zone. Since, as noted earlier, the unsaturated zone m'y a extend to depths of up to 600 neters in certain arid regions of the United States, the staff believes that this 29

favorable condition is a realistic one for both the saturated and unsaturated zones. Therefore, this provision of the technical criteria has not been modified.

III.c.2 Exploratory boreholes: Provisions relating to site characterization are set forth in the final rule at S 60.10 (48 FR 28219). Existing para-graph 60.10(d)(2) requires that the number of exploratory boreholes and shafts be limited to the extent practical consistent with obtaining the information needed for site characterization. Although he recognized the importance of exploring potential sites in the saturated zone with as few drill holes as possible, Roseboom (1983) stated that, in the unsaturated zone, if the host rock already had a high vertical permeability, there was no reason to limit the number of drill holes. Thus Rcseboom noted that, if necessary, a proposed geologic repository could be explored like an ore body or coal bed, with drill holes every few hundred feet on a rectangular grid.

The Commission's view on the importance of not compromising the integrity of a )

site during the site characterization program of testing and exploration was However, if DOE should opt for a site explora-

! stated clearly at 44 FR 70409.

tion and characterization program that includes plans for drilling numerous boreholes, DOE would have the burden of showing that the ability of the site i

to isolate HLW has not been compromised during these activities.

III.c.3 Backfill requirements: Another issue that has been raised pertains to l the necessity of backfill in a geologic repository located within the unsaturated zone. Roseboom (1983) believes that the role of backfill in the unsaturated zone would be the opposite of that in the saturated zone. Backfill material with high sorption that would serve to inhibit the flow of water to and from the waste packages may be highly desirable in the saturated zone. In the unsaturated zone, however, the designers of a geologic repository might wish to promote drainage. Roseboom (1983) stated that within the unsaturated zone,

! backfill should allow ground water to drain readily rather than serve to impede drainage and possibly cause local ponding. He further presented the argument that, if the use of backfill is desired to retard radionuclide migration, it would be effective only if placed at or below the canister level since con-taminated water would move downward. Winograd, as cited in Roseboom (1983),

suggested that if backfill were necessary to preserve structural or waste package integrity, a relatively permeable matarial (e.g. ;obble-sized rock) could be used to enhance drainage.

Proposed 10 CFR Part 60 technical criteria (published for public comment in July 1981) contained design criteria for the underground facility, including backfill materials (46 FR 35292-35293).

After careful consideration of the public comments received on proposed S 60.132, the staff concluded that the approach of specifying several design requirements for the engineered system (including backfill and barriers at shafts) would be i unduly restrictive. Therefore, the final technical criteria contain only the general functional statement that the engineered barriers be designed to assist the geologic setting in meeting the performance objectives for the period following permanent closure (S 60.133(h), 48 FR 28277).

l l

l 30

III.c.4 Waste package design criteria: As defined at S 60.2, the term " waste package" means "the waste form and any containers, shielding, packing and other absorbent materials immediately surrounding an individual waste container" (48 FR 28219). The point has been raised that because of the different nature of the emplacement environment, designs of waste package components for the saturated and unsaturated zones may be quite different. The NRC staff recognizes that several characteristics of the emplacement environment (e.g., oxidation conditions, lithostatic pressure, geochemistry, contact with ground water) may vary significantly between the two hydrogeologic zones. This variation of emplacement environment may necessitate that DOE consider alternative designs for waste packages (including waste form, canisters, and overpack) for geologic disposal in the unsaturated zone. However, 10 CFR Part 60 allows flexibility in developing and selecting any waste package that will meet the performance objectives.

The NRC staff has reviewed the performance objectives that pertain to the waste package (S 60.111 and S 60.113) and believes that the provisions, as currently written, are equally applicable to waste packages emplaced within either the saturated or the unsaturated zone. Similarly, the specific design criteria for the waste package and its components (S 60.135, 48 FR 28227) have been deter-mined to be generally applicable to both zones. Therefore, no changes have been made to SS 60.111, 60.113, or 60.135.

III.c.5 Provisions for ventilation: Roseboom (1983) addressed the issue of restricting the number of ventilation shafts associated with a geologic repository. In the case of the saturated zone, the number of ventilation shafts may be kept at a minimum since the shafts could constitute potential pathways to the accessible environment. In the unsaturated zone, Roseboom (1983) stated that additional shafts for ventilation would not compromise the geologic repository's performance because sealing shafts in the unsaturated zone is much simpler and of less consequence.than in the saturated zone.

Ros(500m cited several potential benefits associated with an increased number of ventilation shafts -- reducing the problem of thermal load in the early phases of the geologic repository, removal of any water vapor during the operational period, drawing large amounts of desert air through the geologic repository to promote even drier conditions, and increasing worker safety by providing alternative sources of ventilation and escape in emergencies.

Hammond (1979) noted that an air-cooled repository need not be designed to bear the load of thermally induced stresses placed on a repository where waste is tightly emplaced and buried in backfill.

The number of ventilation shafts included in any geologic repository will be decided by the designer, 00E. No provision of 10 CFR Part 60 expressly limits the number of ventilation shafts that a geologic repository may contain. What is important is that surface facility ventilation systems comply with the design criteria in 60.132(b) (48 FR 28226) and that the underground facility ventila-tion system be designed in accordance with S 60.133(g) (48 FR 28227). The NRC staff considers the design requirements for the ventilation systems set forth in SS 60.132 and 60.133 to be reasonable and applicable to both saturated and unsaturated zones. As long as the ventilation system complies with provisions 31

of SS 63.111, 60.132 and 60.133 and does not compromise the integrity of the site to host a geologic repository, DOE will have broad flexibility in designing the system. Therefore, the staff does not consider it necessary to amend the provisions of the final technical criteria pertaining to the design of a ventila-tion system for a geologic repository.

IV. CONCLUSIONS l l

The NRC staff has concluded that carefully sited and designed geologic repositories in the unsaturated zone can meet the provisions of 10 CFR Part 60 with the addition of the amendments at 50 FR 29641, as discussed in Chapter III.

Several issues associated with disposal of HLW within the unsaturated zone have been examined, and positive aspects and possible concerns have been identified. 4 In some instances, the possible concerns are generic in nature and would apply l to geologic repositories within either the saturated or the unsaturated zone.

As in the case of geologic repositories in the saturated zone, the favorable and potentially adverse conditions present at a site in the unsaturated zone must be evaluated on a case-by-case basis.

32

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Roseboom, E. H. , Jr. ,1983, Disposal of High-Level Nuclear Waste Above the Water Table in Arid Regions, U.S. Geological Survey Circular 903, Washington, DC, p. 21.

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Salter, P. F. , and G. K. Jacobs,1982, Evaluation of radionuclide transport:

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p. 801.

Serne, R. J., and J. F. Relyea, 1982, The status of radionuclide sorption-desorption studies performed by the WRIT program, PNL-3997.

Shade, J. W., L. L. Ames and J. E. McGarrah, 1983, Actinide and Tc Sorption on Fe-Silicate and Dispersed Clay Colloids, Proceedings of 185th American Chemical Society National Meeting, Division of Nuclear Chemistry and Technology, Seattle, Washington, March 20-25, 1983.

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p. 939-947.

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Torstenfelt, B. and B. Allard, 1984, The retention of redox sensitive waste elements in compacted bentonite, Mat. Res. Soc. Symp. Proc., v. 26,

~

p. 789.

Travis B. J., S. W. Hodson, H.-E. Nuttall, T. L. Cook, and R. S. Rundberg, 1984, Numerical simulation of flow and transport in fractured tuff, Mat-. Res.

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36

)

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37

APPENDIX DEFINITIONS AND CONCEPTS accessible environment means (1) the atmosphere, (2) the land surface, (3) surface water, (4) oceans, and (5) the portion of the lithosphere that is outside the controlled area. (US NRC, 1983) aquifer: A formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs (Lohman et al., 1972) aquitard: A confining bed that retards but does not prevent the flow of water to or from an adjacent aquifer; a leaky confining bed. It does not readily yield water to wells or springs, but may serve as a storage unit for ground water. (American Geological Institute, 1980) capillary fringe: The zone immediately above the water table in which all or some of the interstices are filled with water that is under less than atmospheric pressure and that is continuous with the water below the water table. The water is held above the water table by interfacial forces (for example, surface tension). The capillary fringe is typically saturated to some distance above its base at the water table; upward from the saturated part only progressively smaller pores are filled and the upper limit is indistinct. In some quantitative studies it is convenient to define the upper limit more or less arbitrarily.

conductivity, effective hydraulic [LT 1]: The water flux under unit hydraulic head gradie~nt through a porous medium that contains more than one fluid, such as water and air in the unsaturated zone.

conductivity, hydraulic, [LT 1]: The water flux through a saturated porous medium under unit hydraulic head gradient.

engineered barrier system: The waste packages and the underground facility.

(US NRC, 1983) flux, ground water: The quantity of ground-water flow per unit cross-sectional area per unit time.

gauge pressure: Pressure expressed as a difference from atmospheric pressure.

geologic repository: A system which is intended to be used for, or may be used for, the disposal of radioactive wastes in excavated geologic media. A geologic repository includes (1) the geologic repository operations area, and (2) the portion of the geologic setting that provides isolation of the radioactive wastes. (US NRC, 1983) geologic repository operations area: A high-level radioactive waste facility that is part of a geologic repository, including both surface and subsurface areas, where waste handling activities are conducted.

(US NRC, 1983).

A-1

l ground water: All water which occurs below the land surface, including both the saturated and unsaturated zones. (modified after US NRC, 1985) ground water, perched: Unconfined ground water in a local saturated zone separated from the underlying regional water table by an unsaturated zone.

Perched ground water may be either (1) permanent, where recharge is frequent enough to maintain a local saturated zone above the restricting layer (aquitard) or (2) temporary, where intermittent recharge is not great or frequent enough to prevent the perched water from disappearing from time to time as a result of drainage over the edge of or through the restricting layer. NRC considers perched ground water to be part of the unsaturated zone (modified after Lohman et al., 1972).

ground-water recharge: The flux of water across a water table. (Wilson, 1980) ground water, unconfined: Water in an aquifer that has a water table.

(Lohman et al., 1972) head, static, [L]: The height above a standard datum of the surface of a column of water (or other liquid) that can be supported by the static pressure at a given point. The static head is the sum of the elevation head and the pressure head. (Lohman et al., 1972) head, total,-[L]: The total head of a liquid at a given point is the sum of three components: (1) elevation head, which is equal to the elevation of the point above a datum, (2) pressure head, which is the height of a column of static water that can be supported by the static pressure at the point, and (3) velocity head, which is the height the kinetic energy of the liquid is capable of lifting the liquid (modified after Lohman et al.,

1972).

high-level radioactive waste: (1) Irradiated reactor fuel, (2) liquid wastes resulting from the operation of the first cycle solvent extraction system, or equivalent, and the concentrated wastes from subsequent extraction cycles, or equivalent, in a facility for reprocessing irradiated reactor fuel, and (3) solids into which such liquid wastes have been converted.

(US NRC, 1983) hydraulic head gradient [dimensionless]: The change in static head per unit of distance in a given direction. If not specified, the direction generally is understood to be that of the maximum rate of decrease in head.

infiltration: The downward entry of water into the soil or rock matrix.

(Modified after Soil Service Society of America, 1975) isolation: Inhibiting the transport of radioactive material so that amounts and concentrations of this material entering the accessible environment will be kept within prescribed limits. (US NRC, 1983) matrix: The solid framework of a porous system (e.g., soil, rock). (Wilson, 1980)

A-2

I percolation: The downward movement of water through the unsaturated zone.

I permeability, intrinsic, [L2]: A measure of the relative ease with which j a porous medium can transmit a liquid under a potential gradient. It is a property of the medium alone and is independent of the nature of the liquid and of the force field causing movement. It is a property of the medium that is dependent upon the shape and size of the pores. (Lohman et al., 1972) porosity, [dimensionless]: The ratio of the volume of interstices to total volume for a porous medium. It may be expressed as a decimal fraction or as a percentage, porosity, effe'ctive, [dimensionless]: The amount of interconnected pore space available for fluid transmission expressed as a ratio of the interconnecting interstitial volume to the total volume.

pressure head: The head of water at a point in a porous system, negative for unsaturated systems, positive for saturated systems; quantitatively, it is the water pressure divided by the specific weight of water. (Wilson, 4

1980) saturated zone: See zone, saturated.

static head: See head, static.

total hydraulic head: See head, total.

i' underground facility: The underground structure, including openings and back-fill materials, but excluding shafts, boreholes, and their seals.

(US NRC, 1983) vadose zone: See zone, unsaturated.

water content: The amount of water' stored within a porous matrix, expressed i

as either a volumetric (volume per unit volume) or mass (mass per unit mass) ratio of water to matrix. (Wilson, 1980) water table: That surface in a ground-water body at which the water pressure is atmospheric. (US NRC, 1983) zone, saturated: That part of the earth's crust beneath the regional water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric. (US NRC, 1983) zono, unsaturated: The zone between the land surface and the regional water table. Generally, water in this zone is under less than atmospheric pres-sure, and some of the voids may contain air or other gases at atmospheric pressure.' Beneath flooded areas or in perched water bodies the water pressure locally may be greater than atmospheric. (Modified after Lohman et al., 1972)

I

, A-3

knC PCAM 335 U S. NUCLE 1W iLEGULATOXY COMul55 SON I R hapR T NuYSE R FAss ens @ @y TfDC ### M %o. 8f 44F8 iE2 ' BIBLIOGRAPHIC DATA SHEET NUREG-1046 Fi 1 k

2 TIT 6 AmD5v8T TLE 3 LE AVE 8 LANE Dis sal of High-Level Radioactive Wastes in the Uns urated Zone: Technical Considerations and f. oATE RE,ORi cOuPtETeo Resp se to Comments p NTR veAa l

. AUTROms, Sefember 1985 6 O AT E REPORT ISSvEO C.J. Hack rth, T.J. Nicholson, D.D. Evans * *~'R l

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(*Consu ant) ) October 1985 F P EMORMING ORGANta ' TON NAME ANo MAILING ADDRE55 fracs.oele Comp 8 PROJECT T ASK WORN UNt1 NUY8ER Division of R iation Programs and Earth Sciences 09-l

, , o,0n,,r ,uv n Office of Nucl- Regulatory Research U.S. Nuclear Reg atory Commission Washington, D.C. 0555 10 5.*OP50ReNG ORGANIZ ATsON NAVE 7 MAILtNG AQoRE$$ ffncru elg Caoet its TYPE OF REPORT Technical Same as 7. above. . PE-Oo covtaEo ~~ ~~

July 1983 - September 1985 12 SUPPLEVENT ARY NOTES This report supersedes Draft REG 046, February 1984.

13 485TR ACT (200 *oras or <esas On July 22, 1985, the L . Nuclear Regulatory Commission (NRC) promulgated amendments to 10 CFR Part 60 _o erning disposal of high-level radioactive waste (HLW) in geologic repositori in e unsaturated zone (50 FR 29641). This report contains a discussion of th princip technical issues considered by the NRC staff during the development o these a ndments. It expands or revises certain technical discussions or' inally' pres ted in draft NUREG-1046 (February 1984) based on public comment etters and an nereasing understanding of the physical, geochemical, and hydrog logic processes r rative in unsaturated geologic media.

The following i sues related to - 31 of HLW within the unsaturated zone are discussed: hyd geologic propert. conditions, heat dissipation and temperature, geoch istry, retrievability, . tential for exhunation of the radioactive waste natural causes and by huma intrusion, the effects of future climatic changes n the level of the regiona water table, and transport of radionuclides in e gaseous state.

The chan s to 10 CFR Part 60 in definition siting criteria, and design criteria for t geologic repository operations area a discussed. Other criteria examined by t NRC staff but which were not changed q the rule are the minimum 300-meter de h for waste emplacement, limitations on ex oratory boreholes, back-

• fill require ents, waste package design criteria, and provi ons for ventilation.

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