ML20245C924
| ML20245C924 | |
| Person / Time | |
|---|---|
| Issue date: | 05/24/1989 |
| From: | Ross F NRC |
| To: | |
| Shared Package | |
| ML20245C902 | List: |
| References | |
| REF-WM-1 NUDOCS 8906260328 | |
| Download: ML20245C924 (25) | |
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,.,4-3' DRAFI THE RUIDRKING HISIGRY OF GROUNDWATER TRAVEL TIME:
FRCH THE 1980 ANPR 70 THE 1985 FINAL RUIE Draft Report May 24, 1989 BY Frederick W. Ross Hys upclogist Hydrologic Transport Section i
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TARTE OF CONITNIS BL@
I. R E MORY BACHGROUND......................................... 1 Multipule Barrier Approach.................................... 1 Groundwater Travel time....................................... 2' Disturbed Zone................................................ 3 II. P!JID9EEING 050NOIDGY OF GROUNDWATER 'IRAVEL TIME. . . . . . . . . . . . . 4 Advance ' Notice Ot Proposed Rulenaking. . . . . . . . . . . . . . . . . . . . . . . . . 4 Pv_-f+1 Ru1e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Final Rule.................................................... 11 PrmM Amendments On 'Ibe Unsaturated Zone. . . . . . . . . . . . . . . . . . . 15 Final Ai=J- dinuds On 'Ihe Unsaturated Zone. . . . . . . . . . . . . . . . . . . . . . 17 III. REFERD1CES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 k
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1 gattops. doc y L REGUIA'IORY BACIGOUND Multiole Barrier Anoroach To understand the relationship between the pre-waste-ertplacement groundwater.
travel time regulatory requirement for geologic setting performance after permanent closure and the performance objective for overall system performance after permanent closure, the Nuclear Regulatory rw-innion's multibarrier philosophy tcward regulatiry geologic dim _1 of high-level radioactive waste must be und=:wi.ced first. In the multibarrier approach, the repository system is divided into major subelements or barriers. With the environmental standard proposed by EPA as the measure of performance for the total system, a l mininum post-closure performance stardard for each barrier is assigned. 'Ihis approach is intended to have two advantages over the total system performan approach. First, if the barriers are chosen judiciously, uncertainty in evaluating repository performance can be reduced by requiring barriers to perform in ways that reduce their relative contribution to overall uncertainty. Secand, barriers can be selected on the basis of their ability I
to perform iraieperah-d.ly of one another to enhance confidence that the wastes i
will be isolated for the required period of performance (" defense in depth") .
In implementing the nultibarrier approach, the post closure performance objectives of 20 CFR Part 60 require a geologic repository system emmad of waste packages and an underground facility (ergineered barrier system), and the geologic setting beyard the area effected by waste emplacement (natural i
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barrier). The engineered barriers are intended to be designed so as to provide "substantially ocuplete" containment of waste durire the initial, high-heat period following waste emplacement and to limit the rate at which radioactive materials are released to the geologic system at the end of the containment period as a means of wsating for uncertainty in predictirg E the behavior of the natural system, especially at the time when the natural system is nost disrupted by waste generated heat. In addition, the engineered barrier can be designed to limit the rate of release of radioactive material frun the urv3arground facility after containment failure. Performance of the natural barrier can be relied on after containment failure and when predictions of performance are less uncertain because heat from the enplaced waste is less of a factor. 7b meet these ends,10 GR 60 specifies particular barrier post-closure performance measures for a 300 to 1,000 year waste package containment period, a 10 annual radionuclides release rate frun the underground facility after the containment period, and a 1,000 year pre-waste-emplacement grourx3 water travel time.
Groundwater Travel Time The post-closure performance criterion for the natural barrier, 10 G R 60.113 (a)(2), requires that the geologic repository be located so that pre-waste-emplacement groundwater travel time along the fastest path of likely radionuclides travel frum the disturbed zone to the accessible environment be at least 1,000 years or such other travel time as may be approved or specified by the Ocnnission.
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'Ihe disturbed zone is defined in 10 CTR 60.2 as: " . . . that portion of the controlled area the physical or cha=4r-al properties of which have changed as a result of underground facility u =Loction or. as the result of heat generated
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by the enplaced radioactive wastes such that the resultant change of g w -ties may have a significant effect on the performance of the geologic repository."
h amacalble enviu maat is defined in 10 GR 60.2 as: " .. h atmosphere, land surface, surface water, oceans and the portion of the lithosphere that is outside of the u.n Lulled area."
h controlled area is defined in 10 GR60.2 as a surface location extending horizontally no more than 10 kilometers in any direction from the outer boundary of the undergrourri facility and the underlyiry subsurface.
Disturbed Zone Ccc h L In developing the nultibarrier approach the NRC staff recognized the contribution to uncertainty in predicting repository performance caused by the ocmplex perturbations on tha yolxic and hydrologic enviuanient resulting from the presence of wash. Au +& * .:pository. 'Ihree distinct types of perturbations were identified: stress and mechanical effects of excavation, l
chemical effects frun changes to the chemical equilibrium caused by the )
I addition of wastes, and thermal effects from the radioactive decay heat generated by the enplaced wastes (NRC,1981; page 18) . In considering
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ysttops. doc ; j performance standards for particular barriers, the staff expressed confidence in the ability to design waste packages to contain wastes during the period when uAdulction and waste irxtuced perturbations in close vicinity to the repository are large and would ca.use unacceptably large uncertainty in predicting repository performance. However, in considering the geologic settirg, the staff desired a performance standard that was not subject to many of the sam uncertainties as is repository performance. A regulatory regl'autent for undishW geologic setting performance-groundwater travel time frun the edge of the disturbed zone to the accessible environment--would achieve this objective by bourdirg or eliminating uncertainty relatal to thermal effects on the hydrologic system ard the geochemical impacts on radionuclides transport.
II. RUIDRKING CHRONOIOGY OF GROUNDWATER TRAVEL TIME Advance Notice Of Procosed Rulerrakirn
'Ihe multibarrier approach and associated post-closure performance objectives for particular barriers was initially proposed by the Ucranission in the May 13, 1980 Advance Notice of Proposed Rulemakirg (ANPR) on 10 CFR Part 60 technical criteria for regulating geologic disposal of high-level radioactive waste (45 FR 31393) . 'Ihis action proposed required bal 'iers in the design and construction of a geologic repository to incitrie an erg;neered system of waste packages and an underground facility, ard the geologic environment. It also provided performance requirements, ammirg natural processes ard events I
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gwttops. doc that are likely ("enw+ad") to degrade the ergineered conponents of the geologic repository after permanent closure. For the vaste package and underground facility, the performance requirements were roughly eg;.ivalent to current 10 CFR Part 60 stardards. For the geologic setting, a performance requirement for a 1,000 year radionuclides travel time fran the repository to the a - aim.o envium mult was proposed.
I In proposing these performance standards as a way of reducirg uncertainty and providing reasonable assurance in licensing high-level waste disposal, the Comnission stated that "the two mcst inportant attributes of the natural barrier are that the site should be geologically simple aM stable so that the site can be easily understood and so that there can be confidence that the ability of the site to contain and isolate the wastes will remain viable for long times" (45 FR 31397) . Thus, additional siting requirements were proposed that included a provision that the site shall be selected so that it is not so ocmplex as to preclude thorough investigation ard evaluation of site characteristics inpcrtant to duu4=L.ating that the performance objectives will be met. The proposed siting requirements also specifiM a s,eries of favorable site conditions to enhance the ability of the geologic repository to moet the performance objectives. Included among these favorable conditions were: long groundwater residence times and lorg flow paths between the repository and the amalble environment; inactive groundwater circulation due to low permeability; host rock with very low water content; conditions preventire groundwater intrusion or circulation in host rock and upward groundwater flow between units; conditions that result in low hydraulic I
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..i gradients in host rock and surrounding units; horizontal or downward hydraulic gradients in host rock and surrounding units; ard groundwater residence times under ambient conditions, between the repository and the amancible enviummsit, that avM 1,000 years.
'Its NRC staff technical rationale @m=nt for the ANPR provides no clear indication of how to measure radionuclides travel times or groundwater residence times. However, it does indicate that the radionuclides travel time requirement and the favorable condition for larg groundwater residence times provides a degree of indeixhhit check on performance mialire that can be achieved throtgh dating.of groundwater movement by multiple approaches and the amaa==mant of the et=nical_ evolution of groundwater (NRC,1930a; page 3-26) .
In addition, the staff considered radionuclides travel time to be the type of performance objective that could be determined early-on in site characterization, both in terms of field tests and associated analyses and in performance mialing. As such, it could be used as a tool for site screening (NRC, 1980a; page 3-27).
Proposed Rule On July 8,1981, after considering public -sits on the ANPR, the Caminaion published proposed airsdisits to 10 CFR Part 60 specifying technical criteria for diammal of high-level radioactive wastes in geologic repositories (46 FR 35280). Restating its position on the nultibarrier approach to repository licensing, the Otanission, in describiry the role of the site, stated that it
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recognizes that at see point the design capabilities of the engineered system will be lost and that the geologic setting (the site) must provide for the isolation of wastes frm the enviruanent. This general requirement for the natural barrier was translated into the performance objective requiring that following the containment period, the geologic setting, in conjunction with the engineezed system as long as that system is expected to function, and i
alone thereafter, shall be capable of isolating radioactive waste so that l transport of. radionuclides to the a m ible environment shall be in amounts that conform to the envimmadal starxiards established by the EPA. This evaluation shall be performed assuming anticipated processes and events [as defined in the final rule] (46 FR 35282).
.In addition to the natural barrier performance objective, the proposed rule established mandatory geologic setting siting requirements for structural and tectonic stability since the start of the Quaternary Period; hydrogeologic, geochemical, and gecem1Aic stability since the start of the Quaternary Period; and pre-waste-emplacement groundwater travel times thro:qh the far field to the accessible envimmr.nt of at least 1,000 years. The proposed groundwater travel times siting requirement introducM the coupL of
" pre-vaste-emplacement" groundwater travel through the undisturbed portion of the geologic setting to the a-ible environment as a measure of the
" goodness" of the geologic setting to isolate waste after permanent closure.
Embodied within this new requirement were the mm epus of "far fiald" and
" disturbed zone" that link " pre-waste-emplacement" groundwater travel time to geologic setting performance after permanent closure. In the proposed rule,
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'Ihe ANPR siting requita-d. for a sinple site was deleted. Among the more i
significant points raised by mm.eiters were that the meaning of the term "sinple site" was unclear and that the requirement for a simple site was too restrictive. 'Ibey noted that cartplete knowledge of the site is needed regardless of how difficult or cartplicated characterization may appear to be and that even most simple sites are subject to ccanplicated geologic processes.
Furthermore, they wrote that the most inportant attr ute of a site is that it works to isolate wastes and not that it be sinple to understand so as to facilitate the licensing process (NRC, 1980b; pages 5 and 117).
In developing the mandatory siting requirements for the geologic setting, the NRC staff reasoned that engineered barriers, designed to minimum performance staniards, can provide reasonable assurance that overall system performance will be met for the initial period of time when waste generated heat is greatest. After containment failure, engineered barriers can be designed to limit the release rate of radioactive materials frun the repository. However, once this emirs, the site nust provide whatever additional assurance is needed in order to meet enviiwmeital stardards. Furtherncre, it was reasoned that there exist characteristics of the geologic setting that can be
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" regulated" to ensure that the EPA envituumuial standard is met. Two such characteristics of the geologic settirg are: 1) the transport time of individual radionuclides to the accessible environment and 2) the transport.
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] I time of groundwater to the accessible environment (NRC,1981; pages 40-41). 1 i
Because it avoids the need to model the thermal effect on the hydrologic d 1
system and the geoct=bl ilipacts of radionuclides transport, a minimum f I
groundwater travel time through the undisturbed portion of the geologic setting to the accessible envituumui was chosen as the geologic setting performance measure. 'Ibe staff reasoned that while geochemical retardation is expected to be a mL.uq factor in radionuclides transport and in providing waste isolation, there will be considerable uncertainty in the magnitude of its contribution. 'Ihis uncertainty results frenn the fact that it is very difficult to know how much geochemical retardation will occur, there is 1
currently no agreenent amary the scientific canununity on how much such an evalt:ation can be made, and a rigorous, agreed on correlation between laboratory data and real site performance does not exist. Because of the greater confidence in our ability to measure hydraulic rather than geochemical parameters, and the conservatism that is intr M M , it _ W prudent to select groundwater travel time rather that radionuclides transport as a measure of the geologic settirg's ability to meet the overall performance standard.
. Although the difficulty of characterizing low permeability rock, especially surrounding the repository, was recognized, the staff appears to have envisioned that the major portions of grourxtwater flow paths from the
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it was' stated that evaluations of groundwater travel times requires only the measurement of parameters and nodeling of aquifer flow that is ccmanonly done in water resource analyses; ocmputer codes of these types are .ocnmonly use by the USGS and the oil industry.. Uncertainty in the analysis of groundwater .
travel time was thought to result from limited number 'of boreholes for measuring hydrologic data due to the desire to preserve site integrity, but uncertainty will be less when ocuparsi to measurig geod-ical parameters and S
M 1ing moleoale transport. 'Ihe staff. further stated that in order for this to be a useful regulatory approach, the geologic setting must be stable'to prwide confidence that the waste will continue to' be isolated. - Licensing' issues 'would mainly then be restricted to ensurig that the pig-M repository does not disrupt the hydrologic flow pathways.such that shorter travel times to the envitc..-d. are created, and the adequacy of engineered barriers in dealing with disruptive events and natural pro == that could result in shorter flow pathways (NRC,1981; pages 42-45).
In determining an ewswtiate standard for groundwater travel time, it was reasoned that 100 years would not provide W ete margin to m ys s te for-uncertainty in geochemical factors amarriated with radionuclides migration. A travel time of 10,000 years would eliminate reliance on gevciminical factors entirely, but may not be achievable and may rule out otherwise good repository sites by being werly restrictive. It was decided that a groundwater travel time of 1,000 years is achievable in many hydrologic systans and relies on
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minimal contributions of retardation-to be a conservative nn:sure of geologic I setting perfonnance. (NRC,1981; pages 43-44) .
In developing the u. %L of pre-waste-emplacement groundwater travel time as
-a measure of undisturbed geologic setting perfonnance, the staff did not elaborate on the seemingly parhimJ nature of the disturbed zone um%L.
'Ihat is, within the disturbed zone there are ocuplicated and ill-defined physical and him1 pic-:" effecting the ability of the geologic setting to isolate ' waste, yet unku.mi.anding these same pr-aa= is required to estimate the significance and hence the extent of disturbance.
Final Rule On June 21,1983, after consideration of public u.meuds on the July 8,1981 PW_ Tedinical Criteria, the Nimaion in'mi Final Technical Criteria on the diat=1 of high-level radioactive wastes (48 FR 28194). Comenters generally recognized the usefulness of the nultitarrier approach, but extensively questioned the establishment of fixed numerical values for performance. Numerous u._ ders cited that no logical unus:ction can be danu= Lated to exist between the performance of particular barriers and overall systen performance because the perfonnance values had not been shown to be either mry or sufficient to meet any particular overall starrbrd of performance. Secondly, u. twrs balieved that the prescribed performance ,
standards for particular barriers would restrict flexibility. 'Ihey argued that individual barrier standards should be site-specific (48 FR 28196).
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gwttops. doc In response to these issues, the NRC staff prepared an am ment of how the numerical performance standards for particular barriers contribute to meeting the overall system performance objective. 'Ihis a==a==mant is the basis for the numerical performance standards contained in the Final Rule. In addition, to address the issue _of flexibility, the performance standards for particular barriers were amended to allow the hiaalon to approve or specify, on a case by case basis, sane other radianuclide release rate, design containment period, or groundwater travel time taking into account uncertainties associated with factors affecting the thermal pulse, geocbamimi characteristics, = or other scuroas of urs.miointy in predicting performance (48 FR 28197).
'Ibe meaning of the dish zone for the final rule was modified to am----date the concerns of the Utility Nuclear Waste Management Group and the Degummit of Energy who stated that "the definition of the ' disturbed zone' should specifically refer to the properties of the geologic setting which are of interest insofar as disruption is concerned" (NRC,1983; page 180). In the final rule the disturbed zone definition relates changes in the physical or chemical properties of the controlled area to the performance of the geologic repository as tw-.-.arded by the Ocnnenters (48 FR 28205) .
'1he geologic setting performance objettive requiring that the geologic setting, in conjunction with the engineering barriers, control releases of radionuclides to within limits specified by the EPA standard was deleted from the technical criteria because it merely restated what is inplicit in overall
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system performance. Also deleted wc e che mandatory siting requirements for stability aM for the 1,000 year grountaater travel times. 'Ihe groundwater ;
travel time requinrent was inclwhi as a performance objective for particular i barriers after permanent closure and was amended to include what was only previously inplied, that it applies to the " fastest path of likely radionuclides travel from the dishn+=1 zone to the accessible environment" (48 FR 28210). 'Ihe requirements for stability in the geologic setting since the start of the Quaternary Period were also deleted because, as commenters pointed cut, the geologic setting can only be said to exhibit stability in a relative sense. What the hinaion had inteMed was that the the nature and rate of natural processes be such as to enable recent history to be interpreted and to permit near-term geologic charges to be projected with relatively high aanfidence. 'Ihe new definition of " anticipated processes and events" includes the assumption that processes operatire in the Quaternary Period continue to operate but with the perturbations caused by the presence of emplaced radioactive waste superimposed thereon (48 FR 20210) . In the finale rule, numerical performance standards were established for particular barriers, -ing " anticipated processes and events" (48 FR 28200) .
In discussing " pre-waste-emplacement" grountsater travel time, some commenters 1 were confused about it being used to define performance of the natural barrier after permanent closure and suggested that groun h'ater travel time be expressed in terns of both pre-emplacement and post-emplacement conditions (48 FR 28210). G auenters also raised concerns or questions related to the calculational and verification aspects of the requirement. One cammenter,
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C) i after stating that the performance objective is not precise ard should be clarified, wanted to know if it referred to the average velocity, the leading edge of the pitnae, or to som other veJocity (NRC,1983; page 97) . Another
]l cmmenter questioned the verifiability of groundwater travel tim by stating that there is no sirgle unique groundwater flow path and travel tim for a site. Each pathway can be characterized by its own water travel time, with there being considerable difference frm one pathway to another. The cmmenter stated that if Imc interds to apply the grourdeater travel tim requimisit to the arrival of the first filament of water at the accessible l envimmsit, it should state so. The u.musiter rm - 1'rded that the NRC consider to what extent radionuclides releases alorg the mre direct and rapid ]
1 pathways to the ao-ible envimmsit would significantly contribute to total cumulative releases (NRC,1983; pages 334-335) . Finally, one comenter asked l how groundwater travel time will be estimated for cracked ard fissured areas ]
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In response to these camments, the staff recognized that estimating J groundwater travel time for real environments in the vicinity of potential repository sites is not an easy task because the hydrogeologic conditions desirable for waste MP 1 make many methods to determine flow in aquifers imppropriate. R1rtherncre, they recognized that it is not easy to determine flow through highly heterogeneous and anisotropic environments over di +moes of several kilcraeters or more (NRC,1983; page 147) . However, the staff added that groundwater flow characteristics will significantly affect the performance of any geologic repository, and must be well urderstood prior to
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gwetops.eoc D3A~I any repository p rformance assessment. The travel time requirement will not significantly affect the hydrologic investigations that must be conducted at a potential site (NRC,1983; pages' 334-335) . Regarding the question concerning simulation of flow in fractured rock, the staff r+ -...a-dad the use of equivalent porous continuum models over discrete fracture network nodels (ICC, 1983; page 343). Finally, in response tc' cannenter's cancerns over pre-waste-emplacement groundwater travel time as a measure post-enplacenent performance, the staff replied that such concerns assume that post-emplacement changes [ changes resultirg directly fran waste-emplacement] would be significant. "By definition, however, the portion of the geologic settiIg -
significantly affected by waste emplacement constitutes the ' disturbed zone.'
The groundwater travel time provision applies to transport frun the disturbed zone to the accessible envimmest. This parameter is not dependent upon the-effects of waste emplacement" (48 m 28210) . Uncertainties with respect to the groundwater travel time performance objective were addressed no further in the final rulanaking.
l Prooosed Aire-dista on The Unsaturated Zone l
l Although thc. final technical criteria were stated by the hbion to be generally appropriate to both dip 1 below the water table and disposal in the unsaturated zone, some distinctions were needed (48 m 28203). Rather than prum11 gating specific criteria applicable to the unsaturated zone at the 1
time that the final technical criteria were p2blished, the Camission stated that it preferred to issue such criteria in proposed fann so as to afford 1 - - ___ __-__ -
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I further wrtunity for public wa=2il. Proposed amerrlments developed in I response to this decision were published on February 16,1984 (49 FR 5934) .
'Ihe proposed arerdreits redafined groundwater to include all water beneath the j earth's surface, thereby, exterxiing the scope of the groundwater travel time requirement to ai7 1 in the unsaturated zone. Additionally, the proposed rulemaking action satr3ht public wmmuit on two questions related to groundwater travel time calculations in unsaturated geologic media (49 FR 5937).
In the statement of m nsiderations that a m nied the proposed amendments, the Ocznmission stated that in contrast to grourriaater flow beneath the water table, novement of water in the unsaturated zone is generally discontinuous and strongly dependent on the spatial and tenporal distribution of recharge ;
events and the conductive prtperties of partially saturated geologic media that vary with moisture content. Unsaturated zone materials properties such u reisture content, porosity and matric potential may be extrumely difficult to measure on a representative scale for unsatura.ted porous and fractured geologic media. 'Ihus, the Ommimion concitried that calculations of pre-waste-emplacement groundwater travel time along the fastest path of likely radionuclides travel through the unsaturated zone could have large uncertainties, and hence could be of questionable value in estimatirg the capability of the geologic setting to isolate high-level waste from the accessible enviiwmsit (49 FR 5936). However, the commission concluded that if DOE could deircibltate with reasonable assurance that travel time for grundaater novement through the unsaturated zone can be quantified, then IDE
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should be allowed to include such travel tim when demonstrating compliance with the groundwater travel thne performance objective.
'Ihe cr-ission recognized that it may.be more appropriate to specify another parameter upon which performance may be evaluated for a geologic settirg in the unsaturated zone, or to utilize the. approach set forth in 60.113(b) giving the omnission flexibility to specify variations in the perfonnance objectives on a case-by-case basis as long as the overall system performance objective is satisfied. 'Iherefore, to solicit public input on groundwater travel time in the unsaturated zone the er - iamion posed two questions on this issue. (49 FR 5937). 'Ihese questions requested public ccanent on 1) how grunbater travel tbne in the unsaturated zone could be determined with reasonable assurary.:e, and whether or not the existing groundwater travel time performance objective should be limited to groundwater movement within the saturated zone; and 2) whether groundwater travel time is an appropriate measure of perfonnance for a site within *be unsaturated zone or whether an alternative performance objective would be more appropriate.
Final Aii===du=ud.s On 'Ihe Unsaturated Zone Seven of the fourteen h m==:nters on the proposed amendments to 10 CFR 60 to include specific requirements for high-level waste disposal in the unsaturated zone addressed the two groundwater travel time questions. Because these ccanents were solicited by the NRC, the staff did not believe it was m1y to pro'ide individual responses (NRC,1984; page 5) . However, the staff
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u rew to the ccrumen:ts were sumarized as follcras. Ib the first question the staff stated that ocuments received were "nearly equally divided" into two categories. One group stating that grountaater travel time is difficult to calculate in the unsaturated zone with reasonable assurance because of lack of acceptable methods ard limited socpe of research currently devoted to the question. The secord group (mostly federal agencies) believed that i grountsater travel time in the unsaturated zone could be determined with reasonable assurance.
The DepatLimiuit. of Energy (DOE), for exanple ccrnmented that groundwater velocities and subsequently travel times could be determined frcxn flux 1
measurements ard moisture-characteristics-curves to predict hydraulic properties over a wide range of corditions. DOE noted that reasonable assurance may be gained in groundwater travel time estimates by using the results of state-of-the-art field and laboratory testing, and by bounding 1
l estimates frczn more irriirect methods. DOE also noted that in situ monitoring techniques, including tracer test, are urdergoing development ard may broaden i
the range of rock types and corditions for whicts it is feasible to estimate groundwater velocity and travel time (NRC, 1984; page 9).
l Thus, with respect to question number one, the staff concluded that groundwater travel time can be done in the unsaturated zone, though not mvrily with great precision, provided the proper level of site l characterization ard analysis is corducted. In reaching this conclusion, the staff recognized that theories, rxdels and techniques originally developed for
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2E groundwater investigations in highly porous near-surface soils and unconsolidated rock types would be applied to the consolidated porous and fractured rock types of the deep unsaturated zone. However, the staff also noted DOE's indication that a substantial effort is underway to develop new methodologies and to improve existing techniques for measurity hydrologic parameters and flow to provide the memry input to grourriwater travel time calculations in the unsaturated zone (50 FR 29643).
The staff stated that views on the second question were again equally divided.
Scne ccmnenters asserted that, although not ideal, groundwater travel time may, under certain circumstances, represent an appropriate measure of performance for a geologic settiry in the unsaturated zone. Others argued that groundwater travel time was not atcropriate for the unsaturated zone and suggested alternatives such as a numerical standard based on the volume of groundwater flowing through a repository. The U.S. Geological Survey (USGS),
for exanple, was concerned about estimating groundwater travel time in fractured or other highly hetewr_ircas rock in both saturated as well ne unsaturated media. The USGS contended that either a flux oc travel-time i criterion should be based on an areally integrated or averaged calculation over the cross-sectional area of the repository normal to the direction of expected flux. This would have the effect of reduciry the uncertainty over how the fastest pathway is to be determined, otherwise the fastest pathway would be virtually impossible to calculate (NRC,1984; page 21) . In response j to the USGS, the staff stated that it considers the fastest pathway criterion to be a more appropriate measure of performance than the suggested averaged
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velocity critorion. Variations in the hydrologic regime will be addressed in the evaluation of un&xtainties surrounding groundwater travel time calculations (NRC, 1984;.page 22).
Following consideration of various alternatives suggested by commenters, the Ccmnission hela that groundwater travel time represents a more appropriate l parameter upon which the performance of the geologic setting can be evaluated than any of the suggested alternatives because a prescribed groundwater travel I
time can be generically applied and will provide a conservative estimate of a ,
I minimum radionuclides release time to the accessible environment. However, the j
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staff noted that the Ctanission still retains the option of applying the ;
provisions of 60.113(b) to a particular geologic setting when such action is l deemed appropriate (50 FR 29643). 'Ihe staff rn - era Sed that the himion I maintain its original position as set forth in 49 FR 5936, that if DOE can demonstrate with reasonable assurance that travel times for groundwater movement through the unsaturated zone can be quantified, then DOE should be allowed to include such travel times *v_n dettonstrating compliance with the groundwater travel time regulatory requirement. However, the staff recognizes that for the unsaturated zone it may be more appropriate in scoe cases for the j Cammission to utilize the approach set forth in 60.113(b) which provides the i
himion with the flexibility to specify variations in performance l
objectives on a case-by-case basis as long as the overall system performance objective is satisfied (50 FR 29644).
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- I A.J Although no charge was made explicitly to the groundwater travel time regulatory requinma L, the proposed definition of the term " groundwater" set j forth in 10 GR 60.2 would clarify that groundwater travel time is equally .
r applicable to geologic repositories within either the saturated or unsaturated l zone. Shnilarly, the re?--.arded amerdimit to the siting criteria 1
(60.122(b)(7)) would have the effect of making pre-waste-emplacement l groundwater travel time along the fastest path of likely radionuclides travel frun the disturbed zone to the accessible errvirement which substantially exceeds 1,000 years a favorable corviition for high-level waste disposal within either hydrogeologic zone (50 FR 29645).
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.III. REFERENCES 45 3 94 "10 CTR 60 Technical Criteria for Regulating Geologic Disposal High-Invel Radioactive Waste: Advance Notice of Fin- =-3 Rulemaking," U.S.
- Nuclear Regulatory rw-ianion, 31393, May 13,1980.
i 46 E 130 "10 CFR 60 Diam =al .of High-Isvel Radioactive Wastes in Geologic Repositories: Fu-- *1 Rule," U.S. Nuclear Regulatory cwanimaion, 35280, July
- 8, 1981.
48 3 120 "10 CFR 60 D4P-a1 of High-Invel Radioactive Wastes in Geologic Repositories: Final Rule," U.S. Nuclear regulatory Ctanission, 28194,. June 21, 1983.
49 3 33 "10 CFR 60 Diamaa1 of High-Isvel Radioactive Wastes in Geologic Repositories: FxW Rule," U.S. Nuclear Regulatory cannission, 5934,
' February 16, 1984.
50 3140 "10 CFR 60 Diam =al of High-Isvel Radioactive Wastes in Geologic Repositories: Final Rule," U.S. Nuclear Regulatory (W=nimaion, 29641, July 22, 1985.
U.S. Nuclear Regulatory Otmanission,1980a ' W cal Support W = ntation for the Siting Requirements in USNRC 10 CFR Part 60 Disposal of High-Isvel
o f
I \ .. ,
gWtops.dc>c d -
Radioactive Wastes in Geologic Repositories," USNRC Working Draft, April 7, 1980.
U.S. Nuclear Regulatory n,minaion,1980b, " Analysis of Public Cmments on the Advance Notice' of Proposed Rulemaking-10 CTR Part 60, ' Technical Criteria for Regulating Geologic Diap'aal High-level Radioactive Waste," USNRC Draft, E* M 2 a 29, 1980.
U.S. Nuclear Regulatory h inaion, 1981, " Rational for Performance Objectives arxi Required Characteristics of the Geologic setting," in SECY-81-267,10 CFR Part 60 "Discosal of Hiah-Isvel Radioactive Wastes in Geoloaic Repositories:
Technical Criteria," Enclosure J, USNRC, April 27, 1981.
U.S. Nuclear Regulatory n = incion, 1983, " Staff Analysis of public C mments on Prtposed Rule 10 CFR Part 60, 'Di p 1 of High-Isvel Radioactive wastes in Geologic Repositories',"USNRC Report NUREG-0804, July 1983.
U.S. Nuclear Regulatory Ommission,1984, " Staff Analysis of Public Caments on the PIW Amendmencs to 10 CFR Part 60 Related to the Unsaturated Zone and Draft NUREU-1046," in SECY-85-109, Amendments to 10 CFR Part 60-Disposal of Hiah-level Radioactive Wastes in Geolccic Repositories," Enclosure D, USNRC, Mart:h 27, 1985.