ML20151V934

Draft Generic Technical Position Guidance for Determination of Anticipated Processes & Events & Unanticipated Processes & Events

ML20151V934
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Issue date:	08/17/1988
From:	NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
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ML20151V905	List: ML20151V900 ML20151V934
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DRAFT GENERIC TECHNICAL POSITION ,

i GUIDANCE FOR DETERMINATION OF ANTICIPATED PROCESSES AND EVENTS

-AND UNANTICIPATED PROCESSES AND EVENTS l

Division of High-level Waste Management l Office of Nuclear Material Safety and Safeguards 1 i

U.S. Nuclear Regulatory Comission l l

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i CONTENTS i

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1. INTR 000CTION............................................... 1.

1.1 Purpose.............................................. 1 1.2 Scope................................................ 1 1.3 Plan of Development.................................. 2

2. REGULATORY BACKGROUND...................................... 3

3. TECHNICAL P0SITION......................................... 4

4. DISCVSSI0N................................................. 7 4.1 Regulatory Background................................ 7 4.2 NRC Approach - Rationa10............................. 9 5.-NRC APPROACH - EXAMPLES.................................... 14

6. REFERENCES USED............................................ 19 APPENDIX A - GL0SSARY......................................... 20 l

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ORAFT GENERIC TECHNICAL POSITION GUIDANCE FOR DETERMINATION OF ANTICIPATED PROCESSES AND EVENTS AND UNANTICIPATED PROCESSES AND EVENTS

1. INTPODUCTION 1.1 Purpose This Generic Technical Position (GTP) provides guidance on the methodologies the NRC staff consider should be used to evaluate processes and events that could occur after closure of a high-level radioactive waste repository so that these processes and events can be cateaorized into anticipated processes and events and unanticipated processes and events.I

1.2 Scope This GTP is primarily intended to provide guidance on the methodologies and rationale that NRC staff will use for categorizing naturally initiated 1 As was stated in the Federal Register Notice, Vol. 51, No. 118, l pp 22288-22300, dated June 19, 1986, the NRC staff considers that the Environmental Protection Agency (EPA) term "undisturbed performance" and I the NRC term "anticipated processes and events" imply equal conditions, i and that the sumation of all anticipated processes and events, and j unanticipated processes and events, is equal to EPA's term "all  ;

significant processes and events." If, at some later date, the NRC adopts I the EPA terminology, such a change wnuld have no effect on the intent of the rule or the guidarce given in this GTP.

l 2 The period in which the intended performance objectives of 160,112 and

$60.113 are to be met is during the period after permanent closure. As anticipated processes and events and unanticipated processes and events only apply to these performant.e objectives, they are terms which only apply to the post-closure period. l I

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processes and events. The NRC staff recognizes that the first step in developing the scenarios that will need to be addressed, in determininn compliance with 10 CFR Part 60, is the identification of a comprehensive set of processes and events that may be important at the site being considered.

Scenario development is, and has been, the subject of many investigations, and will not be addressed in detail in this GTP. Instead, this GTP will provide l guidance on how the staff would categorize these processes and events into "anticipated processes and events" and "unanticipated processes and events" so that compliance with those parts of 10 CFR Part 60, such as $60,113, which require differentiation of the various processes and events, can be evaluated. '

This GTP will also discuss "human-induced" processes and events, and to a minor degree will also address the category of "processes and events which are not sufficiently credible to warrant consideration."

1.3 Plan of Develop.?ent This GTP will provio. 1e following information:

1. A brief synopsis of 10 CFP Part 60, as it relates to anticipated processes and events and unanticipated processes and events;

2. An identification of those portions of the rule which use these processes and events in desion and analysis, to demonstrate compliance with the performance objectives of 10 CFR Part 60;

3. The position of the NRC staff; l

4. A discut;sion of the rationale that has caused the staff to adopt this position;  !

5. Examples that demonstrate how the process of categorization of j anticipated processes and events and uranticipated processes and events l

would be implemented;

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6. The references used; and l

7. A glossary of terms that have beer, defined in 10 CFR 960.2 and used within this position.

2. REGULATORY RACKGROUND Within $60,113(a)(1), it is specified that the engineered barrier system be designed so that assuming anticipated processes and events: (a) containment of high level waste will be substantially complete during the period when radiation and thennal conditions within the engineered barrier system are dominated by fission product decay; (b) release of radionuclides from the engineered barrier system will be a gradual process; and (c) specific ' numerical performance objectives are achieved. Therefore, without specifically stating the premise, the primary function of the category of processes and events defined as anticipated is to specify, in part, the design basis for the engineered system and waste package, during the post-closure period.

In addition to the requirements specified in 660,113, anticipated processes and events must be considered, alono with unanticipated processes and events, in evaluation of compliance with the overall containment requirements of 40 CFR Part 191, as specified in 960,112. In the license application, the applicant must present an evaluation of the perfomance of the geologic repository for the period after permanent closure, assuming anticipated processes and events, along with a similar evaluation assuming unanticipated processes and events (960.21(c)(1)(ii)(C)). The NRC has not yet formally adopted the EPA standard because it is subject to ongoing litigation. However, if the final standard is '

5 similar to the vacated standard, it will be necessary to consider anticipated l processes and events in the evaluation of compliance with the special sources ,

of groundwater and individual radiation dose protection requirements.

As stated above, unanticipated processes and events must be considered, along with anticipated processes and events, in evaluating compliance with the I

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overall containment requirements of 40 CFR Part 191, as specified in 10 CFR 960,112. In the license application. the applicant must perfom en evaluation of the performance of the geologic repository, assuming unanticipated processes and events. Based on these analyses, as stated in 960,113(c),theNRCmay specify additional. requirements to satisfy the overall system performance requirements (960,112) as they relate to unanticipated processes and events.

4 The requirements for the unanticipated processes and events thus serve several purposes. These analyses help to provide assurance that the multiple barrier concept is valid, and provide a means of evaluating the margin of safety designed into the engineered barrier system.

3. TECHNICAL POSITION It is the staff's position that:

1. An "anticipated event" and a natural "unanticipated event" should consider the maximum event which has occurred within the geologic settinq 3 during the Quaternary Period.4 The "anticipated event" should be assumed J

3 The definition of geologic setting in 10 CFR 60.? discusses the "systems of the region in which the geologic repository operations area is located." For the purpose of this GTP, the staff considers these systems ,

as a regularly interacting or interdependent group of items forming a '

unified whole within the region of the geologic repository operations l area, such as a groundwater basin or an area of similar tectonic processes. It will be accessary, therefore, to define which systems are j relevant, and their geographic extent. j I

l 4 The Quaternary Periot iegan approximately 1.8 million years ago, and I extends to the present. This Period consists of the Pleistocene Epoch, which lasted from approximately 1.8 million years before the present to

! approximately 10,000 years before the present, and the Holocene Epoch,

! which lasted from approximately 10,000 years before the present, to the l l

present. The infomal tem "late Quaternary" is being used in this GTP to approximate the tire period since the start of the Late Pleistocere.

approximately 700,000 years before the present, to the present. As such, the term specifically includes the Holocere.

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S to occur at the location, or along the controlling geologic structure, where this event occurred during the Quaternary Period. An "unancicipate'd event" should be transposed within the geologic setting to any credible location at which it could occur, to determine its effect on waste ,

isolation.

2. An "anticipated proc.ess" should consider a reasonable and conservative projection of the rate of the process that is occurring or that has occurred, within the geologic setting during the Quaternary Period. The variation around the average rate of the process should be considered, with emphasis on the rate of this process during the late Quaternary. A natural "unanticipated process" should consider the maximum sustained rate of this process during the Quaternary Period. As with anticipated and unanticipated events, an anticipated process should be assumed to occur at the location where the process is occurring, or has occurred. An unanticipated process should be assumed to occur at any credible location within the geologic setting, to determine its effect on waste isolation.

3. Both the "anticipated" and "unanticipated" processes and events must consider the potential modifications to such processes and events caused by past, ongoing or projected human-induced processes and events that will not be under DOE's control.

4. Human-induced processes and events that may occur on lands under DOE's jurisdiction, either within or outside the controlled area, are considered unanticipated.

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5. Natural processes and events that are generally treated as "random" j processes and events will nomally be considered as anticipated processes l and events. l

6. The processes and events used in the perfomance assessment and in i design must take into account, and be adjusted to consider, modifications

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6 to these processes and events that could occur due to the perturbations from the effects of waste emplacement and the uncertainty in the various projections.

In evaluating the various natural processes and events that may affect the site during the post-closure period the staff will ne3d to use professional judgment in considering the available scientific information about these processes and events. The staff will not simply use average or maximum values, but will consider the natural geologic variability including, but not limited to:

1. The spatial and temporal variability of the processes and events

2. The periodicity of the processes and events Additionally, although the Quaternary record for the region of the geologic setting - especially the late Quaternary record - is the main basis for providing the evidence for consideration of the various processes and events, the staff will also consider:

1. The pre-Quaternary record for those processes and events which have a l cycle which may not be adequately reflected in the Quaternary record. i

2. Any changes in the processes and events which are evident in the Quaternary record.

3. Evidence from analogues in other geological settings, experinental data, or the results of modeling and sensitivity exercises, j When evaluating the effects of human-induced processes and events, if l potentially e.dver'e condition 160,122(c)(17) is present, the program of exploration, assumed in the analysis, should be based on a comparison of drilling histories of similar deposits. If potentially adverse condition I 560.122(c)(17) is not present, the outdance that had been presented in Appendix B to 40 CFR Part 191 (subsecuently vacated), on "Frequency and Severity of l

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7 InadvertentHumanIntrusionintoGeologicRepositories,"shouldbefollowed.f6 The evaluations should also consider the express assumptions contained in the definition of "unanticipated processes and events" in 160.2.

4. DISCUSSION l

4.1 Regulatory Background i

"Anticipated processes and events" have been defined in 10 CFR Part 60 as natural processes and events that are reasonably likely to occur over the period of concern. They are processes and events taking place in the geologic setting, or those that occurred in the geologic setting during the Ouaternary I Period. They are external to the engineered barrier system, but they may have an effect on the performance of the engineered barrier system 7 An assessment 5 Potentially adverse condition 660.122(c)(17) is defined as follows:

"the presence of naturally occurring materials, whether identified or 3 undiscovered, within the site, in such form that: i (i) Economic extraction is currently feasible or potentially feasible l during the foreseeable future; oi 1 (ii) Such materials have greater gross value or net value than the average for other areas of similar size that are representative of and located within the geologic setting.

6 As explained previously, the NRC has not yet formally accepted the EPA standard because it is subject to ongoing litigation. As the referenced  ;

portion of the standard was not subject to challenge, the NRC expects that i this portion of the final standard will remain unchanged.

7 The waste package, for example, must consider the natural anticipated processes and events, such as the geochemical conditions, in the design and analysis. Co rosion of the waste package may occur under these conditions and is a perturbation which may modify the "anticipated" geochemical conditions. Corrosion is, therefore, not an anticipated process or event, but cay be a consequence of anticipated processes and events. In analyzing the effects of corrosion, the applicant must demonstrate that, under the anticipated processes and events, the amount and type of corrosion expected will not prevent the Comission from making

8 of anticipated processes and events must begin with an evaluation of the processes and events that are and have been occurring in the geologic setting.

To the extent reasonable from the evidence, it will he assumed that those processes operating in the geologic setting during the Quaternary Period continue to operate with modifications due to the perturbations caused by waste emplacement. (See 560.122 b(1) and Ref.1, Federal Register, June 21, 1983, p 28200.)

In contrast, "unanticipated processes and events" have been defined in 10 CFR 560.2 as a category of post-closure processes and events, either natural or induced by human activity, which are considered not reasonably likely to occur durino the period of intended performance, but which are sufficiently credible to warrant consider ation. As with "anticipated processes and events," the assessment must first begin with an evaluation of the processes and events which are and have been occurring in the geologic setting.

1 In detemining how to provide working guidance on the identification and l differentiation of natural anticipated processes and events and unanticipated processes and events, the staff has carefully reviewed the Federal Register which describes the Commission's reasoning during preparation of the final rule 10 CFR 60. Many public coments on the proposed rule 10 CFR Part 60 suggested a probabilistic basis for classifying processes and events as anticipated processes and events and unanticipated processes and events, respectively. The j Comission rejected this approach, observing: "Identification of anticipated and unanticipated processes and events will recuire considerable judgement and will not be amenable to accurate quantification, by statistical analysis, of their probability of occurrence." Further, it was noted that "There can only be estinates rather than rigorous demonstrations of probabilities of occurrence." (See Ref. 1, Federal Reaister, June 21, 1983, p 28200.)

a finding of reasonable assurance that the waste package can meet the required perfomance objectives.

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4.2 NRC Approach - Rationale The staff considers that an acceptable approach to the identification of natural processes and events which must be included in the design and analysis 0

of the geologic repository must start from a deteministic basis, after careful consideration of the geologic record. This will require professional judgment which must take into account the inherent uncertainty in the understanding of the various processes and events. After the processes and events that require consideration have been determined, probabilities of these processes and events need only be estimated insofar as is necessary to determine compliance with the containment requirements specified in 10 CFR 960,112. That is, while it is recognized that probabilistic risk assessment techniques may be used by the DOE to help define the risk associated with the 9

various compliance requirements and performance objectives of $60.113 ,

probabilities will probably not need to be estinated to determine compliance.

, The staff considers that the geologic record of the geologic setting for the Quaternary Period should provide the basis for determining the classification of events and processes. This is consistent with both the wording and intent of 10 CFR Part 60, and is also in agreement with several scientific studies 8 Although comor usage will differentiate between "probabilistic" and "deterministic" methods, it is recognized that in the geologic sciences the deteministic methods used also must often employ "empirical" methods, along with considerable technical judgment.

9 If the final EPA standard, 40 CFR Part 191, is similar to the vacated standard, and is implemented by incorporation into 10 CFR Part 60, it will .

probably not be necessary to assign probabilities to determine compliance l with the special sources of groundwater and individual radiation dose protection requirements, j i

10 that suggest that the Ouaternary record, and especially the late Quaternary record, provides the best basis for projecting future geologic events (e.g.,

see Ref. 2, Allen, 1975).

There are many natural processes and events external to the engineered barrier system that will need to be considered during the post-closure period.

However, natural processes and events of concern for a high-level geologic repository can be grouped as either tectonic, climatological, geochemical, hydrological, or geomorphic processes and events. These five basic groups can be further consolidated into those that are driving processes and events that can cL- .e other processes and events, and those that are resultant processes and events. In simplest form, it can be stated that only tectonic and climatological processes and events are driving processes and events; all others are basically resultant processes and events. In other words, if the general geologic conditions such as lithology, structure and mineralogy of the geologic setting are known, along with the present state (either static or dynamic) of the hydroicgical, geochemical and geomorphological systems, then any expected change to, or response of, these systems could only occur if there were a change in the tectonic or climatological systems. The conditions of the base line hydrological system can only be modified if disturbed by some outside i

force, such as change in precipitation (climate); uplift (tectonics); or faulting (tectonics). Changes to the geochemistry system can also only be produced through the change in some outside condition such as a change in temperature (climate or tectonics) or a change in groundwater flow (climate or tectonics). Changes to the tectonic and climatological systems, however, can occur without impetus from other systems even though some chanaes may be induced from outside the system.

There are certain processes and events that do not fall conveniently into one group, e.g., strean capture changing a hydrological flow regime. The staff considers, in this example, that the erosion that would cause stream capture is primarily controlled by tectonic and climatological processes and events, if such conditions as structure and litholoay are assumed to remain constant.

11 Thus, although it is recognized that there are many other factors that could be considered, all scenarios for the post-closure period require an understanding of the driving processes and events. If these driving processes and events can be categorized into anticipated processes and events, and unanticipated processes and events, then all resulting processes and events can be categorized.

Many studies have shown that tectonic and climatological processes and events have spatial and temporal variability. To make reasonable profections for the next 10,000 years, it will be necessary to understaad the range in the variability, the periodicity of the various cycles, and the present condition of the system under investigation with respect to the periodicity of the cycle.

If these variabilities were understood perfectly, projection would be relatively simple. There is great uncertainty, however, both in our understanding of and in our ability to measure the various processes and events. The "anticipated" and "unanticipated" processes and events should, therefore, reflect the variability of the processes and events and must be expressed conservatively, comensurate with the uncertainty.

The inherent uncertainty in our understanding of the various processes and events has resulted in the convention of referring to many processes and events as "random." A pertinent case is the assignment of a random or floatiro ,

earthquake to some area. Earthquakes are not true random events, but events with a clearly defined causative mechanism. However, our ebility to detect the causative mechanism before the actual occurrence of the event is poor in many areas. To compensate for our inability to detect such causative mechanisms, it has been common practice to evaluate the characteristics of the geologic area and, based on these characteristics, assign a random or floating event. The need to assign floating events in the design and evaluation of the structure under consideration is roughly inversely proportional to our knowledge of the causative mechanism and our ability to detect this mechanism. In the eastern United States, where our ability to detect causative earthquake rechanisms is extremely poor, most nuclear facilities have had to assume a random earthquake l

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nuclear facilities.

It is necessary to differentiate between the reasonably likN y "anticipated processes and events," and those less likely processes an6 -eats which are deemed not reasonably likely, but are yet sufficiently cri Nr to warrant consideration (i.e., "unanticipated" processes and events). Therefore, the NRC will be concerned with the distinctior between those processes and events that are reasonably likely, those that are less reasonably likely, and processes and events that are not sufficiently credible to warrant consideration.

The staff considers that, without geologic evidence to the contrary, it is reasonably likely that the processes that have operated in the geologic setting during the Quaternary Period will continue to operate, or coulo operate, during the period of performance. It is also reasonably likely that events similar to events that have occurred in a specific location, or along a specific structure within the geologic setting during the Quaternary Period, could recur at that location or along the specific controlling structure during the period of performance. Such processes and events would therefore be classified as "anticipated." Processes and events that have not occurred in the geologic setting during the Ouaternary Period would nomally not be considered by the staff to be sufficiently credible to warrant consideration. After examination of the geologic record, in the geologic setting, processes and events that fall between the reasonably likely (anticipated), and not sufficiently credible to warrant consideration, would be considered "unanticipated processes and events." Such processes and events are: those that have occurred in the region of the geologic setting., and could reasonably be transposed to other areas within the geologic setting; those processes and events that are not known to have occurred in the geologic setting, during the Quaternary Period, but that have a cycle that could credibly result in a recurrence during the period of perfomance; those processes and events for which there is little scientific

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l 13 basis for ruling out an occurrence; and those processes ano events that would directly result from another unanticipated process or event.

The NRC staff considers that significant human-induced processes and events should be classified as "unanticipated." The staff notes that 10 CFR Part 60 is clear that human activities within the controlled area are considered to be unanticipated. However, it could be argued that human activities outside the controlled area need to be considered when evaluating anticipated processes and events. If, for example, the site lies in an area that has ongoing irrigation, continuation of this activity might possibly be so considered. The NRC staff las rejected this argument because of two provisions of the rule, 960,121 and

$60.113(c). Based on the provisions of $60,121, the applicant would be required to have appropriata controls outside the controlled area, if adverse human actions could significantly reduce the geologic repository's ability to achieve isolation. If the Commission considers that additional constraints are necessary to protect the public health and safe ^y, even with the controls imposed by DOE, then $60,113(c) allows the Commission to impose additional requirements, as may be necessary, to satisfy the overall system performance objective os it relates to unanticipated processes and events. Therefore, the staff considers that if the activity in question lies within an area that can come under the applicant's control, the flexibility these tw provisions of the rule provide does nct require the additional constraint of considering human-induced activities in the evaluation of anticipated processes and events.

The staff also recognizes that certain human-induced activities that are not under the applicant's control can have a significant effect on waste isolation.

One such JCtivity Could be the introduction of pollutants to the atmosphere, causing modifications to the climatic cycles. Such activities must be accounted for in determining the final anticipated and unanticipated processes  !

and events for performance evaluation and design, i

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5. NRC APPROACH - EXAMPLES

[The following examples are intended for illustrative purposes. The stated positions are subject to change in the light of new technical information that might call the staff's current understanding into question.)

If we first consider the distinction between "anticipated processes and events" and those processes and events that are not sufficiently credible to warrant consideration, it has been shown, through various studies, that the Pasco Basin is in a region of compressional tectonics, which appears to have been initiated during the Miocene (approximately 15 million years before the present) and is I continuing. The staff would consider it reasonably likely, and therefore "anticipated," that during the 10,000 year containment performance period, a site in the Pasco Basin should continue to be in a compressional tectonic )

regime. The staff would not consider it credible that a new tectonic regime woulJ form in the next 10,000 years, and would consider such a scenario not I sufficiently credible to warrant consideration.

If we expand on this example, the various references on the Pasco Basin suggest that there have been only minor variations in the magnitude of the stress and  !

resultant strain since approximately the Miocene (See Ref. 3, Rockwell, 1983).

Therefore, both the anticipated and unanticipated processes and events would )

occur under a compressional regime. The "anticipated" rate of tectonic shortening would be evaluated by the staff by comparing variations in stress / strain since the Miocene with the trend of the data on tectoni'.

shortening for the late Quaternary. A conservative projection for the next 10,000 years would be made to evaluate the "anticipated" amount of stress and/or strain that would be expected to occur during the 10,000 year post-closure period. More weight would normally be given to the late

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Quaternary evidence. It must be recognized, however, that relatively little information is available for the late Quaternary in this area. Therefore, the projections may need to rely more heavily than in the ideal case on the pre-Quaternary geologic record.

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• 1 15 The "unanticipated" rate of tectonic shortening would be evaluated using the maximum rates that have been projected for any sustained period since the inception of the present stress regime. Without geologic evidence to the contrary, the staff would use this rate to evaluate the resultant "unanticipated" stress and/or strain. However, if it can be shown that there has been a fundamental change in the tectonic characteristics of the geologic setting, such that it is not credible to use the entire record since thc inception of the present stress regime, the data for the events and processes prior to the change in the stress regime would be eliminated from consideration. Only the tectonic shortening data since the change in the fundamental characteristics of the tectonic regime would be use i to determine the unanticipated amount of stress and/or strain.

The Yucca Mountain site lies in an area between the dominant extension tectonics of the Basin and Range province system, and the area of dominant right-lateral shear tectonics related to the California tectonic system. The site is, therefore, in a tectonically complex area that will require differentiation of the effects of the processes and events of two different j systems. However, either strike-slip faulting or normal faulting would be considered reasonably likely, whereas pure thrust faulting, associated with compressional tectonics, would be considered not sufficiently credible to warrant consideration.

It is recognized that various climatic extremes existed during the Pleistocene, and that the world is presently in an interglacial stage. There are several  !

theories available regarding climatic proiection, ranging from extrapolation of I the past climatic cycles to projection of the various cycles with the superposition of the "greenhouse effect" resulting from the addition of carbon dioxide and other pollutants to the atmosphere. To evaluate the "anticipated" climatic processes and events, the NRC would assess the variation in the climatic data and the present position of the climatic cycle, and use these various projections to evaluate the resultant effect on waste isolation. While the entire Quaternary Period needs to be evaluated, the late Quaternary record

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16 would be the record most relied upon by the staff. The scenarios for anticipated processes and events for the post-closure should conservatively reflect modifications to the projected climatic trend, resulting from such phenomena as the greenhouse effect, if the climatic model assumed in the analysis warrants consideration of such effects. Although this example deals with the climatological phenomena, the estimates of precipitation derived from this exercise would serve as the basis for determining "anticipated" changes to the groundwater flow regime due to the anticipated climatic changes.

Further examination of the various references on Pleistocene climates, such as Imbrie and Imbrie (1979)(see Ref. 4), shows that their best projection for the next 10,000 years does not reach the climatic extremes these authors extrapolated from the Quaternary data. If we assume, for the purpose of this example, that the reference cited describes an acceptable model, then the staff would normally consider, without geologic evidence to the contrary, that it is credible, but not reasonably likely, that the extremes could again be reached )

in the next 10,000 years. These extremes would be considered as an l unanticipated climatic process and should be further evaluated. I Dissolution is a geochemical process. The rate of the process, however, is still being primarily controlled by climatic processes and therefore dissolution can be treated as in the above example. By examination of the late Quaternary record especially, rates of dissolution can be determined. From these rates projections for the next 10,000 years can be made for the j anticipated processes and events assuming the "anticipated" climate. Extreme l rates, which have not occurred in the site area, but which have occurred in the geologic setting, could be used for determining the unanticipated processes and events. By use of these rates, projections for both areal dissolution and point source dissolution can be determined. If the anticipated rates indicate that disruption of the repository could occur, this disruption would be considered in the anticipated scenarios. If the unanticipated rates of dissolution could cause the disruption, then the resulting disruption should be considered in the unanticipated scenarios.

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Thus far, the discussion has only considered the processes and events by themselves. The scope and nature of the processes and events considered must l be modified, however, to take into account the perturbations caused by the l emplacement of radioactive waste. These perturbations may be nf several types.  !

They may include, for example, the changes in the physical and chemical characteristics of the geologic setting, attributable to the heat generated by the emplaced waste. The construction of the underground facility, the shafts, boreholes, and seals may modify the hydrological pathways and must be taken into account. Similarly, the presence of the waste package, backfill, ard other engineered components could affect groundwater chemistry and must also be considered.

If, for example, the thernal conditions that will be present in the un6rground facility in sites that are tectonically quiescent are being considered t he resultant anticipated temperature used in design would consider the heet generated by the waste package and the geothermal gradient. If, however, there have been instances of volcanic activity during the Quaternary Period, a recurrence of volcanic activity would be considered an anticipated event. The resultant temperature that would need to be considered for design would evaluate the effect of this anticipated volcanic heat source, in conjunction with the heat generated from the waste package, to determine tiie resultant snticipated temperature.

As a final example, it may be worthwhile to expand upon the matter of volcanism at Yucca Mountain. Based on present knowledge, it can be shown that multiple episodes of basaltic volcanism have occurred within the geologic setting during the Quaternary. Therefore, tTsaltic volcanism would be considered an anticipated event. Silicic vot.taism has not occurred during the Quaternary Period, and would be considereu ..t c sufficiently credible to warrant consideration. According to infornietion presented in Kane and Bracken (1983)(see Ref. 5), the basaltic cones west of Yucca Mountain in Crater Flat appear to lie along a northeast trending magnetic anomaly. Characterization activities may show that this linear anomaly represents a controlling

18 structural feature, and that similar structural features do, in fact, occur in the area of other cones, such as the Lathrop Wells basaltic cone. If such proves to be the case, it would be assumed to be reasonably likely that future volcanic events would occur along these controlling structural features. The anticipated volcanic event would, therefore, be assumed to occur along these structural features. To determine the unanticipated volcanic event, the staff would consider both the results of such probabilistic studies as Crowe, et al.,

(1982)(see Ref. 6), and the evidence regardino the presence of structural features similar to the structural features related to the known instances of basaltic volcanism. If it can be demonstrated that volcanogenic structures are present in the area, both the structural features and the probabilistic evaluation would be considered, in determining compliance with 960,117. If, instead, related structures are present, aside from those considered in determination of the anticipated events, only the probabilistic calculations would be cor..idered. The staff considers that such an approach would allow an adequate consideration of volcanism, which reflects the uncertainty in understanding the process.

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6. REFERENCES USED I

1. U.S. Nuclear Regulatory Commission, "Disposal of High-level l Radioactive Wastes in Geologic Repositories, Technical Criteria," Federal l Register, Vol. 48, No. 120, June 21, 1083, pp. 28194-28229.

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2. Allen, C.A., "Geologic Criteria for Evaluating Seismicity," Geological i Society of America Bull . ,- Vol . 86, pp.1041-1057,1975.-

3. Rockwell International, "Preliminary Interpretation of the Tectonic Stability of the Reference Repository Location, Cold Creek Syncline, Hanford Site," RH0-BW-ST-19P, 1983.

4 Imbrie, John and Imbrie, K.P., "Ice Ages - Solving the Mystery,"

Enslow Publishers, Short Hills, N.J.,1979.

5. Kane, M.F., and Bracken, R.E., "Aeromagnetic Map of Yucca Mountain and Surrounding Regions, Southwest Nevada," United States Geologic Survey Open File Report, USGS-0FR-83-616, 1983.-

6. Crowe, B.M., Johnson, M.E., and Beckman, R.J., "Calculation of the Probability of Volcanic Disruption of a High-level Radioactive Waste Reposi'.ory within Southern Nevada, USA," Radioactive Waste Management and the Nuclear Fuel Cycle, Vol. 3, No. 2, pp. 167-190, 1982.

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20 APPENDIX A: GLOSSARY "Anticipated processes and events" means those natural processes and events that are reasonable likely to occur during the period the intended performance obiective must be achieved. To the extent reasonable in the light of the geologic record, it shall be assumed that those processes operating in the geologic setting during the Quaternary Period continue to operate but with the perturbations caused by the presence of emplaced radioactive waste superimposed thereon.

"Barrier" means any material or structure that prevents or substantially delays movement of water or radionuclides.

"Engineered barrier system" means the waste packages and the underground facility.

"Geologic setting" means the geologic, hydrolooic, and geochemical systems of the region in which a geologic repository operations area is or may be located.

"Site" means the location of the controlled area.

"Unanticipated processes and events" means those processes and events affecting the geologic setting that are judged not to be reasonably likely to occur during the period the intended performance objective must be achieved, but which are nevertheless sufficiently credible to warrant consideration.

Unanticipated processes and events may be either natural processes or events or processes and events initiated by human activities other than those activities licensed under this part. Processes and events initiated by human activities may only be found to be sufficiently credible to warrant consideration if it is assumed that: (1) The monuments provided for by this part are sufficiently 9 All definitions have been taken directly from 10 CFR 660.2.

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21 permanent to serve their intended purpose; (?) the value to future generations of potential resources within the site can be assessed adequately under the applicable provisions of this part; (3) an understanding of the nature of radioactivity, and an appreciation of its hazards, have been retained in'some functioning institutions; (4) institutions are able to assess risk and to take remedial action at a level of social organization and technological competence equivalent to, or superior to, that which was applied in initiatina the processes or events concerned; and (5) relevant records are preserved, and remain accessible, for several hundred years after permanent closure.

"Underground facility" means the underground structure, including openings and backfill materials, but excluding shafts, boreholes, and their seals.

"Waste package" means the waste form and any containers, shielding, packing and other absorbent materials imediately surrounding an individual waste container.

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