ML19340E818

Low Level Radwaste Modeling Needs from NRC Perspective

ML19340E818
Person / Time
Issue date:	12/22/1980
From:	Hawkins E, Rich Smith NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS)
To:
References
NUDOCS 8101150876
Download: ML19340E818 (9)

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LOW LEVEL RADI0 ACTIVE WASTE MODELING NEEDS R

FROM THE USNRC PERSPECTIVE R. Dale Smith W

Edward F. Hawkins 17 3

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5 is Low-Level Waste Licensing Branch A

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Division of Waste Management U.S. Nuclear Regulatory Commission Washington, D.C.

20555 ABSTRACT The NRC is developing a low-level waste regulation to be issued for public comment by April 30, 1981. Publication of the final rule and final EIS is expected in 1982. This regulation,10 CFR Part 61 will apply only to near surface disposal. Two basic approaches have been considered in developing specific regulations:

a prescriptive approach and a perfomance objective approach.

To accomplish the licensing and regulation of low-level waste disposal facilities, the information that will be needed and the methodologies to be used to assess performance are being established.

Regulatory guides will cover site selection and characterization, facility design and operations, waste clar.ification, waste form perfomance, site closure, post-operational surveillance, funding and monitoring.

In developing licensing review procedures, initial efforts are to establish analytical modeling capabilities. We are establishing our modeling needs, determining models availability, obtaining potentially useful models and adopting those that meet our needs. As a test case, the Barnwell facility will be modeled and its performance assessed. Our analyses will then be compared with those of our contractor, ORNL and the Licensee.

The NRC schedule for promulgation of a low-level waste regulation calls for issuance of the notice of proposed rulemaking along with a supporting draft EIS by April 30, 1981. Publication of a final rule and final EIS is expected in 1982 and will depend on the nature and extent of comments received and whether a formal hearing is held.

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4 Part 61 will apply to the near surface disposal of waste by such means as shallow-burial, engineered structures, and deeper burial.

The disposal of waste by other methods (e.g., deep mined cavity or other deep burial) will not be dealt with in this rulemaking action and will be considered at a later time through a separate rulemaking.

In developing specific regulations, we have considered two basic approaches:

a prescriptive approach and a perfomance objective approach.

In the fomer approach, specific detailed requirements for design and operation of a waste disposal facility would be set out in the regulations. Prescriptive standards would specify the particular practices, designs, or methods which are to be employed--for example, the thickness of the cover material (the cap) over a shallow land burial disposal trench, or the maximum slepe of the trench walls.

Setting of prescriptive standards requires a considerable amount of detailed knowledge about potential designs, techniques, and proce-dures for disposing of waste in order to prescribe which designs, techniques, and procedures are best and also assumes that the state..

of-art in waste disposal.is developed to the point where there are clear choices to be made among all the potential approaches.

A regulation oriented toward perfomance objectives, on the other hand, would establish the overall objectives to be achieved in waste disposal and would leave flexibility in how the objectives would be achieved. The perfomance standards would specify levels of impact which should not be exceeded at a disposal facility in order to provide protection against radiological hazards.

NRC staff believes that neither a purely prescriptive nor a perfomance objective approach will be satisfactory in Part 61. A combinat' ion of both approaches is needed such that performance ob-jectives are established to define the overall perfomance expected in dispoul while at the same time, certain specific minimum tech-nical criteria would be established.

NRC's overall goal is to assure protection of the public health and safety.

In considering radioactive waste disposal, this goal would appear to fall into two time frames. That is, we must be concerned with protection of workers and the.public during the short-term operational phase and protection of the public over the long

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tem after operations cease. Thus, the near-tenn performance objective will be to assure that the disposal facility will be operated in com-pliance with existing standards as set out in Part 20 of our regula-tions. Although routine operational releases are not expected at a disposal facility, they might occur in those cases where waste pro-cessing or volume reduction operations are conducted at a disposal site.

In such cases we would propose to apply the same limits that would apply if these operations were conducted at fuel cycle facili-ties (40 CFR Part 190).

Assuring safety over the long term involves two considerations:

1) protection of an individual who might unknowingly contact the waste at some point in the future; and 2) protection of the general public froa potential releases to the environment. We believe that intentional intrusion into the waste--e.g., an archaeologist reclaiming artifacts-cannot reasonably be protected against. After active institutional controls cease, however, one or a few individuals could inadvertently disturb the waste through such activities as construction or farming.

It is important to note that consideration of intrusion involves hypothetical exposure pathways which are assumed to occur. Actual intrusion into the waste may never occur. But, for purposes of Part 61, intrusion is considered such that if it should occur the one or few individuals contacting the waste should not receive an unacceptable exposure. With respect to the unknowing intruder, we are presently considering use of the Part 20 maximum individual dose limit of 500 mrea/yr to assure protection of such an i ntruder.

With respect to migration and protection of the ground water, we are evaluating a range of exposure guidelines from 1 to 25 mrem /yr that would t e applied at the site boundary. We expect our analyses 1

will result in a limit around 4-5 mrem /yr.

I'd like to review what these performance objectives mean in terms establishing minimum technical requirements; that is, the prescriptive part of the regulation. These requirements would in-volve placing controls on the various parts or barriers of the overall disposal " system" and will determine which wastes are ac-ceptable for near-surface disposal and which wastes are not and must therefore be disposed of by other methods providing much greater confinement. The principal parts or barriers of an overall LLW disposal system that are readily identifiable and will be addressed

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4 in the prescriptive requirements are:

the characteristics of the waste and its packaging; the characteristics of the site into which the waste is placed; the methods by which the site is utilized, the waste emplaced and the site closed; and the degree and length of institutional control, surveillance, and monitoring of the site after closure.

It is instructive to start with the intruder perfomance objec-tive since although it deals with a hypothetical pathway, it provides a basis for several limiting requirements and establishes two

" classes" of waste suitable for near-surface disposal.

There are two principal means of controlling potential exposures to an intruder--use of institutional controls and use of natural or engineered barriers which would make it more difficult for a potential intruder to contact the waste.

Institutional controls can be further separated into two types:

(1) " active" controls, which require per-formance of some action by a person or agency-e.g., controlled access to the site, including barries to entry (fences) and periodic inspec-tion and monitoring of the disposal site by a regulatory or other governmental agency; and (2) " passive" controls, which are not specifically dependent upon human actions-e.g., government ownership of land, redundant records and restrictions on land use. Natural or engineered barriers could include deeper burial or use of caissons backfilled with concrete.

Neither institutional controls nor engineered barriers can be relied upon to completely prevent human intrusion. Although we have not completed our final analyses we expect to allow reliance on active institutional controls to prevent intrusion for about 100 years. This will mean that certain wastes (e.g., those with short half-lives) will not present an undue risk to an intruder when active institutional controls are removed. The active controls will be followed by several hundred additional years of passive controls.

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r Passive controls, although certainly minimizing potential for inad-vertent intrusion may not be completely effective in preventing it.

A second class of waste would then be that which may present an undue risk to an intruder when active controls are removed.

For this waste additional measures must be taken to reduce this risk. This can be done by introducing an additional barrier to intrusion, such as deeper burial or engineered barriers. These additional measures will not be relied on for more than 500 years; thus waste which could potentially result in an exposure to an intruder greater than 500 mrem after 500 years is not acceptable for near surface disposal.

A similar type of analysis for migration cannot be performed in as direct a manner. While intruder exposures are relatively non-site specific, potential ground-water releases are very site specific and are more difficult to treat in as generic a fashion as the intruder.

In order to carry out a site specific analysis, the variables beyond the control of the site operator need to be fixed to some extent, Of principal importance is the long-term stability of the waste and disposal facility. Unless the waste and the disposal site are stable over time, there is no way to predict the long-tem radiological impacts of disposal, or the activities (maintenance, monitoring, etc.)

and associated costs required to maintain potential impacts to low levels. This would suggest that all waste should be placed in a solid, non-compressible form for disposal in the near surface.

However, many wastes--particularly trash waste streams--contain very low levels of activity and present little or no concern from the standpoint of long-tem migration. Thus, a more reasor,able require-ment would be that compressible waste containing low-levels of activity should be segregated 'and disposed of separately. All other waste would be placed in a solid, non-compressible form (e.g., solidifica-tion in the case of resins and filter sludges) and the disposal facility should be designed and operated to enhance stability mer the long-tem. Certain nuclides, concentrations of nuclides or waste forms may also require individual consideration from the stand;oint l

of migration on a site specific basis. This might include, for example, a large one-time shipment of a very mobile radionuclide or a lage quantity of chelating agent in a single shipment. Thus, migratfon l

considerations establish " classes" of wastes that require special disposal considerations.

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't We expect to establish a common sense base of siting requirements that could be consistently applied throughout the country. The require-ments would essentially eliminate certain limited areas from consider-ation due to undesirable characteristics leaving large areas in each region where acceptable sites could be found. The kinds of require-ments we would expect to establish include:

1) Avoid areas of fractured bedrock;

2) Avoid siting in wetlands, high hazard coastal areas, flood -

plains, swamps or other types of very wet or potentially very wet terrain;

3) The land surface should be relatively stable structurally and geanorphically;

4) The hydrogeologic conditions of the site should be simple enough for reliable residence time prediction to be made; and

5) The site should provide long travel times for radionuclides from disposal trenches to the biosphere.

1 The completion of Part 61 does not, however, mean that our job is done. To accomplish the licensing and regulation of low-level waste disposal facilities, we are also establishing the information needed and the methodologies we will use to assess the performance of low-level waste disposal facilities. These efforts are concentrated in two activities--the development of regulatory guides to support the regulation and the development of licensing review procedures. The regulatory guides are being initiated more or less in parallel with our efforts on licensing review procedures. As we now envision it, regulatory guides will be developed in the areas of site selection, site characterization, facility design and operations, waste classi-fication, waste form performance, site closure, post-operational surveillance, funding and monitoring. As with the regulation, these guides will be directed at disposal by means of shallow and inter-mediate depth land burial.

Future guides, as needed, will be directed at other methods of disposal.

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1 To develop our licensing review procedures, that is, the way we will review applications for low-level waste disposal, we are initially concentrating on methodologies we will use to assess per-fomance. The first step irl,this development is to establish our capability to perform analytical modeling. Although NRC has extensive experience in modeling and sone modeling has been done of lou-level waste disposal in the past, a systematic approach is now being taken to establish a program to assure that our analytical capabilities are in place as soon as possible. To accomplish this, we have em-barked on an effort to establish our modeling needs, detemine the availability of models to meet these needs, obtain prospective models, develop our operational capability of the selected models, and test the models on existing or proposed sites. Let me make it clear that it is not our intention to develop a new set of models. Rather, we will investigate models that are already in use and adapt, or adopt them for our use.

If our investigations reveal that there are no acceptable models to meet a specific need, we will then initiate a program of model development.

Our first task will be to develop screening criteria to be used..

to select potential mcdels that can be used to simulate low-level waste sites and the effects of construction, operation and ultimate closure. Models will need to be able to simulate the geological environment, ground and surface water pathways, air pathways, biotic and vegetation pathways, radionuclide transport and the assessment of potential radiological and non-radiological impacts. Model s will also be screened in terms of their level of detail and our need for such detail.

In some cases site screening and gross evaluation type of models may be all that are required; whereas for other cases very detailed, highly accurate models may be needed to fully assess impacts. Perhaps in a few cases a model may need to be developed that is applicable to a specific site. The screening criteria must also take into account the need for models in a wide variety of geologic and climatic settings, e.g., the obvious differences be-tween arid and humid sites. This screening will also be done in the context of the time and computer systems available to us.

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Through our own efforts and those of our contractors and consul-tants, our search for available and acceptable models will begin with a review of those that NRC already has.

We will also investigate what other agencies are doing and what they have available. Very specifical-ly, this is one of the primary reasons we are here at this workshop.

We are very anxious to find out what you are doing. Our efforts will also include the work going on at the National Laboratories, the universities by private industry and any other source that we can find.

As we accumulate models for potential use, we will evaluate and categorize them for our particular needs in terms of their usefulness to us for making licensing decisions. Key items in our evaluations will be:

their applicability (humid vs. arid sites, saturated and unsaturated flow, single and nultiple aquifers, solute transport, dose assess-ment,etc.);

the details of the model (data requirements; simple, conservative vs. detailed, more realistic; difficulty of use, model options, computer requirements, etc.);

4 availability of docunentation (users and programmers manuals, sample runs, examples, etc.); and validation and verification of models (theoretical proofs, pre-vious applications to field conditions, reproducibility, re-peatability,e^c.).

It is obvious that this is a major undertaking both in terms of time and staff effort. However, it is necessary if we are to be able to make knowledgeable licens199 decisions in a timely manner. To enable us to focus on a specific goal, we have elected to concentrate our initial efforts on performing an assessment of the Barnwell Low-Level Waste Disposal Facility in South Carolina as a test case.

There are several reasons why we made this choice.

First, we have agreed to assist the state of South Carolina this next year in the environmen-tal assessment of the Barnwell site as a part of their review for the renewal of the state license for the faciltiy. Second, we have con-tracted with Dak Ridge National Laboratory to assist us in this ef-fort, including performance assessment of the facility. Theirs and the licensees analyses should provide a good canparison to our efforts.

Last, since the Barnwell facility has been operating for some time l

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.9 and there are other nuclear facilities nearby that have been studied extensively, we feel that data requirements for the selected models should probably not be a liniiting factor. This should provide us more flexbility in model selection.

Once we have completed one test case (Barnwell) we will then turn over efforts to expanding our capabilities to other types of sites and situations. Hopefully, we will not have to go back to the beginning of the exercise since we should have learned a great deal the first time through and should have accumulated some applicable information.

In conclusion, I would like to summarize the modeling needs from our perspective:

1) We have identified the need for models that are directed at enabling us to make licensing decisions.

2) We have determined that the ability to perform assessments must be contained within our own staff.

3) Our modeling capabilities must be able, eventually, to evaluate the full range of potential sites, facilities, designs and operations that may occur. We have, however, elected to first concentrate on models that are poten-tially applicable to the Barnwell facility, which will be used as a test case.

4) We have determined that the best course for us is to adopt, or adapt, existing models to our use to the fullest extent possible.

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