United States Science Advisory EPA-SAB-RAC-ADV-99-006

Environmental Board February 1999

Protection Agency Washington, DC www.epa.gov/sab

 

AN SAB ADVISORY:

Modeling of Radionuclide

Releases from Disposal of

Low Activity Mixed Waste

ADVISORY ON THE OFFICE OF

RADIATION AND INDOOR AIR’S

DRAFT PROPOSALS ON

MODELING OF RADIONUCLIDE

RELEASES FROM DISPOSAL OF

LOW ACTIVITY MIXED WASTE BY

THE RADIATION ADVISORY

COMMITTEE
February 22, 1999

EPA-SAB-RAC-ADV-99-006

Honorable Carol M. Browner

Administrator

U.S. Environmental Protection Agency

401 M. Street, S.W.

Washington, DC 20460

Re: Advisory on Modeling of Radionuclide Releases from Disposal of Low Activity

Mixed Waste (LAMW)

Dear Ms. Browner:

The enclosed Advisory was developed by the Radiation Advisory Committee (RAC) of

the Science Advisory Board (SAB) in response to a request from the Office of Radiation and

Indoor Air (ORIA) to provide advice on the modeling of low activity mixed radioactive waste

(LAMRW) and specifically to respond to a three element Charge, summarized below (the detailed

Charge may be found in Section 2.2 of the enclosed report:

a) Does the EPA dose assessment reasonably cover the range of hydrogeologic and

climatic settings that might be used for disposal of low-activity mixed waste?

b) What modeling time frame does the Committee recommend be used to project

potential doses from disposal of low-activity mixed waste? Does a 1000-year time

frame provide an adequate technical basis for setting regulatory limits?

c) Is it reasonable to assign a “high” release rate for the duration of the simulation? 

Does the SAB advise an alternative approach? 

The RAC held a public meeting on November 17, 18, and 19, 1998 at which it was briefed

by, and had technical discussions, with ORIA staff, as well as a writing session by the Committee. 

A public teleconference was also held on December 15, 1998. The advisory generated by this

meeting responds both to the three Charge questions, as well as addressing other issues identified

during the public meeting.

ORIA conducted analyses for three sites, based on the hydrogeologic and climatic

characteristics of well-characterized Department of Energy (DOE) sites selected from a group of

ten such sites analyzed in a screening study. The Committee concluded that these sites do not

necessarily cover the range of conditions that might be encountered at current or future RCRA-C

facilities. For example, they include no sites outside the conterminous United States, and

probably do not cover the range of possibilities for depth to groundwater, existence of snowpack
2

and freeze-thaw cycles, or soil composition in which RCRA-C facilities are or could be sited. 

Moreover, the modeling as conducted does not consider such other site characteristics as soil and

waste pH, redox conditions, the presence of reactive organic matter, and biological activity. The

Committee recommends that ORIA conduct at least screening analyses of the extent to which

these factors could affect radionuclide migration rates. Furthermore, Charge a), as framed by

ORIA seems overly narrow. Although it is desirable to select sites to bound important potential

site conditions and characteristics, the ultimate goal should be to bound probable site

performance. ORIA should consider enhancing its approach to include additional site

characteristics. 

The Committee concluded that a sensitivity analysis of the modeling time frame could

provide potentially useful information on the variation of peak dose with time horizon, especially

for less mobile radionuclides with long half-lives. The Committee did not reach consensus,

however, on how ORIA should weigh projections for the longer periods against the more reliable

projections for shorter periods. All members agreed that under the modeling assumptions used by

ORIA, peak doses for some radionuclides might occur after 1,000 years and be significantly

higher than at 1,000 years. While some members stated that this scientific conclusion was

sufficient to extend the modeling time frame to 10,000 years or beyond, others emphasized that

uncertainties inherent not only in the modeling assumptions but also about future scientific and

social changes could make the results irrelevant. Consequently, the Committee’s advice is limited

to suggesting issues that should be considered in ORIA’s eventual decisions about LAMW. 

Among them are:

a) Characterization of the candidate wastes with respect to concentrations and total

inventories of long-lived radionuclides,

b) Harmonization of the modeling time frame between radioactive and hazardous

wastes,

c) Consideration of uncertainties about not only the validity of the technical

assumptions but also in the relevant medical and social conditions far in the future,

d) Consideration of issues about site ownership, and

e) Appropriate degree of conservatism given the policy intent of the proposed

LAMW disposal option.

The Committee believes that changes in assumptions about the time course of concrete

degradation might significantly affect predictions of the time to peak dose or its magnitude. The

assumption of a constant rate of release relative to the remaining inventory of a radionuclide could

either overestimate or underestimate peak dose, depending on the half life of the parent

radionuclide and the characteristics of its progeny. The Committee recommends that a simulation
3

be performed to verify, at least qualitatively, that the constant loss rate assumption is reasonably--

but not excessively--conservative. Other assumptions, such as an exponential deterioration

model, could also be considered, although selection of a technically defensible deterioration rate

could be problematic because it could be a complicated function of physical, geochemical, and

thermal factors.

The Committee’s major findings and recommendations concerning those issues not

included in the Charge include:

a) ORIA should better justify the choice of the PRESTO model as the principal tool

for the analysis. Benchmarking of the model through comparison with other

models of similar intent would be useful, as would demonstration that its scope of

applicability is appropriate.

b) ORIA should consider using a classification of radionuclides according to half-life

as a refinement of the “unity rule” for determining the acceptability of wastes with

more than one important radionuclide.

c) ORIA should consider whether the total quantity of waste to be emplaced in a

RCRA-C facilities, not just its radionuclide concentrations, should be part of the

decision process. Moreover, the potential presence of non-stabilized hazardous

wastes in a solid waste management unit (SWMU) near to LAMW RCRA-C

SWMUs should be considered.

d) Various modeling assumptions should be re-examined, including ones about

segregated vs. random emplacement, the amount of waste potentially subject to

disposal as LAMW, the composition of such wastes, and the classification of

radionuclides as mobile or immobile. 

e) Given the wide range of environments that might be found at current or future

RCRA-C sites, ORIA might be well advised to propose two sets of concentration

criteria, one valid for relatively dry sites with deep groundwater and another for

wet sites. The latter might be able to accept only wastes with short-lived

radionuclides.

f) The modeling effort may be an opportunity for EPA to examine the similarities and

differences in radioactive waste and hazardous waste control systems and waste

acceptance criteria and to harmonize them to the extent feasible. Additional

classes of waste could also be analyzed within the modeling structure.

g) ORIA should take care not to create an incentive for a generator to produce a

waste classifiable as LAMW that would otherwise be simply Low-Level

Radioactive Waste (LLRW). It should also examine whether redefining the waste
4

would change the liability distribution between waste generators and disposal

facility operators.

The RAC appreciates the opportunity to provide this advisory to you and we hope that it

will be helpful. We look forward to the response of the Assistant Administrator for Air and

Radiation to the advisory in general and to the specific comments and recommendations in this

letter in particular.

Sincerely,

/signed/

Dr. Joan M. Daisey, Chair

Science Advisory Board

/signed/

Dr. Stephen L. Brown, Chair

Radiation Advisory Committee
i

NOTICE

This report has been written as part of the activities of the Science Advisory Board, a

public advisory group providing extramural scientific information and advice to the Administrator

and other officials of the Environmental Protection Agency. The Board is structured to provide

balanced, expert assessment of scientific matters related to problems facing the Agency. This

report has not been reviewed for approval by the Agency and, hence, the contents of this report

do not necessarily represent the views and policies of the Environmental Protection Agency, nor

of other agencies in the Executive Branch of the Federal government, nor does mention of trade

names or commercial products constitute a recommendation for use.
ii

ABSTRACT

On November 17-19, 1998, the Science Advisory Board’s Radiation Advisory Committee

conducted an advisory of the Office of Radiation and Indoor Air's (ORIA) modeling of low

activity mixed waste, including: dose assessment over a wide range of disposal site-specific

hydrogeologic and climatic settings; the 1000 year modeling time frame; and using a “high”

release rate from concrete for the modeling. 

The Committee found that the sites modeled do not necessarily cover the range of

conditions that might be encountered at RCRA-C facilities. It recommends that ORIA should

further assess the impact of site-specific conditions to bound probable site performance better. 

While the Committee did not reach consensus on the modeling time frame, it recommends that

ORIA consider: conducting a sensitivity analysis to address the variation of peak dose with time;

improving its waste characterization; the relationship between radioactive and hazardous waste

modeling time frames; uncertainties in its technical assumptions and future medical and social

conditions; site ownership; and its degree of conservatism given the intent of the proposal. The

Committee recommends that ORIA perform a simulation to verify that its assumptions about the

releases from concrete are reasonably conservative. 

Beyond the Charge, the Committee recommends that ORIA: better justify choosing the

PRESTO model; consider classifying radionuclides according to half-life; consider whether the

total quantity of waste as well as its radionuclide concentrations should be part of the decision

process; re-examine certain modeling assumptions; propose concentration criteria addressing

"dry" and "wet” sites; and compare control systems and acceptance criteria for radioactive and

hazardous wastes.

KEY WORDS: low activity mixed wastes; hazardous waste facilities; Resource

Conservation and Recovery Act, Subtitle C (RCRA-C); low level

radioactive waste; modeling of potential releases of radionuclides.
1

 Did not attend meetings, but provided comments in the review.

iii

U.S. ENVIRONMENTAL PROTECTION AGENCY

SCIENCE ADVISORY BOARD

 RADIATION ADVISORY COMMITTEE

November 17-19, 1998, and December 15, 1998

CHAIR

Dr. Stephen L. Brown, Director, Risks of Radiation and Chemical Compounds, Oakland, CA

MEMBERS AND CONSULTANTS

Dr. William J. Bair, Retired, Richland, WA 

Dr. Vicki M. Bier, Associate Professor of Industrial Engineering and Engineering Physics,

University of Wisconsin, Dept. of Industrial Engineering, Madison, WI 

Dr. June Fabryka-Martin,

1

 Staff Scientist, Los Alamos National Laboratory, Los Alamos

National Laboratory, Los Alamos, NM 

Dr. Thomas F. Gesell, Professor of Health Physics and Director, Technical Safety Office, Idaho

State University, Pocatello, ID 

Dr. F. Owen Hoffman, President and Director, SENES Oak Ridge, Inc., Center for Risk

Analysis, Oak Ridge, TN 

Dr. Janet Johnson, Senior Radiation Scientist, Shepherd Miller, Inc., Ft. Collins, CO 

Dr. Donald Langmuir,1 President, Hydrochem Systems Corp., Golden, CO 

Dr. Jill Lipoti, Assistant Director for Radiation Protection Programs, Div. of Environmental

Safety, Health and Analytical Programs, New Jersey Dept. of Environmental Protection,

Trenton, NJ 

Dr. Ellen Mangione, M.D., M.P.H., 1 Director, Disease Control & Environmental Epidemiology

Division, Colorado Department of Public Health and Environment, Denver, CO 

Dr. Paul J. Merges, Director, Bureau of Radiation and Hazardous Site Management, Division of

Solid & Hazardous Materials, New York State Department of Environmental

Conservation, Albany, NY 

Dr. John W. Poston, Sr., 1 Professor and Head, Dept. of Nuclear Engineering, Texas A&M

University, College Station, TX
2

 Did not participate in this review.

3

Provided editorial support in the preparation of this report, but did not participate in the review.

iv

Dr. Genevieve S. Roessler, Radiation Consultant, Elysian, MN 

EHC Liaison

Dr. David G. Hoel, Distinguished University Professor, Department of Biometry &

Epidemiology, Medical University of South Carolina, Charleston, SC 

EEC Liaisons

Dr. Hilary I. Inyang,1 Director, Center for Environmental Engineering and Science

Technologies (CEEST), University of Massachusetts, Lowell, MA

Dr. Calvin C. Chien, Senior Environmental Fellow, E.I. DuPont Company, Wilmington, DE 

Former Chair

Dr. James E. Watson, Jr.,2 Professor, Department of Environmental Sciences and Engineering,

University of North Carolina at Chapel Hill, Chapel Hill, NC

Science Advisory Board Staff

Dr. K. Jack Kooyoomjian, Designated Federal Officer, Science Advisory Board, U.S. EPA,

Washington, DC

Mr. Samuel Rondberg3,Designated Federal Officer, Science Advisory Board, U.S. EPA,

Washington, DC

Ms. Diana L. Pozun, Management Assistant, Science Advisory Board, U.S. EPA, Washington,

DC
v

TABLE OF CONTENTS

1. EXECUTIVE SUMMARY ................................................. 1

2. INTRODUCTION ........................................................ 6

2.1 Background ...................................................... 6

2.2 Charge to the SAB ................................................. 7

3. DETAILED FINDINGS AND RECOMMENDATIONS ........................... 8

3.1 Extent of the EPA Dose Assessment (Question a) .......................... 8

3.2 Suitability of the 1000-year Time Frame (Question b) ...................... 11

3.2.1 Scientific Issues ........................................... 12

3.2.2 Future Health Risk and Land Ownership Considerations ............. 14

3.2.3 Public Acceptance ......................................... 14

3.2.4 Economic Considerations .................................... 15

3.3 Modeling Release Rates (Question c) .................................. 15

3.3.1 Technical Issues ........................................... 16

3.3.2 Science Policy Issues ....................................... 18

3.4 Additional Issues .................................................. 19

3.4.1 Modeling Structure ......................................... 19

3.4.2 Science Policy Issues ....................................... 21

3.4.3 Other Policy Issues ......................................... 23

APPENDIX A - DETAILED COMMENTS ..................................... A-1

APPENDIX B - TOTAL SYSTEM PERFORMANCE ASSESSMENT APPROACH ..... B-1

REFERENCES ........................................................... R-1
1

1. EXECUTIVE SUMMARY

The Office of Radiation and Indoor Air (ORIA) is considering a proposal to allow certain

commercial low activity mixed wastes (LAMW) to be disposed of in facilities permitted as

hazardous waste facilities under the Resource Conservation and Recovery Act, Subtitle C

(RCRA-C). LAMW is waste that is currently classified as hazardous under RCRA and as

radioactive waste under the Atomic Energy Act (AEA). Only a subset of the waste classified as

Class A low level radioactive waste (LLRW) would qualify, and the RCRA-C facility would have

to obtain a (presumably simplified) license for radioactive waste from the Nuclear Regulatory

Commission or an Agreement State.

In support of its decision, ORIA is conducting modeling of potential releases of

radionuclides from such wastes to establish radionuclide concentration criteria for waste

acceptance. Before proceeding further, ORIA sought advice from the Science Advisory Board’s

(SAB) Radiation Advisory Committee (RAC) on three questions regarding modeling procedures

and assumptions. The Committee’s advice on these questions and on some related broader issues

is summarized below.

Charge question (a) asked:

Having relied on extensive site characterization data from existing and potential lowlevel radioactive waste disposal landfills, does the EPA dose assessment reasonably

cover the range of hydrogeologic and climatic settings that might be used for disposal of

low-activity mixed waste?

ORIA conducted analyses for three sites based on the hydrogeologic and climatic

characteristics of well-characterized Department of Energy (DOE) sites selected from a group of

ten such sites analyzed in a screening study. The Committee concluded that these sites do not

necessarily cover the range of conditions that might be encountered at current or future RCRA-C

facilities. For example, they include no sites outside the conterminous United States, and

probably do not cover the range of possibilities for depth to groundwater, existence of snowpack

and freeze-thaw cycles, or soil composition in which RCRA-C facilities are or could be sited. 

Moreover, the modeling as conducted does not consider the effects of such other site

characteristics, such as soil and waste pH, redox conditions, the presence of reactive organic

matter, and biological activity. The Committee recommends that ORIA take a broader look at the

range of radionuclide leaching and transport behaviors that might occur at real RCRA-C sites. 

Furthermore, Question a), as framed by ORIA, seems overly narrow. While it is desirable to

select sites to bound important potential site conditions and characteristics, the ultimate goal

should be to bound probable site performance. Based on the approach being taken, it may be

difficult to develop predictions of the long-term performance of a mixed waste disposal site. 

ORIA should also consider using more comprehensive decision schemes such as Total Systems

Performance Assessment. However, the Committee is not recommending that the modeling
2

exercise include site characteristics that would clearly be found unacceptable for future RCRA-C

facilities.

Charge question (b) asked:

What modeling time frame does the Committee recommend be used to project potential

doses from disposal of low-activity mixed waste? Does a 1000-year time frame provide

an adequate technical basis for setting regulatory limits?

The Committee concluded that a sensitivity analysis of the modeling time frame could

provide potentially useful information on the variation of peak dose with time horizon, especially

for less mobile radionuclides with long half-lives. The Committee did not reach consensus,

however, on how ORIA should weigh projections for the longer periods against the more reliable

projections for shorter periods. All members agreed that under the modeling assumptions used by

ORIA, peak doses for some radionuclides might occur after 1,000 years and be significantly

higher than at 1,000 years. While some members stated that this scientific conclusion was

sufficient to extend the modeling time frame to 10,000 years or beyond, others emphasized that

there are uncertainties not only in the modeling assumptions but also about future scientific and

social changes that could make the modeling results irrelevant. Consequently, the Committee’s

advice is limited to suggesting issues that should be considered in ORIA’s eventual decision. The

issues fall into four categories: scientific, future land use, public acceptance, and economic

viability, and are explained in detail in Section 3. Major considerations include the following

items:

a) ORIA should attempt to characterize the wastes that might fall under the proposed

disposal option. If these wastes rarely contain long-lived radionuclides, then a

shorter modeling time frame is more easily supported, while a substantial

complement of long-lived radionuclides would argue for a longer time frame or a

constraint on the waste to prevent disposal of such radionuclides.

b) ORIA and other offices of EPA should consider the issue of harmonizing the time

horizons used in analyzing the risks of radionuclides and hazardous wastes. For

instance, lead has an infinite half life and has many other characteristics in common

with long-lived radionuclides.

c) Uncertainties about not only radionuclide releases and pathways of exposure but

also medical and social conditions grow greatly as the modeling time frame is

extended. Does modeling to 1,000 or 10,000 years help or hinder public

acceptance of the results?

d) The requirements for site ownership differ between radioactive waste disposal and

hazardous waste disposal, with radioactive wastes being more likely to remain

under government control. Should this difference argue for a longer modeling

time frame, consistent with other analyses of radioactive waste?
3

e) The proposed alternative for LAMW disposal will be useful to waste generators

only to the extent that they can dispose of waste less expensively and/or more

easily than under current regulations. Excessive conservatism in modeling

assumptions, including the time frame, may exclude most waste from the proposed

disposal option. On the other hand, if modeling to 1,000 years or less does not

convince the public to accept radioactive wastes into RCRA-C facilities, the

contemplated rulemaking will also be in vain.

Charge question (c) asked:

Given the modeling approach described [in the Agency’s presentation], and the available

knowledge regarding concrete durability and modes of degradation, is it reasonable to

assign a “high” release rate for the duration of the simulation? Does the SAB advise an

alternative approach, such as assuming a lower release at the start and increasing it

incrementally over the modeling period, thereby mimicking the gradual deterioration of

the concrete?

The Committee believes that changes in assumptions about the time course of concrete

degradation might significantly affect predictions of the time to peak dose or its magnitude. The

assumption of a constant rate of release relative to the remaining inventory of a Radionuclide, 

even when its quantity and half-life are taken into consideration, could either overestimate or

underestimate peak dose, depending on the half life of the parent radionuclide and the

characteristics of its progeny. The Committee recommends that a simulation be performed to

verify, at least qualitatively, that the constant loss rate assumption is reasonably--but not

excessively--conservative. Other assumptions, such as an exponential deterioration model, could

also be considered. The Committee recognizes that selection of a technically defensible

deterioration rate could be problematic because it could be a complicated function of physical,

geochemical, and thermal factors. The Committee also notes that the concrete deterioration

assumptions interact with the containment failure assumptions; the timing of water infiltration

through the cap and leakage through the liners could affect the rate of concrete deterioration as

well as its significance. Finally, ORIA should consider how to deal with wastes that are not

ordinarily disposed of in concrete containment. Will such wastes simply be excluded from

consideration under the contemplated rule? If not, are the assumptions about concrete

degradation conservative in comparison with the expected behavior of the actual containment?

During the public meetings, the Committee identified and discussed several issues not

incorporated in the Charge; these issues are discussed below.

The Committee reached some conclusions and related recommendations about the LAMW

modeling approach that go beyond the original three questions of the Charge. They include

comments on the overall modeling structure as well as science policy and environmental policy

questions.
4

 By "random," ORIA means emplacement that is not limited to a particular portion of the RCRA-C cell. No formal randomization

process is implied.

4

The Committee believes that ORIA should better justify the choice of the PRESTO model

as the principal tool for the analysis. Benchmarking of the model through comparison with other

models of similar intent would be useful, as would demonstration that its scope of applicability is

appropriate.

ORIA should consider using a classification of radionuclides according to half-life as a

refinement of the “unity rule” for determining the acceptability of wastes with more than one

important radionuclide. Otherwise, radionuclides with vastly different times of peak dose may be

inappropriately combined.

As currently structured, the decision criteria for waste acceptance appear to be based only

on concentration. ORIA should consider whether the total quantity of waste to be emplaced in

RCRA-C facilities should also be part of the decision process. Moreover, the potential presence

of non-stabilized hazardous wastes in a solid waste management unit (SWMU) near-to the

RCRA-C facilities should be considered, as they may affect the stability of the concrete

containment or the mobility of some radionuclides.

Various modeling assumptions should be re-examined, including ones about segregated

vs. random4 emplacement, the amount of waste potentially subject to disposal as LAMW, the

composition of such wastes, and the classification of radionuclides as mobile or immobile. In

addition, the time-step modeling must be based on rigorously random, stratified sampling, with

attention devoted to choosing modeling time steps that are appropriate to the dynamics of

radionuclide decay, concrete degradation, and waste movement.

Given the wide range of environments that might be found at current or future RCRA-C

sites, ORIA might be well advised to propose two sets of concentration criteria, addressing the

interaction of waste and site. One set could be designed to be valid for relatively dry sites with

deep groundwater and the other for wet sites. The latter sites might be able to accept only wastes

with short-lived radionuclides.

The Committee sees this modeling effort as an opportunity for EPA to examine the

similarities and differences in radioactive waste and hazardous waste control systems and to

harmonize them to the extent feasible. At the same time, ORIA could study, within the same

modeling framework, a range of waste types beyond that currently being considered, for example

wastes containing RCRA hazardous and technologically enhanced naturally occurring radioactive

material (TENORM) radioactive constituents. ORIA could also study harmonization of the stillto-be-determined “reference doses” with risk criteria used for hazardous chemical wastes.

Because a waste that contains both chemical and radioactive constituents is potentially

more hazardous than one that contains only one or the other (at the same concentrations), and
5

because the cost of disposal for hazardous wastes is usually less than for radioactive wastes,

ORIA should take care not to create an incentive for a generator to classify improperly a waste as

LAMW that would otherwise be simply LLRW. It should also examine whether redefining the

waste would change the liability distribution between waste generators and disposal facility

operators.
6

2. INTRODUCTION

2.1 Background

When a waste is designated as both “hazardous” because of its chemical or physical

properties and “radioactive” because of the presence of radionuclides--a “mixed waste”-- the

options for its disposal are limited under current regulatory requirements promulgated by the U.S.

Environmental Protection Agency (EPA or “the Agency”) under the Resource Conservation and

Recovery Act (RCRA) and by the U.S. Nuclear Regulatory Commission (NRC) under the Atomic

Energy Act (AEA), respectively. Although certain mixed wastes can be disposed at a commercial

facility in Utah, and although Federal or State regulatory agencies may allow disposal of certain

types of mixed wastes elsewhere under special circumstances; finding a disposal alternative at a

tolerable cost is often difficult, particularly for small-volume generators such as universities,

medical facilities, and other research laboratories. The EPA is therefore seeking alternatives that

would facilitate the safe disposal of these wastes. 

As a start, the Agency is investigating the merits of a regulation that would permit certain

low-activity radioactive mixed wastes (LAMW) to be disposed of in facilities permitted as

hazardous waste facilities under RCRA Subtitle C (RCRA-C). Such a site would be required to

obtain a radioactive waste license under NRC (or agreement state) jurisdiction. The intent of the

initiative is to encourage simplification of the radioactive materials license requirements for

RCRA-C sites willing to accept LAMW that meet specific dose criteria considered protective of

human health and the environment. The contemplated regulation would establish acceptance

criteria, generally stated as concentrations of specific radionuclides in the waste, that if met would

allow the disposal of the mixed waste in a RCRA-C facility. 

EPA’s Office of Radiation and Indoor Air (ORIA) is currently conducting a modeling

exercise to determine what those criteria might be in order to meet radiation dose limits yet to be

finalized. To conduct that modeling, ORIA needs to make a number of assumptions about how

the radionuclides might be released and how people living near or even on the site might become

exposed to them. It also needs to set limits on the range of possible conditions that it will include

in the modeling of human health risks. Using overly conservative assumptions and procedures or

an extremely low reference dose might vastly overstate risks and prevent most or all mixed wastes

generators from realizing the benefits of disposal under the new regulation even if such disposal

were truly of little concern; using less conservative assumptions, however, might allow disposal of

some wastes under conditions that do not provide sufficient protection. The criteria are intended

to prevent any person from receiving an unacceptable radiation dose during any year following

emplacement. The highest estimated annual dose after emplacement, or "peak dose," is therefore

the limiting variable.
7

2.2 Charge to the SAB

Given the issues noted above, the ORIA requested that the Science Advisory Board’s

(SAB) Radiation Advisory Committee (RAC) provide advice on its modeling efforts by

responding to the following three Charge questions:

Question a) Having relied on extensive site characterization data from existing and

potential low-level radioactive waste disposal facilities, does the EPA dose

assessment reasonably cover the range of hydrogeologic and climatic

settings that might be used for disposal of low-activity mixed waste?

Question b) What modeling time frame does the Committee recommend be used to

project potential doses from disposal of low-activity mixed waste? Does a

1000-year time frame provide an adequate technical basis for setting

regulatory limits?

Question c) Given the modeling approach described [in the Agency’s presentation], and

the available knowledge regarding concrete durability and modes of

degradation, is it reasonable to assign a “high” release rate for the duration

of the simulation? Does the SAB advise an alternative approach, such as

assuming a lower release at the start and increasing it incrementally over

the modeling period, thereby mimicking the gradual deterioration of the

concrete?
8

3. DETAILED FINDINGS AND RECOMMENDATIONS

In general, the Agency has provided a modeling approach that is consistent with its

previous analyses of radioactive waste issues and that may be reasonable for establishing

acceptance criteria for the disposal of mixed wastes in RCRA-C disposal cells. The proposal is,

for the most part, clearly described in the materials presented to the Committee, which struck a

good compromise between being comprehensive and being brief (See U.S. EPA/ORIA 1998a

through 1998m). The Committee provides below some suggestions that may help improve the

modeling effort for the task of managing LAMW.

The Committee is supportive of the Agency’s intent to find a partial solution to the vexing

problem of disposing of wastes that contain both RCRA-regulated hazardous constituents and

AEA-regulated radioactive constituents. It is entirely reasonable that some level of radionuclide

concentrations should be acceptable for disposal in RCRA-C waste management units. The

challenge is to define criteria that provide sufficient protection from long-term risks yet qualify a

substantial portion of mixed wastes for disposal under less onerous requirements than currently

used for low-level radioactive waste (LLRW) disposal.

The Committee’s responses to the three Charge questions appear below. However, the

Committee believes that those responses are to some extent conditioned on choices about

broader--and probably more important--issues in the modeling and regulation of LAMW disposal. 

Therefore, we offer in Section 3.4 comments beyond the Charge. We also provide, in Appendix

A, some detailed comments designed to improve any Technical Support Document that is

eventually issued in conjunction with a proposed regulation.

3.1 Extent of the EPA Dose Assessment (Question a)

Charge question (a) asked for comment on EPA's proposed dose assessment, focusing on

its adequacy across the wide range of climatic and hydrogeological settings which might be

encountered in disposing of low-level wastes.

The site characterization does not entirely cover the range of settings for disposal sites. 

The Committee believes that some existing RCRA-C sites and possible future RCRA-C sites in

the states of Alaska and Hawaii, as well as the territories of Puerto Rico, Guam, and trusteeships

are not adequately bounded by ORIA’s use of three existing DOE sites to cover the range of

hydrogeologic and climatic settings. In order to use the ten selected DOE sites as a point of

departure, EPA must demonstrate that they are similar to the 22 currently operating RCRA-C

disposal facilities, as well as possible future facilities outside the conterminous USA. The

similarity must not only include hydrogeologic and climatic characteristics, but other siting and

design issues such as
9

a) Are any of the existing and potential future RCRA-C sites in the 100-year or 500-

year flood plain? How should ORIA deal with potential future changes in the

flood plain area due to upstream development, etc.?

b) Do the sites have similar depths below the waste to the groundwater table? What

is the thickness, nature, and extent of infiltration in the unsaturated zone?

c) Are the sites similar with respect to the hydraulic conductivity of their unsaturated

zones? Are other geochemical processes important to the movement of

radioactive materials?

d) Are the sites similar with respect to fracturing of the underlying strata?

e) Are seismic characteristics that might influence the stability of containment similar

between the RCRA-C and DOE sites?

f) Are the ecological systems of the two sets of sites similar? These systems can

influence the probability of cap degradation.

g) Are the containment designs of the RCRA-C facilities similar to the DOE facilities?

h) Do the DOE sites experience similar freeze-thaw and snow pack buildup, as the

RCRA-C facilities?

i) Are dry sites with shallow ground water depth represented by the DOE sites? If

ORIA believes that such sites are not relevant, it should explain why not.

j) Conditions in, and properties of, the waste and surrounding geological materials

control the rate of release and transport of chemical and radioactive contaminants

from the waste to the accessible environment. Important site considerations are

the compositions, proportions and distribution of radioactive and hazardous wastes

at a site, waste and soil redox and pH conditions, the presence of reactive materials

(i.e., highly sorptive), and biological activity. Do the ten DOE sites take into

account the expected variability of these wastes, site conditions and properties?

Without this information, ORIA cannot assume that its “bounding analyses” truly bounds

the RCRA-C sites. However, the Committee is not recommending inclusion of site characteristics

that would clearly be found unacceptable for future RCRA-C facilities.

Furthermore, Question (a) (See Section 2.2) as framed by ORIA seems overly narrow. 

While it is desirable to select sites to bound important potential site conditions and characteristics,

the ultimate goal should be to bound probable site performance. ORIA should consider

enhancing its approach to include additional site characteristics (See discussion of the Total
5

Cementitious materials in the waste, if fresh, will buffer the (near-field) pH near 10.

10

Systems Performance Analysis approach in Appendix B). Model predictions of the long-term

performance of potential LAMW sites should be validated as much as possible against the actual

performance of existing waste sites. In some cases, there are sufficient historic data to validate

30-year predictions of hazardous waste site performance, and a few decades of performance data

for several radioactive waste sites are available. However, ORIA is proposing to make

predictions of radionuclide releases for 1,000 yrs or more. Should it be using performance data

for existing waste sites as a basis for modeling predictions for proposed LAMW sites?

In the overview document (U.S. EPA/ORIA 1998a, 1998b), the EPA presents model

calculations to predict the concentrations of different radionuclides assumed to be mobile or

immobile in the typical site. All waste sites are apparently considered oxidizing, based on the

radionuclides that ORIA classifies as mobile (e.g., U, Np, Se, Tc). This assumption will probably

be true in cold and/or dry settings. However, in wet (high rainfall), warm climates in the presence

of reactive organic wastes or organic-rich soils, (near-field) subsurface conditions are more likely

to be reducing, which could make these radionuclides immobile. For other radionuclides,

reducing conditions might enhance mobility. 

In the generic characterization document (U.S. EPA/ORIA, 1998f), it appears that ORIA

has considered an extensive list of site-specific physical and hydroponic parameters in the

PRESTO modeling calculations to predict performance of the ten DOE sites in the screening step. 

It has assumed single values for all parameters in the calculations, whereas a probability

distribution of values using a Monte Carlo approach, for example, would give more realistic

results and provide an estimate of the uncertainty.

Apparently the only geochemical inputs to predictive modeling are the radionuclidespecific distribution coefficients (Kd

) (U.S. EPA,/ORIA, 1998i, p. 2), which the document states

will vary with soil type. Values of Kd for individual actinide elements in particular can vary by up

to five orders of magnitude depending not only on soil type (i.e., soil mineralogy and particle

size), but also on soil and waste pH,5 redox conditions, the presence of reactive organic matter,

and biological activity (Langmuir, 1997). The chemical and biological reactivity of the mixed

waste (of its organic, inorganic and radioactive substances) and its interaction with surrounding

soils will control the release rate of the radionuclides. The climatic and general hydrogeologic

conditions at two sites could be identical. However, differences in the composition, reactivity and

relative amounts of chemical and radioactive materials in different wastes, and differences in the

behavior of reactive minerals present in the soil and underlying groundwater systems, could result

in vast differences among sites in the timing and amounts of specific radionuclides that arrive at

adjacent wells.

As or more important than depth to bedrock and thickness of the unsaturated zone (UZ),

is the nature and extent of infiltration through the UZ, which will reflect the structure and

stratigraphy of ‘soils’ in the UZ. For example, does flow at and under the waste site occur in
6

The decay of cellulosic materials to form ligands that can complex the actinides, for example.

7

Temperature accelerates biological activity and waste material breakdown.

8

see http://www.epa.gov/crdlvweb /databases/datahome.htm

11

coarse-grained materials (permeable sands and gravels, for example), or in less permeable silts and

clays? At sites with poorly developed or thin soils as on hill slopes or in mountainous areas,

waste sites may be emplaced in partially weathered bedrock, in which case infiltration of leachates

carrying radionuclides may move easily down fractures in the bedrock to groundwater. A dual

permeability modeling approach is needed in sites with fractures; an “equivalent continuum”

model is not reliable for travel-time calculations.

The only geochemical process mentioned is the ion exchange behavior of soils, which is

only one of the geochemical processes or conditions of importance to radionuclide releases. 

Other processes include specific adsorption by solids in the waste or soil (for trace radionuclides,

generally more important than ion exchange), precipitation or co-precipitation of radionuclides

with solids, waste and soil redox conditions, the presence of substances that can form strong

complexes with radionuclides, increasing their mobilities, among others. Important soil and waste

characteristics that control radionuclide mobilities include: (1) reactive organic matter content6

and its amount relative to that of the radionuclides (as noted above); (2) temperature7

; (3) oxygen

flux rate; (4) moisture content and infiltration rate, and (5) the presence of solids in the soil that

are sorptive for actinides (e.g., Fe and Mn oxides, phosphates). All of these characteristics can

change along the flow path from source to well.

Although the Committee recognizes that ORIA is attempting a generic analysis that

cannot take into account all the site-specific characteristics of real sites or the properties of real

combinations of wastes, we recommend that ORIA take a broader look at the range of

radionuclide leaching and transport behaviors that might occur at real RCRA-C sites. Discussions

with modelers in the RCRA or Superfund programs could be helpful in this regard, as would a

perusal of the EPA Environmental Sciences Division’s web page on databases and software.8

This source contains several modeling structures that might help in exploring this issue. At a

minimum, ORIA should discuss what steps it has taken to determine the effect of the variables

mentioned above on model results and why it believes that using a short list of DOE site

characteristics in the PRESTO model will be sufficient.

3.2 Suitability of the 1000-year Time Frame (Question b)

The Committee concluded that a sensitivity analysis of the modeling time frame could

provide potentially useful information on the variation of peak dose with time horizon, especially

for less mobile radionuclides with long half-lives. The Committee did not reach consensus,

however, on how ORIA should weigh projections for the longer periods against the more reliable

projections for shorter periods. The factors that contributed to the Committee’s lack of

consensus centered on the consideration of four issues – scientific, future land use, public
12

acceptance, and economic viability. The comments below reflect considerations that should

influence ORIA’s ultimate choices about time frame. Because these comments represent the

different perspectives of individual Committee Members, they do not signify a Committee

consensus.

 3.2.1 Scientific Issues

The Committee addressed seven major scientific issues, outlined below

a) Capturing the time of peak dose: Clearly, some radionuclides with long halflives and low mobility will not attain maximum concentrations in groundwater

within 1,000 years, especially for the “dry” scenario with a very thick unsaturated

zone . The New York State Department of Environmental Conservation

performance assessment modeling in support of a LLRW disposal facility used a

10,000-year time frame. The results of this modeling showed the peak off-site

dose to the public occurred 1,200-2,000 years after closure for most near surface

disposal scenarios modeled. Whether or not a failure to capture the time of peak

dose will be significant from a scientific standpoint depends on the scenario and

policy assumptions that accompany the model (See land use and concrete

containment discussions). If the model time frame were extended to 10,000 years,

it would capture the peak dose for most if not all radionuclides, and it should also

account for the peak dose from in-growth of decay products.

b) Limited life of controls: On the basis of design conservatism of facilities and the

stress system in the near-surface environment in which they are operated, the

controls that may be exerted on radionuclide doses by RCRA-C facilities layers can

be assumed to be zero after a few hundred years. Maintenance activities under a

“perpetual care” commitment by the facility managers and natural hazards can

influence the specific time range. By the time the service life is attained, the

fatigue-type stresses that are common in the near-surface environment (freezethaw, dry-wet cycles, expansion-contraction, weathering, burrowing by animals,

possible physical erosion of covering soils, etc.) would have created and/or

extended flaws in the system to change the fluid entry and release mode from

permeation to pipe-flow. If the analysis is extended to 1,000 years or more, the

influence of containment would then be entirely ignored in the analysis of releases

after 300-400 years.

c) Definitions of failure: One of the most difficult questions to answer on

containment system performance is, “What is failure?” The failure of a component

(cover or liner) may not necessarily mean the failure of the system that contains

many components. Are the components connected structurally or functionally in

parallel or in series? Structural failure may not always mean functional failure. 

For the purposes of this project, ORIA should focus on a definition of system
13

functional failure that considers failure as the release of radionuclides sufficient to

cause a dose above the dose criterion in a person residing at a specified location

adjacent to the site.

d) Variable time frame as a function of LAMW composition: The range of the

possible radiological compositions of candidate wastes is not well defined. It

appears that there is greater uncertainty about which of the long-lived

radionuclides may be present in commercial mixed waste than about the short-lived

radionuclides. If there were no long-lived radionuclides, there would be no need

for modeling beyond 500 years. However, if long-lived radionuclides are present,

and the modeling out to 10,000 years demonstrates that the dose standard would

be exceeded at the time of the peak dose, the radionuclide con Therefore, one way

for ORIA to limit the modeling time frame is to verify that certain long-lived

radionuclides would not be present in commercial mixed waste, or alternatively

specify that they must not be present in wastes destined for RCRA-C disposal. 

Which radionuclides may be subject to such consideration could be identified by

conducting runs at several time scales (e.g., 100, 300, 1,000, 3,000, and 10,000

years) and comparing peak doses for the various runs.

e) Conservatism of modeling assumptions: Modeling beyond 1,000 years may be

unrealistically conservative and unrealistically uncertain. Among the conservative

assumptions are a) that all waste sites are always oxidizing, which ensures that

certain radionuclides are always modeled as mobile regardless of the waste form or

waste composition, and b) that no reactions or processes within the waste, in the

unsaturated zone, or in the groundwater system other than radioactive decay and

simple adsorption by soil will prevent the release of radionuclides to the

environment. In the presence of concrete, many of the so-called “mobile”

radionuclides in the waste would become less mobile, at least for short times.

f) Effect of climate on long-term predictions: A more comprehensive performance

assessment analysis might show that for some sites, radionuclide releases can be

predicted to be low and not an issue for times up to 1,000 years. Such sites are

likely to be in arid climates with thick, well drained unsaturated zones. It might be

necessary to limit LAMW disposal to such sites. In wet, humid climates, the

performance of sites for near-surface disposal of LAMW cannot be predicted

reliably for time spans beyond a few hundred years. Thus, radionuclides with

longer half lives, such as Np, would still be a problem.

g) Consistency with other waste regulations: The Agency should also consider

what time frame it would use if it regulated hazardous waste disposal with riskbased rather than technology-based rules. An illustrative example would be lead,

which has an infinite half-life, physical-chemical properties similar to many of the

naturally occurring radionuclides, and a dose-response relationship without an
14

established threshold. Certain carcinogenic heavy metals also could be considered. 

While NRC regulations require site characteristics be considered for a minimum of

500 years, most performance assessments for LLRW sites are run to 10,000 years. 

Should hazardous waste environmental fate analyses be modeled to 10,000 years?

 3.2.2 Future Health Risk and Land Ownership Considerations

The Committee discussed two primary issues:

a) Long-term societal changes: One thousand years may be unrealistically long

given the likelihood of substantial changes in public health and social conditions. 

Currently, cancer is the endpoint for health risk from radionuclides. It is certainly

possible that within a 1,000-year time frame, cancer will become a disease of less

concern because of advances in prevention or treatment; cancer as an endpoint

could become largely irrelevant. Likewise, society may view radiation risk in a

much different way. By suggesting modeling out to 10,000 years, we may not

even comprehend what forces will dominate the pathways for exposure. It is when

we consider future societal decision-making that the 10,000-year modeling

scenario appears speculative indeed.

b) Land ownership issues: Part 61 licensed LLRW sites must be owned by Federal

or State government following closure and a post-closure observation period. All

DOE facilities are on Federal government owned sites. It is very important to

consider site ownership by the Federal and State government when discussing

modeling time frames. While no government has lasted 10,000 years and

government agencies do not always meet their obligations as environmental

stewards, government ownership of a site is a clear commitment by a civilization to

maintaining the integrity of a LLRW disposal site over time.

RCRA-C facilities have no such requirement. Profit-making corporations own RCRA-C

facilities. They have different responsibilities to their shareholders, and any post-closure land use

scenario is possible, constrained only by local planning and zoning boards’ interpretation of

RCRA guidance. 

3.2.3 Public Acceptance

Although the Charge to the SAB is specific to providing an adequate technical basis for

setting regulatory limits, the Committee members have enough experience with public acceptance

of LLRW disposal facilities to comment on this aspect of deciding on appropriate modeling time

frames. It appears that modeling can be a useful tool for describing the behavior of radionuclides

at an RCRA-C site. Particularly important is the fact that the site is already in operation accepting

hazardous waste, and was not sited using Part 61 characteristics. Therefore it must be made

crystal clear to the public that the site will operate within radiation standards. If the site cannot be
15

shown to be “safe” at the time of the peak dose, it is likely not to win public acceptance. So if the

site is modeled to 1,000 years, the public may not be convinced that it should approve the disposal

of radioactive material there. However, if conservative assumptions show that, at the time of

peak dose, the site will still be adequate to protect public health, public acceptance is more likely. 

If the RCRA-C sites that are permitted to accept LAMW commit to “perpetual care,” acceptance

would be further enhanced.

Table 2 on pages 6 through 8 of the Modeling Time Frame document (U.S. EPA/ORIA,

1998g) has 30 of 48 radionuclides peaking at 1,000 years in the 1,000-year modeling period,

demonstrating that the peak dose was not captured during the 1,000-year time frame. Attachment

3 on page 3.1 of the same document has 129I and 99Tc peaks that are much higher at 10,000 years

than at 1,000 years. The public will surely notice that modeling to 1,000 years does not show

peak dose for all radionuclides. Therefore, modeling to 10,000 years would be useful.

On the other hand, the public may be accepting of model predictions of fate and transport

for a time frame of 100 years. The use of 10,000 years could raise serious questions concerning

the degree of uncertainty and possibly the credibility of the entire approach for estimating future

health risks.

 3.2.4 Economic Considerations

The entire premise of the EPA’s proposed LAMW disposal rule is to provide a lower cost

option for RCRA-C facilities to apply for Part 61 licenses so that they could also accept mixed

waste. This proposed rule will assist some research facilities, medical facilities, and utilities by

providing additional disposal options for mixed wastes. These facilities currently store this waste

and are waiting for a place to dispose of it in a cost-effective manner.

If the modeling to 10,000 years puts such constraints on the radionuclide concentrations

that no RCRA-C facility applies for the Part 61 license, the purpose for the proposed rule is lost. 

RCRA-C facilities have to be able to make a profit on the waste or they will not accept it. 

However, if modeling to 1,000 years does not convince the public to accept radioactive

wastes into RCRA-C facilities, the rulemaking will also be in vain. Currently, the most expensive

item in siting a new disposal facility is winning public acceptance. 

3.3 Modeling Release Rates (Question c)

The last Charge question asked the RAC to comment on EPA's approach to modeling

release rates over time, taking into consideration the available knowledge regarding concrete

durability and modes of degradation.

The Committee concluded that the key issue in this third question was the impact of the

deterioration assumption on the calculation of the peak annual dose.
16

 3.3.1 Technical Issues

In order to determine the limits for radionuclides in mixed waste to be disposed in RCRAC facilities, doses were calculated for unit concentrations of specific nuclides in the wastes. 

Doses were modeled assuming that the waste was solidified in concrete, using an inexpensive,

normal Portland cement. This assumption could be conservative as long as this type of treatment

is the minimum RCRA requirement. Use of special types of cement/concrete mixtures that would

minimize the potential for degradation, i.e., sulfate resistant cement (U.S. EPA/ORIA, 1998l,

citing SMI, 1998) or specific construction and reinforcement as described in the New York State

Final Generic Environmental Impact Statement for Low-Level Waste Landfills (NY,1993) could

be addressed in site-specific radioactive materials license conditions.

The release rates from concrete used in developing the dose estimates are based on the

assumption that the concrete degrades over time, releasing all radionuclides at a constant

(relative) rate. This rate was assumed to be 0.001 per year, based on the diffusion coefficient for

tritium in concrete. The release rate assumes a first-order reaction; that is, the rate of release is

proportional to the inventory remaining in the waste form. Values of 0.0005 per year and 0.005

per year for the relative release rate were also used in the dose calculations. The initial

concentrations in groundwater were determined using radionuclide-specific distribution

coefficients (Kd) for the concrete/water partitioning, and were derived from the literature. The

retardation factor in the concrete was applied in the calculation in the same manner as it is in the

calculation of retardation in soils.

The rate of release has a significant effect on the calculation of a peak dose via the

groundwater pathway. For 3H, the fractional increase in dose is approximately equal to the

fractional increase in release rate for tritium. For 129I, however, the dose increased by a factor less

than 2 when the release rate was increased by a factor of 5, indicating a non-linear response for

this radionuclide. ORIA states, however, that changing the release rate does not affect the year in

which the peak dose occurs for this radionuclide.

ORIA also states that the constant release rate assumption results in peak doses not

significantly different from the doses that would be calculated using a variable release rate, i.e., a

slow initial rate increasing over time as the concrete degrades. While degradation rates for

concrete could not be quantitatively determined from the available literature, progressive

degradation is a more reasonable scenario (U.S. EPA/ORIA, citing SMI, 1998). The Committee

recommends that a simulation be performed to verify, at least qualitatively, that the constant loss

rate assumption is reasonably--but not excessively--conservative, for all radionuclides. 

For example, an exponential deterioration model can be used to scale its structural

properties with respect to time (e.g., porosity).. Then for each time segment within the modeling

time frame, the relevant magnitude of the structural parameters can be fed into the selected

leaching model to estimate release rates. Monolith leaching models such as those that relate to
17

the American Nuclear Society’s American National Standards Institute (ANSI) 6.1 test can be

adapted for use. 

The concrete deterioration model also interacts with the containment failure models. 

There is no scientific basis for ORIA’s assumption that at the time the liner fails completely

(100% failure), the cover would have failed only by 10%. Actually, one could argue that the

cover is likely to fail before the liner because it is subject to extremes and reversals in temperature

and moisture conditions and access by burrowing animals and plant roots. A deterioration pattern

could be analyzed based on the transport-related characteristics of the containment system. Then,

depending on the time frame of concern, the relevant magnitude of the parameter could be fed

into the model instead of specifying an arbitrary failure rate or ratio (See Figure 1).

Although RCRA-C facilities incorporate polymeric membranes in both the cover (cap) and

lining systems, ORIA’s analysis focuses only on soil layers. Significant decreases in radionuclide

release rates would be obtained if engineering barriers were included in the analysis. Essentially,

the barrier system is the second line of defense for the radionuclides, after the concrete

encapsulation itself. Error in the release rates would propagate throughout the entire analysis,

including the calculation of dose rates.

For radionuclides with short half-lives, the doses could be significantly overestimated by

assuming a high constant loss rate. In the initial phases, when the concrete remains intact and the

loss rate is, in fact, very low, the short-lived radionuclides would decay before release, thus

producing very little, if any, dose. On the other hand, for longer-lived radionuclides that decay to

progeny with higher bioaccumulation potential and dose coefficients (e.g., 230Th to 226Ra), the

assumption that the loss rate is constant may not be conservative, depending on the effect of

retardation (Kd) as the radionuclides move through the concrete and soil matrices.

The emphasis on concrete degradation assumes that the treatment option for all of the

mixed waste potential candidates for disposal at RCRA sites would be solidification in concrete. 

For some wastes, such as contaminated soils, lead bricks or lab trash, solidification may not be the

optimal treatment. Under the Land Disposal Restrictions (LDR), moreover, some hazardous

wastes require treatments other than solidification in concrete. The Agency should either exclude

such wastes from the rulemaking on LAMW or assure the required LDR treatment is more

restrictive than solidification in concrete. For example, is the requirement for amalgamation of

mercury more restrictive over 1,000 or 10,000 years than solidification in concrete? The concrete

waste form may not be the most conservative assumption. Even for concrete, the possible

influence of the hazardous waste constituents on concrete stability should be considered.
18

Figure 1 Barrier Performance Assessment through Mathematical Modeling and

Monitoring (Provided courtesy of Dr. Hillary Inyang, Chair, SAB/EEC).

 

3.3.2 Science Policy Issues

The question of whether the most conservative assumptions for the disposal options are

the most effective in reducing overall risk should be addressed in the background considerations

for the RCRA mixed waste standards. Unduly restrictive limits for disposal at RCRA-C facilities

may significantly reduce the effectiveness of this option. Wastes that currently cannot or should

not be disposed at licensed LLW sites or conventional mixed waste facilities are generally stored

at or near the site of generation. Although such wastes are subject to inspections and other

requirements designed to ensure their safety during storage, indefinite accumulation and storage

of mixed waste at the site of generation clearly runs counter to current waste management

philosophy. Waste stored at the generator site is usually closer to workers and the public than 

would be the situation with disposed waste. This proximity increases the probability of human

contact with the waste as a consequence of normal handling, accidents and natural disasters and

thus may increase the health risks over what they would be if the wastes were disposed of at

RCRA-C facilities. The goal of this effort should be to reduce overall risk. Therefore,

unnecessary conservatism in setting disposal limits may be counter-productive.
19

The Committee recommends that ORIA investigate the types of waste that might fall

under the provisions of the proposed rule and the conditions under which they are currently stored

and, to the extent possible, characterize the health risks of plausible failure scenarios. The risk

assessment principles should be similar to those used in the present modeling exercise, in the sense

of using similar degrees of conservatism. Ideally, both analyses would have elements of a

quantitative uncertainty analysis. Then, the probability of improving the overall risk balance for

different sets of waste acceptance criteria could be studied. (Note: this comment applies to any

assumption posited on the grounds of conservatism.) 

3.4 Additional Issues

 3.4.1 Modeling Structure

ORIA has selected the PRESTO model to conduct the acceptance criteria modeling. 

Although the Committee understands that PRESTO has a long history of use in the Agency and

that ORIA has considerable confidence in the applicability of its results for the intended purpose,

we advise that ORIA be prepared to justify its use in the Technical Support Document that will

eventually be produced to support any rulemaking. Such justification might include answers to

the following questions:

a) Has it been benchmarked (e.g., through BIOMASS or another effort for crosscomparison of models)?

b) Can it adequately simulate the role of geochemical processes in enhancing or

retarding radionuclide transport (not just Kd and radioactive decay)?

c) Can it consider the effects of variations in the composition and form of mixed

waste as emplaced on radionuclide releases?

d) Does it account for the ingrowth of decay products after disposal of the parent

radionuclide for which the criteria are calculated? (Decay products are not

included in dose tabulations in some EPA publications.)

e) Is it particularly suited for the spatial scale of the analysis (e.g., a potable water

well 50 m from the edge of the RCRA cell)?

f) Has it been updated and enhanced to be consistent with the current state-of-the-art

of multimedia health risk models? With RCRA modeling assumptions?

Any other assurances of its applicability and reliability would also be welcome. For

example, ORIA might consider using a second relevant model as a check on the doses predicted

for unit concentration of each radionuclide. Reasonable agreement would support the choice of
20

PRESTO. Disagreement, however, would require ORIA to investigate the causes of the

disagreement and make informed choices about any needed adjustments to its modeling.

ORIA stated that it would use the “unity rule” for judging the acceptability of LAMW

containing more than one relevant radionuclide. Under the unity rule, the concentration of each

radionuclide in a candidate waste would be divided by the corresponding criterion concentration

and these fractions would be summed over all radionuclides. If the sum of fractions is less than

1.0, the waste would be found acceptable for disposal. This rule may be unnecessarily restrictive

for certain combinations of radionuclides. For example, if a waste contained both 3H and 14C at

their respective concentration limits based on the reference dose, application of the unity rule

would prohibit its disposal at a wet RCRA-C facility (e.g., one with characteristics similar to

Fernald). However, the peak dose for 3H is reached in about 100 years but the peak dose for 14C

is not reached until 1,000 years or more after the liner is breached. Therefore, the dose to an

exposed individual could not exceed the reference dose in any year. To overcome this unintended

consequence, radionuclides could be classified according to similar peak dose time frames and the

unity rule applied only within those time classes. The Committee recognizes that such a

procedure would increase the complexity of the contemplated rule and might appear to be in

conflict with similar NRC procedures. 

Presumably, the eventual Technical Support Document will describe more clearly the

process of determining acceptance criteria in concentration units starting with a policy choice

about the acceptable reference dose. If the Committee understands ORIA’s intent correctly,

concentration would be the principal if not the only determining criterion for waste acceptance. 

However, it seems likely that the potential peak dose for a given concentration in waste would

also depend on the total quantity of wastes of that concentration deposited in the modeled site. 

ORIA appears to be currently assuming that only a small fraction of the total waste in the RCRAC cell would be LAMW at the criterion concentration. But there appears to be no restriction

against a larger fraction nor any credit given for a smaller fraction. ORIA might consider

additional limitations on the total quantity of any one long-lived Radionuclide allowable at a

RCRA-C facility of given capacity. If it does so, it should also consider the implications of such a

limitation on the unity rule.

Some pathways of exposure seem to have been excluded from the modeling. For

example, it is not clear that evolution of radon or other volatiles from potable water use in the

home has been treated, and it is clear that soil ingestion and dermal contact with soil have not

been included, even though the on-site residential (“immobile”) scenario postulates excavation of

soil from the disposal cell during construction. While dermal absorption of inorganic hazardous

chemicals is usually unimportant, the same is not always true for soil ingestion, which is frequently

more important than soil re-suspension, a modeled pathway

Many of the existing RCRA-C disposal facilities have SWMUs that contain non-stabilized

hazardous wastes. EPA should consider whether or not these wastes could enhance the

degradation of LAMW containment or the mobility of radionuclides after release. Even for the
21

licensing of new facilities with stabilized wastes, the potential for enhanced mobility from the codisposal of hazardous and radioactive wastes should be considered.

For the mobile (groundwater) scenario, there are different results for the assumption of

segregated vs. random emplacement. However for the immobile (intruder) scenario, separate

calculations are not made for assumptions of segregated vs. random emplacement. Because the

immobile scenario postulates an intruder, exposure would seem to be strongly dependent upon

whether the waste was segregated or placed randomly, suggesting that separate calculations are

appropriate. The detailed assumptions for the segregated vs. random emplacement should be

better described.

On p.2 of the Overview (U.S. EPA/ORIA, 1998b), EPA apparently is using a 1990 NRC

survey published in 1992 as NUREG/CR-5938 (U.S. NRC, 1992) to estimate the volume of

commercial mixed waste generated in the U.S.. Pollution prevention and waste minimization

efforts have significantly reduced annual mixed waste volumes since then. On the other hand, the

decontamination and decommissioning of nuclear power plants and naval reactors in the next ten

years may result in a significant increase in commercial mixed waste volumes.

On p.11 of the Overview (U.S. EPA/ORIA, 1998b), 3H and 14C in waste gases are

assumed to be negligible due to prior treatment by incineration. Based on the large volumes of

wastes containing these radionuclides being generated at commercial sites, this assumption needs

to be justified. Also, EPA should consider modeling root uptake of these radionuclides in cover

vegetation.

On p.13 of the Overview (U.S. EPA/ORIA, 1998b), it is stated that a subset of

radionuclides from Table 4 were included in the mobile modeling scenario. 90Sr was not included

in the list of the mobile radionuclides, and was listed in Table 7 as a Radionuclide that resulted in

no doses from mobile or immobile scenarios. EPA should reexamine the scenarios in view of

90Sr's historic problems at LLRW sites.

 3.4.2 Science Policy Issues

Given the large range in the results for the different environments modeled, one has to

question the utility of a single set of Radionuclide concentration limits, which would, of necessity,

have to apply to the worst possible environment. The large differences for the “dry” and “wet”

sites suggest an approach of developing at least two sets of concentration limits. For example,

Type 1 sites could include sites in arid climates with deep, well drained unsaturated zones and

sites in semi-arid climates in groundwater recharge zones with deep, well drained soils if the latter

were capped with compacted clay, contoured to export runoff, and vegetated to minimize

infiltration. It would be easier to predict the long-term performance of such sites than sites in

wetter settings. Type 1 sites could perhaps accept long-lived radionuclides. Type 2 sites would

be those in wetter settings with a shallower groundwater table. Because of the difficulty of

defensibly predicting their performance for long times, and the greater risk of radionuclide
22

releases, Type 2 sites might be limited to the disposal of LAMW containing only short-lived

radionuclides. In addition, it should be made possible for an applicant to demonstrate, through

site-specific modeling, that the dose limit could be met with alternative concentrations. Again, the

Committee recognizes the additional complexity of such a procedure and that it might appear to

be in conflict with similar NRC procedures.

In its eventual technical product, ORIA should attempt to tie together the modeling

assumptions and health risk criteria between the hazardous chemical constituents and the

radioactive constituents of the mixed waste. The proposal must be seen to be consistent with the

objectives of the current RCRA protection system even if not with its specific requirements. The

Technical Support Document should provide background on the similarities and differences

between chemical and radiation risk management and how the modeling dealt with them.

As described to the Committee, the LAMW proposal would apply to a rather limited

range of wastes generated by “commercial” activities and classified as both RCRA hazardous

waste and AEA Class A LLRW, and the only new disposal alternative would be in a RCRA-C

facility. However, the modeling effort has many additional potential uses, including decisions

about:

a) disposal of wastes containing RCRA hazardous and TENORM, which are not

regulated under the AEA,

b) disposal of wastes containing Toxic Substances Control Act (TSCA) substances

and either AEA or TENORM radioactive constituents,

c) disposal of qualifying Class A radioactive wastes that do not contain RCRA

hazardous components at RCRA-C facilities, and

d) defining “de-minimis” concentrations for radionuclides that would enable nonhazardous very low level radioactive wastes to be disposed in RCRA-D (sanitary)

landfills.

The Committee recommends that these potential uses be kept in mind when making

modeling choices for the current proposal. ORIA should also examine the forthcoming NCRP

report on waste classification for other possible uses. Having a wider range of modeling uses will

better justify the time and effort spent, which may otherwise seem excessive for the limited

quantities of waste currently thought to fall under the proposed rule. In making this suggestion,

the Committee is well aware that there may be legal, economic, or political reasons for EPA to

disallow disposal of some of these wastes in RCRA facilities even if they meet the dose criterion.

The Committee recommends that the eventual Technical Support Document discuss how

the Agency has responded not only to these comments but also to the RAC’s 1985 recommendations (SAB, 1985).
23

 3.4.3 Other Policy Issues

The EPA overview document notes that “reference doses” -- presumably the doses that

will limit the allowable concentrations of radionuclides in the qualifying waste--are still to be

determined. The Committee notes that the goal of risk harmonization would be furthered if such

doses corresponded to risk levels consistent with those that would be used as guidelines for the

regulation of hazardous chemical waste (SAB, 1992).

A waste that contains radioactive materials AND hazardous constituents is clearly more

hazardous than a waste that contains the same levels of only the radioactive or only the hazardous

components. On purely scientific grounds, it is not logical to allow a mixed waste to be deposited

in a RCRA-C facility when a purely radioactive waste of identical radionuclide content would not

be permitted. The Committee recognizes that EPA may wish to discourage disposal of purely

radioactive waste that would otherwise meet the acceptance criteria for LAMW because it might

disturb established low-level radioactive waste policies or Compact relationships. Such a policy,

however, might encourage waste generators to manage wastes so that they could be disposed of

as LAMW instead of LLRW.

The cost of disposal of LLRW at a Part 61 licensed site is much higher than that for

hazardous wastes at RCRA-C facilities. In addition, access to the nation’s LLRW disposal sites

for many states is very tenuous. Thus, LLRW generators may find disposal of LLRW as LAMW

in a RCRA-C facility is cheaper and possibly their only disposal option. Avoiding this perverse

incentive may prove difficult under the LAMW rule as currently contemplated.

It is not clear to the Committee that EPA has considered whether the LAMW rule would

cover mixed radioactive and TSCA wastes (PCBs, asbestos) or mixed wastes containing uranium

mill tailings.

The Agency should consider whether accepting radioactive wastes at a RCRA-C facility

changes the liability between radioactive waste generators and disposal facility operators. The

issue of “taking possession of the radioactive materials” in the LAMW needs to be resolved.

Since RCRA-C facilities currently accept foreign hazardous wastes, will mixed wastes

from other nations be allowed to be disposed of under the LAMW rule? In particular, can

LAMW from Canada and Mexico be excluded under NAFTA? How does this affect EPA’s

estimate of the quantity of waste that might fall under the proposed rule?

Part 61.56(a)(7) allows gaseous LLRW disposal under 1.5 atmospheres at 20 degrees

Celsius. Will the LAMW rule allow for the disposal of mixed gaseous wastes?
A-1

APPENDIX A - DETAILED COMMENTS

a) In any final documentation of the modeling, SI units should be used with the traditional

units in parentheses.

b) In the final work product, the Agency should be sure that undue certainty is not implied by

the use of too many significant figures.

c) Clarify what is meant by dose. Several different wordings appeared in the Committee’s

briefing materials, most of which appeared to be synonymous. If differences are intended,

the nomenclature should be clearly defined. If not, uniformity is desirable.

d) The meaning of commercial is not obvious. It seems to be used in one sense to qualify the

facility and in another to qualify the waste. The use of this term should be defined.

e) If the principal focus remains on wastes that are regulated under the AEA, consider

eliminating from the lists radionuclides such as 40K that probably are never considered

germane to AEA control.

f) Only cancer is considered a relevant endpoint for risk assessment. ORIA should be careful

to justify why it believes that other potential effects of radiation, such as birth defects, are

not relevant at the dose criterion eventually selected.

g) Some of the data in Attachment 1 for the Idaho National Engineering Environmental

Laboratory (INEEL) appear to be in error, unless a great deal of conservatism is intended. 

The thickness of the vadose zone at INEEL is much greater that 41 m, unless perched

water bodies are meant, or basalt is excluded. At the current radioactive facility at

INEEL, the depth to the aquifer is nearly 200 meters. The thickness of the aquifer is much

greater than 12 m, and in fact is so thick that its thickness is unknown. The depth of the

aquifer is much greater than 12 m, in fact it is so deep that it is unknown. Is 12 m an

artificial thickness? Although the INEEL was not chosen as one of the three sites used to

bound the various scenarios, these possible discrepancies suggest that the data used for the

model parameters at the other sites should be examined critically, or at least explained. 

Similarly, the climatic description for Rocky Flats appears questionable. Note also

inconsistencies between Attachment 1 and Table 2 of the Overview regarding depth of

vadose zone (U.S. EPA/ORIA, 1998b).

h) In Table 2 on page 9 of the overview, the meaning of 31/300 for “end of events” is unclear

(U.S. EPA/ORIA, 1998b). 

i) In Table 2 on page 5 of the generic characterization document, the qualitative descriptions

should be accompanied by numeric ranges to avoid confusion (U.S. EPA/ORIA, 1998f).
B-1

APPENDIX B - TOTAL SYSTEM PERFORMANCE ASSESSMENT

APPROACH

The proposed high level waste repository for commercial spent fuel at Yucca Mountain

would be emplaced hundreds of meters below the land surface above the water table in a relatively

dry and geologically stable setting. Even so, predicting the fate of radionuclides to be buried in

the repository for times of 1,000 yrs or longer has been highly contentious. After many years of

being criticized because their predictive approach did not properly consider and weigh the relative

importance of all important site variables and processes, the DOE adapted a comprehensive Total

System Performance Assessment (TSPA) modeling approach. This approach has led to

predictions of site performance including radionuclide release rates in terms of their probability. 

ORIA might consider using a similar approach, being careful to select the important

variables that will influence future radionuclide release rates from mixed waste sites. The

importance to radionuclide release rates of the detailed composition and behavior of the wastes

themselves or of waste and waste product interactions with surrounding geological materials

cannot be overemphasized. Predicting the performance of waste sites that contain organics,

metals, metalloids and radionuclides for times of 100 to 1,000 yrs or longer must be based on a

probability analysis which will reflect increasing uncertainty of performance for longer times. A

TSPA modeling approach provides the formal structure on which to hang such a predictive

analysis. TSPA calculations also direct the user towards the most important properties of a

possible disposal site and the most important reactions, for example that control radionuclide 

release rates. The public may also be more accepting of performance predictions developed and

defended through TSPA (TRW, 1995). The DOE TSPA model is relatively complex. The

Electric Power Research Institute (EPRI) has developed a simpler TSPA model which may be

adequate for ORIA’s purposes (EPRI, 1990).
C-1

APPENDIX C - GLOSSARY OF TERMS AND ACRONYMS

AEA Atomic Energy Act

ANS American Nuclear Society

ANSI American National Standards Institute

BIOMASS Biomass Modeling and Assessment Program, sponsored by the International

Atomic Energy Agency and others. It addressees radiological issues associated

with accidental and routine releases of radionuclides into the environment, and

solid waste management. It covers three themes related to environmental

modeling and safety assessment, which are radioactive waste disposal,

environmental releases, and biosphere processes. 

14C Carbon-14, an isotope of carbon with an atomic mass of 14 and a half-life of 5715

years 

DOE U.S. Department of Energy (U.S. DOE)

EEC Environmental Engineering Committee (of the U.S. EPA/SAB/EEC)

EHC Environmental Health Committee (of the U.S. EPA/SAB/EHC)

EPA U.S. Environmental Protection Agency (U.S. EPA, EPA, or “the Agency”)

EPRI Electric Power Research Institute

Fe Iron (Ferric, Ferrous Oxides)

3H Tritium (A radioactive isotope of Hydrogen with an atomic mass of 3 and a halflife of 12.5 years)

129I Iodine-129, an isotope of iodine with an atomic mass of 129 and a half-life of 16

million years 

INEEL Idaho National Engineering Environmental Laboratory 

Kd Distribution/Retardation Coefficient

40K Potassium, an isotope of potassium with an atomic mass of 40

LAMW Low Activity Mixed Waste

LAMRW Low Activity Mixed Radioactive Waste

LDR Land Disposal Restrictions

LLRW Low Level Radioactive Waste

LLW Low Level Waste 

m Meter

Mn Manganese

NAFTA National Free Trade Agreement 

NCRP National Council on Radiation Protection and Measurements

NORM Naturally-Occurring Radioactive Material

Np Neptunium (A naturally radioactive element with an atomic number of 93. The

longest-lived isotope is 237Np) 

NRC U.S. Nuclear Regulatory Commission (U.S. NRC)

NUREG/CR U.S. Nuclear Regulatory Commission/ Commission Report 

ORIA Office of Radiation and Indoor Air (U.S. EPA)
C-2

PATHRAE Computer code used to assess the maximum annual dose to a critical population

group resulting from waste disposal. It is a member of the PRESTO-EPA family

of codes and emphasizes two areas: (1) the addition of exposure pathways

pertaining to on-site workers during disposal operations, off-site personnel after

site closure, and reclaimers and inadvertent intruders after site closure; and (2) the

simplification of the sophisticated dynamic submodels to quasi-steady state

submodels.

Peak Dose Highest annual committed effective dose projected to be received by an off-site

receptor at a specified location.

PCB Polychlorinatedbiphenyl

pH Negative log of hydrogen ion concentration

PRESTO A family of codes developed to evaluate doses resulting from the disposal of lowactivity radioactive waste. These codes include PRESTO-EPA-CPG (assesses

annual effective dose equivalents to a critical population group), PRESTO-EPADEEP (assesses cumulative population health effects resulting from the disposal of

low-activity waste using deep geologic repositories), PRESTO-EPA-BRC

(assesses cumulative population health effects to the general population residing in

the downstream regional basin as a result of the disposal of low-activity waste in

an unregulated sanitary landfill), PRESTO-EPA-POP (assesses cumulative

population health effects to the general population residing in the downstream

regional basin on a low-level waste site), and PATHRAE (see above). 

226Ra Radium-226, an isotope of radium with an atomic mass of 226 and a half-life of

1,599 years, produced from alpha-decay of thorium-230 

RAC Radiation Advisory Committee (of the U.S. EPA/SAB/RAC)

RCRA Resource Conservation and Recovery Act, including its various subtitles

SAB Science Advisory Board (of the U.S. EPA/SAB)

Se Selinium

SI International System of Units (System Internationale)

SMI Shepherd-Miller, Inc.

SWMU Solid Waste Management Unit

90Sr Strontium, the strontium isotope with a mass of 90, having a half-life of 28 years

TENORM Technologically Enhanced Naturally Occurring Radioactive Material

99Tc Technetium (Tc) as an element, or Technetium-99, an isotope of technetium with

an atomic mass of 99 and a half-life of 213,000 years

230Th Thorium, as an element or isotope (e.g., 228Th, 230Th, 232 Th, 234 Th)

TSCA Toxic Substances Control Act

TSPA Total System Performance Assessment

U Uranium, including the isotopes 235U with an atomic mass of 235 and a half-life of

7.13x108 years, and the most common 238U with an atomic mass of 238 and a halflife of 4.51x109

 years.

U.S. United States

UZ Unsaturated Zone
R-1

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High-Level Waste Repository Evaluation,” EPRI NP-7057 Project 3055-2 Final Report,

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Level Waste Program, Nuclear Power Division, Palo Alto, CA, (October)

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NY (New York Department of Environmental Conservation). 1993. Final Generic

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R-2

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R-3

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(December)
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