**ENVIRONMENTAL PROTECTION AGENCY**

**40 CFR PART 197**

**\[EPA-HQ-OAR-2005-0083; FRL-\]**

**\[RIN 2060-AN15\]**

**PUBLIC HEALTH AND ENVIRONMENTAL RADIATION PROTECTION STANDARDS FOR
YUCCA MOUNTAIN, NEVADA**

**AGENCY:** Environmental Protection Agency (EPA)

**ACTION:** Final Rule

**SUMMARY:** Number range CHAPTERWe, the Environmental Protection Agency
(EPA), are promulgating amendments to our public health and safety
standards for radioactive material stored or disposed of in the
potential repository at Yucca Mountain, Nevada. Section 801(a) of the
Energy Policy Act of 1992 (EnPA, Public Law No. 102-486) directed us to
develop these standards. Section 801 of the EnPA also required us to
contract with the National Academy of Sciences (NAS) to conduct a study
to provide findings and recommendations on reasonable standards for
protection of the public health and safety. The health and safety
standards promulgated by EPA are to be "based upon and consistent with"
the findings and recommendations of NAS. On August 1, 1995, NAS released
its report (the NAS Report), titled \"Technical Bases for Yucca Mountain
Standards.\" These standards (the 2001 standards) were originally
promulgated on June 13, 2001 (66 FR 32074). In promulgating our
standards, we considered the NAS Report as the EnPA directs.

On July 9, 2004, in response to a legal challenge led by the State of
Nevada and the Natural Resources Defense Council, the U.S. Court of
Appeals for the District of Columbia Circuit vacated portions of our
standards that addressed the period of time for which compliance must be
demonstrated. The Court ruled that the compliance period of 10,000 years
was not "based upon and consistent with" the findings and
recommendations of the NAS and remanded those portions of the standards
to us for revision. These remanded provisions are the subject of this
action.

This final rule incorporates multiple compliance criteria applicable at
different times for protection of individuals and in circumstances
involving human intrusion into the repository. Compliance will be judged
against a standard of 150 microsieverts per year (μSv/yr) (15 millirem
per year (mrem/yr)) committed effective dose equivalent at times up to
10,000 years after disposal and against a standard of 3.5 millisieverts
per year (350 mrem/yr) committed effective dose equivalent at times
after 10,000 years and up to 1 million years after disposal. This final
rule also includes several supporting provisions affecting DOE's
performance projections. DOE will calculate the arithmetic mean of the
distribution of doses, will calculate those doses using updated
scientific factors, and will incorporate specific direction on analyzing
features, events, and processes that may affect performance.

Section 801(b) of the EnPA requires the Nuclear Regulatory Commission
(NRC) to modify its technical requirements for licensing of the Yucca
Mountain repository to be consistent with the standards promulgated by
EPA. In 2001, NRC incorporated EPA's Yucca Mountain standards into its
licensing regulations and the compliance period provision of these was
similarly vacated by the Court of Appeals. NRC must revise its licensing
regulations to be consistent with our amended standards. The Department
of Energy (DOE) plans to submit a license application providing a
compliance demonstration. The NRC will determine whether DOE has
demonstrated compliance with NRC's licensing regulations, which must be
consistent with our standards, prior to granting or denying the
necessary licenses to dispose of radioactive material in Yucca Mountain.

**DATES:** *Effective Date:* This final rule is effective on \[insert
date that is 30 days from date of publication\].

**ADDRESSES:** Number range CHAPTER EPA has established a docket for
this action under Docket ID No. **EPA-HQ-OAR-2005-0083**. All documents
in the docket are listed on the
[www.regulations.gov](http://www.regulations.gov/) web site. Although
listed in the index, some information is not publicly available, e.g.,
Confidential Business Information (CBI) or other information whose
disclosure is restricted by statute. Certain other material, such as
copyrighted material, is not placed on the Internet and will be publicly
available only in hard copy form. Publicly available docket materials
are available either electronically through
[www.regulations.gov](http://www.regulations.gov/), for purchase or
access from sources identified in the docket (Docket Nos.
EPA-HQ-OAR-2005-0083-0086 and EPA-HQ-OAR-2005-0083-0087), or in hard
copy at the Air and Radiation Docket, EPA/DC, EPA West, Room 3334, 1301
Constitution Ave., NW, Washington, DC. The Public Reading Room is open
from 8:30 a.m. to 4:30 p.m., Monday through Friday, excluding legal
holidays. The telephone number for the Public Reading Room is (202)
566-1744.

**FOR FURTHER INFORMATION CONTACT:** Number range CHAPTERRay Clark,
Office of Radiation and Indoor Air, Radiation Protection Division
(6608J), U.S. Environmental Protection Agency, 1200 Pennsylvania Ave.,
NW, Washington, D.C. 20460-0001; telephone number: 202-343-9601; fax
number: 202-343-2305; e-mail address: clark.ray@epa.gov.

**SUPPLEMENTARY INFORMATION:**

Number range CHAPTER

**I. General Information**

A. [Does this Action Apply to Me?]{.underline}

Number range CHAPTER The DOE is the only entity regulated by these
standards. Our standards affect NRC only because, under Section 801(b)
of the EnPA, 42 U.S.C. 10141 n., NRC must modify its licensing
requirements, as necessary, to make them consistent with our final
standards. Before it may construct the repository or accept waste at the
Yucca Mountain site, DOE must obtain a license from NRC. DOE will be
subject to NRC's modified regulations, which NRC will implement through
its licensing proceedings.

B. [How Can I View Items in the Docket?]{.underline}

1\. [Information Files.]{.underline} EPA is working with the Lied
Library at the University of Nevada-Las Vegas
([http://www.library.unlv.edu/about/hours.html](http://www.library.unlv.edu/about/hours.html#desks))
and the Amargosa Valley, Nevada public library
([http://www.amargosalibrary.com](http://www.amargosavalley.com/Library.html))
to provide information files on this rulemaking. These files are not
legal dockets; however, every effort will be made to put the same
material in them as in the official public docket in Washington, DC. The
Lied Library information file is at the Research and Information Desk,
Government Publications Section (702-895-2200). Hours vary based upon
the academic calendar, so we suggest that you call ahead to be certain
that the library will be open at the time you wish to visit (for a
recorded message, call 702-895-2255). The other information file is in
the Public Library at 829 East Farm Road in Amargosa Valley, Nevada
(phone 775-372-5340). As of the date of publication, the hours are
Monday through Friday (9 a.m.-5 p.m.) and Saturday (9 a.m.-1 p.m.). The
library is closed on Sunday. These hours can change, so we suggest that
you call ahead to be certain when the library will be open.

2\. [Electronic Access.]{.underline} An electronic version of the public
docket is available through the Federal Docket Management System at
www.regulations.gov. You may use
[www.regulations.gov](http://www.regulations.gov/) to view comments,
access the index listing of the contents of the official public docket,
and to access those documents in the public docket that are available
electronically. To access the docket go directly to
[http://www.regulations.gov](http://www.regulations.gov/) and select
"All Documents." In the ID window, type in the docket identification
number EPA-HQ-OAR-2005-0083 and click on "Submit." Please be patient
since the search could take several minutes. This will bring you to the
"Docket Search Results" page. From there, you may access the docket
contents (e.g., EPA-HQ-OAR-2005-0083-0002) by clicking on the icon in
the "Views" column.

C. [Can I Access Information by Telephone or Via the
Internet?]{.underline}

Yes. You may call our toll-free information line (800-331-9477) 24 hours
per day. By calling this number, you may listen to a brief update
describing our rulemaking activities for Yucca Mountain, leave a message
requesting that we add your name and address to the Yucca Mountain
mailing list, or request that an EPA staff person return your call. In
addition, we have established an electronic listserv through which you
can receive electronic updates of activities related to this rulemaking.
To subscribe to the listserv, go to
https://lists.epa.gov/read/all_forums. In the alphabetical list, locate
"yucca-updates" and select "subscribe" at the far right of the screen.
You will be asked to provide your e-mail address and choose a password.
You also can find information and documents relevant to this rulemaking
on the World Wide Web at
[[http://www.epa.gov/radiation/yucca.]{.underline}](http://www.epa.gov/radiation/yucca.)
The proposed rule for today's final rule appeared in the Federal
Register on August 22, 2005 (70 FR 49014). We also recommend that you
examine the preamble and regulatory language for the earlier proposed
and final rules, which appeared in the [Federal Register]{.underline} on
August 27, 1999 (64 FR 46976) and June 13, 2001 (66 FR 32074),
respectively.

D. [What Documents are Referenced in Today's Final Rule?]{.underline}

We refer to a number of documents that provide supporting information
for our Yucca Mountain standards. All documents relied upon by EPA in
regulatory decision-making may be found in our docket
(EPA-HQ-OAR-2005-0083). Other documents, e.g., statutes, regulations,
and proposed rules, are readily available from public sources. The
documents below are referenced most frequently in today's final rule.

Item No. (EPA-HQ-OAR-2005-0083-xxxx)

0044 "Safety Indicators in Different Time Frames for the Safety
Assessment of Underground Radioactive Waste Repositories," International
Atomic Energy Agency TECDOC-767, 1994

0045 "Regulatory Decision Making in the Presence of Uncertainty in the
Context of Disposal of Long Lived Radioactive Wastes," International
Atomic Energy Agency TECDOC-975, 1997

0046 "The Handling of Timescales in Assessing Post-Closure Safety:
Lessons Learnt from the April 2002 Workshop in Paris, France," Nuclear
Energy Agency (Organisation for Economic Co-operation and Development),
2004

0051 "Geological Disposal of Radioactive Waste," International Atomic
Energy Agency Draft Safety Requirements (DS154), April 2005

0061 "Principles and Standards for Disposal of Long-Lived Radioactive
Wastes," Neil Chapman and Charles McCombie, Elsevier Press, 2003

0062 "An International Peer Review of the Yucca Mountain Project
TSPA-SR," Joint Report by the OECD Nuclear Energy Agency and the
International Atomic Energy Agency, OECD, 2002

0076 Technical Bases for Yucca Mountain Standards (the NAS Report),
National Research Council, National Academy Press, 1995

0077 "Assessment of Variations in Radiation Exposure in the United
States," EPA Technical Support Document, July 2005

0085 "Assumptions, Conservatisms, and Uncertainties in Yucca Mountain
Performance Assessments," EPA Technical Support Document, July 2005

0086 DOE Final Environmental Impact Statement, DOE/EIS-0250, February
2002

xxxx Response to Comments Document for Final Rule, EPA-xxxx, December
2006

**Acronyms and Abbreviations**

Number range CHAPTER We use many acronyms and abbreviations in this
document. These include:

BID-background information document

CED-committed effective dose

CEDE-committed effective dose equivalent

CFR-Code of Federal Regulations

DOE-U.S. Department of Energy

DOE/VA-DOE's Viability Assessment

EIS-Environmental Impact Statement

EnPA-Energy Policy Act of 1992

EPA-U.S. Environmental Protection Agency

FEIS-Final Environmental Impact Statement

FEPs-features, events, and processes

FR-[Federal Register]{.underline}

GCD-greater confinement disposal

HLW-high-level radioactive waste

HSK-Swiss Federal Nuclear Safety Inspectorate

IAEA-International Atomic Energy Agency

ICRP-International Commission on Radiological Protection

KASAM- Swedish National Council for Nuclear Waste

LLW-low-level radioactive waste

MCL-maximum contaminant level

MTHM-metric tons of heavy metal

NAPA- National Academy of Public Administration

NAS-National Academy of Sciences

NEA-Nuclear Energy Agency

NEI-Nuclear Energy Institute

NRC-U.S. Nuclear Regulatory Commission

NRDC-Natural Resources Defense Council

NTS-Nevada Test Site

NTTAA-National Technology Transfer and Advancement Act

NWPA-Nuclear Waste Policy Act of 1982

NWPAA-Nuclear Waste Policy Amendments Act of 1987

OECD-Organization for Economic Cooperation and Development

OMB-Office of Management and Budget

RMEI-reasonably maximally exposed individual

SSI-Swedish Radiation Protection Authority

SNF-spent nuclear fuel

SR-Site recommendation

TRU-transuranic

TSPA-Total System Performance Assessment

UK-United Kingdom

UMRA-Unfunded Mandates Reform Act of 1995

U.S.C.-United States Code

WIPP LWA-Waste Isolation Pilot Plant Land Withdrawal Act of 1992

**Outline of Today's Action**

I. What is the History of This Action?

A. Promulgation of 40 CFR part 197 in 2001

B. Legal Challenges to 40 CFR part 197

II\. Summary of Proposed Amendments to 40 CFR part 197 and Public
Comments

A. How Did We Propose to Amend Our 2001 Standards?

B. What Factors Did We Consider in Developing Our Proposal?

C. In Making Our Decisions, How Did We Incorporate Public Comments on
the Proposed Rule?

D. What Comments Did We Receive?

III\. What Final Amendments Are We Issuing With This Action?

A. What Dose Standards Will Apply?

1\. What is the Dose Standard for 10,000 Years After Disposal?

2\. What is the Dose Standard Between 10,000 Years and 1 Million Years?

3\. How Did We Consider Background Radiation in Developing the Peak Dose
Standard?

4\. How Does Our Rule Protect Future Generations?

5\. How Did We Consider Uncertainty and Reasonable Expectation?

6\. What is Geologic Stability and Why is it Important?

7\. Why is the Period of Geologic Stability 1 Million Years?

8\. How Will NRC Judge Compliance?

9\. How Will DOE Calculate the Dose?

B. How Will This Final Rule Affect DOE's Performance Assessments?

IV\. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

B. Paperwork Reduction Act

C. Regulatory Flexibility Act

D. Unfunded Mandates Reform Act

E. Executive Order 13132: Federalism

F. Executive Order 13175: Consultation and Coordination with Indian
Tribal Governments

G. Executive Order 13045: Protection of Children from Environmental
Health & Safety Risks

H. Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution, or Use

I. National Technology Transfer and Advancement Act

J. Congressional Review Act

**I. What is the History of This Action?**

Number range CHAPTER

Radioactive wastes result from the use of nuclear fuel and other
radioactive materials. Today, we are revising certain standards
pertaining to spent nuclear fuel, high-level radioactive waste, and
other radioactive waste (we refer to these items collectively as
\"radioactive materials\" or \"waste\") that may be stored or disposed
of in the Yucca Mountain repository. (When we discuss storage or
disposal in this document in reference to Yucca Mountain, we note that
no decision has been made regarding the acceptability of Yucca Mountain
for storage or disposal as of the date of this publication. To save
space and to avoid excessive repetition, we will not describe Yucca
Mountain as a \"potential\" repository; however, we intend this meaning
to apply.) Pursuant to Section 801(a) of the Energy Policy Act of 1992
(EnPA, Public Law No. 102-486), these standards apply only to facilities
at Yucca Mountain.

Once nuclear reactions have consumed a certain percentage of the uranium
or other fissionable material in nuclear reactor fuel, the fuel no
longer is useful for its intended purpose. It then is known as \"spent\"
nuclear fuel (SNF). It is possible to recover specific radionuclides
from SNF through "reprocessing," which is a process that dissolves the
SNF, thus separating the radionuclides from one another. Radionuclides
not recovered through reprocessing become part of the acidic liquid
wastes that the Department of Energy (DOE) plans to convert into various
types of solid materials. High-level waste (HLW) is the highly
radioactive liquid or solid wastes that result from reprocessing SNF.
The SNF that does not undergo reprocessing prior to disposal remains
inside the fuel assembly and becomes the final waste form.

In the U.S., SNF and HLW have been produced since the 1940s, mainly as a
result of commercial power production and defense activities. Since the
inception of the nuclear age, the proper disposal of these wastes has
been the responsibility of the Federal government. The Nuclear Waste
Policy Act of 1982 (NWPA, 42 U.S.C. Chapter 108) formalizes the current
Federal program for the disposal of SNF and HLW by:

> \(1\) making DOE responsible for siting, building, and operating an
> underground geologic repository for the disposal of SNF and HLW;
>
> \(2\) directing us to set generally applicable environmental radiation
> protection standards based on authority established under other
> laws[^1]; and
>
> \(3\) requiring the Nuclear Regulatory Commission (NRC) to implement
> our standards by revising its licensing requirements for SNF and HLW
> repositories to be consistent with our standards.

This general division of responsibilities continues for the Yucca
Mountain repository. Thus, today we are promulgating amendments to our
public health protection standards at 40 CFR part 197 (which are,
pursuant to EnPA Section 801(a), applicable only to Yucca Mountain,
rather than generally applicable). The NRC will issue implementing
regulations for these standards. The DOE plans to submit a license
application to NRC. The NRC then will determine whether DOE has met
NRC's regulations and whether to grant or deny a license for Yucca
Mountain.

In 1985, we established generic standards for the management, storage,
and disposal of SNF, HLW, and transuranic (TRU) radioactive waste (see
40 CFR part 191, 50 FR 38066, September 19, 1985), which were intended
to apply to any facilities utilized for the storage or disposal of these
wastes, including Yucca Mountain. In 1987, the U.S. Court of Appeals for
the First Circuit remanded the disposal standards in 40 CFR part 191
([NRDC v. EPA, 824 F.2d 1258 (1st Cir. 1987)]{.underline}). As discussed
below, we later amended and reissued these standards to address issues
that the court raised. Also in 1987, the Nuclear Waste Policy Amendments
Act (NWPAA, Public Law 100-203) amended the NWPA by, among other
actions, selecting Yucca Mountain, Nevada, as the only potential site
that DOE should characterize for a long-term geologic repository. In
October 1992, the Waste Isolation Pilot Plant Land Withdrawal Act (WIPP
LWA, Public Law 102-579) and the EnPA became law. These statutes changed
our obligations concerning radiation standards for the Yucca Mountain
candidate repository. The WIPP LWA:

> \(1\) reinstated the 40 CFR part 191 disposal standards, except those
> portions that were the specific subject of the remand by the First
> Circuit;
>
> \(2\) required us to issue standards to replace the portion of the
> challenged standards remanded by the court; and
>
> \(3\) exempted the Yucca Mountain site from the 40 CFR part 191
> disposal standards.

We issued the amended 40 CFR part 191 disposal standards, which
addressed the judicial remand, on December 20, 1993 (58 FR 66398). The
EnPA, enacted in 1992, set forth our responsibilities as they relate to
Yucca Mountain. In the EnPA, Congress directed us to set public health
and safety radiation standards for Yucca Mountain. Specifically, section
801(a)(1) of the EnPA directed us to \"promulgate, by rule, public
health and safety standards for the protection of the public from
releases from radioactive materials stored or disposed of in the
repository at the Yucca Mountain site.\" Section 801(a)(2) directed us
to contract with the National Academy of Sciences (NAS) to conduct a
study to provide us with its findings and recommendations on reasonable
standards for protection of public health and safety from releases from
the Yucca Mountain disposal system. Moreover, it provided that our
standards shall be the only such standards applicable to the Yucca
Mountain site and are to be based upon and consistent with NAS\'s
findings and recommendations. On August 1, 1995, NAS released its
report, \"Technical Bases for Yucca Mountain Standards\" (the NAS
Report) (Docket No. OAR-2005-0083-0076).

[A. Promulgation of 40 CFR part 197 in 2001]{.underline}

Following the direction in the EnPA, we developed standards specifically
applicable to releases from radioactive material stored or disposed of
in the Yucca Mountain repository. In doing so, we gave special
consideration to both the NAS Report and our generic standards in 40 CFR
part 191, and also considered other relevant information, precedents,
and analyses.

We evaluated 40 CFR part 191 because those standards were developed to
apply to any site selected for storage and disposal of SNF and HLW, and
would have applied to Yucca Mountain had Congress not directed
otherwise. Thus, we believed that 40 CFR part 191 already included the
major components of standards needed for any specific site, such as
Yucca Mountain. However, we recognized that all the components would not
necessarily be directly transferable to the situation at Yucca Mountain,
and that some modification might be necessary. We also considered that
some components of the generic standards would not be carried into
site-specific standards, simply because not all of the conditions found
among all potential sites are present at each site. See 66 FR
32076-32078, June 13, 2001 (Docket No. OAR-2005-0083-0042), for a more
detailed discussion of the role of 40 CFR part 191 in developing 40 CFR
part 197.

We also considered the findings and recommendations of the NAS in
developing standards for Yucca Mountain. In some cases, provisions of 40
CFR part 191 were already consistent with NAS's analysis (e.g., level of
protection for the individual). In other cases, we used the NAS Report
to modify or draw out parts of 40 CFR part 191 to apply more directly to
Yucca Mountain (e.g., the stylized drilling scenario for human
intrusion). See the NAS Report for a complete description of findings
and recommendations (Docket No. EPA-HQ-OAR-2005-0083-0076).

Because our standards are intended to apply specifically to the Yucca
Mountain disposal system, in a number of areas we tailored our approach
to consider the characteristics of the site and the local populations.
Yucca Mountain is in southwestern Nevada approximately 100 miles
northwest of Las Vegas. The eastern part of the site is on the Nevada
Test Site (NTS). The northwestern part of the site is on the Nevada Test
and Training Range (referred to in our proposal as the Nellis Air Force
Range). The southwestern part of the site is on Bureau of Land
Management land. The area has a desert climate with topography typical
of the Basin and Range province. Yucca Mountain is made of layers of
ashfalls from volcanic eruptions that happened more than 10 million
years ago. There are two major aquifers beneath Yucca Mountain. Regional
ground water in the vicinity of Yucca Mountain is believed to flow
generally in a south-southeasterly direction. The DOE plans to build the
repository about 300 meters below the surface and about 300 to 500
meters above the water table. For more detailed descriptions of Yucca
Mountain's geologic and hydrologic characteristics, and the disposal
system, please see chapter 7 of the 2001 BID (Docket No.
OAR-2005-0083-0050) and the preamble to the proposed rule (64 FR
46979-46980, August 27, 1999, Docket No. OAR-2005-0083-0041).

We proposed the original standards for Yucca Mountain on August 27, 1999
(64 FR 46976). In response to our proposal, we received more than 800
public comments and conducted four public hearings. After evaluating
public comments, we issued final standards (66 FR 32074, June 13, 2001).
See the Response to Comments document from that rulemaking for more
discussion of comments (Docket No. OAR-2005-0083-0043).

The standards issued in 2001 as 40 CFR part 197 included the following:

-   A standard to protect the public during management and storage
    operations on the Yucca Mountain site;

-   An individual-protection standard to protect the public after
    disposal from releases from the undisturbed repository;

-   A human-intrusion standard to protect the public after disposal from
    releases caused by a drilling penetration into the repository;

-   A set of standards to protect ground water from radionuclide
    contamination caused by releases from the disposal system after
    disposal;

-   The requirement that compliance with the disposal standards be shown
    for 10,000 years;

-   The requirement that DOE continue its projections for the
    individual-protection and human-intrusion standards beyond 10,000
    years to the time of peak (maximum) dose, and place those
    projections in the Environmental Impact Statement (EIS) for Yucca
    Mountain;

-   The concept of the Reasonably Maximally Exposed Individual (RMEI),
    defined as a hypothetical person whose lifestyle is representative
    of the local population living today in the Town of Amargosa Valley,
    as the individual against whom the disposal standards should be
    assessed; and

-   The concept of a "controlled area," defined as an area immediately
    surrounding the repository whose geology is considered part of the
    natural barrier component of the overall disposal system, and inside
    of which radioactive releases are not regulated.

More detail on these aspects of the 2001 final rule may be found at 66
FR 32074-32134, June 13, 2001, and 70 FR 49019-49020, August 22, 2005.

[B. Legal Challenges to 40 CFR part 197]{.underline}

Various aspects of our standards were challenged in lawsuits filed with
the U.S. Court of Appeals for the District of Columbia Circuit in July
2001. Oral arguments were conducted on January 14, 2004. These
challenges and the outcome are described briefly here, emphasizing the
aspects leading to today's final rule, and in more detail in the
preamble to the proposed rule (70 FR 49014, August 22, 2005).

The State of Nevada, the Natural Resources Defense Council (NRDC), and
several other environmental and public interest groups challenged
several aspects of our final standards on the grounds that they were
insufficiently protective and had not been adequately justified. The
focus of this challenge was the 10,000-year compliance period. Nevada
and NRDC claimed that EPA's promulgation of standards that apply for
10,000 years after disposal violated the EnPA because such standards are
not "based upon and consistent with" the findings and recommendations of
the NAS. NAS recommended standards that would apply to the time of
maximum risk, within the limits imposed by the long-term geologic
stability of the site, and stated that there is "no scientific basis for
limiting the time period of the individual-risk standard to 10,000 years
or any other value."

The D.C. Circuit Court's ruling was issued on July 9, 2004. The Court
dismissed most challenges by Nevada and NRDC, as well as those filed by
the Nuclear Energy Institute (NEI). However, on the question of EPA's
10,000-year compliance period, the Court upheld the challenge, ruling
that EPA's action was not "based upon and consistent with" the NAS
Report, and that EPA had not sufficiently justified its decision to
apply compliance standards only to the first 10,000 years after disposal
on policy grounds. [Nuclear Energy Institute v. Environmental Protection
Agency]{.underline}, 373 F.3d 1 (D.C. Cir. 2004) ([NEI]{.underline})
(Docket No. OAR-2005-0083-0080). On that point, the Court stated that:

> NAS's conclusion that EPA "might choose to establish consistent
> policies" is of little importance...And although our case law makes
> clear that a phrase like "based upon and consistent with" does not
> require EPA to hew rigidly to NAS's findings, EnPA Section 801(a)
> cannot reasonably be read to allow a regulation wholly inconsistent
> with NAS recommendations.

[NEI]{.underline}, 373 F.3d at 30.

Similarly, the Court rejected EPA's reasoning that the requirement of 40
CFR 197.35 that DOE project performance to the time of peak dose and
place those projections in the Yucca Mountain EIS addressed the intent
of the NAS recommendation by ensuring that assessments would not be
arbitrarily cut off at some earlier time:

> Although EPA's addition of this provision might well represent a nod
> to NAS, it hardly makes the agency's regulation consistent with the
> Academy's findings. NAS recommended that the compliance period extend
> to the time of peak risk, yet EPA's rule requires only that DOE
> [calculate]{.underline} peak doses and expressly provides that "\[n\]o
> regulatory standard applies to the results of this analysis."

[Id.]{.underline} at 31, emphasis in original.

While the Court suggested that under different circumstances the
Agency's standard might have been upheld, it nevertheless rejected the
Agency's limitation of the compliance period to 10,000 years:

> In sum, because EPA's chosen compliance period sharply differs from
> NAS's findings and recommendations, it represents an unreasonable
> construction of section 801(a) of the Energy Policy Act. Although
> EnPA's "based upon and consistent with" mandate leaves EPA with some
> flexibility in crafting standards in light of NAS's findings, EPA may
> not stretch this flexibility to cover standards that are inconsistent
> with the NAS Report. Had EPA begun with the Academy's recommendation
> to base the compliance period on peak dosage and then made adjustments
> to accommodate policy considerations not considered by NAS, this might
> be a very different case. But as the foregoing discussion
> demonstrates, EPA wholly rejected the Academy's recommendations. We
> will thus vacate part 197 to the extent that it requires DOE to show
> compliance for only 10,000 years following disposal.

[Id.]{.underline} at 31.

Finally, the Court concluded that "we vacate 40 CFR part 197 to the
extent that it incorporates a 10,000-year compliance period..."
([Id.]{.underline} at 100.) The Court did not address the protectiveness
of the 150 μSv/yr (15 mrem/yr) dose standard applied over the
10,000-year compliance period, nor was the protectiveness of the
standard challenged. It ruled only that the compliance period could not
be found consistent with or based upon the NAS findings and
recommendations, and therefore was contrary to the plain language of the
EnPA.

As the Court noted, NAS stated that it had found "no scientific basis
for limiting the time period of the individual-risk standard to 10,000
years or any other value," and that "compliance assessment is
feasible...on the time scale of the long-term stability of the
fundamental geologic regime -- a time scale that is on the order of
10^6^ years at Yucca Mountain." As a result, and given that "at least
some potentially important exposures might not occur until after several
hundred thousand years...we recommend that compliance assessment be
conducted for the time when the greatest risk occurs" (NAS Report pp.
6-7).

However, NAS also stated "although the selection of a time period of
applicability has scientific elements, it also has policy aspects that
we have not addressed. For example, EPA might choose to establish
consistent policies for managing risks from disposal of both long-lived
hazardous nonradioactive materials and radioactive materials" (NAS
Report p. 56).

**II. Summary of Proposed Amendments to 40 CFR part 197 and Public
Comments**

The primary goal of our proposal was to satisfy the Court decision and
NAS recommendation to assess compliance at the time of maximum dose
(risk). Therefore, our proposed amendments centered on the extension of
the compliance period to capture the peak projected dose from the Yucca
Mountain disposal system "within the limits imposed by the long-term
stability of the geologic environment" (NAS Report p. 2).

A. How Did We Propose to Amend Our 2001 Standards?

We considered carefully the language and reasoning of the Court's
decision to determine its applicability to each element of our 2001
standards. As originally promulgated in 2001, 40 CFR part 197 contained
four sets of standards against which compliance would be assessed. The
storage standard applies to exposures of the general public during the
operational period, when waste is received at the site, handled in
preparation for emplacement in the repository, emplaced in the
repository, and stored in the repository until final closure. The three
disposal standards apply to releases of radionuclides from the disposal
system after final closure, and include an individual-protection
standard, a human-intrusion standard, and a set of ground-water
protection standards.

The Court's ruling vacated only one aspect of 40 CFR part 197, the
10,000-year compliance period. Therefore, the storage standard, which is
applicable only for the period before disposal, is not affected by that
ruling. Further, the Court recognized that the ground-water protection
standards were issued as an expression of EPA's overall ground-water
protection policies, and were not among the standards addressed by the
NAS, either in form or purpose ("...NAS treated the compliance-period
and ground-water issues quite differently...NAS made no 'finding' or
'recommendation' that EPA's regulation could fail to be 'based upon and
consistent with'" (NEI, 373 F.3d at 46-47)). Thus, the Court viewed the
ground-water standards as independent of any NAS recommendation.
Therefore, we concluded that the Court's vacature of the 10,000-year
compliance period, which was explicitly tied to those recommendations,
does not extend to the ground-water provisions. As a result, we did not
propose to amend the ground-water protection standards. Nothing in
today's final rule affects those standards.

We proposed to revise only the individual-protection and human-intrusion
standards, along with certain supporting provisions related to the way
DOE must consider features, events, and processes (FEPs) in its
compliance analyses. In addition, we proposed to adopt updated
scientific factors for calculating doses to show compliance with the
storage, individual-protection, and human-intrusion standards. We
requested comments only on those aspects of the individual-protection
and human-intrusion standards which were to be amended. Specifically, we
proposed to:

-   Extend the compliance period for the individual-protection and
    human-intrusion standards to 1 million years after disposal
    (closure), consistent with NAS estimates regarding the "long-term
    stability of the geologic environment";

-   Retain the dose standard of 150 μSv/yr (15 mrem/yr) committed
    effective dose equivalent (CEDE) for the first 10,000 years after
    disposal, as promulgated in 2001;

-   Establish a dose standard of 3.5 mSv/yr (350 mrem/yr) CEDE for the
    period between 10,000 years and 1 million years;

-   Clarify that the arithmetic mean of projected results will be
    compared to the dose standard for the initial 10,000 years, and
    specify use of the median of projected results between 10,000 and 1
    million years;

-   Retain the probability threshold (1 in 10,000 chance of occurring in
    10,000 years, or 1 in 100 million chance of occurring per year)
    below which "very unlikely" FEPs may be excluded from consideration;

-   Allow FEPs above the probability threshold to be excluded if they
    would not significantly affect the results of performance
    assessments in the initial 10,000 years;

-   Require consideration of seismic and igneous events causing direct
    damage to the engineered barrier system during the 1 million-year
    period;

-   Require consideration of the effects of increased water flow through
    the repository resulting from climate change, which could be
    represented by constant conditions between 10,000 and 1 million
    years;

-   Require consideration of the effects of general corrosion of the
    engineered barriers between 10,000 and 1 million years; and

-   Require use of updated scientific factors, based on ICRP
    Publications 60 and 72, to calculate dose for comparison with the
    storage, individual-protection, and human-intrusion standards.

B. What Factors Did We Consider in Developing our Proposal?

Our primary concern in extending the compliance period to 1 million
years is the increasing uncertainty associated with numerical
projections of radionuclide releases from the Yucca Mountain disposal
system and subsequent exposures to the Reasonably Maximally Exposed
Individual (RMEI). This uncertainty affects not only the projections
themselves, but also the interpretation of the results. There is general
agreement in the international community that dose projections over
periods as long as 1 million years cannot be viewed in the same context
or with the same confidence as projections for periods as "short" as
10,000 years. As a result, the nature of regulatory decision-making
fundamentally changes when faced with the prospect of a compliance
demonstration over 1 million years. International guidance from the
International Atomic Energy Agency (IAEA) and Nuclear Energy Agency
(NEA), as well as geologic disposal programs in other countries,
recognize this difficulty and accommodate it by viewing longer-term
projections in a more qualitative manner, to be balanced and
supplemented by other considerations that would provide confidence in
the long-term safety of the disposal system. In effect, numerical
projections are given less weight in decision-making at longer times.
Such approaches discourage comparison of projections against a strict
compliance limit.

This uncertainty was the overriding reason for limiting the compliance
period to 10,000 years in our 2001 rule. We did require DOE to continue
projections through the time of peak dose, but did not require them to
be judged against a compliance standard. By doing so, we essentially
adopted the approach favored by the international community. However,
while we considered this approach to be consistent with the intent of
the NAS recommendation to assess compliance at the time of maximum dose
(risk) and the committee's acknowledgment that policy considerations
would also play a role in determining the compliance period, the Court
concluded that it was inconsistent with the recommendation itself. We
determined that the most direct way to address the Court's ruling would
be to establish a compliance standard for the time of peak dose, within
the period of geologic stability at Yucca Mountain, which NAS judged to
be "on the order of one million years" (NAS Report p. 2).

We also recognize that our role as the standard-setting agency limits
our ability to specify how NRC should weigh those standards in
licensing. Therefore, we must consider that any standard established for
the 1 million-year period plays the same role in our regulation as do
standards applicable for 10,000 years. As a result, we do not believe
that extending the 10,000-year individual-protection standard of 15
mrem/yr to apply for 1 million years adequately accounts for the
considerations outlined above. In fact, we made such a statement in our
2001 final rule: "Setting a strict numerical standard at a level of risk
acceptable today for the period of geologic stability would ignore this
cumulative uncertainty and the extreme difficulty of using highly
uncertain assessment results to determine compliance with that
standard." (66 FR 32098, June 13, 2001) We turned back to the
international literature for advice regarding appropriate points of
comparison for doses projected over hundreds of thousands of years. A
number of sources suggested that natural sources of radioactivity would
provide an appropriate benchmark for such comparisons. We also found
that the variation in background radiation across the United States
covered a wide range (from roughly 100 mrem/yr to 1 rem/yr), primarily
because of local variation in radon exposures. We chose for our proposal
a level of 350 mrem/yr, which is close to the national average
background radiation exposure, but specifically represented the
difference between estimated background levels in Amargosa Valley and
the State of Colorado. This level was proposed for both the
individual-protection and human-intrusion standards as offering both a
reasonable level of protection and a sound basis for regulatory
decision-making when exposures are projected to occur hundreds of
thousands of years into the future. Selecting such a level also provides
an indication that exposures incurred by the RMEI in the far future from
the combination of natural background radiation and releases from the
Yucca Mountain disposal system would not exceed exposures incurred by
residents of other parts of the country today from natural sources
alone.

Uncertainty in long-term projections also influenced other aspects of
our proposal. Given the probabilistic nature of performance assessments,
it is inevitable that some combinations of parameter values will result
in very high doses. Although there may be only a few results that are
very high, extreme results have the potential to exert a strong
influence on the arithmetic mean, which could make the mean less
representative of overall performance. This possibility may be increased
by the natural tendency to introduce additional conservatisms as a way
to account for uncertainties. We expressed a preference for a
statistical measure that would not be strongly affected by either very
high- or low-end estimates, believing it appropriate to focus on the
"central tendency" of the distribution, where the bulk of the results
might be expected to be found. We proposed the median of the
distribution as the representation of central tendency. Because it is
always located at the point where half the distribution is higher and
half lower, the median depends only on the relative nature of the
distribution, rather than the absolute calculated values. Given our
concerns about specifying a peak dose compliance value against which
performance would be judged, we believed the median would provide a
reasonable test of long-term performance.

Our consideration of FEPs also was affected to some extent by
uncertainty, as well as by conclusions of the NAS committee. In our
proposal, the overall probability threshold for inclusion of FEPs
remained the same as in the 2001 rule, which we believe provides a very
inclusive initial screen that captures both major and minor factors
potentially affecting performance. Uncertainty plays a role in the sense
that very gradual or infrequent processes and events may begin to
influence performance only at times in the hundreds of thousands of
years, when the overall uncertainty of assessments is increasing. The
additional uncertainty introduced by these slow-acting FEPs led us to
propose the exclusion of FEPs if they were not significant to the
assessments in the initial 10,000 years. We believed this would still
provide for robust assessments that would address the factors of most
importance over the entire 1 million-year period. We did consider in our
proposal whether significant FEPs might not be captured using this
approach. In evaluating whether excluded FEPs might become more probable
or more significant after 10,000 years, and therefore should not be
eliminated, we identified general corrosion as a FEP that is certain to
occur and represents a significant failure mechanism at longer times,
even though it is less significant in the initial 10,000 years.

We also consulted the NAS Report for advice on handling long-term FEPs.
NAS identified three "modifiers" that it believed could be reasonably
included in assessments: seismic events, igneous events, and climate
change. We developed provisions addressing these FEPs that incorporated
the views expressed by the NAS committee. For seismic and igneous
events, we proposed that DOE focus its attention on events causing
direct damage to the engineered barriers. We took this approach because
failure of the engineered barrier system, particularly the waste
packages, is the predominant factor in determining the timing and
magnitude of the peak dose, and is the overriding uncertainty in
assessing performance of the disposal system. To address climate change,
we required DOE to focus on the effects of increased water flow through
the repository, which is the climatic effect with most influence on
release and transport of radionuclides. We determined that such a focus
would provide the basis for a reasonable test of the disposal system,
and that climate change beyond 10,000 years could be represented by
constant conditions, which eliminates unresolvable speculation regarding
the timing, magnitude, and duration of climatic cycles over this time
frame. We also directed that NRC should establish the exact nature of
future climate characteristics to be used in performance assessments.
NRC subsequently issued a proposal to specify deep percolation into the
repository (70 FR 53313, September 8, 2005).

Finally, we proposed to update the factors used to calculate dose for
the storage, individual-protection, and human-intrusion standards. Our
generic standards in 40 CFR part 191, and by inference our Yucca
Mountain standards in 2001, specified the factors associated with
Publications 26 and 30 of the International Commission on Radiation
Protection (ICRP). Since we issued 40 CFR part 191, ICRP has modified
the models and associated organ-weighting factors to more accurately
calculate dose. ICRP's recommendations are embodied in its Publications
60 and 72. We used this newer method in 1999 to develop our Federal
Guidance Report 13, "Cancer Risk Coefficients from Exposure to
Radionuclides" (Docket No. EPA-HQ-OAR-2005-0083-0072). Where possible,
we believe it is appropriate to adopt the latest scientific methods.

C. In Making Our Final Decisions, How Did We Incorporate Public Comments
on the Proposed Rule?

Section 801(a)(1) of the EnPA requires us to set public health and
safety radiation protection standards for Yucca Mountain by
rulemaking.[^2] Pursuant to Section 4 of the Administrative Procedure
Act (APA), regulatory agencies engaging in informal

rulemaking must provide notice of a proposed rulemaking, an opportunity
for the public to comment on the proposed rule, and a general statement
of the basis and purpose of the final rule.[^3] The notice of proposed
rulemaking required by the APA must "disclose in detail the thinking
that has animated the form of the proposed rule and the data upon which
the rule is based." (*Portland Cement Association* v. *Ruckelshaus*, 486
F. 2d 375, 392--94 (D.C. Cir. 1973)) The public thus is enabled to
participate in the process by making informed comments on the proposal.
This provides us with the benefit of "an exchange of views, information,
and criticism between interested persons and the agency." (*Id.*)

There are two primary mechanisms by which we explain the issues raised
in public comments and our reactions to them. First, we discuss broad or
major comments in the succeeding sections of this preamble. Second, we
are publishing a document, accompanying today's action, entitled
"Response to Comments" (Docket No. EPA-HQ-OAR-2005-0083-xxxx). The
Response to Comments document provides more detailed responses to issues
addressed in the preamble. It also addresses all other significant
comments on the proposal. We gave all the comments we received, whether
written or oral, consideration in developing the final rule.

D. What Public Comments Did We Receive?

The public comment period ended November 21, 2005. We received more than
300 individual submittals, although any particular submittal could
contain many specific comments. We also received many more submissions
as part of mass comment efforts, in which organizations encourage
commenters to use prepared texts or comment on specific aspects of the
proposal. All, or representative, comments are available electronically
through the Federal Document Management System (FDMS), available at
[http://www.regulations.gov](http://www.regulations.gov/). See the
"General Information" section of this document for instructions on how
to access the electronic docket. Some submittals may be duplicated in
FDMS, as a commenter may have used several methods to ensure the
comments were received, such as fax, email, U.S. mail, or directly
through FDMS.

A significant number of comments addressed the proposed peak dose
standard of 3.5 mSv/yr (350 mrem/yr), which would apply between 10,000
and 1 million years. Most commenters opposed our proposal, arguing that
it is much higher than any previous standard, is not protective, is not
equitable to future generations, and is based on inappropriate use of
background radiation data. Many commenters also took issue with our
proposal to use the median of the distribution of results as the
statistical measure between 10,000 and 1 million years, viewing this
measure as inconsistent with NAS recommendations to use the mean.
Commenters also viewed the median as too "lax" and likely to discount
scenarios that would result in high exposures. We also received comment
on our proposal to address FEPs beyond 10,000 years, with some comments
expressing the opinion that we had inappropriately constrained the
analyses, leaving out potentially significant FEPs. Some commenters
disagreed with our general premise that uncertainty increases with
assessment time, and that we should take uncertainties into account when
considering standards applicable to the far future. These specific
comments, and our responses to them, will be discussed in more detail in
Section III and in the Response to Comments document associated with
this action (Docket No. EPA-HQ-OAR-2005-0083-xxxx).

Some commenters also questioned our conclusion that extending the
compliance period is the appropriate way to respond to the Court ruling.
These commenters point out that the Court's opinion could be interpreted
to permit us to justify the approach taken in our 2001 standards. They
cite statements by the Court such as "\[i\]t would have been one thing
had EPA taken the Academy's recommendations into account and then
tailored a standard that accommodated the agency's policy concerns" and
"\[h\]ad EPA begun with the Academy's recommendation to base the
compliance period on peak dosage and then made adjustments to
accommodate policy considerations not considered by NAS, this might be a
very different case" (*NEI*, 373 F.3d at 26 and 31, respectively) to
support the thesis that the Court's judgment was based primarily on the
presentation of our case, rather than the substance. In the commenters'
view, the Court would have been receptive to our arguments had they been
presented differently, and the Court provided a clear "road map" to
justify keeping our original standards in place. In addition, these and
other commenters viewed extending the compliance period to 1 million
years as not justifiable either scientifically or as a matter of public
policy. While it is clear that we share many of the concerns expressed
by these latter commenters regarding the meaning and implementability of
a 1 million-year compliance period, we believe this is in fact the most
appropriate approach in view of the language in the Court's decision and
the weight accorded by the Court's decision to the committee's technical
recommendations concerning the period of geologic stability. As we
stated in our proposal, "it is not clear how EPA's earlier explanation
of its policy concerns might be reconciled with NAS's technical
recommendation." (70 FR 49032) Accordingly, as the Court suggested, in
today's final rule we have taken steps to implement the NAS technical
recommendation with regard to the length of time for the compliance
period and to "accommodate" our policy concerns in the provisions
related to the peak dose standard, statistical measure of compliance,
and FEPs.

We received some comments that suggested we should have provided more or
better opportunities for public participation in our decision making
process. For example, that we should have rescheduled public hearings,
extended the public comment period, and provided alternatives to the
public hearing process. We provided numerous opportunities and avenues
for public participation in the development of these standards. For
example, we held public hearings in Washington, DC; Las Vegas, NV; and
Amargosa Valley, NV. We also opened a 60-day public comment period and
met with key stakeholders before and during that time. In response to
requests from stakeholders, we extended the public comment period by 30
days and held an additional public hearing in Las Vegas. We conducted
targeted outreach to Native American tribal groups and have fully
considered all comments received through December 31, 2005, after the
end of the extended public comment period. These measures are in full
compliance with the public participation requirements of the
Administrative Procedure Act.

Several comments supported our role in setting standards for Yucca
Mountain. Other comments thought that aspects of our standards duplicate
NRC's implementation role. We believe the provisions of this rule
clearly are within our authority and they are central to the concept of
a public health protection standard. We also believe our standards leave
NRC the necessary flexibility to adapt to changing conditions at Yucca
Mountain or to impose additional requirements in its implementation
efforts, if NRC deems them to be necessary.

We also received many general comments, and others addressing topics
that are outside the scope of our authority under the EnPA. Several
commenters simply expressed their support for, or opposition to, the
Yucca Mountain repository. The purpose of our standards is to ensure
that any potential releases from the disposal system do not result in
unacceptably high radiation exposures. Our standards make no judgment
regarding the suitability of the Yucca Mountain site or whether NRC
should issue a license for the site. Such a decision is beyond the scope
of our statutory authority.

Some comments suggested our standards should explicitly consider
radiation exposures from all sources because of the site's proximity to
the Nevada Test Site (NTS) and other sources of potential contamination.
We are aware of the other such sources of radionuclide contamination in
the area. However, our mandate under the EnPA is to set standards that
apply only to the storage or disposal of radioactive materials on the
Yucca Mountain site, not to these other sources.

A number of commenters suggested that we should explore alternative
methods of waste disposal, such as neutralizing radionuclides. Comments
also expressed concern regarding risks of transporting radioactive
materials to Yucca Mountain. Considerations like these all are outside
the scope of our authority. Congress delegated to us neither the
authority to postpone the promulgation of these standards in favor of
the development of other disposal methods nor the regulation of
transportation of waste to Yucca Mountain.

Many comments touched on issues related to our authority and standards,
but outside the limited scope of this rulemaking. In particular, many
comments urged us to extend the ground-water protection limits to the
time of peak dose within the 1 million-year compliance period. Many of
these commenters disagreed with our position that the ground-water
standards were not the subject of the Court's ruling, and that in fact
the Court left us with discretion regarding the content and application
of those standards. Others believed that we are obligated to accept
comments on this topic, since we were proposing not to change the
standards. We stated clearly in our proposal that we were not
soliciting, and would not consider, comments on this issue.

**III. What Final Amendments Are We Issuing With This Action?**

This section describes the provisions of our final rule and summarizes
public comments on various aspects of our proposal. Today's final rule
establishes the dose standards applicable for a period up to 1 million
years after disposal, the statistical measures used to determine
compliance with those standards, the methods to be used to calculate the
dose, and the requirements for including features, events, and processes
(FEPs) in the performance assessments. The discussion that follows
addresses the factors we considered in developing our final rule.

**III.A. What Dose Standards Will Apply?**

Today's final rule includes an individual-protection standard consisting
of two parts, which will apply over different time frames. One part of
the standard, which will apply over the initial 10,000 years after
disposal, consists of the 150 μSv/yr (15 mrem/yr) committed effective
dose equivalent (CEDE) individual-protection standard promulgated in
2001 as 40 CFR 197.20. The other part of the standard, which we
described in our proposal, will apply beyond 10,000 years to the time of
peak dose, up to a limit of 1 million years after disposal. A dose limit
of 3.5 mSv/yr (350 mrem/yr) CEDE will apply to the long-term peak dose
projections. (Hereafter, these dose limits will be generally referred to
as "15 mrem/yr," and "350 mrem/yr," respectively.) We believe this
approach establishes a peak dose standard for this longer period that is
protective of public health and safety, while also appropriately
recognizing the relative manageability of uncertainties at such
disparate times, and the resulting level of confidence that can be
derived from performance projections.

Many commenters stated their belief that neither the NAS Report nor the
D.C. Circuit's decision allows us to apply different standards covering
different time periods within the overall compliance period. These
commenters take the position that the Court's vacature of the
10,000-year compliance period in the 2001 standards precludes us from
having a standard that applies for that initial period, and only permits
a single dose limit applicable for the entirety of the extended
compliance period. We disagree with these commenters for several
reasons. As we noted in our proposal, there was no legal challenge and
the Court made no ruling on the protectiveness of our standards up to
10,000 years. Further, the Court ruled that we must address peak dose,
but did not state, and we do not believe intended, that we could not
have additional measures to bolster the overall protectiveness of the
standard. As the Court noted, the EnPA requires that EPA "establish a
set of health and safety standards, at least one of which must include
an EDE-based, individual protection standard" ([NEI]{.underline}, 373
F.3d at 45, Docket No. EPA-HQ-OAR-2005-0083-0080), but does not restrict
us from issuing additional standards. Thus, as long as we issue "at
least one" standard addressing the NAS recommendation regarding peak
dose, we are not precluded from issuing other, complementary, standards
to apply for a different compliance period. The Court's concern was
whether we had been inconsistent with the NAS recommendation by not
extending the period of compliance to capture the peak dose "within the
limits imposed by the long-term stability of the geologic environment."
(NAS Report p. 2) Today's final rule defines the period of geologic
stability for purposes of compliance as ending at 1 million years after
disposal. We believe the decision to establish multiple compliance
standards applicable during this period, one of which is the required
"EDE-based" individual protection standard applying to the peak dose
during the period of geologic stability between 10,000 years and 1
million years, falls well within our policy discretion and is supported
by scientific considerations concerning the impact of uncertainties in
projecting doses over extremely long time frames, as discussed in
Section III.A.5 of this document ("How Did We Consider Uncertainty and
Reasonable Expectation?").

Although NAS itself did not directly address the approach of having
separate standards to apply over different time periods, we believe this
approach is not inconsistent with the intent of the committee. As
discussed in more detail in Section III.A.4 ("How Does Our Rule Protect
Future Generations?"), the committee contrasted an approach in which "a
health-based risk standard could be specified to apply uniformly across
time and generations" with "some other expression of the principle of
intergenerational equity" to be determined by "social judgment." (NAS
Report pp. 56-57) We believe the committee clearly recognized the
potential for a compliance standard that changes as the time period
covered by the assessment increases to be one possible outcome of the
rulemaking process. We also find it useful to consider the testimony
before the Senate Environment and Public Works Committee on March 1,
2006, by Mr. Robert Fri, chair of the NAS committee. We emphasize that
Mr. Fri was testifying in his individual capacity and was not
representing the NAS committee; however, we believe his testimony
provides assistance in interpreting the NAS committee's statements. Mr.
Fri pointed out that "the specification of the time horizon and the
selection of the person to be protected are intimately connected." As a
result, he noted that retaining the RMEI as the receptor (which the NAS
committee recognized as more conservative than its preferred
probabilistic critical group) while at the same time extending the
compliance period "runs the risk of excessive conservatism," potentially
putting the rule where the "committee specifically did not want to be."
He noted that the committee had considered and rejected such an
approach. (NAS Report pp. 100-103) Mr. Fri viewed our proposal of a
higher dose limit between 10,000 and 1 million years as a way "to avoid
becoming overly conservative." Therefore, while he offered no opinion on
the *level* of the proposed post-10,000-year standard, he indicated
that, in his opinion, our *approach* was not in conflict with the
committee's intention, and would in fact move us closer to the
committee's overall goal. He concluded by stating "the committee
recognized that EPA properly had considerable discretion in applying
policy considerations outside the scope of our study to the development
of the health standard for Yucca Mountain." (See generally NAS Report p.
3) We believe the decision to establish multiple compliance standards
falls well within our policy discretion and in that context the
10,000-year individual-protection standard is analogous to our
ground-water protection standards.

**III.A.1. What is the Dose Standard for 10,000 Years After Disposal?**

Today's final rule retains the standard promulgated in 2001 as §197.20,
which requires that DOE demonstrate a reasonable expectation that the
RMEI will not incur annual doses greater than 15 mrem from releases of
radionuclides from the Yucca Mountain disposal system for 10,000 years
after disposal. DOE will make this demonstration using the arithmetic
mean of performance assessment results (see Section III.A.8, "How Will
NRC Judge Compliance?" for further discussion of the mean). We believe
this is appropriate, protective, and will maintain consistency with our
generic standards at 40 CFR part 191 (now applied to the WIPP) and other
applications in both our regulations for hazardous materials and
internationally for radioactive waste. Further, NAS stated that the
"range \[of 10^-5^ to 10^-6^ per year for risk\] could therefore be used
as a reasonable [starting point]{.underline} for EPA's rulemaking" (NAS
Report p. 49, emphasis in original). By maintaining the 15 mrem/yr
standard for 10,000 years we clearly establish a "starting point" for
assessing compliance that is consistent with both the NAS
recommendations and our overall risk management policies, and serves as
a logical foundation for us to incorporate concerns regarding far future
projections.

As we stated in our proposal, an important reason for retaining a
standard applicable for the first 10,000 years is to address the
possibility, however unlikely, that significant doses could occur within
10,000 years, even if the peak dose occurs significantly later, as DOE
currently projects. We received some comments suggesting that DOE's
estimates of waste package performance are overly optimistic and that
significant early package failures are possible, if not to be expected.
Some commenters incorrectly argued that we had inappropriately
"ratified" DOE's projections of waste package performance and our
proposal "would provide essentially no protection for the period before
10,000 years," because early failure of a system licensed against a 350
mrem/yr peak dose standard would have greater consequences than would
early failure of a system licensed against a 15 mrem/yr standard. We
recognize that DOE's estimates of waste package integrity rely heavily
on extrapolations of laboratory testing data, which involves significant
uncertainties, especially when considering time frames well in excess of
all practical experience. It is not possible to claim unequivocally that
no information will come to light that might cause a reassessment of the
containers' behavior and its effect on disposal system performance.
However, while DOE must defend its estimates in licensing, our
rulemaking is not dependent on resolution of this issue. DOE will have
to demonstrate that there is a reasonable expectation that the dose to
the RMEI will not exceed 15 mrem/yr in the first 10,000 years after
closure. Thus, the addition of the peak dose standard in no way weakens
the protection provided by our 2001 standards, since disposal system
performance must still be assessed against the 15 mrem/yr limit.
Significant numbers of earlier-than-expected waste package failures in
reality will challenge the capabilities of the disposal system,
regardless of the level of the peak dose standard. Should evidence arise
that legitimately challenges DOE's projections that waste package
lifetimes will exceed 10,000 years, the 15 mrem/yr standard for that
initial period assures that a level of performance equivalent to that
required by 40 CFR part 191 must still be demonstrated at Yucca
Mountain. The peak dose standard adds a new level of protection for the
post-10,000-year period that was not defined in our 2001 standards. We
believe it important to structure our regulations to preclude the chance
that protection at Yucca Mountain would be less than that provided for
WIPP or the Greater Confinement Disposal facility (GCD, which is a group
of 120-feet deep boreholes, located within NTS, which contain disposed
transuranic wastes). It would be inappropriate to apply a standard
designed to accommodate the uncertainties in projections many tens to
hundreds of thousands of years into the future to projections within
10,000 years, when uncertainties are more manageable.

**III.A.2. What is the Peak Dose Standard Between 10,000 and 1 Million
Years After Disposal?**

Today we are finalizing our proposed peak dose standard of 3.5 mSv/yr
(350 mrem/yr), which will apply for the period between 10,000 years and
1 million years after closure of the facility. In our proposal, we
discussed several factors that we considered to be important in setting
a dose standard for the time of peak dose within the period of geologic
stability. We emphasized the cumulative and increasing uncertainty in
projecting potential doses over great time periods, and argued against
viewing projected doses as predictions of disposal system performance.
This is consistent with the position taken by the NAS committee: "The
results of compliance analysis should not, however, be interpreted as
accurate predictions of the expected behavior of a geologic repository."
(NAS Report p. 71) We believe a higher dose standard for the period
beyond 10,000 years is both protective of public health and safety and
appropriate given the increased uncertainties in projecting releases
from the Yucca Mountain disposal system.

We have also considered how the role of quantitative projections in
making compliance decisions must change as the times covered by those
projections increases. We noted that emphasizing incremental dose
increases when such increases may be overwhelmed by fundamental
uncertainties inappropriately takes attention away from an evaluation of
the overall safety of the disposal system. In our view, the role of the
peak dose standard in the overall decision of disposal system safety
must be consistent with the relative confidence that can be placed in
quantitative projections over extremely long times. We have recognized
the strong consensus in the international radioactive waste community
that dose projections extending many tens to hundreds of thousands of
years into the future can best be viewed as qualitative indicators of
disposal system performance, rather than as firm predictions that can be
compared against strict numerical criteria. We agree that confidence in
the way the projections were performed, and supporting qualitative
information, may be more important to an overall judgment of safety at
longer times. However, since our task is to establish a firm regulatory
limit, rather than a qualitative standard or dose target, we believe a
higher peak dose standard for the period between 10,000 and 1 million
years is justified in the context of regulatory decision-making. We
continue to believe, as we stated in our 2001 rulemaking, "Setting a
strict numerical standard at a level of risk acceptable today for the
period of geologic stability would ignore this cumulative uncertainty
and the extreme difficulty of using highly uncertain assessment results
to determine compliance with that standard" (66 FR 32098, June 13, 2001,
Docket No. EPA-HQ-OAR-2005-0083-0042).

As in our proposal, we considered the range of variation in background
radiation across the United States in arriving at the final peak dose
standard. Given the extremely long time frame under consideration, we
believe variations in background radiation across the United States
provide a reasonable and logical context for evaluating long-term
disposal system safety. In that context, our goal was to establish a
peak dose standard, such that total exposures to the Reasonably
Maximally Exposed Individual (RMEI) from the combination of background
radiation and releases from the Yucca Mountain disposal system would be
no greater than exposures incurred by residents of other parts of the
country from natural sources alone. The specific basis for the final
peak dose standard is described in detail in Section III.A.3 of this
document ("How Did We Consider Background Radiation in Developing the
Peak Dose Standard?").

We believe that a standard of 3.5 mSv/yr (350 mrem/yr) appropriately
satisfies our statutory and judicial mandates by blending the
considerations outlined above with current and historical thinking
regarding risks associated with background radiation, while recognizing
the conceptual difficulties inherent in projecting and evaluating
potential events hundreds of thousands of years into the future.

We received many comments questioning both the legality and the
protectiveness of our proposed standards. As described previously in
Section III.A, commenters stated that the NAS Report and Court decision
required us to retain a single dose standard (i.e., 15 mrem/yr) for the
entire 1 million-year compliance period, equivalent to the period of
geologic stability defined in our rule. Commenters pointed out that the
350 mrem/yr level was well above the range identified by NAS as a
starting point for our rulemaking (which ranged from about 2 to 20
mrem/yr), and therefore stated that only the 15 mrem/yr level could be
considered consistent with the committee's recommendation. Similarly,
some commenters interpreted the Court ruling to require us to adjust the
time period covered by the existing 15 mrem/yr standard, which was not
challenged. We do not believe this interpretation to be correct. It
should be emphasized that the NAS provided only a "reasonable starting
point" for our rulemaking, and that none of the regulatory precedents
considered by NAS applied for periods approaching 1 million years. (NAS
Report pp. 5 and 49, respectively) In fact, NAS explicitly declined to
recommend a level of protection, recognizing that this was a matter best
left to EPA to establish through rulemaking: "We have not recommended
what levels of risk are acceptable...The specific level of acceptable
risk cannot be identified by scientific analysis, but must rather be the
result of a societal decision-making process. Because we have no
particular authority or expertise for judging the outcome of a properly
constructed social decision-making process on acceptable risk, we have
not attempted to make recommendations on this important question." (NAS
Report p. 20) Indeed, NAS explicitly acknowledged "that determining what
risk level is acceptable is not ultimately a question of science but of
public policy." (NAS Report p. 5) Further, NAS noted that the final
outcome of the rulemaking might diverge substantially from the starting
point suggested by NAS: "Finally we have identified several instances
where science cannot provide all of the guidance necessary to resolve an
issue...In these cases, we have tried to suggest positions that could be
used by the responsible agency in formulating a proposed rule. Other
starting positions are possible, and of course *the final rule could
differ markedly from any of them*." (NAS Report p. 3, emphasis added)
Thus, we agree with NAS that the selection of a level for the peak dose
standard is one of the regulatory policy issues left to EPA's discretion
by the EnPA.

We also find it instructive to consider again the personal Senate
testimony of NAS committee chair Robert Fri, as described in Section
III.A ("What Dose Limits Will Apply?"). Mr. Fri noted that simply
extending the compliance period in our 2001 rule to 1 million years
"runs the risk of excessive conservatism" and could place our standard
where the "committee specifically did not want to be." He recognized
that a higher standard at the time of peak dose would be one way to
reduce that conservatism. Mr. Fri was not prepared to address the
consistency of our proposed dose level with the NAS findings and
recommendations; however, he indicated that, in his view, retaining the
15 mrem/yr standard at the time of peak dose would not be consistent
with those findings and recommendations if other aspects of our rule
remained unchanged (specifically, the choice of receptor). We find this
perspective noteworthy, in that it suggests that there are circumstances
in which applying 15 mrem/yr throughout the 1 million-year compliance
period could result in a standard directly contrary to the committee's
overall goals, which emphasized the use of "cautious, but reasonable"
assumptions and care in the use of "pessimistic scenarios and parameter
values." (NAS Report pp. 100 and 79, respectively) Further, we do not
believe the Court's decision can be seen to provide direction
independent of the NAS Report; rather, the Court's underlying purpose
was to ensure that our standards would be consistent with the
committee's findings and recommendations, as required by the EnPA.

In considering appropriate dose standards for periods approaching 1
million years, we also considered the development of our generic
standards in 40 CFR part 191. In both our 1985 and 1993 rulemakings, we
emphasized that the 10,000-year compliance period for both the
containment requirements and individual-protection limit would lead to a
combination of site characteristics and engineered barriers that would
be capable of providing containment and isolation of the waste for these
long periods of time. We did not, however, anticipate that such
performance could be maintained indefinitely. Our generic technical
analyses, in fact, suggested that significant releases and doses to
individuals could result at later times, depending on the
characteristics of the site in question and the presumed location of the
receptor. For example:

> The Agency examined potential doses to individuals, considering
> various times in the future, from waste disposal systems in several
> different geologic media. In most of the cases studied, radionuclide
> releases resulting in exposures to individuals did not occur until
> more than 1,000 years after disposal due to the containment
> capabilities of the engineered barrier systems. Beyond 1,000 years,
> but prior to 10,000 years, as the engineered barriers begin to
> degrade, releases resulting in doses on the order of a few rems per
> year appeared for some of the geologic media studied...For other,
> better geologic media, the Agency's generic analyses estimate no
> releases for 10,000 years. The Agency believes that selecting a
> 10,000-year time for the requirements, rather than a 1,000-year time
> frame, will encourage the selection of better sites and/or the design
> of more robust engineered barrier systems capable of significantly
> impeding radionuclide releases. These actions, in turn, will serve to
> reduce the individual risks associated with the disposal of
> radioactive waste.

58 FR 66401, December 20, 1993.

We note that sites whose natural features did not provide strong
containment were not necessarily considered unsuitable, but we
recognized that in those instances, the focus would have to be on "the
design of more robust engineered barrier systems capable of
significantly impeding radionuclide releases." We believe that it is
unrealistic to assume that these sites would then exhibit better
performance after the failure of those barriers than they would in the
initial 10,000-year period. Consequently, we believe the potential for
doses higher than 15 mrem/yr to individuals in the far future has always
been implicit in the concept of geologic disposal. Reliance on
engineered barriers cannot be assumed for time frames approaching 1
million years, nor do we believe it is reasonable to judge the safety of
a disposal system over such time frames against a level of performance
consistent with the initial containment period.

Comments on the protectiveness of our proposal pointed out that 350
mrem/yr is much higher than any previous EPA regulation, resulting in
risks outside the range of 10^-4^ to 10^-6^ lifetime chance of
developing a fatal cancer typically applied by the Agency across
programs and pollutants. Many further cited estimates of cancer
incidence or fatality as high as 1 in 36 or greater. Using current EPA
cancer risk coefficients, we estimate that members of a population
receiving an extra 350 mrem/yr over a lifetime would have an additional
cancer mortality risk of 1 to 2 in 100 (i.e., 1 to 2%). However, we
deliberately did not provide risk estimates associated with 350 mrem/yr
in our proposal because the selection of that level, which is to apply
over an unprecedented time period, was not based on considerations of
risk in the same way as previous standards have been developed, nor do
we believe it should be viewed in the same way. Rather, it takes into
account larger, less quantifiable factors such as the uncertainties
involved in projecting doses over 1 million years and the meaning that
can be assigned to such projections, as well as the relative importance
they should assume, in a regulatory context. Further, in considering how
the overall safety of a geologic disposal system can be portrayed over
times approaching 1 million years, we consulted various international
sources, which suggested that natural sources of radiation can provide
an appropriate benchmark for public health protection over very long
times. From a global perspective, doses in the range of natural
background radiation do not threaten life or limit the ability of future
generations to pursue their interests (see Section III.A.3, "How Did We
Consider Background Radiation in Developing the Peak Dose Standard?" and
70 FR 49036-49039 for more discussion of background radiation). Finally,
it must be emphasized that the 350 mrem/yr level applies to the RMEI,
who is described as a person subject to doses at the high end of the
local population. Most residents in the vicinity of Yucca Mountain would
receive much lower doses from the disposal system than the RMEI, if any
dose at all.

A number of comments compared our proposal to international practices
and concluded that our standard would be "the weakest standard in the
world" or otherwise inconsistent with those practices. Most commenters
offered no specific examples or contrary examples to support those
conclusions. In general, we find few similarities in the details of the
international approaches that are directly applicable, and no obvious
basis for comparing the different approaches. At the same time, we did
find broad points of similarity in the overall approach to long-term
projections, and referred to organizations such as IAEA and NEA, as well
as specific countries, such as Sweden. The more typical approach
internationally is to require compliance with quantitative performance
assessment for only a limited period of time (in some cases, less than
10,000 years). Longer-term doses may be compared to dose or risk targets
or reference levels, but are viewed more as qualitative indicators of
performance, to be weighed in conjunction with other qualitative
arguments for confidence in the overall safety of the facility. At
longer times, the weight given to quantitative projections typically
decreases. We attempted such an approach in our 2001 rulemaking, which
gave NRC flexibility to consider longer-term dose projections as it
thought appropriate within the licensing process (i.e., NRC would decide
how much meaning or weight should be assigned to those projections). We
considered that a better approach than establishing a compliance limit
for times approaching 1 million years, given the increased uncertainties
associated with projections over such times.

Today's final rule is responsive to the NAS recommendations and to the
D.C. Circuit's decision, but is atypical for such situations in actually
prescribing a compliance limit for very long times. It is also atypical
in the sense that it is a site-specific standard for which a license
application is actively being prepared, whereas most countries have not
progressed beyond the identification of candidate sites and may have
significantly different legislative and regulatory frameworks in place.
Therefore, it is not directly comparable to international situations.
However, we did consider the international perspective regarding
uncertainties and the confidence that can be placed in very long-term
projections for regulatory decision-making. We believe a higher peak
dose standard is justified on both counts, particularly since we are
establishing a dose limit, rather than a target or reference level that
could be exceeded for unspecified reasons. Further, as discussed in more
detail in the next section, we also considered international views
regarding the use of natural sources of radiation as a framework for
evaluating long-term dose projections. We believe our peak dose standard
of 350 mrem/yr is protective, consistent with international views, and
appropriately accommodates those views within the overall context of
reasonable expectation. More detailed discussion of specific
international approaches may be found in Section 4 of the Response to
Comments document for this final rule.

**III.A.3. How Did We Consider Background Radiation In Developing the
Peak Dose Standard?**

As noted above, we considered a variety of factors in selecting our
final peak dose limit, with a strong emphasis on its consistency with
the range of variation of background radiation across the United States.
Many of the comments we received criticized our proposed use of
background radiation as a benchmark for evaluating human-caused
exposures in the very far future. Besides taking issue with specifics of
our approach, as discussed below, commenters expressed the strong
opinion that exposures cannot be considered "safe" just because they are
natural, and a high level of natural exposures in one location does not
justify allowing additional exposures to another population.

As described in this section, the way in which we have incorporated
considerations of background radiation into our decision has changed
somewhat. However, we still believe it provides a reasonable perspective
from which to judge the overall acceptability of the Yucca Mountain
disposal system over a period of 1 million years, as well as providing a
context for consideration of the uncertainties involved in projecting
doses at such long times. From that perspective, doses in the range of
background radiation do not threaten life or limit the ability of future
generations to pursue their interests. We cited a number of
international sources suggesting that such comparisons are appropriate
as uncertainties increase over long times (70 FR 49036-49039). For
example, IAEA has stated that, for time frames extending from about
10,000 to 1 million years, "it may be appropriate to use quantitative
and qualitative assessments based on comparisons with natural
radioactivity and naturally occurring toxic substances." ("Safety
Indicators in Different Time Frames for the Safety Assessment of
Underground Radioactive Waste Repositories," IAEA-TECDOC-767, p. 19,
1994, Docket No. EPA-HQ-OAR 2005-0083-0044) The IAEA also suggests that
"\[i\]n very long time frames...uncertainties could become much larger
and calculated doses may exceed the dose constraint. Comparison of the
doses with doses from naturally occurring radionuclides may provide a
useful indication of the significance of such cases." ("Geological
Disposal of Radioactive Waste," Final Safety Requirements Document
WS-R-4, Section A.7, p. 37, 2006, Docket No. EPA-HQ-OAR-2003-0085-xxxx)
In this context, the "dose constraint" referred to by IAEA is akin to
our 15 mrem/yr standard. As discussed previously in Section III.A.2, the
typical international approach is not to set a strict regulatory limit
at times beyond 10,000 years, as we are doing today. As indicated by the
citation above, this could lead to situations in which the initial
regulatory limit is exceeded, but an overall judgment of safety is
supported by other considerations, which take on more importance.

In developing our proposal, we compiled average background radiation
exposure data for individual states across the country (see "Assessment
of Variations in Radiation Exposure in the United States," technical
support document for the 2005 proposed amendments, Docket No.
EPA-HQ-OAR-2005-0083-0077, updated for this final rule as described
below). For our purposes, background radiation included cosmic,
terrestrial, and indoor radon exposures. After considering that data,
the geographic distribution, and the significant variation represented,
we determined that our overall approach would be to select a level such
that total exposures to the RMEI from the combination of background
radiation and releases from the Yucca Mountain disposal system would be
no greater than exposures incurred by residents of other parts of the
country from natural sources alone.

Following that approach, we focused our proposal on a specific
comparison of estimated background radiation in Amargosa Valley and the
State of Colorado. Using Amargosa Valley as one point of comparison
allows us to provide some assurance that the RMEI location would be
adequately taken into account. We estimated the background level in
Colorado to be 700 mrem/yr; however, because our data compilation
provided only statewide averages, we did not have data specific to
Amargosa Valley (although state averages varied significantly, localized
data is even more variable and affected by the more limited data
points). We consulted DOE's 2002 FEIS, which showed only estimates for
Amargosa Valley consistent with national averages, totaling 300 mrem/yr.
We adjusted that figure on the basis of EPA's 1993 studies of radon
potential, which indicated that Nye County has a higher radon potential
than Clark County, which contains two-thirds of the state's population.
We then compared the 700 mrem/yr estimate for Colorado with the adjusted
estimate of 350 mrem/yr for Amargosa Valley, which resulted in a
difference of 350 mrem/yr between the two locations. We discussed
specific locations because we believed this type of comparison would be
easier to understand than a more generalized discussion of variation in
background radiation across the United States. We also thought it
illustrated very well our underlying premise for the proposed peak dose
limit, which is that exposures from Yucca Mountain in the very far
future should be held to a level such that total exposures to the RMEI
would not exceed exposures incurred today by residents of other parts of
the country from natural sources alone.

However, a significant number of comments questioned our comparative
background approach on the grounds that we had incorrectly included
indoor radon in our definition of background radiation, that our data
was otherwise flawed, or that the basis of comparison we used was
inadequately justified. On the question of indoor radon, many commenters
argued that it should not be considered "natural" background radiation
because it is an "artifact of construction" (some comments referred to
indoor radon as "man-made," which is clearly incorrect), because it is
extremely variable, and because this approach is at odds with EPA's
program to encourage radon mitigation, and in fact assumes the failure
of that program. We agree that radon concentrations are highly variable,
and emphasized that fact in our proposal as a reason to rely more on
statewide averages. Further, we agree that indoor radon exposures are
influenced by the type of building considered (e.g., if it has a
basement or is a multi-story apartment house) as well as by the amount
of time inhabitants spend in the relatively high radon concentration
areas. Again, because these factors make it difficult to precisely
correlate concentrations to exposures, we believe a wider base of data
is desirable. However, we do not agree that indoor radon should be
excluded from the definition of background radiation. Indoor radon is
the most significant daily exposure incurred by the majority of the
population and is likely to be the primary differential in considering
relative exposures between locations. Organizations such as ICRP, NCRP,
and UNSCEAR commonly discuss indoor radon in the context of background
radiation. [As for EPA's radon abatement program, we noted in our
proposal that EPA does not recommend action be taken at concentrations
below 4 pCi/l (which we have typically translated to 800 mrem/yr), and
recommends that building owners consider appropriate actions only
between 2 and 4 pCi/l (about 400-800 mrem/yr). When establishing a dose
standard that will apply for up to 1 million years to a hypothetical
RMEI, we believe it is reasonable to consider indoor radon to define
representative variations in current background radiation]{.mark}.

We received credible information that our estimated background radiation
for Amargosa Valley, which was adjusted from what were essentially
"average" figures, is significantly higher than available monitoring and
lifestyle information would support. For example, the Desert Research
Institute has conducted monitoring that suggests the average background
radiation in Amargosa Valley is closer to 110 mrem/yr for terrestrial
and cosmic radiation exposure (Docket No.
EPA-HQ-OAR-2005-0083-0364.2-1). However, it has also been noted that a
significant proportion of the residents of Amargosa Valley live in
mobile homes, which could affect indoor radon levels. We also note the
recent publication of a study in the October 2006 edition of *Health
Physics* by Dr. Dade Moeller ("Comparison of Natural Background Dose
Rates for Residents of the Amargosa Valley, NV, to those in Leadville,
CO, and the States of Colorado and Nevada," co-authored by Lin-Shen
Sun). Dr. Moeller is a well-known health physicist and past chair of the
Health Physics Society. Dr. Moeller also presented his results at a
public meeting of the NRC's Advisory Committee on Nuclear Waste (ACNW)
in November 2005, and a preliminary version of his paper was submitted
with public comments by the Department of Energy. Several other
commenters referred to aspects of Dr. Moeller's study.

Dr. Moeller explores the various factors affecting background radiation
doses in more detail than we did in our supporting document ("Assessment
of Variations in Radiation Exposure in the United States," Docket No.
EPA-HQ-OAR-2005-0083-0077, updated for this final rule as described
below). He suggests that a more appropriate comparison with Amargosa
Valley would be the town of Leadville, Colorado, which is comparable in
population. He believes this provides a better basis for comparison than
the average for the state as a whole. He also located information
indicating that the vast majority of residents of Amargosa Valley live
in mobile homes (roughly 91%), which we did not account for in our
estimate. This could significantly affect the indoor radon levels
encountered. Dr. Moeller calculates that the overall average exposure in
Amargosa Valley would be reduced by more than 60% from our estimate. He
concludes that the difference in background radiation between Amargosa
Valley and Colorado is 254 mrem/yr (compared to our estimate of 350
mrem/yr), and the difference between Amargosa Valley and Leadville (his
preferred comparison) is 396 mrem/yr, about 14% higher than our proposed
dose standard.

Two factors may have influenced Dr. Moeller's estimates relative to our
own. Dr. Moeller employs the Lawrence Berkeley National Laboratory
(LBNL) radon database, which is derived from data collected by EPA in
the early 1990s, but cannot be directly compared to the data we used.
Dr. Moeller also employed a radon dose conversion factor lower than
ours, which he cites as consistent with UNSCEAR and forthcoming NCRP
recommendations. The factor we employed for our proposal is that
published by NCRP in its initial studies of background radiation in
Publications 93 and 94. Much work has been done in this area, but there
is no consensus that the earlier factors are outdated. However, for the
states of Colorado and Nevada, where our estimates are directly
comparable, we see no consistent difference. Dr. Moeller's estimate for
Colorado is roughly 45% lower than ours (386 compared to 700 mrem/yr),
while his estimate for Nevada is almost exactly the same as ours (227
compared to 222 mrem/yr). As noted above, his estimate for Amargosa
Valley based on site-specific considerations is considerably lower than
ours (derived from DOE estimates), but is also lower than indicated by
actual monitoring data (excluding indoor radon). Dr. Moeller finds
considerable uncertainty associated with his estimates, primarily in
conjunction with the radon component, which is not surprising.

Given the comments we received and Dr. Moeller's work, we revised our
estimates of background radiation using the radon conversion factor
employed by Dr. Moeller ("Assessment of Variation in Radiation Exposure
in the United States," technical support document for the 2006 final
amendments, Docket No. EPA-HQ-OAR-2005-0083-xxxx). As expected, our
state estimates were reduced proportionally to the fraction of
background radiation represented by radon. However, the range of
estimates remains greater than 350 mrem/yr, and comparison with Dr.
Moeller's conclusions continues to show inconsistencies. While our
revised estimate for Colorado is now almost identical to Dr. Moeller's
(387 to 386), our estimate for Nevada is less than two-thirds his value
(141 to 227). In addition, when considering two different approaches to
estimating radon exposures, our estimates for background radiation
exposures for Amargosa Valley residents range from approximately 110
mrem/yr to 160 mrem/yr (Docket No. EPA-HQ-OAR-2005-0083-xxxx).
Therefore, while we cautioned in our proposal that background radiation
rates are highly variable (particularly the indoor radon component), and
that no definitive or comprehensive source of data exists, we have
considered this additional information and explored other data sources
to determine whether a better or more complete data set might be
available. Ultimately, however, we conclude that the data in our record
is reasonable and as credible as that from other sources, although we
did examine the LBNL radon database for a specific purpose, as noted
below.

Finally, we received a number of comments questioning our rationale for
selecting Colorado as the appropriate end-point for this comparison, as
well as suggestions for other locations. Most simply expressed concern
that a state at the high end of the background range was used as a
reference point and believe our statements of similarity between the two
locations are not well-reasoned. As noted above, Dr. Moeller believed
the town of Leadville, Colorado would represent a more appropriate
comparison, citing similarities in population and "altitude and
accompanying relatively high cosmic radiation dose rate" as reasons for
his selection. He believed the use of the statewide average would not be
appropriate because it would not explicitly consider locations with
higher than average dose rates. Commenters also suggested using
variations in background radiation in the vicinity of Yucca Mountain,
and pointed out that the variation within the state of Nevada is greater
than 350 mrem/yr, which we confirmed by examining the LBNL radon
database. By this reasoning, a resident of Amargosa Valley could remain
in the state and incur 350 mrem/yr additional background radiation. We
note that Dr. Moeller considered the comparison with the statewide
average for Nevada, as suggested by some comments, to be "a very
questionable option" in his presentation to the ACNW.

Taken together, these comments on the quality of the data used and the
justification for comparison of specific locations illustrate the
difficulty we have had in formulating a standard based on variation in
background radiation. Unfortunately, as we stated earlier, there is no
definitive or comprehensive source of background radiation data, and all
available datasets have limitations. Further, we presented the proposal
of 350 mrem/yr in the context of the difference in background radiation
between two specific locations. We believed this would provide readers
with a clear understanding of the implications of our proposal, and a
way to evaluate those implications from the perspective of daily life.
It is clear that many readers found this comparison unsatisfying. The
comparison was intended to be illustrative, not definitive, and we did
not intend to invite debate regarding which location is most similar to
Amargosa Valley.

In issuing our final rule, therefore, we believe it is more effective to
address the question of variation in background radiation in a wider
context, without reference to specific locations. From that perspective,
350 mrem/yr is within that variation, whether considered nationally,
regionally (e.g., western states), or within Nevada itself. The 350
mrem/yr level is also comparable to the widely-accepted "average" U.S.
natural background of 300 mrem/yr as described by NCRP. We view this
difference as well within the margin of uncertainties in estimates of
background radiation. Finally, we believe this level continues to
fulfill the overall objective expressed in our proposal: exposures
incurred by the RMEI from the combination of background radiation and
releases from Yucca Mountain would be no greater than exposures incurred
by residents elsewhere from natural sources alone.

Some comments criticized our citations to international sources
regarding comparisons with natural radioactivity as a benchmark for
long-term doses. Comments maintain that we misrepresented statements
from these sources regarding fractions of background radiation at very
long times, recasting them to support a much larger dose standard at
relatively short times. The comments point out, for example, that a
reference at an NEA workshop to "a dose constraint derived from natural
background levels" for periods up to 100,000 years (70 FR 49036)
actually considered 10% of worldwide variation (excluding indoor radon),
or roughly 30 mrem/yr, as the "derived" level (Chapman, Neil, "Long
Timescales, Low Risks: Rational Containment Objectives that Account for
Ethics, Resources, Feasibility and Public Expectations -- Some Thoughts
to Provoke Discussion," available in the proceedings of the NEA Workshop
on "The Handling of Timescales in Assesssing Post-Closure Safety of Deep
Geological Repositories," April 16-18, 2002, Docket No.
EPA-HQ-OAR-2005-0083-xxxx). We would point out, however, that the cited
reference goes on to suggest that beyond 100,000 years, the objective
would be "that the eventual redistribution of the residual radioactivity
in the environment by erosion and other natural processes should be
indistinguishable from regional variations in natural terrestrial
radioactivity in near-surface rocks, soils, and waters: with 'regional'
taken in the broad sense of, for example, Europe or North America." In
that period, "it must be recognized and accepted that the potential
exists for uranium ore deposits, or spent fuel or HLW repositories, to
give rise locally to doses that are higher than the global average for
natural radiation (\~2.5 mSv/a)." We do not wish to debate the meaning
of "indistinguishable"; however, in this approach, a distinction is
clearly being suggested between radiation levels in the vicinity of the
repository and those on a much larger scale. We continue to believe
these types of statements can be interpreted as generally supporting the
proposition that, if a dose standard is to be applied over long times,
it is reasonable for the dose standard to change as the time period
covered by the assessment increases. Further, we believe these sources
provide even stronger support for the proposition that the context in
which doses are considered necessarily changes over time. The source
cited above also suggests similar changes are necessary in the way in
which "the spirit of current radiological protection principles could be
applied."

We do not anticipate that all readers will agree with our rationale for
the final peak dose standard or with our interpretation of these various
sources, nor is it likely they all agree with each other. There will be
disagreement regardless of the content of our final standards. For
example, we received a number of comments on both our 1999 and 2005
proposals stating that a 15 mrem/yr standard is insufficiently
protective. We believe the approach we have taken is a reasonable one
that appropriately balances the need for a fixed quantitative long-term
peak dose limit standard consistent with NAS and judicial direction, the
limitations in quantitative performance assessment methodologies, and
the need for a definite marker against which to judge compliance in a
regulatory process. We believe our final peak dose standard is both
protective and provides the basis for a reasonable test of the disposal
system over such extended time frames.

**III.A.4 How Does Our Rule Protect Future Generations?**

We received extensive comment on our proposal from the perspective of
its potential impact on future generations as compared to the current or
next few generations. Commenters on this point questioned our reasoning
behind proposing a higher dose standard for the far future, and
disagreed with our interpretation of literature on the subject.
Ultimately, most commenters expressed the view that there is no
justification for the level of protection to be different from today's
level, whether it is 10,000 or 1 million years (or even longer) from
now.

EPA remains committed to the principle of intergenerational equity,
which holds (in part) that the risks from a current action should not be
greater to future generations than would be acceptable today. A strict
reading of this principle initially would lead to the conclusion that
the same level of protection must apply at all times, or for as long as
the action presents risks. However, we believe that peak dose limits
over periods approaching 1 million years should be viewed as
qualitatively different from limits applied at earlier times; in other
words, the basis for judgment at different times is not the same. We
believe the peak dose standard we proposed, and are establishing in our
final rule today, appropriately considers this differing basis for
judgment and provides the necessary protections for far future
generations.

In particular, we have tried to understand how the concept of
intergenerational equity is viewed when applied to periods up to 1
million years, because only in the context of radioactive waste
management has there been serious consideration of such time frames. For
example, does the idea of "risks no greater than would be acceptable
today" take on a different meaning over periods during which human
evolutionary change may occur? Many commenters expressed the view that
it does not. However, as we discussed in our proposal, a number of
regulatory and scientific bodies suggest that it may be appropriate to
relate longer-term standards to background radiation levels, which
strictly speaking would be "greater than would be acceptable today" from
a waste management practice, but are not routinely considered as a major
risk factor in collective and individual decision-making.

In addition, while the concept of intergenerational equity is of
sufficient importance to underlie two of the nine fundamental
radioactive waste management safety principles endorsed by IAEA ("The
Principles of Radioactive Waste Management," Safety Series 111-F, 1995,
in particular Principles 4 and 5, which relate to protection of future
generations and burdens on future generations, respectively) and has
been incorporated into the Joint Convention on the Safety of Spent Fuel
Management and the Safety of Radioactive Waste Management (an
international agreement ratified by more than 30 countries, including
the U.S.), we have viewed it as necessary to consider other principles
that have been put forward in the context of making decisions with
implications for the future, with particular attention to those relevant
to radioactive waste management. The arguments for maintaining a single
level of protection for all times as an expression of intergenerational
equity are well-known. We were also interested in examining arguments
that intergenerational concerns could be accommodated, and equity
achieved, by approaching the problem in other ways. This led us to
consider documents prepared by the National Academy of Public
Administration (NAPA) and Swedish National Council for Nuclear Waste
(KASAM). NAPA is a Congressionally-chartered organization whose purpose
is to provide assistance to government in assessing and effectively
addressing issues of governance, including future implications of
contemplated actions. KASAM was created by the Swedish government in
1985 to provide an independent review of issues related to nuclear
waste.

We emphasize that we do not question whether there is an obligation to
future generations, but we believe there is no consensus regarding the
nature of that obligation, for how long it applies, whether it changes
over time, or how it can be discharged. Regarding radioactive waste
management and geologic disposal, there is general agreement that
assurances can be provided that the protections offered will be similar
to those acceptable today for periods approximating 10,000 years, which
is a very long time. However, as one considers times in the hundreds of
thousands of years, can similar assurances be offered when, as we
believe, the underlying bases for those assurances has fundamentally
changed? What form can those assurances take (i.e., can we reasonably
make assurances regarding our ability to distinguish among and control
incremental radiation exposures over long times)? Can they provide the
same level of confidence? We are establishing today a standard that
would not affect the quality of life for future generations. We believe
this is a reasonable level of commitment for such long times, given the
complexities of the situation and what we see as our responsibility to
establish a level of compliance, not a soft target or reference level
that could be exceeded for unspecified reasons and by unspecified
amounts.

Some comments criticized our discussion of the literature from
international sources, believing we misrepresented these sources as
stating that dose assessments should not be conducted over times
approaching 1 million years. We believe these comments confuse two
concepts, the conduct of dose assessments and the establishment of dose
standards. We believe we accurately represented international sources on
both points. These sources do generally take the position that numerical
assessments eventually lose their utility (e.g., "calculations of dose
and risk should not be extended to times beyond those for which the
assumptions underlying the models and data can be justified," NEA, cited
at 70 FR 49027). This sentiment is in complete agreement with NAS
statements regarding geologic stability: "After the geologic environment
has changed, of course, the scientific basis for performance assessment
is substantially eroded and little useful information can be developed."
(NAS Report p. 72, see also Section III.A.5 of this document) However,
even for shorter periods when assessments can provide insights into
disposal system performance, the typical approach internationally is not
to hold the results of those assessments to strict numerical limits, but
to view them more as qualitative indicators of performance (see, for
example, 70 FR 49026-49027). This approach, which we adopted in our 2001
rule, acknowledges that the nature of dose projections changes over
time, so that comparison of those projections to strict numerical limits
may not be the most meaningful indicator of equity over long time
frames.

A number of other commenters cite the statements of the NAS committee
regarding intergenerational equity to support their position that a
higher dose level for longer times is contrary both to that principle
and the NAS recommendation. We disagree, and have discussed the second
point in some detail in Section III.A.2 ("What is the Peak Dose Standard
Between 10,000 and 1 Million Years After Disposal?"). Regarding the
question of intergenerational equity, we cited the NAS discussion in our
proposal (page 49036). In citing NRC and IAEA sources, the NAS wrote:

> A health-based risk standard could be specified to apply uniformly
> over time and generations. Such an approach would be consistent with
> the principle of intergenerational equity that requires that the risks
> to future generations be no greater than the risks that would be
> accepted today. Whether to adopt this *or some other expression of the
> principle of intergenerational equity* is a matter for social
> judgment.

NAS Report pp. 56-57, emphasis added.

We generally agree with the NAS statement. A single dose standard
applicable at all times would typically be consistent with the principle
of intergenerational equity. However, as we noted in Section III.A.2,
there may be some reason to believe that a 15 mrem/yr peak dose limit in
our rule could be viewed as "overly conservative" from the NAS
perspective and not consistent with the intent of the committee. In such
a case, it must be considered whether such a conclusion would have
implications for the appropriate expression of the principle of
intergenerational equity. Further, NAS clearly acknowledges that "some
other" approach could also be consistent with that principle. We believe
it is reasonable to conclude that "some other" approach must include
situations where the same dose standard does not apply at all times. The
rulemaking process we are following is the accepted way for "social
judgment" to be incorporated into regulations.

Determining whether a dose limit is adequately protective of both
current and future generations must also consider the ability of
performance assessments, and those who interpret them, to distinguish
between differing repository designs, as well as different
conceptualizations of total system performance over very long time
frames. In our view, it makes little sense to assert that a 15 mrem/yr
dose limit for the period within 10,000 years is more "protective" than
a higher limit much later in time if, in the time frame of hundreds of
thousands of years, the uncertainties in projecting disposal system
performance cannot easily make distinctions at such incremental levels.
As we stated in our proposal, "In our view, the 350 mrem/yr level and
these other values are within a range of values for which projections
might well be indistinguishable after several hundred thousand years.
That is, when taking increasing uncertainties into account in the very
long term, the effects of factors that would distinguish projections of
100, 200, and 350 mrem/yr within a 10,000-year time frame are more
difficult to identify clearly at very long times, so that such
projections may be qualitatively identical to each other and to the
level of performance represented by projections of 15 mrem/yr at 10,000
years." (70 FR 49038) Where fundamental uncertainties have significant
effects, decisions about overall safety based on incremental doses may
be less defensible.

We believe this is a very real challenge at Yucca Mountain. As discussed
in more detail in Section III.A.5 ("Uncertainty and Reasonable
Expectation"), we estimate that uncertainties in transport through the
natural barrier system alone contribute roughly two orders of magnitude
to the spread of projected doses within the period of geologic
stability, when starting with a defined situation at 10,000 years where
uncertainty in projections is already present (Docket ref). We believe
that a peak dose standard of 350 mrem/yr, which is comparable to average
background radiation exposures and well within the variation of such
exposures across the United States today, represents a protective and
reasonable approach that appropriately balances the influence of
uncertainty on long-term projections with the demands of
intergenerational equity. More discussion of this topic may be found in
Section 9 of the Response to Comments Document for this final rule
(Docket No EPA-HQ-OAR-2005-0083-xxxx).

**III.A.5 Uncertainty and Reasonable Expectation**

In our proposal, we stressed the uncertainties inherent in projecting
disposal system performance over times as long as 1 million years to
support our proposal for a higher peak dose standard beyond 10,000
years. Such uncertainties, we argued, make it more difficult to
distinguish among incremental projected doses and influence the judgment
that those projections will meet a standard with "reasonable
expectation." We concluded that, in light of increased uncertainty, "the
concept of reasonable expectation underlying our standards implies that
a dose limit for that very long period that is higher than the 15
mrem/yr limit that applies in the relatively 'certain' pre-10,000-year
compliance period could still provide a comparable judgment of overall
safety." (70 FR 49029)

Many commenters disputed this conclusion, contending that our emphasis
on uncertainty, reasonable expectation, and a related concept,
implementability by NRC in a licensing process, is intended to disguise
our true intent, which is to set a standard that Yucca Mountain can
pass. A number of commenters took the position that, while our concern
about uncertainty may be legitimate, the only legitimate response is to
say the site cannot be licensed in the face of such overwhelming
uncertainty. Still other commenters challenged our position that
uncertainty generally increases as the time covered by the assessment
increases (or as the time of peak dose moves farther out in time). They
cite statements by the NAS such as "analyses that are uncertain at one
time might not be so uncertain at a later time; for example, the
uncertainties about cumulative releases to the biosphere that depend on
the rate of failure of the waste packages are large in the near term but
are smaller later, when enough time has passed that all of the packages
will have failed." (NAS Report pp. 29-30) Some commenters also pointed
to numerous NAS statements regarding use of "bounding" assumptions as an
indication that the committee did not believe that uncertainties become
more difficult to manage at longer times.

On this last point, we believe it should be clear that NAS did view
overall uncertainties as increasing with time: "We recognize that there
are significant uncertainties in the supporting calculations and that
the uncertainties increase as the time at which peak risk occurs
increases." (NAS Report p. 56) On the role of bounding assessments, we
have been more cautious, as described in our proposal (70 FR 49021,
49029, 49042). We do believe that bounding analyses have value, but that
value can be compromised if the analyses are excessively conservative in
the assumptions underlying the analysis or the spread of parameter
values chosen for the analysis. One purpose of bounding analyses is to
assess reasonably conservative scenarios in order to provide confidence
that actual doses will be lower than projected. However, uncertainty
associated with conceptual models or data can drive the use of bounding
analyses, although reliance on them can come at the expense of more
realistic scenarios that may contribute more to the understanding of
site performance. NAS also took this position in stating "care should be
given as to how one could combine the robust, bounding estimate type of
assessment with a probabilistic analysis." (NAS Report p. 79) In this
regard, we also disagree with commenters who advocated deliberately
increasing the amount of conservatism in the modeling as a way to
address uncertainty. While we agree that some conservatism is inevitable
and may be desirable, we do not believe judgments of disposal system
safety should focus on scenarios selected to be extreme.

In general, as we discussed at length in our proposal, there is overall
agreement that uncertainties in long-term dose projections increase,
which decreases confidence in numerical projections and makes it more
questionable to rely on them as the basis for regulatory
decision-making. The typical response to this internationally is to
require strict compliance with a dose or risk limit for only a few
thousand years. Numerical results for much longer periods are considered
as indicative of disposal system performance, but do not have
quantitative standards associated with them. However, we believe the
appropriate response to the Court decision is to establish a numeric
dose limit against which compliance can be assessed at the time of peak
dose, within the period of geologic stability. That dose limit must be
protective, meaningful, implementable, and consistent with the NAS
Report, the Court ruling, and the principles of reasonable expectation.
It is incumbent on us, in meeting these goals, to consider how the
factors affecting long-term dose projections, including uncertainty,
influence the selection of the peak dose limit.

In responding to comments on this issue, we considered how it might be
possible to demonstrate the increase in projected uncertainties and
provide a quantitative estimate of the degree of increased uncertainty
that might be encountered. To examine the long-term propagation of
uncertainty in dose projections, we used a simplified Yucca Mountain
site performance assessment model and constructed a hypothetical
disposal system that would, under site conditions, produce a mean dose
to the RMEI of 15 mrem/yr at 10,000 years. That is, we estimated the
number of waste package failures that would be necessary to produce a
disposal system operating at the "edge of compliance" at 10,000 years.
This disposal system, which would still meet the performance standard at
10,000 years, was the reference base case for our uncertainty analyses.
The number of "failed" waste packages needed to produce the reference
case dose (a mean of 15 mrem/yr at 10,000 years) was calculated using
the site model and parameters, and assumed the components of the
engineered barrier did not function to provide containment (i.e., the
titanium drip shields designed to divert water from the waste packages,
as well as other components of the engineered barrier system, were
removed from the model). Further, upon "failure" of a waste package, the
entire inventory of that package was assumed to be available for
dissolution and transport.

To assess the progressive effects of uncertainty, the number of "failed"
packages was limited to the number necessary to produce 15 mrem/yr at
10,000 years, and the site model was used to make dose projections from
10,000 years (the reference base case) through the period of peak dose
within the period of geologic stability. Thus, the system established as
a starting point for the peak dose projections was one in which some
degree of release and transport to the RMEI had already taken place,
providing a basis for judging how the continuation of these processes
would change the results over time. These analyses therefore examined
only the effects of uncertainties from the natural barrier portion of
the disposal system, since additional waste package failures were not
considered. It should be recognized that the base case was determined
using probabilistic methods, so the results at 10,000 years already
showed some effects of uncertainty, as indicated by the range of
projected doses. We found that the uncertainty in dose projections, from
the base case (at 10,000 years) to peak dose (as measured by the spread
in dose estimates between the 5 and 95 percentiles at these times),
increased by approximately two orders of magnitude. These results showed
quantitatively that uncertainty in performance projections does increase
with time for the Yucca Mountain system, and supports the premise that
increasing uncertainty reduces the degree of confidence that can be
assumed for very long-term performance assessments. The increasing
uncertainty in dose projections over very long time periods lessens the
ability of performance assessment modeling to meaningfully distinguish
between alternative (and equally "likely") "futures" represented by
individual model simulations, and ultimately to distinguish between
alternate models and assumptions for site performance assessments.

Although we were primarily interested in the relative uncertainty of the
dose projections, we also note that the mean peak doses calculated, for
various variations of modeling parameters and assumptions, were found to
be in the range of approximately 300 -- 400 mrem/yr. This result offers
a significant insight into the degree of uncertainty growth, in that the
increases were not excessive, pushing the reference mean peak dose into
the many rem/yr range or higher at peak dose, nor were the uncertainties
low enough (such that the separation of high-end results from the
remainder of the distribution is more limited) such that the calculated
mean peak doses remained in the tens of mrem/yr range. While it does not
directly inform our selection of the peak dose limit in today's final
rule, this observation suggests that the 350 mrem/yr peak dose limit
could, as we stated in our proposal, be "qualitatively identical...to
the level of performance represented by projections of 15 mrem/yr at
10,000 years" (70 FR 49038), and supports our reasoning that for very
long periods within the geologic stability period, a dose limit based on
variations in background radiation levels is a reasonable approach to
setting a dose limit, considering the increasing uncertainties affecting
performance projections and the associated difficulty in interpreting
them. In this sense, we believe our uncertainty analyses do provide some
confirmatory evidence for the line of reasoning used to set the
post-10,000-year peak dose limit. From that perspective, we do not
believe that longer-term limit should be perceived as a "loosened"
standard relative to the 10,000-year standard. More detail on the site
model we used, parameter databases, sensitivity analyses and discussion
of the results, is provided in the technical reports describing this
work (docket references).

It should be understood that these assessments do not explore how the
disposal system will actually perform over time, and should not be
directly compared to DOE's performance assessments, which include the
engineered components of the repository and show results from the
integrated disposal system. The disposal system examined in our analyses
was a hypothetical one developed exclusively to examine the effects of
uncertainty in the performance of the natural barrier over time and the
consequences on dose projections. Our study indicates that, while the
timing of waste package failures is perhaps the uncertainty with the
greatest overall effect on projected peak dose, the natural barrier
system also contributes significant uncertainty, contrary to some
comments. For the actual performance of the disposal system, the
engineered barriers will function in addition to the natural barrier to
provide containment and isolation, and the entire inventory of waste
packages in the repository will contribute to the long-term behavior of
the disposal system. It is not possible to predict exactly how
uncertainty will influence dose projections for the actual disposal
system, since all the variables will be in play in such assessments. Our
analyses are useful to demonstrate quantitatively that uncertainties do
increase over time and provide an "order-of-magnitude" estimation of the
effects, as well as to show that uncertainties lessen the ability of
performance assessments to meaningfully distinguish between alternative
"futures" and performance scenarios. Regarding the quantitative
estimates of uncertainty, it should also be noted that there is a
difference between relative uncertainty and absolute uncertainty in
projecting potential health effects, which also plays into
interpretation of results. For example, a spread of doses from 0.1
mrem/yr to 10 mrem/yr represents two orders of magnitude. A spread of
doses from 10 mrem/yr to 1,000 mrem/yr also represents two orders of
magnitude, yet the absolute uncertainty in the latter case is clearly
greater. Finally, the results of our analyses indicate that our
rationale for selecting a peak dose limit comparable to background
radiation levels is not unreasonable when compared to the magnitude of
uncertainties and their effects on projected doses.

**III.A.6 What is Geologic Stability and Why is it Important?**

Underlying the NAS recommendation to assess compliance at the time of
maximum risk is the concept of geologic stability (i.e., peak dose
should be assessed "within the limits imposed by the long-term stability
of the geologic environment," NAS Report p. 2). NAS viewed this as an
important consideration in assessing performance, both analytically and
in regulatory review. Indeed, NAS discussed two important kinds of
uncertainty in describing this concept, which are spatial and temporal
uncertainty. The committee concluded that spatial uncertainties will
always exist no matter what time frame is used for the performance
assessments. Temporal uncertainties, on the other hand, will vary over
different time frames, and the presence of such uncertainties indicates
the advisability of defining a "period of geologic stability," during
which performance projections can be made with some degree of
confidence. For time periods where conditions at the site would change
dramatically in a relatively short time, projections of site conditions
would be highly speculative, and consequently performance assessments
would have very limited if any validity. It is important to understand
that "stable" in this context is not synonymous with "static and
unchanging." Rather, NAS recognized that many "physical and geologic
processes" are characteristic of any site and have the potential to
affect performance of the disposal system. NAS concluded that these
processes could be evaluated as long as "the geologic system is
relatively stable and varies in a boundable manner" (NAS Report p. 9).
Thus, the site itself could be anticipated to change over time, but in
relatively narrow ways that can be defined ("bounded"). Implicit in the
NAS recommendation is the idea that the maximum risk might occur outside
the period of geologic stability, but assessments performed at that time
would have little credibility and would not be a legitimate basis for
regulatory decisions: "After the geologic environment has changed, of
course, the scientific basis for performance assessment is substantially
eroded and little useful information can be developed." (NAS Report p.
72)

NAS judged this period of "long-term stability" to be "on the order of
one million years." (NAS Report p. 2) We describe in Section III.A.7
("Why is the Period of Geologic Stability 1 Million Years?") the policy
judgment on our part to explicitly equate the period of geologic
stability with 1 million years. More important, however, is to
understand the relationship among the regulatory definition, the
physical reality of the site, and the performance assessment models. In
reaching its conclusion, NAS considered information available on the
site properties and the processes as they currently operate. This
provides a basis for understanding how the site functions today, but
would not be sufficient to project that understanding for periods of
millions of years into the future. To do that, NAS also considered
information obtained through studies of the geologic record at the site,
to see if evidence existed for times when processes were either
fundamentally different or they operated at different rates. This is
similar to our recommendation that DOE consider at least the last two
million years (the Quaternary period) in characterizing FEPs. In fact,
examination of the Quaternary geologic record is an important component
in understanding the evolution of the geologic setting over time. NAS
expressed confidence that neither the processes active at the site, nor
the site itself, had changed in fundamental ways over the Quaternary
Period and longer, and probably would continue to behave much as it does
today for the next million years. NAS therefore suggested that
conditions could be bounded with reasonable confidence for periods "on
the order of one million years."

Models used to assess performance need to incorporate a description of
the bounds under which the model can be considered valid, so as to avoid
physically impossible situations, as well as assure that the conceptual
models upon which the performance assessments are based reasonably
represent the way the site is expected to behave over the period of
stability. They must be defined so that significant changes to the
properties of the site and physical and geologic processes are not
projected inadvertently to create conditions of "geologic instability."
That is, they must avoid crossing over into sets of conditions that
would in reality not be a geologically stable situation, or are outside
the bounds under which the model can be considered valid. Here again the
examination of the geologic record at the site provides the means of
constructing the models to adequately make simulations of future
performance that reflect the range of expected conditions at the site
over the regulatory compliance period. Parameter distributions used in
the simulations, which are the fundamental input information used to
make the dose assessments, should not be limited only to data collected
for the present situation at the site, but should consider how those
parameter values could change over the period of stability. Expert
judgment where appropriate, based upon site-specific information and
broader understanding of how these processes operate in general, plays
an important role in defining such modeling input data.

The geologic record is the primary source of information on the question
of geologic stability and was considered by NAS in reaching its
conclusions about the geologic stability period. We believe that the
geologic record at the site clearly supports the position that the site
will be stable over the course of the next million years. Conclusions
based on extrapolation beyond what can be supported in the geologic
record should be avoided.

**III.A.7 Why is the Period of Geologic Stability 1 Million Years?**

Today's final rule includes a compliance period of 1 million years,
during which DOE must demonstrate compliance with the
individual-protection and human-intrusion standards. As discussed at
length in our proposal and more briefly in Sections I and II of this
document, our rulemaking is in response to the D.C. Circuit decision
vacating the 10,000-year compliance period in our 2001 rule. The Court
concluded that the 10,000-year compliance period was not based upon and
consistent with recommendations of the NAS, as the EnPA required. The
NAS recommended "that compliance with the standard be assessed at the
time of peak risk, whenever it occurs, within the limits imposed by the
long-term stability of the geologic environment, which is on the order
of one million years." (NAS Report p. 2) NAS found that "compliance
assessment is feasible for most physical and geologic aspects of
repository performance on the time scale of the long-term stability of
the fundamental geologic regime," and accordingly "there is no
scientific basis for limiting the time period of an individual-risk
standard." (NAS Report p. 6) As a matter of policy, we believe it is
appropriate and necessary to define a compliance period within which our
standards apply. This section discusses the considerations that led us
to conclude that a compliance period of 1 million years is appropriate
from a policy perspective and consistent with NAS statements regarding
geologic stability at Yucca Mountain.

As discussed in Section III.A.6 ("What is Geologic Stability and Why is
it Important"?), the NAS introduced the concept of geologic stability in
its report and referred to it repeatedly in its discussions (NAS Report,
e.g., pp. 9, 55, 69, 71, and 72). In discussing the physical properties
and geologic processes leading to the transport of radionuclides away
from the repository, the NAS committee concluded "that these physical
and geologic processes are sufficiently quantifiable and the related
uncertainties sufficiently boundable that the performance can be
assessed over time frames during which the geologic system is relatively
stable or varies in a boundable manner." (NAS Report p. 9) While
variation of site characteristics over time produces some uncertainty
(NAS Report p. 72), NAS believed that such changes could be bounded
during the period of geologic stability of the site (NAS Report p. 77),
i.e., as long as the conditions do not change significantly. NAS also
noted that "\[a\]fter the geologic environment has changed, of course,
the scientific basis for performance assessment is substantially eroded
and little useful information can be developed." (NAS Report p. 72)
While NAS made no additional qualification on what constituted
"significant" changes, it made numerous references in its report to a
stability period for the site "on the order of one million years." The
committee concluded that during this period it would be feasible to make
projections of repository site conditions. We concur and believe that
assessments can be made and bounded where uncertainty exists, and
consequently performance assessments can be developed with adequate
confidence for regulatory decision-making within the context of the
requirements adopted in today's final rule. We discuss some additional
qualifications to this proposition in the remainder of this section.

While the NAS characterized the length of the geologic stability period
in loose terms ("on the order of"), we believe it is appropriate to fix
the stability period duration as a matter of regulatory policy. We find
support on this point from NAS: "It is important, therefore, that the
'rules' for the compliance assessment be established in advance of the
licensing process." (NAS Report p.73). We believe, therefore, as a
matter of regulatory philosophy and policy, that a relatively loosely
defined stability period "on the order of" one million years is not
sufficiently specific for regulatory purposes, i.e., implementing our
standards and reaching a compliance decision. Indeed, NAS clearly
considered that the compliance period could be one of the "rules" that
should be established for compliance assessments. (NAS Report p. 56)
Some commenters suggested that the period of geologic stability could be
longer (or interpreted "on the order of one million years" as possibly
as long as ten million years), and said our rule should allow
consideration of longer timescales if justified by considerations of
geologic stability. The actual period of geologic stability at Yucca
Mountain is unknowable, and we disagree that an open-ended compliance
standard is justified over such time frames. We believe that the
applicant (DOE) and the compliance decision-maker (NRC) must have
definitive markers to judge when compliance is demonstrated, and that a
loosely defined time frame does not provide such a marker for
implementation of our standards in a licensing process. We believe that
the geologic stability period of 1 million years that we have defined
provides the necessary marker, and is within our discretion to set as a
matter of policy. (See generally NAS Report p. 3) To do otherwise we
believe would leave the licensing process in a potentially untenable
situation of dealing with possibly endless debate over exactly when a
peak dose occurs in relation to a compliance period time limit. Such
debate can arise because of the inherent uncertainty that exists in
characterizing the complex processes and variables involved in
projecting performance of the disposal system over very long periods of
time. As the NAS explained, "although the selection of a time period of
applicability has scientific elements, it also has policy aspects we
have not addressed." (NAS Report p. 56)

As commenters have pointed out, the rate of waste package failure is a
dominant factor in determining when the peak dose for a probabilistic
assessment will occur. With all the parameters (and the uncertainty in
their values over time) involved in a total system performance
assessment, as well as the assumptions necessary to select processes
involved in projecting performance, it is quite possible that
significant debate could result in the licensing process over selection
of the parameter values and the resulting timing of the peak dose
results. We do not believe such debate is warranted because it would not
advance the goal of providing a reasonable test of the disposal system.
We also believe that the 1 million year stability period provides the
needed definitive marker for judging the time over which the standards
apply and is an appropriate exercise of our policy discretion.

Throughout our proposal and in this final rule we have cited a
significant number of international references to support policy
judgments such as the one discussed here. Readers may recall that we
cited such references suggesting that dose projections beyond 1 million
years have little credibility and believe that we used those arguments
to justify proposing the 1 million-year compliance period (70 FR 49036,
August 22, 2005). We did not explicitly discuss in the proposal our
reasons for selecting 1 million years as the compliance period and
equating it to the period of geologic stability, other than references
to the NAS language that it is "on the order of" 1 million years.
However, these sources do generally reflect widespread acceptance of the
proposition that quantitative performance projections at very long time
frames have limited utility for regulatory decision-making, and that 1
million years may be a reasonable reference point beyond which such
projections either should not be required or should be considered only
in their broadest sense.[^4] Further, while it should be clear that we
agree with the thrust of those international sources regarding the
effects of uncertainty on long-term dose projections and the relative
level of confidence that can be placed in them for decision-making, we
believe the peak dose standard in today's final rule appropriately
accommodates those considerations and is protective, meaningful,
implementable, and provides a reasonable test of the disposal system
that is consistent with the NAS Report, D.C. Circuit decision, and the
principles of reasonable expectation.

To support these general policy arguments, which would lead us to
consider a time period of approximately 1 million years as an
appropriate regulatory time frame, it is necessary to address NAS's
scientific judgments. While NAS did not define with precision the period
of time that the geologic environment likely would remain stable, for
purposes of our regulation we believe scientific information can be
relied upon to support a firm definition of that period as ending at 1
million years after disposal. Further, we believe that equating a
specific time period with the "period of geologic stability" is a
site-specific decision, as NAS's statements regarding geologic stability
were wholly in the context of Yucca Mountain. (See, for example, NAS
Report p. 69: "The time scales of long term geologic processes at Yucca
Mountain are on the order of 10^6^ years"; and NAS Report p. 85: "The
geologic record suggests this time frame is on the order of about 10^6^
years.") Therefore, we have considered how the natural processes and
characteristics at the Yucca Mountain site would support defining the
period of geologic stability as ending at a specified time after
disposal. In considering the natural setting, many comments expressed
the view that the site's natural characteristics are so conducive to
rapid release and transport of radionuclides, only the waste packages
and other engineered barriers would make it possible for significant
doses to be delayed much beyond 10,000 years. We believe it is therefore
also appropriate to consider the geologic stability period from the
perspective of a reasonable length of time to allow significant waste
package failure, which is the limiting factor in projecting doses within
a specific time period, as discussed earlier. Natural processes and
events would contribute to both the package failures and to the
subsequent transport of radionuclides, even if such failures occur
relatively late in the period under consideration.

A consideration of the past history of the site, in the areas of igneous
and seismic activity, also supports a 1 million year stability period.
Information compiled by the NRC (Docket No. EPA-HQ-OAR-2005-0083-0373)
concerning basaltic igneous activity around the site shows that this
type of activity has been the only activity around the site through the
Pliocene (beginning roughly 5.4 million years ago), and that the volume
of eruptive activity (both tuff and basaltic material) has decreased
continually over the last 10 million years (Coleman et al., 2004, Docket
No. EPA-HQ-OAR-2005-0083-0378). From the identification of surface
features as well as indicators of buried remnants of past volcanic
activity, the episodes of basaltic activity around the site can be shown
to have occurred in clusters of events around 1 million and 4 million
years ago (Hill, 2004, Docket No. EPA-HQ-OAR-2005-0083-0373). The
occurrence of these clusters indicates that the nature and extent of
past volcanic activity can be reasonably well characterized and that
annual probabilities for such events can be reasonably estimated from
the geologic record around the site. Annual probabilities of volcanic
disruptions to the repository have been estimated by various
investigators, and range from as high as 10^--6^ to as low as 5.4 x
10^-10^ (Coleman et al, 2004, Docket No. EPA-HQ-OAR-2005-0083-0378).

Further, while geologic stability may be viewed as being affected
primarily by large-scale events, accumulations of small-scale changes
over very long time periods also have the potential to alter the
geologic setting and affect the technical basis for performance
assessments. Tectonic events have such a potential at Yucca Mountain.
Rates of displacement on the nearest potentially significant fault in
the region average about 0.02 mm/yr. (DOE, Science & Engineering Report,
2002, p. 4-409, Docket No. EPA-HQ-OAR-2005-0083-0069) This means that in
10,000 years, there could be 20 cm (0.65 ft) of displacement, a
relatively small change not likely to affect performance of the geologic
system. However, in 1 million years, the same rate of movement results
in 20 m (65 ft) of displacement on the fault. Using the larger estimates
of movement within the range of potential movement, displacement could
be as much as 30 m (100 ft) over 1 million years. Such changes in the
geologic setting at Yucca Mountain have the potential to erode the
scientific basis for performance assessment [so as to render the
assessment of little value to decision-makers]{.mark}.

NAS also stated that "we see no technical basis for limiting the period
of concern to a period that is short compared to the time of peak risk
or the anticipated travel time." (NAS Report p. 56) This statement
suggests that the stability period must be long enough to allow FEPs
that pass the probability and significance screens to demonstrate their
effects, if any, on the results of the performance assessments, even
from waste package failures occurring relatively late in the period. In
contrast to the accumulated small-scale changes discussed above,
larger-scale seismic events are more likely to contribute directly to
radionuclide releases through the effects of ground motion. Strong
seismic events could damage waste package integrity by causing
emplacement drift collapse or vigorous shaking of the packages
themselves. Earthquake recurrence intervals for the site indicate that
strong events could reasonably be assumed to test waste package
integrity at various times within the 1 million-year period (Docket No.
EPA-HQ-OAR-2005-0083-0374 and 0379). In addition, we note that estimates
of ground water travel time from the repository to the RMEI location is
on the order of thousands of years (see the BID for the 2001 final rule,
Docket No. EPA-HQ-OAR-2005-0083-0050). At these rates, the effects of
disruptive volcanic and seismic effects on releases would not be delayed
from reaching the RMEI location during the stability period, e.g. added
releases from a low probability seismic event at 800,000 years would
have ample time to be captured by the performance assessments. Based on
these considerations, the 1 million-year period is a sufficiently long
time frame to evaluate the potential consequences of both gradual
processes and disruptive events on disposal system performance.

In summary, for regulatory policy as well as site-specific scientific
considerations, we believe that fixing the period of geologic stability
for compliance assessments at 1 million years provides a reasonable test
for the disposal system performance. We believe a fixed time period is
necessary both to provide a definitive marker for compliance
decision-making and to prevent unbounded speculation surrounding the
factors affecting engineered barrier performance and the ultimate timing
of peak dose projections. Examination of site characteristics indicates
that the influences of natural processes and events on release and
transport of radionuclides would be demonstrated even for waste package
failures occurring relatively late in the period. We believe that
setting a 1 million year limit is a cautious but reasonable approach
consistent with the NAS position on bounding performance assessments for
uncertain elements affecting disposal system performance. Finally,
explicitly defining the period during which our standards apply will
focus attention on times for which the geologic setting and associated
processes are more quantifiable and boundable, rather than entering
debate on disposal system performance in time periods where the
fundamental geologic regime may have sufficiently changed so that the
"scientific basis for performance assessment is substantially eroded and
little useful information can be developed." (NAS Report p. 72)

**III.A.8 How Will NRC Judge Compliance?**

Today's final rule includes a modification of our proposal that NRC use
the median of the distribution of projected doses from DOE's
probabilistic performance assessments to determine compliance with the
350 mrem/yr peak dose standard between 10,000 and 1 million years. After
consideration of public comments, today's final rule directs NRC to use
the arithmetic mean of the distribution of projected doses to determine
compliance with the peak dose standard, provided that the value of the
arithmetic mean is less than or equal to the 75^th^ percentile value of
the distribution of results. If the arithmetic mean is greater than the
75^th^ percentile value at the time of peak dose, the 75^th^ percentile
value shall be used instead. To determine compliance with the 15 mrem/yr
standard applicable for the first 10,000 years after closure, NRC shall
use the arithmetic mean of the distribution of projected doses without
qualification.

In reaching this decision, we considered comments raising legal,
technical, and policy points. We believe the use of the arithmetic mean
beyond 10,000 years, constrained by the 75^th^ percentile of the
distribution of probabilistic dose projections, appropriately balances
both the commenters' and our concerns, consistent with the principles of
reasonable expectation, as described in the following discussion.

The legal basis for our proposal was challenged by commenters who
focused on a statement by the NAS committee: "We recommend that the mean
values of calculations be the basis for comparison with our recommended
standards." (NAS Report p. 123) This is the entirety of the statement,
which appeared in the final section of the report describing
commonalities with 40 CFR part 191. Unlike its other recommendations,
the committee did not provide any scientific or technical basis for use
of the mean. Similarly, the committee did not discuss how its
recommendation to assess compliance at times "on the order of one
million years" might influence applicability of the mean (when 40 CFR
part 191 applied for 10,000 years). Specifically, NAS did not address
the statistical nature of probabilistic analyses, nor did it indicate
that technical or policy considerations might come into play if
projections are extremely skewed or otherwise suggest the mean would not
be representative of expected performance. Given its context, lack of
amplifying discussion, and location in the report, we have not viewed
this statement as comparable to the other recommendations made by the
committee. We did receive some comments making the case that our
proposal to use the median could be consistent with the NAS statement,
in the sense that, as noted above, the committee's recommendation was in
the context of a standard covering only 10,000 years, but did not show
any appreciation that the distribution of projections covering hundreds
of thousands of years might differ in significant ways from those
shorter-term projections. Nevertheless, in addition to public comments
urging us to use the mean, the committee's use of the word "recommend,"
coupled with the D.C. Circuit's interpretation of the consistency of our
2001 rule with the NAS recommendations, makes the median a less
attractive option for our final standard. As a result, we have decided
today to start with the mean as the compliance measure, as recommended
by NAS, and qualify its use to address important policy concerns.

Many commenters also took issue with our proposal to use the median on
technical and policy grounds. Most commenters objected to our proposal
on the grounds that the median would be a less stringent measure than
the arithmetic mean (i.e., the median would be lower than the arithmetic
mean), and therefore compliance would be easier to demonstrate. We agree
that the arithmetic mean value likely will be higher than the median
value of the distribution of calculated doses, given the nature of these
long-term projections. However, it does not necessarily follow that the
median would not be protective of public health and safety. As we
discussed in our 2001 rulemaking, even for periods of 10,000 years we
pointed out that scenarios resulting in very high dose estimates had the
potential to strongly influence the mean value. In such cases, we warned
that "as the only alternative for a compliance measure, the mean in some
cases may be interpreted too restrictively." (66 FR 32125, June 13,
2001)

In that 2001 rulemaking, we stated that the mean value of the
distribution would be the "literal mathematical interpretation" of
"reasonable expectation" (66 FR 32125). Further, as we noted in our 2005
proposal, NAS used the term "expected value of a probabilistic
distribution" to define a value to compare to the regulatory standard.
For a probabilistic analysis in which parameter values are typically not
single values but are distributed and sampled randomly, and the
calculated results are weighted by their probabilities, the "expected
value" of the resulting distribution is most often equated to the
arithmetic mean. In the context of disposal system performance, the
arithmetic mean would then be considered to represent the "expected
performance" of the system.

As we discussed in our 2005 proposal, however, the arithmetic mean can
be strongly influenced by the values at the high end of the
probabilistic distribution, known as the extreme values. As a result,
depending on the nature of the extreme values, the arithmetic mean may
give a distorted picture of expected performance. A single data point at
the very high end could potentially affect the arithmetic mean more
strongly than multiple data points that are more centrally located
within the distribution. The arithmetic mean may then reflect a very few
data points with very high values. When compared to a normal
(bell-shaped) or other symmetrical distribution, where values are
distributed equally around the most common value, a distribution in
which most data points have values higher than the most common value
(the "peak"), and which contains some extreme values at the high end,
will appear stretched toward the high end, so that the upper-end "tail"
is longer. Such a distribution is considered "positively skewed." Such a
characteristic is typical of long-term disposal system projections, as
the random selection of parameter values and the very long times under
consideration are likely to result in a few outcomes with unusually high
doses.

As skewness in the distribution increases, it becomes increasingly
likely that the arithmetic mean will become farther removed from the
bulk of the observed data points. In such cases, the "expected value"
may not actually be close to the result that would be "expected" if
another calculation were performed. It may in fact be found in a part of
the distribution with very few results, if the upper-end "tail" is very
long. We pointed out that this effect, as applied to long-term disposal
system performance, could be unrealistically conservative (tending to
overstate the risk) and thus would not be consistent with "reasonable
expectation." In essence, an arithmetic mean value could drive
regulatory decision-making on the basis of very unlikely combinations of
parameter values and not on projected performance. In our 2001
rulemaking, we suggested that in cases where the arithmetic mean is
highly influenced by extreme values, use of the median could in fact be
more consistent with "reasonable expectation." (66 FR 32125)

Some comments took our statements as justifying the use of the median on
the grounds that DOE's modeling will be excessively (and improperly)
conservative. That is, commenters believed we had already concluded that
the modeling would be conservative, and chose the median as a way to
compensate for this problem. That was not our intent; however, we
acknowledge that we stressed this point, and that our primary reason for
not proposing the arithmetic mean was the concern that a limited number
of high-end estimates would disproportionately skew the mean toward
those extreme values. We cautioned strongly in our proposal against
introducing excessive conservatism into either the models or the
parameter value distributions, and do not agree with the implication of
many commenters that a more conservative compliance measure is by
default preferable as a regulatory policy. Excessive conservatism is not
desirable and can lead to assessments based on a system that is unlikely
to exist. The decisions that follow from such assessments would then be
focused on extreme situations. Upon further consideration, therefore,
and in light of the NAS language, we believe the more appropriate
approach is to apply the arithmetic mean with the qualification
described here and emphasize the "reasonable expectation" concept as
explicitly discouraging reliance on extreme assumptions, while
encouraging "cautious but reasonable" projections. For similar reasons,
we reject comments calling on us to require that the single maximum
possible projected dose be used to determine compliance. This approach
would rely only on the extreme worst case results, discarding altogether
the performance information to be gained from other, equally likely
outcomes, as well as the overall character of the realization resulting
in the maximum projected dose. It should be clear that this approach is
inconsistent with "reasonable expectation," the intent of the NAS
committee, and previous EPA radioactive waste standards (i.e., 40 CFR
parts 191 and 194, which are our generic standards and WIPP-specific
compliance certification criteria, respectively). The principles of
"reasonable expectation" require that uncertainties be recognized and
reliance on extreme situations be avoided.

Having determined now that we will apply the arithmetic mean between
10,000 and 1 million years, we have considered how best to address our
concerns regarding the potential influence of extreme values on the
mean. We proposed the median as a way to address these concerns and to
meet the overall goal of "reasonable expectation," which would focus on
ensuring that the statistical measure captures the area of the curve
where the results are most likely to fall. In determining how we might
qualify the arithmetic mean in a way that would be consistent with the
NAS recommendation and at the same time address the policy issues NAS
did not consider, we looked to the committee report for further insight.
The report clearly recognizes that results should not be driven by
extremes. For example, the committee stated that "unrealistic
assumptions are inappropriate" and noted that "\[t\]he situation to be
avoided, therefore, is an extreme case defined by unreasonable
assumptions regarding the factors affecting dose and risk, while meeting
the objectives of protecting the vast majority of the public." (NAS
Report pp. 103, 5, and 52) While these points were made in the context
of a discussion of the appropriate receptor to which the standard would
apply, they also are consistent with adoption of a performance measure
that is not overly sensitive to extreme results caused by conservative
or bounding assumptions. At the same time, the committee recognized that
in some areas bounding analyses might be all that are possible. When the
NAS committee concluded that long-term changes, such as those caused by
climate, seismicity, and volcanism, could be addressed in a peak dose
performance assessment, it also recognized that these analyses may have
to be based on "bounding assumptions." Specifically, the NAS committee
concluded that "the probabilities and consequences of modifications
generated by climate change, seismic activity, and volcanic eruptions at
Yucca Mountain are sufficiently boundable so that these factors can be
included in performance assessments that extend over periods on the
order of about 10^6^ years." (NAS Report p. 91)

Once the time frame for peak dose performance projections is extended
into the very long term, the confidence that can be placed on either the
high- or low-end release scenarios becomes progressively more difficult
to estimate even though a "bounding" approach may simplify calculations.
Consequently, using the arithmetic mean in compliance decisions could
potentially over-emphasize high-end release results. We believe placing
appropriate bounds on the arithmetic mean is consistent with "reasonable
expectation." Using the arithmetic mean with the upper level constraint
discussed above allows the recognition of growing uncertainties and the
potential for individual realizations to produce unrealistic physical
situations leading to extremely high dose estimates, while at the same
time specifying characteristics of the distribution that must apply at
all times to avoid being overly affected by extreme data points.

For these reasons, the basic approach of our final rule issued today
will require NRC to use the arithmetic mean of the distribution of
results for comparison with the individual-protection standard at all
times. However, in assessing the peak dose between 10,000 and 1 million
years, NRC shall only use the arithmetic mean if it does not exceed 75%
of the projected results. At the time of peak dose within 1 million
years, if the arithmetic mean exceeds 75% of the results projected at
that time, NRC shall use the 75^th^ percentile value for comparison with
the post-10,000-year standard. It should be clear that we are not
adopting the 75^th^ percentile value as the general measure for
comparison with the standard in all circumstances; instead, that value
only comes into play if the arithmetic mean is above that percentile in
the distribution. In other words, compliance would be shown if the mean
is below the standard, even if the 75^th^ percentile value is above the
standard. As a point of comparison, the arithmetic mean at the time of
peak dose (roughly 476,000 years) in DOE's FEIS was at approximately the
70^th^ percentile.

In arriving at this approach, we weighed both our concerns regarding
extreme influences and the concerns of commenters regarding the
protectiveness of our proposal. In our proposal, we emphasized the
concept of central tendency in judging whether a specific statistical
measure could adequately represent the overall nature of the
distribution. We proposed the median as a reasonable representation of
"central tendency," in that it is a measure that always corresponds to
the value at the mid-point of the distribution of projected results,
where one might expect to see the bulk of projected doses. In general,
the "central tendency" is a way to consider where the "middle" of a
distribution can be found, in order to gauge where most of the results
are "likely" to fall. As described previously, the arithmetic mean is
typically considered the best measure of central tendency when the
distribution is relatively symmetrical; however, as symmetry decreases
and the distribution becomes increasingly skewed (appearing stretched to
the high side), the median is often relied upon. As an illustration of
this concept that will be familiar to readers, the median (i.e., the
50^th^ percentile) is generally considered a better indicator of
economic conditions such as housing prices or income, because a
relatively small number of very high values can give a misleading
picture if the mean is used. Because the published results of disposal
system performance assessments generally tend to be skewed in this way,
we believe the sensitivity of the arithmetic mean to extreme values
would make its unqualified use in this context inconsistent with the
principles of reasonable expectation. We are not aware of any consensus
regarding the transition point at which the distribution becomes overly
skewed and the arithmetic mean becomes unrepresentative of performance
assessment results, or whether such a point exists. However, in our
judgment, the 75^th^ percentile provides a realistic measure at which
the arithmetic mean could still be considered reasonably representative
for the reasons discussed above, should it reach that level.

In today's final rule, we see the establishment of the 75^th^ percentile
as clearly providing assurance that the "central tendency" will remain
the focus of the compliance determination, while also limiting the
potential for extreme results to raise the compliance measure to levels
that might be considered less representative overall. By accounting for
projected doses above the 50^th^ percentile (the median), this approach
also clearly improves upon the protectiveness of our proposal by
incorporating up to an additional one-quarter of the results. The margin
between the 50^th^ and 75^th^ percentile values could be more or less
significant depending on a number of factors, including the skewness of
the distribution. As an example that is illustrative only and should not
be taken as indicative of future projected doses, if we consider again
results from DOE's FEIS, the arithmetic mean at the time of peak dose
was about 155 mrem/yr (\~70^th^ percentile), while the median was at
about 55 mrem/yr. Taken together, we believe the use of the arithmetic
mean, with the bounds established today, will provide a reasonable test
of the disposal system over very long times.

We emphasize that limiting the use of the arithmetic mean to the 75^th^
percentile is a policy decision on our part. NAS recommended that we use
the mean as the compliance measure (NAS Report p. 123); however,
scientific considerations alone do not dictate how to apply that measure
(e.g., whether or how to limit the application of the arithmetic mean).
From a technical perspective, the difficulty of comparing the arithmetic
mean to a dose limit does not increase when the mean is above the 75^th^
percentile. However, our primary concern has always been whether dose
projections at very long times are meaningful in themselves, and how
much they can be relied upon for decision-making in the face of rising
uncertainty. As the times covered by assessments increase, we believe
the assumptions and concepts embedded in the models become increasingly
important to informing the regulators, and the numbers generated by the
models become less so.

We believe this policy decision is consistent with both the NAS Report
and the Court ruling. We refer to the statements of the Court: "It would
have been one thing had EPA taken the Academy's recommendations into
account and then tailored a standard that accommodated the agency's
policy concerns"; "Had EPA begun with the Academy's recommendation to
base the compliance period on peak dosage and then made adjustments to
accommodate policy considerations not considered by NAS, this might be a
very different case" (NEI, 373 F.3d at 26 and 31, respectively).

We believe this view also represents the intent of the NAS committee,
which stated that "\[t\]he challenge is to define a standard that
specifies a high level of protection but that does not rule out an
adequately sited and well-designed repository because of highly
improbable events." (NAS Report p. 28) As described previously in
Section III.A ("What Dose Standards Will Apply?"), in personal testimony
before the Senate Environment and Public Works Committee on March 1,
2006, Mr. Robert Fri, chair of the NAS committee, pointed out that "the
specification of the time horizon and the selection of the person to be
protected are intimately connected." As a result, he believed that
retaining the RMEI as the receptor (which the NAS committee recognized
as more conservative than its preferred probabilistic critical group)
while at the same time extending the compliance period "runs the risk of
excessive conservatism," effectively putting us in a position where the
"committee specifically did not want to be." He noted that the committee
had considered and rejected such an approach. (NAS Report pp. 100-103)
He viewed our proposal of a higher dose limit between 10,000 and 1
million years as a way "to avoid becoming overly conservative." He
concluded by stating "the committee recognized that EPA properly had
considerable discretion in applying policy considerations outside the
scope of our study to the development of the health standard for Yucca
Mountain." (See generally NAS Report p. 3) In our view, determining how
to judge compliance with a dose limit is also part of the
standard-setting process, and therefore must be responsive to the same
policy considerations as the selection of the dose limit itself.

It is worth noting that the NAS committee member who favored a more
conservative receptor, mentioned in Mr. Fri's testimony, also disagreed
with the committee's recommendation of the mean ("expected value") as
the statistical measure of compliance. (NAS Report p. 179) This member
believed use of the mean did not adequately capture the uncertainties in
projected doses over long time periods. He therefore proposed that the
standard should include a dose range within which some percentage of
projected doses should be required to fall. This approach is similar to
the statistical distribution that we discussed in our proposal (70 FR
49033-49034, August 22, 2005), which we also considered because it would
effectively consider the uncertainties in the entire distribution. The
NAS member suggested the 95^th^ percentile value as the point of
comparison (that is, 95% of projected doses would have to be within the
specified interval). In effect, the 95^th^ percentile value would become
the dose standard. Mr. Fri's response made clear that the committee
viewed the member's proposed subsistence farmer receptor as potentially
"extreme" rather than "cautious but reasonable." (NAS Report p. 188)
Without knowing the dose range under consideration, one cannot say
whether use of the 95^th^ percentile value might similarly be considered
extreme. However, it may be reasonable to consider whether using the
95^th^ percentile value to determine compliance over the time frames in
question would be consistent with the committee's guidance to be
"cautious but reasonable."

We note that the measure of compliance we are adopting today also
contains elements of the statistical distribution approach discussed in
the preamble to our proposed rule. That approach would have required the
bulk of DOE's projected results to fall within a specified range. No
more than a certain percentage of results could be above the high end or
below the low end of that range. Applying the arithmetic mean with a
constraint at the 75^th^ percentile does not truly duplicate the
statistical approach because the percentage of results that may exceed
the standard is dependent on the relationship of the mean to the rest of
the distribution. However, the approach in today's final rule conveys an
advantage similar to the statistical approach in that it recognizes
growing uncertainties but constrains how much of a role that uncertainty
should play in the compliance demonstration by specifying
characteristics of the distribution that must apply at all times to
avoid being overly affected by extreme results. In some sense, the use
of the median could also be viewed in this way, as it represents the
selection of a specified percentile value (i.e., the 50^th^ percentile)
that must not exceed the standard. At the same time, today's approach
avoids our concern with the statistical distribution approach by
explicitly incorporating the calculated doses into the determination of
the performance measure. We also believe it is more transparent to the
public and provides more clarity regarding the level of disposal system
performance.

To reiterate, we believe the arithmetic mean, bounded by the 75^th^
percentile, is a reasonable measure to apply for the period between
10,000 and 1 million years, as well as consistent with the NAS
recommendation. However, we emphasize again that it may not be if the
assumptions and data used in assessments are selected to over-estimate
the projected doses. It is not that we see no role for conservatism. On
the contrary, we noted in our proposal that "conservatism in long-term
performance projections may be unavoidable in practice" (70 FR 49042).
In some cases, assumptions or parameter values may be labeled as
conservative when there is insufficient evidence to support that claim,
meaning that they may actually be seen as realistic if more information
was available. Rather, it is the consistent reliance on conservatism
when choices present themselves that should be viewed with caution. It
is understandable that limitations in data and models used to represent
highly complex systems with many interacting components create an
inclination to assume those components will perform less effectively
than they should. Both applicant and regulator may take comfort in
seeing conservative assessments comply with the regulatory standard,
indicating that a margin of safety is present. This position was also
expressed by NAS in its frequent references to "bounding" analyses.
However, neither applicant nor regulator should be reassured if there is
embedded conservatism that is difficult to identify and extract so it
can be analyzed. From our perspective, it is valuable and important for
conservatisms to be recognized, understood, and accounted for in
decision-making. We refer again to a statement by the joint IAEA-NEA
Peer Review of DOE's Site Recommendation TSPA:

> At a fundamental level, it is useful to resort to a probabilistic
> analysis of a system evolution in time if a realistic model can be
> attempted but legitimate uncertainties persist. However, if the
> starting model is built *a priori* to be conservative, exercising it
> probabilistically has little or no added value, as one would still
> obtain conservative results. In the TSPA--SR a hybrid
> conservative/probabilistic methodology is used, which causes
> assumptions and reality to be mixed in a confusing way. *In the future
> it may be appropriate to present: (i) A probabilistic analysis based
> on a realistic or credible representation; and (ii) a set of
> complementary analyses with different conservatisms, in order to place
> the best available knowledge in perspective.* These ancillary analyses
> could be given a probabilistic weight as well. This should satisfy the
> regulatory requirements whilst providing a better basis for dialogue
> and decision-making.

pp. 54-55, emphasis in original (70 FR 49028).

We believe the point that should be taken from this statement is that it
is proper for DOE's treatment of conservatisms and uncertainties to be
transparent, so that all interested parties in the licensing review can
evaluate how these issues have been addressed (see also NAS Report p.
69: "transparency in the use of assumptions is critical to evaluating
the calculated risk"). We also believe it is appropriate to reiterate
the role of NRC in implementing "reasonable expectation" in its
licensing review. That review will not simply consist of a comparison of
projected doses with the regulatory standard. Rather, the principles of
"reasonable expectation" will guide NRC's review of DOE's modeling
efforts. Application of these principles should ensure that the analyses
do not rely on extreme assumptions or parameter values, whether
conservative or optimistic. Instead, the full range of reasonable and
defensible parameter values will be emphasized in the performance
assessments and considered in reaching a compliance determination.
Transparency in describing these assumptions and parameter values, and
the uncertainties associated with them, will aid in reaching that
determination.

We considered other methods to address our concerns regarding the
potential influence of extreme values on the statistical measure. One
method that is applied in some situations is to "trim" the distribution
by removing a certain number of values at both the high and low ends of
the distribution, then calculating the "trimmed mean." We decided not to
pursue this approach, primarily because there is no clear basis at this
time to justify trimming the distribution or to specify where the
distribution should be trimmed. Similarly, were we to leave the trimming
to NRC's judgment, we would feel it appropriate to provide guidance, if
not direction in our rule, for NRC to use in identifying extreme values.
The process to be followed in that case is also not clear to us. On this
point, we believe every realization is an equally likely representation
of disposal system performance. That is, one cannot view a zero result
as more or less likely than the highest projected dose, and each extreme
has the same probability of being the "actual" performance as does the
50^th^ (or 75^th^) percentile projection. Therefore, each realization
independently carries important information that should not be
discounted. In that sense, it can be argued that no result should be
considered faulty or subject to elimination simply because it is
extreme, if it is calculated using the same probabilistic parameter
values as more "realistic" results. This is why it is important to
understand the reasons for a seemingly anomalous result, rather than
just eliminating it because it appears anomalous.

It may be argued that limiting the arithmetic mean to the 75^th^
percentile value would in fact effectively eliminate high-end values
from consideration. However, we believe there is a critical distinction
between our approach and the "trimmed mean" approach described above.
The "trimmed mean" approach eliminates values at the "tails" of the
distribution (both high- and low-end) before calculating the mean. As a
result, the mean considers only a subset of the entire distribution. In
the approach we are adopting today, the entire distribution is used to
calculate the mean and identify the 75^th^ percentile value. The
influence of high-end values, including those that may be considered
extreme, is fully incorporated into this result. The arithmetic mean in
this case is likely to be higher than the "trimmed mean" because,
depending on the trimming approach and the relative number and magnitude
of the values being trimmed, the high-end values eliminated from the
calculation could have more influence on the mean than would the low-end
values. The 75^th^ percentile value would be expected to demonstrate
similar characteristics under the two approaches. We believe it is
appropriate for those high-end values to be considered in deriving the
compliance measure, but do not believe it is appropriate for the
compliance determination to be driven by a relatively small number of
realizations. We believe this is consistent with the NAS committee's
view, cited earlier, that "\[t\]he challenge is to define a standard
that specifies a high level of protection but that does not rule out an
adequately sited and well-designed repository because of highly
improbable events." (NAS Report p. 28)

We received a few other comments on our proposed use of the median,
which we will touch on briefly here. Many commenters expressed the
concern that using the median would allow half the projected results to
exceed the dose standard. That is true; however, it would also require
half the projected doses to be below the standard. It should be pointed
out that, regardless of the dose level that is selected, using the
arithmetic mean also allows some percentage of results to exceed the
standard, as recognized by the dissenting NAS committee member's
preferred approach. As the degree of skewness decreases from what is
more typical of disposal system performance assessments, and the
distribution approaches normality, the percentage of results exceeding
the mean increases. When the distribution is normal, the mean and median
are coincident as the most representative measure, and half the results
exceed the mean.

A number of commenters incorrectly concluded that using the median would
allow half the population to receive doses in excess of the standard.
This is not the case. Projected doses are based on the RMEI, who is a
hypothetical individual representative of the current population and
lifestyle in Amargosa Valley. The RMEI is not a population concept, nor
is the RMEI an "average" person. Rather, characteristics of the RMEI,
specifically location and use of ground water, are selected to place
that person among the most highly-exposed members of the population. The
vast majority of people in the vicinity of Yucca Mountain would be
expected to receive much lower doses, if any.

Finally, some commenters believe our proposal gave a misleading picture
of the "true" peak dose limit we proposed, in the sense that the
difference would no longer be simply that between 15 mrem/yr and 350
mrem/yr, but also the difference between using the arithmetic mean for
the first 10,000 years and the median for the period beyond 10,000
years. A number of commenters claimed that a median dose of 350 mrem/yr
would be comparable to an arithmetic mean of 1,000 mrem/yr or more.

Regarding the relationship of the arithmetic mean to the median, as
noted above, given the nature of the projections of disposal system
performance, we do expect that the arithmetic mean will be higher than
the median. However, it is not possible to know how much higher unless
the results of the analysis are known. It appears that many of the
commenters have extrapolated from results presented in the 2002 FEIS,
which showed the arithmetic mean at about 155 mrem/yr and the median at
about 55 mrem/yr, and have speculated that [if]{.underline} the median
were 350 mrem/yr, then the arithmetic mean [would be]{.underline} about
1,000 mrem/yr (1 rem/yr). We believe such speculation is not appropriate
because it assumes a complete performance projection that has not yet
been made for the Yucca Mountain disposal system. We note that DOE's
post-10,000-year projections to support its actual license application,
which will be based upon assumptions and approaches specified in our
final rule, may differ in significant ways from its FEIS projections,
which were meant only to be illustrative in the very long term, but we
cannot forecast the results of those analyses.

We believe our action today to require NRC to determine compliance with
our standards using the arithmetic mean of the distribution of projected
doses, and to limit application of the arithmetic mean to the 75^th^
percentile at the time of peak dose between 10,000 and 1 million years,
fully considers and is consistent with the NAS recommendation, and
appropriately addresses our policy concerns, public comments on this
issue, and historic information to place our action in context (e.g.,
previously published performance assessment results).

**III.A.9 How Will DOE Calculate the Dose?**

Today's final rule requires DOE to calculate the annual committed
effective dose equivalent (CEDE) for comparison to the storage,
individual-protection, and human-intrusion standards using the
radiation- and organ-weighting factors in ICRP Publication 60 ("1990
Recommendations of the ICRP"), rather than those in ICRP Publication 26
("1977 Recommendations of the ICRP"). As we described in our proposal,
this action will incorporate updated scientific factors necessary for
the calculation, but will not change the underlying methodology. We
explained in some detail the use of the terms "effective dose
equivalent" and "effective dose" in the EnPA, the D.C. Circuit decision,
the ICRP publications, and our previous actions to support our position
that use of the weighting factors in ICRP 60 (and its follow-on
implementing Publication 72) is consistent with calculation of effective
dose equivalent, as required by the EnPA. (70 FR 49046-49047)

We received some comment disagreeing with our conclusion that use of the
term "effective dose equivalent" is consistent with the use of the ICRP
60 weighting factors. As we discussed in our proposal, we believe a
close reading of ICRP 60 supports our interpretation that effective dose
equivalent and effective dose are synonymous concepts. ICRP defined two
weighting factors in ICRP 26, the radiation weighting factor, W~R~, and
the tissue weighting factor, W~T~. In ICRP 26, the tissue weighting
factor was presented as a rigid construct with defined values for
specific organs. In ICRP 60, the tissue weighting factor was redefined
as a set of recommended values for an expanded set of organs, and it was
explained that the attributes of the tissue weighting factor include the
components of detriment cited by the comments (fatal and non-fatal
cancers, length of life lost, and hereditary effects). However, ICRP
made a clear distinction between its renaming of the doubly weighted
dose quantity from "effective dose equivalent" (ede) to "effective dose"
(E) and its redefining of W~T~. The association of effective dose
equivalent with the ICRP 26 tissue weighting factors is thus
coincidental but not required. We cited ICRP to that effect in our
proposal:

> The weighted equivalent dose (a doubly weighted absorbed dose) has
> previously been called the effective dose equivalent but this name is
> unnecessarily cumbersome, especially in more complex combinations such
> as collective committed effective dose equivalent. The Commission has
> now decided to use the simpler name effective dose, [E.]{.underline}
> The introduction of the name effective dose is associated with the
> change to equivalent dose, *but has no connection with changes in the
> number or magnitude of the tissue weighting factors*...

ICRP Publication 60, p. 7, paragraph 27, Docket No.
EPA-HQ-OAR-2005-0083-0087, emphasis added.

Similarly, ICRP also states:

> The values of both the radiation and tissue weighting factors depend
> on our current knowledge of radiobiology *and may change from time to
> time*. Indeed, new values are adopted in these recommendations....It
> is appropriate to treat as additive the weighted quantities used by
> the Commission but assessed at different times, despite the use of
> different values of weighting factors. The Commission does not
> recommend that any attempt be made to correct earlier values. *It is
> also appropriate to add values of dose equivalent to equivalent dose
> and values of effective dose equivalent to effective dose without any
> adjustments*.

ICRP Publication 60, p. 9, paragraph 31, Docket No.
EPA-HQ-OAR-2005-0083-0087, emphasis added.

In summary, we believe the intent of Congress in specifying effective
dose equivalent is that the Yucca Mountain standards be based on a
doubly weighted dose quantity, not that the assessment of that quantity
be tied to factors developed at a particular time, when newer science
indicates those factors should be updated. We use effective dose
equivalent for consistency with the terminology used in the EnPA, but
are adopting in today's final rule the current recommended values for
W~T~. Our approach is thus fully consistent with both the current ICRP
recommendations and the EnPA.

Today's final rule does incorporate a change to the proposed definition
of "committed effective dose equivalent" in §197.2 to make it consistent
with language in Appendix A regarding the potential use of future ICRP
recommendations. We received some comment suggesting that the appendix
should not include specific weighting factors, but state only that doses
are to be calculated in accordance with the methods of ICRP 60/72. The
commenter believes this is appropriate because NRC's proposed licensing
requirements included the tissue weighting factors, but not the
radiation weighting factors. Further, the commenter points out that dose
coefficients in ICRP 72 (and Federal Guidance Report 13) consider a
somewhat different set of organs than do the tissue weighting factors.
We prefer not to adopt the commenter's suggestion, which we believe
could lead to questions regarding the appropriate factors to use. We
note that ICRP 60, unlike ICRP 26, is not tied to a specific set of
weighting factors, and allows for the possibility that users will
substitute their own preferred set of factors. Stating only that the
methods of ICRP 60/72 be used to calculate dose, without the additional
stipulations in the appendix, would not provide sufficient clarity on
this point. Therefore, we are adding language to the definition in
§197.2 to the effect that NRC can direct that other weighting factors be
used to calculate dose, consistent with the conditions presented in
Appendix A. We believe this will effectively address the commenter's
concern.

**III.B How Will Today's Final Rule Affect DOE's Performance
Assessments?**

Today's final rule requires DOE to demonstrate compliance with the
individual-protection standard through use of performance assessment. A
performance assessment is developed by first compiling lists of features
(characteristics of the disposal system, including both natural and
engineered barriers), events (discrete and episodic occurrences at the
site), and processes (continuing activity, gradual or more rapid, and
which may occur over intervals of time) anticipated to be active during
the compliance period of the disposal system. These items are
collectively referred to as "FEPs" (features, events, and processes).
Once FEPs are identified, they are evaluated for their probability of
occurrence (i.e., how likely they are to occur during the compliance
period) and their effect on the results of the performance assessment
(i.e., do they significantly affect projected doses from the disposal
system). Addressing these aspects of performance assessment for a
compliance period of 1 million years was a central aspect of our
proposal and is the focus of this section.

After considering public comments, we are retaining §197.36 as proposed,
with two modifications. First, the probability threshold for FEPs to be
considered for inclusion in performance assessments conducted to show
compliance with §§197.20(a)(1), 197.25(b)(1), and 197.30 is now stated
as an annual probability of 1 in 100 million (10^-8^ per year). Because
the same FEPs included in these performance assessments will also be
included in performance assessments conducted to show compliance with
§§197.20(a)(2) and 197.25(b)(2), the same probability threshold applies
in all cases. Second, we are adding a provision to address a potential
effect of seismicity on hydrology that was identified by NAS. The final
rule now requires the potential effects of a rise in the ground-water
table as a result of seismicity to be considered. If NRC determines such
effects to be significant to the results of the performance assessment,
it shall specify the extent of the rise for DOE to assess.

Our 2001 rule set forth three basic criteria for evaluating FEPs for
their potential effects on site performance and their incorporation into
the scenarios used for compliance performance assessments (§197.36).
These criteria retained the same limitations originally established in
40 CFR part 191, which were developed to apply to any potential
repository for spent nuclear fuel, high-level waste, or transuranic
radioactive waste. The approach in part 191 provided a reasonable way to
address this issue. We believe that approach remains reasonable for the
site-specific Yucca Mountain standard, and we believe it is desirable to
maintain consistency between the two regulations for geologic
repositories in the basic criteria for evaluating FEPs. The criteria for
evaluating FEPs are:

-   A probability threshold below which FEPs are considered "very
    unlikely" and need not be included in performance assessments;

-   A provision allowing FEPs above the probability threshold to be
    excluded from the analyses if they would not significantly change
    the results of performance assessments; and

-   An additional stipulation that "unlikely" FEPs need not be
    considered in performance assessments conducted to show compliance
    with the human-intrusion and ground-water protection standards.

As an initial step, a wide-ranging set of FEPs that potentially could
affect disposal system performance is identified. The term "potentially"
is key here, because at this early stage, the list is deliberately
broad, focusing more on "what could happen" rather than "what is likely
to happen at Yucca Mountain." Under the 2001 rule, each of these FEPs is
then examined to determine whether it should be included in an
assessment of disposal system performance over a 10,000-year period by
evaluating the probability of occurrence at Yucca Mountain and, as
appropriate, the effects of the FEP on the results of the performance
assessment. Based on these evaluations, a FEP may be excluded from the
assessment of disposal system performance on the basis of probability,
or if the results of the performance assessments would not be changed
significantly by its exclusion.

We included in our proposal provisions describing how FEPs should be
incorporated into assessments of disposal system performance during the
period of geologic stability, defined as ending at 1 million years after
closure. Our purpose was to build upon the provisions applicable to the
10,000-year compliance period in our 2001 rule to address the
complexities introduced by extending the compliance period to 1 million
years. In general, the database of FEPs applicable to Yucca Mountain
should be the same, regardless of the period covered by the assessments.
In developing our proposal, however, we considered how these general
provisions might change when the compliance period extends to 1 million
years. We also proposed specific provisions to address climate change,
seismicity, and igneous events, which were identified by NAS as
potential "modifiers" whose effects could be bounded within the period
of geologic stability.

Some commenters questioned whether our authority to establish public
health protection standards for Yucca Mountain extended to specifying
how FEPs must be considered, contending that this function properly lies
with the implementing authority (NRC). We disagree. While NRC clearly
has authority to specify such provisions, it is also within our purview
to stipulate such conditions as are necessary to place our regulations
in context and ensure they are implemented as we intended. For analyses
covering 1 million years, it is important to focus on those factors most
affecting performance, if necessary by excluding other aspects that are
more likely to have little or no significance. We believe this approach
is consistent with the direction of the NAS. NAS was charged with
providing advice to EPA on "reasonable standards for protection of
public health and safety" (EnPA Section 801(a)(2)). NAS provided its
findings and recommendations in the context of standards to be developed
by EPA, including discussion of FEPs, for example: "the radiological
health risk from volcanism can and should be subject to the overall
health risk standard to be required for a repository at Yucca Mountain."
(NAS Report p. 95) Further, NAS discussed the question of uncertainty in
quantifying physical and chemical processes and their operation over
long time periods and the inevitability of "residual, unquantifiable
uncertainty," stating "\[t\]he only defense against it is to rely on
informed judgment." (NAS Report p. 80) Therefore, we believe it
appropriate to specify, where necessary, additional provisions for the
treatment of FEPs in disposal system assessments to avoid boundless
speculation. We have explained our understanding of the proper use of
bounding performance scenarios, and we believe we are consistent with
the NAS on this point. Bounding assessments addressing uncertainty in
understanding the long-term behavior of the site should be constructed
using informed judgment, not speculative assumptions without credible
supporting evidence.

Two of the criteria for evaluating FEPs, probability and significance of
the impacts on performance assessments, are of primary importance in
considering how the provisions applicable to the 10,000-year period
might change when the compliance period is extended to 1 million years.
In the proposed rule, we concluded that the 10,000-year FEPs screening
could serve as an adequate basis for longer-term assessments because it
is sufficiently inclusive to be appropriate for the entire 1
million-year compliance period, while at the same time reasonably
bounding the scenarios that must be considered over the longer time
frame. We thought our statements in the preamble on this point were
sufficiently clear, but we understand that the way we structured §197.36
of the proposal, essentially separating the two time periods, may have
caused some confusion. For example, we did not intend to indicate or
imply that the post-closure performance assessments would consist of two
separate and dramatically different calculations, with each having
distinctly different scenario construction, parameter value
distributions, or other attributes. Regardless of the standard against
which compliance is being judged, the probability of occurrence and the
significance of the impacts on performance assessment are the two
primary criteria for including a FEP in the compliance analysis. The
initial screening provides a database of FEPs, which is then used for
both the 10,000-year and post-10,000-year peak dose analyses, with some
additional stipulations for the period beyond 10,000 years. The
discussion that follows addresses each of these screening criteria in
turn.

**Probability**

In the proposed standards, we defined the probability threshold for
"very unlikely" FEPs as a 1 in 10,000 chance of occurrence within 10,000
years, or roughly a 1 in 100 million (10^-8^) chance per year of
occurring. In today's final rule, the probability threshold is now
stated only as an annual probability of 1 in 100 million (10^-8^). We
believe it is appropriate to clarify that FEPs have associated
probabilities of occurrence that generally are independent of time. That
is, the database of FEPs deemed sufficiently probable would serve
equally well as the basis for assessments covering 1,000, 10,000,
100,000, or 1 million years. These probabilities of occurrence are
established by examining the geologic record and considering potential
mechanisms for components of the repository and its natural setting to
undergo changes. FEPs with a probability of occurrence greater than 1
chance in 100 million per year should be considered for inclusion in the
performance assessments to show compliance with the 10,000-year
standards, and the same FEPs included in those assessments should be
used to develop the performance assessment scenarios to be analyzed for
the peak dose performance assessments between 10,000 and 1 million
years. We believe that this is an inclusive threshold level that fully
considers a range of low-probability FEPs, while at the same time
limiting speculation over highly improbable FEPs. We believe the
probability screening threshold provides the foundation for a reasonable
test of the disposal system, as discussed further below.

Although we discussed the meaning of the probability threshold in some
detail in our proposal, we emphasize it again as the foundation for
constructing the performance assessment. A 1 in 100 million annual
probability of occurrence, when considered over a 10,000-year period,
includes FEPs with a cumulative chance of occurring of one one-hundredth
of one percent (0.01%). Similarly, over 1 million years, the cumulative
probability increases to only a one percent (1%) chance of occurrence
within that time frame. We believe that the database of information
necessary to assess FEPs at this low probability is the same as that
necessary for examining their importance over the entire 1 million-year
compliance period. We believe this probability criterion leads to an
inclusive set of potential FEPs for both the 10,000-year and peak dose
assessments, and in our view would support a reasonable test of the
disposal system that encompasses the climate change, seismic, igneous,
and corrosion scenarios specified in our proposal.

In our proposed rule, we concluded that the 10,000-year FEPs screening
could serve as an adequate basis for longer-term assessments because it
is sufficiently inclusive to be appropriate for use in developing
performance scenarios applicable to the entire 1 million-year compliance
period. That is, we did not propose to require DOE to consider FEPs with
an annual probability lower than 10^-8^ to accommodate the lengthened
compliance period. We believe excluding FEPs with less than a 1% chance
of occurrence in 1 million years is consistent with the principles of
reasonable expectation. We believe that lowering the annual probability
level below 10^-8^ would allow for speculative scenarios to be
considered in the peak dose performance assessment, which would be
neither reasonable nor justifiable, as explained below.

Some commenters disagreed, stating that, because we are extending the
compliance period by a factor of 100, the probability threshold for
excluding FEPs should also be extended by a factor of 100, resulting in
a threshold of 1 chance in 10 billion of occurrence per year. Similarly,
we received some comments questioning altogether the need for or
validity of a probability threshold. The comments suggest that, because
the effects are weighted by the probability of occurrence, any potential
FEP, no matter how unlikely, should be characterized and assessed
because its influence will be mitigated by its low probability. They
cite NAS to the effect that "all these scenarios need to be quantified"
with respect to probability and consequence. (NAS Report p. 72)
Therefore, the commenters conclude that our concerns about introducing
excessive speculation are unfounded. We disagree. We addressed this
topic in our proposal, in the expectation that we would be encouraged to
adjust the probability threshold by two orders of magnitude (i.e.,
widening the probability range by a factor of 100) to account for the
similarly lengthened compliance period. We believe that simply extending
the approach of using a one in 10,000 probability over a 1 million-year
period to give 1 in 10 billion chance per year of occurring (10^-10^)
would be so speculative as to be unreasonable (70 FR 49052). Nor do we
believe it would be consistent with the NAS's view that the overall goal
was "to define a standard that specifies a high level of protection but
that does not rule out an adequately sited and well-designed repository
because of highly improbable events." (NAS Report p. 28) Further, NAS
itself suggested situations in which scenarios need not be quantified.
NAS discusses, in the context of volcanism, a 10^-8^ annual probability
of occurrence as a level that "might be sufficiently low to constitute a
negligible risk" below which "it might not be necessary to consider" how
the event might contribute to releases from the disposal system. (NAS
Report p. 95) We believe this example is instructive, given that
volcanism is the single scenario resulting in direct release of
radioactive material from the repository into the biosphere, resulting
in relatively immediate exposures. We believe it is reasonable to extend
the concept expressed by NAS as "negligible risk" to FEPs whose
influences are seen in the gradual release and transport of
radionuclides over long periods of time. Therefore, we believe that
lowering the probability threshold, or eliminating it altogether, would
be inconsistent with the important NAS cautions to focus assessment
efforts on FEPs that can be bounded within the limits of geologic
stability.

In our view, were we to lower or eliminate the probability threshold, it
would be necessary to consider and describe FEPs that may have been
present or occurred in the initial years of the planet's existence.
Similarly, FEPs with an annual probability of 10^-10^ may be only
hypothetical, since the age of the Earth is generally considered to be
"only" 4.6 x 10^9^ years, suggesting that planetary-scale changes might
have a 50% chance of occurring within the probability window. Indeed,
the volcanic rocks comprising Yucca Mountain and its surroundings are
only on the order of 10-12 million years old (\~10^7^ years). In
determining the probability of particular FEPs, the geologic record at
the site is the source of information to identify what FEPs have
occurred at the site in the past and may occur in the future (through
the period of geologic stability). Since the host rock formations at the
site are only about 10 million years old, an annual probability cut-off
of 10^-10^ would mean that probability estimates for some FEPs would
have to be made in spite of the fact that there is no evidence for their
occurrence at the site in the past. As it is, the 10^-8^ probability
threshold presents a significant challenge to characterize FEPs with
some degree of confidence, given the limits of today's science and
technology. ICRP makes a similar point in its 2006 draft
recommendations: "The use of probability assessment is limited by the
extent that unlikely events can be forecast. The estimates of annual
probabilities of initiating events much less than 10^-6^ must be treated
with doubt because of the serious uncertainty of predicting the
existence of all the unlikely initiating events." (Docket No.
EPA-HQ-OAR-2005-0083-xxxx, Section 8.3, paragraph 319) Overall, we
believe events with a lower annual probability than 10^-8^ would
introduce speculation beyond what is appropriate to define a reasonable
test of disposal system performance.

We also received comments stating that maintaining the probability
screening criteria for the extended compliance period undermines our
arguments for increasing uncertainty. To the contrary, we believe the
physical meaning of the probability threshold (0.01% chance of
occurrence within 10,000 years, but a 1% chance within 1 million years)
appropriately incorporates the concept of uncertainty increasing with
time, while still applying a substantially conservative screening
criterion.

We believe that the guidance we have provided for executing a FEPs
evaluation and screening process assures that it is executed in a
thorough manner. For example, we have stated that the geologic record
through the Quaternary Period (a period extending back approximately 2
million years from today) at and around the site should be examined to
identify relevant FEPs. While we believe that the Quaternary Period
offers the most reliable data for identifying and characterizing site
geologic FEPs, we do not believe that evidence preserved in older
portions of the geologic record should be ignored in the FEPs
identification process. We did not mean to imply that DOE need only
consider the previous 10,000 years when developing evidence for the
probability of occurrence of future events. Rather, our statements
regarding the Quaternary Period as an appropriate geologic record were
intended to confirm that, where available, reliable geologic records for
earlier time periods should be consulted. For example, determining the
probability of seismic and igneous events would make use of the geologic
record at the site for as far back in time as reliable estimates of past
events can be made so that defensible probability estimates can be made.
We believe the Quaternary Period offers the best information to quantify
the probabilities and consequences of geologic FEPs relevant to site
performance. However, we did not intend that significant information
about FEPs be ignored simply because that information appears in the
geologic record at the site prior to the Quaternary Period.

In fact, a longer portion of the geologic record has been examined by
DOE and NRC in developing FEP probabilities. For example, to determine
the nature and frequency of volcanic activity around Yucca Mountain,
volcanic activity around the site through the Quaternary Period was
extensively examined, as well as volcanic activity prior to that time
(ACNW Workshop on Volcanism at Yucca Mountain, September 22, 2004
--Docket No. EPA-HQ-OAR-2005-0083-0373 and 0378). We believe that the
information necessary to evaluate FEPs against the probability threshold
we established (10^-8^ annual probability) will be extensive, and that
increasing the compliance period from 10,000 to 1 million years does not
require that additional studies be performed beyond those necessary to
derive the FEPs probabilities under the screening process done for the
10,000-year time frame assessments. As we have noted previously, the
database for evaluating FEPs probabilities is the same, whether the time
frame is 10,000 years or 1 million years.

On this last point, we stress that the revised §197.36(a) issued today
should not be interpreted as compelling DOE to extend the databases for
its technical justifications. We are restating the probability screening
criterion, not recasting the entire framework for the analysis. We
recognize that in any licensing process the burden of proof is on the
applicant to demonstrate that the necessary factors and influences have
been evaluated. It must also be recognized that there will always be
limits to the ability of science and technology to characterize FEPs and
their effects on the disposal system. However, NAS has stated that many
of these processes and their uncertainties are boundable. In our
judgment, given the capabilities of today's science and technology, it
would be contrary to the principle of reasonable expectation to require
DOE to affirm the same level of confidence in assessments covering 1
million years as it would for a much shorter 10,000-year analysis.

Similarly, we believe that this clarification does not create the
prospect of speculative scenarios of very low probability (from
combinations of FEPs) being proposed, thereby opening the performance
assessments to unbounded speculation. For example, if two low
probability independent FEPs were proposed to occur simultaneously
because of the longer time horizon under consideration, the probability
of that combination would be the product of their respective
probabilities. In other words, the probability of the combined FEPs
occurring during the same year will be much lower, by possibly orders of
magnitude, than the probability of either FEP occurring individually.
Therefore, since the contributions of various FEPs (or scenarios) to the
dose assessments is the product of their respective probabilities and
consequences, the consequence of the combined FEPs would need to be
inversely proportionally higher, typically by orders of magnitude, than
the combined consequences of the individual FEPs considered separately,
in order to make a significant change in the overall dose assessment.

We did receive some comment suggesting that we had inappropriately
excluded the type of volcanic events that created the Yucca Mountain
tuff some 12 to 14 million years ago, instead focusing on the past
several million years. However, as we stated in our proposal, the
geologic record of the past several million years in the area around the
site indicates that basaltic volcanism is the type of volcanism that has
occurred recently and has the potential to recur in the future. The
earlier events were of a much different, cataclysmic nature, producing
rock units more than 6000 ft (1800 m) thick. The type of volcanic
activity that created Yucca Mountain and the surrounding area has not
recurred over the approximately 10 million years since the deposits were
originally laid down and is extremely unlikely to occur within the next
1 million years (Docket No. EPA-HQ-OAR-2005-0083-0050, pp. 7-42 through
7-49). Further, we question whether such cataclysmic events could be
reasonably considered to fall within the bounds of geologic stability as
envisioned by NAS. Inclusion of such events in the peak dose assessment
up to 1 million years would be inconsistent with the intent of the NAS
when it noted that long-term performance can be assessed (because
physical and geologic processes are sufficiently quantifiable, and the
related uncertainties sufficiently boundable) when the geologic system
is relatively stable or varies in a boundable manner. (NAS Report p. 9)
However, NAS noted that "\[a\]fter the geologic environment has changed,
of course, the scientific basis for performance assessment is
substantially eroded and little useful information can be developed."
(NAS Report p. 72) We believe that volcanism of that magnitude would
result in fundamental change of the geologic environment and would not
represent a reasonable test of the disposal system. Therefore, we
continue to see no basis for requiring this type of event be included in
the performance assessment.

Some may view our approach using a single probability threshold for
determining which FEPs should be considered for inclusion in the
performance assessments as inconsistent with the application of
different dose standards for the initial 10,000 years and the period up
to 1 million years. We do not see an inconsistency primarily because the
nature and effects of uncertainty on event probability and dose
projections are dissimilar. The overall uncertainty in projecting doses
using a model simulating the complex interplay of the disposal system
components over long times, each of which has inherent uncertainties in
their characteristics, and the associated difficulty in relying on such
projections for regulatory decisions, should not be confused with the
uncertainty implied in assigning a probability of occurrence to a
particular FEP, which in many cases derives from an examination of the
geologic record at the site. We have noted the difficulty in
extrapolating performance to very long times, and believe it is
appropriate to address this difficulty by establishing a somewhat
higher, but still protective, dose limit for the period beyond 10,000
years. FEP probabilities are assigned based on observations that may
cover long periods of time, such as for geologic processes, or from
laboratory testing and the extrapolation of such results to conditions
that may exist in the distant future, such as for corrosion processes.
In today's final rule, the FEP probability threshold that must be
considered in developing performance assessments represents a policy
judgment about how such events should be addressed in order to meet the
regulatory challenge recognized by NAS, supported by technical reasoning
about the nature of the site database for identifying and characterizing
FEPs.

**Significance**

The second criterion for evaluating FEPs, the evaluation of the
significance of the impacts on performance assessment, allows FEPs above
the probability threshold to be excluded from the analyses if they would
not significantly change the results of performance assessments. In
other words, this evaluation is intended to identify those FEPs whose
projected probability would otherwise make them candidates for inclusion
in the performance assessment, but whose effect on repository
performance (however probable) can be demonstrated not to be
significant. We are retaining the provisions presented in the proposed
rule related to screening FEPs for their effects on the performance
assessment results, and, for the reasons discussed below, are adding an
additional provision regarding the analysis of seismic FEPs in
§197.36(c).

Today's final rule continues to focus on seismic and igneous events that
cause direct damage to the engineered barrier system (e.g., repository
drifts and waste packages). Regardless of other effects of these events
on the disposal system, the timing and degree of waste package
degradation has a significant effect on peak dose. The longevity of
waste packages, when considering periods of hundreds of thousands of
years, is uncertain and dependent on a number of factors. Therefore, the
aspect of primary interest in evaluating seismic and igneous FEPs is
their potential to breach waste packages and make radioactive material
available for transport by infiltrating water (or, in the case of
volcanic events, for direct release into the biosphere).

We recognize that setting forth the significance screening criterion in
§197.36(a) of our proposal as pertaining to the 10,000-year period could
be construed as creating a situation in which important long-term
processes could be excluded altogether from the analysis if they were
not significant in the earlier period. However, we do not believe it is
reasonable to interpret the significance criterion in this way. We have
taken specific steps to ensure that significant long-term FEPs will be
considered in the assessments. Consistent with NAS, we have addressed
the long-term effects of seismic, igneous, and climatic FEPs. In
addition, as described below, we examined FEPs affecting the engineered
barriers and have directed that the effects of general corrosion on the
barrier system be evaluated. Further, contrary to some comments, we
explicitly required that FEPs included in the 10,000-year analysis must
continue to be included for the longer-term (10,000 years to 1 million
years) assessment. That is, FEPs included in the initial 10,000-year
assessments will continue to operate throughout the period of geologic
stability. These FEPs are already identified as appropriate for
inclusion, and include fundamental physical and geologic processes that
play roles in the release and transport of radionuclides, regardless of
the time period covered by the assessment.

As noted above, to further bolster the significance screening criterion,
in our proposal we considered whether some FEPs eliminated from
consideration during the first 10,000 years should be included in the
longer-term assessment if they would have a significant bearing on
performance at later times, even if they could legitimately be dismissed
for the initial 10,000-year period. We focused our attention on FEPs
affecting the engineered barriers since, as noted above, waste package
failure is the dominant factor in the timing and magnitude of the peak
dose, and is the primary reason for considering time frames up to 1
million years. To illustrate one consideration, thermal conditions in
the repository change dramatically within the initial 10,000-year
period, affecting the relative importance of some FEPs during and after
the thermal pulse. However, FEPs involved in release and transport of
radionuclides would generally be the same, regardless of when the waste
package fails. Further, while FEPs associated with the natural
characteristics of the site are active today or can be observed in the
geologic record, FEPs related to engineered barrier longevity involve
extrapolation of shorter-term testing data. The degree to which natural
FEPs can contribute to the breaching of waste packages is dependent to a
large extent on the condition of those packages over time, making FEPs
specific to the engineered barriers of particular importance. We took
this approach for two reasons. First, we needed to clearly outline the
reasons why a FEP that could be excluded on the basis of significance
from the performance assessments for the initial 10,000-year period
might potentially need to be re-considered for the lengthened compliance
period. Second, we wanted to further our goal of issuing an
implementable standard by limiting potentially unconstrained speculation
over the longer compliance period. By discussing the considerations
involved in evaluating FEPs that could be previously excluded, we hoped
to lay out clearly the reasoning that could be used to justify inclusion
of additional FEPs beyond those identified by the NAS committee.

We identified only one such FEP, general corrosion of the waste packages
and engineered barriers, which we explicitly addressed in our proposal
because it is likely to be a significant degradation process at later
times. We identified this FEP as being significant at times greater than
10,000 years because we believe it is the principal process FEP that
could lead to \"gross breaching\" of the waste package over those
extended time frames. Processes and events that could lead to \"gross
breaching\" are of greatest significance to long term performance
because, as noted by the NAS, \"canisters are likely to fail initially
at small local openings through which water might enter, but out of
which the diffusion of dissolved wastes will be slow until the canister
is grossly breached.\" (NAS Report p. 86) It is the time of \"gross
breaching\" that determines the time of more rapid release of dissolved
wastes from the repository and hence may have a significant effect on
the time and magnitude of the peak dose within 1 million years. Although
the general corrosion process is slow, tends to decrease with decreasing
temperature, and may not lead to significant releases for the first
10,000 years (depending on DOE\'s design of the waste package), we
believe this single FEP is significant enough over the long term to
require inclusion in the assessment of performance during the time of
geologic stability, regardless of the screening decision in the first
10,000 years. Further, consideration of the uncertainties involved in
extrapolating general corrosion data for the proposed waste package
materials supports the inclusion of this potentially highly significant
process ("Assumptions, Conservatisms, and Uncertainties in Yucca
Mountain Performance Assessments," Docket No. EPA-HQ-OAR-2005-0083-0085,
Section 5.4.1). Therefore, we believe that general corrosion, as well as
those FEPs related to seismicity, igneous activity and climate change
identified by NAS, require explicit inclusion in the assessments during
the time of geologic stability

We did, as one commenter pointed out, consider providing NRC more
latitude to identify FEPs if they would significantly affect the peak
dose. After further consideration, we decided against this approach,
believing the provisions outlined above and the specification of general
corrosion would adequately address this situation, provide a reasonable
test of disposal system performance, and give DOE the necessary
assurance that the important factors have been explicitly identified in
the rule. As we noted above, we identified general corrosion of
engineered barriers as a FEP potentially significant to the peak dose,
and specified its inclusion because it is likely to be a significant
degradation process at later times. Similarly, consistent with the NAS
recommendations, we have specified the inclusion of climate change,
seismicity, and igneous scenarios. We view the requirement to include
general corrosion, as well as the climate, seismic, and igneous
scenarios identified by NAS, as leading to an effective and extensive
assessment, which can fairly be represented as a reasonable test of the
disposal system. As we discussed in our proposal, the search for
additional FEPs that might be significant at some point beyond 10,000
years can rapidly become highly speculative and limited in benefit.
Therefore, we continue to believe that our approach represents "informed
judgment" and a reasonable test of repository performance over time
frames as long as 1 million years.

We also note that DOE submitted, as part of its comments on the proposed
rule, the results of analyses based on a simplified peak dose model
(Docket No. EPA-HQ-OAR-2005-0083-0352, Appendix 1). DOE states that it
had compiled a database of FEPs, independent of compliance period, and
evaluated them for inclusion in a 10,000-year analysis. DOE
"subsequently re-evaluated the FEPs over the period beyond 10,000 years"
and concluded that those FEPs excluded on the basis of significance
within 10,000 years would also not have significant effects on
performance projections beyond 10,000 years. DOE reached its conclusion
both for FEPs excluded "on a low consequence basis that is not affected
by time" and for "gradual and continuing processes" that "are time
dependent."

As part of its comments, DOE submitted an analysis that identified three
reasons why gradual and/or infrequent FEPs excluded on the basis of
significance within 10,000 years would also not have significant effects
on performance projections beyond 10,000 years: (1) an excluded FEP was
determined to be of secondary importance to the primary significant
degradation FEP, which was included in the analysis; (2) the inclusion
of the FEP would tend to lower the peak dose during the time of geologic
stability because it resulted in earlier and more diffuse releases
(hence the exclusion of the FEP would be conservative from a peak dose
perspective); or (3) the FEP is correlated in some way with temperature
(e.g., in the rate with which it operates), so it would be less
significant at later times due to the lower temperature in the
repository over time. (Docket No. EPA-HQ-OAR-2005-0083-0352, Appendix 1,
Section 6.1 and Table 24) DOE considered FEPs of this nature associated
with both the engineered and natural barrier systems. DOE concluded, for
example, that some longer-term processes, such as general corrosion, may
contribute to waste package failure, and disruptive seismic events may
contribute to rockfall and other physical mechanisms leading to
[release]{.mark}

We also considered public comments on this topic. Most commenters who
disagreed with our proposal cited the limited data available on various
corrosion mechanisms that could affect the waste packages. Many of these
commenters seem to believe that we have excluded all corrosion
mechanisms except general corrosion. This is not the case. We have
explicitly directed that general corrosion be considered because it is
likely to be the most significant such process at longer times; however,
other corrosion mechanisms (such as localized corrosion) are more likely
in the early period after disposal when temperatures inside the
repository are high. If DOE determines these processes to be
insignificant within 10,000 years, they are not likely to be more
significant than general corrosion at later times. If they are included
in the 10,000-year analysis, they must be included in the longer-term
assessments. One commenter highlighted our discussion of criticality as
excluding one of the "most worrisome threats to the repository" over the
long term. We cited an NRC technical study to support our conclusion
that such an event is unlikely to be significant to the results of the
assessments. Further, the DOE reference cited above concludes that all
criticality scenarios fall below the probability screening threshold. An
alternative view on the FEPs screening process was expressed in a report
by the Electric Power Research Institute (EPRI): "Thus, the current EPA
screening limit is very conservative compared to the \[Negligible
Incremental Dose\] level suggested by \[NAS\]. It is likely that there
are many FEPs that DOE has already included in their analysis using the
EPA approach that would not have been included if the
\[NAS\]-recommended approach had been followed. Given that many
additional FEPs are already included, it should be unnecessary to
include any additional FEPs if the regulatory compliance period is
extended beyond 10,000 years." ("Yucca Mountain Licensing Standard
Options for Very Long Time Frames," April 2005, pp. 3-5 and 3-6, Docket
No. EPA-HQ-OAR-2005-0083-0087) Taking all of this information into
account, we continue to believe it is reasonable that, with the
exception of the specific FEPs identified in 197.36(c), a FEP determined
to be insignificant in the first 10,000 years may continue to be
excluded in the post 10,000-year analyses.

As we noted above, we are modifying the proposed rule regarding the
provisions related to seismic events in §197.36(c). We noted in our
proposal the NAS statement that "\[w\]ith respect to the effects of
seismicity on the hydrologic regime, the possibility of adverse effects
due to displacements along existing fractures cannot be overlooked" but
that "such displacements have an equal probability of favorably changing
the hydrologic regime." (NAS Report p. 93) We argued that these effects
would likely be minimal given the many small-scale changes that would be
possible in the connectivity of the fracture networks, and that these
effects would likely be small compared to the effects of climate change
on the hydrologic behavior of the disposal system We did not mean to
imply that the seismic and climate events would involve the same
hydrologic characteristics and processes or produce the same effects on
the ground-water flow regime, but that the effects of one were likely to
outweigh the effects of the other. While we still believe that is
likely, we have concluded, after further consideration, that the issue
of hydrologic effects resulting from seismic events needs to be examined
in sufficient detail to address the point made by NAS. We believe the
effects of fault displacement on the hydrologic regime will be
adequately addressed by the variation in parameters such as hydraulic
conductivity (i.e., evaluating reasonable variation in ground-water flow
parameters, whether seismically-induced or not, will illustrate the
range of effects that might result from seismicity). However, NAS also
identified another seismic effect on hydrology, namely the potential for
transient rise in the ground-water table. In this instance, NAS did not
simply state that such potential could be bounded, but noted
site-specific studies suggesting "a probable maximum transient rise on
the order of 20 m or less" (NAS Report p. 94). Therefore, we now require
that the effects of a rise in the ground-water table as a result of
seismicity be considered. We are not specifying the extent of the rise
to be considered, but leave that conclusion to be determined by NRC. In
this case, however, we are also allowing NRC to make a judgment as to
whether such a rise in ground water would be significant to the results
of the performance assessment. If NRC determines that such a reasonably
bounded scenario would not be significant, DOE would not be required to
evaluate its effects.

We believe deferring to NRC on this point is the appropriate approach.
The above quote from page 93 of the NAS Report makes it clear that
changes to the hydrologic regime from seismic events would be equally
likely to enhance or reduce transport of radionuclides. However, it
would seem unlikely for changes to occur that would all combine to
enhance transport to the saturated zone and then through the controlled
area, such that concentrations of radionuclides at the RMEI location
would be significantly increased. It seems more likely that localized
changes would occur, which in sum would not significantly increase
overall transport of radionuclides. Further, as noted above, we believe
these seismically-induced changes are likely to be approximated by the
normal variation in flow parameters. It may be that changes in the
hydrologic system from climate change[, including elevation of the
ground-water table,]{.mark} may be quantitatively more significant than
such changes resulting from seismic activity. We believe NRC is better
positioned to make judgments regarding the significance and extent of
such changes. We note that a dozen years of site characterization,
scientific study, and performance assessments have been conducted since
the NAS Report in 1995. NRC has conducted its own analyses as well as
participated in ongoing technical exchanges with DOE over this period.
We view deferring to NRC's judgment in this case as comparable to the
approach we have taken with climate change. In that instance, we
outlined the primary issues and overall approach, but specified that NRC
would establish the details required to implement our standard.

Finally, we are retaining the provision related to climate change as it
was proposed. We believe this is a reasonable approach, which allows NRC
to characterize climate change beyond 10,000 years using constant
conditions. This approach has the advantage of avoiding speculation
regarding the timing and magnitude of climatic cycles, while addressing
the important aspects of climate change. We received some comments that
appear to have misinterpreted our proposal. Some comments suggested that
our citation of the NAS statement to the effect that "climate changes on
the time scale of hundreds of years would probably have little if any
effect on repository performance" (NAS Report p. 92) as implying that we
are "ignoring longer-term changes" such as "glacial periods covering
thousands of years." This represents a fundamental misunderstanding of
our proposal, which would allow the future climate to be represented by
what is essentially a glacial transition period lasting 990,000 years,
but in any event placed no limits on the duration of periods of
increased precipitation. Similarly, some commenters expressed the view
that we "required" the future climate to be represented by constant
conditions, or that we were suggesting that a single climate be used in
all realizations. On the contrary, we cited the NAS conclusion that "a
doubling of the effective wetness" might be significant as one
justification for stating that it would be reasonable to represent
far-future climate by constant conditions. Today's final rule,
consistent with our proposal, leaves it to NRC to determine the
parameter values that would define the future climate, including
influential parameters other than precipitation, such as temperature.
Our specification of the outcome of "increased water flow through the
repository" provides NRC with the flexibility to specify basic
parameters, such as precipitation and temperature, that must be assumed
by DOE, or to derive estimates of water flow directly. This is
consistent with our current belief that the dominant mechanisms and flow
paths for water to move from the surface through the repository and
beyond should be determined by NRC rather than EPA. Further, we
anticipated that "constant climate conditions" would be used as another
parameter in the probabilistic assessment. That is, each realization
would select its constant conditions from among a distribution of such
conditions. This is exactly the approach that NRC has taken in its
proposal, i.e., that a range of deep percolation values be used (70 FR
53313-53320, September 8, 2005).

Some commenters disagreed with the approach of specifying constant
climate conditions leading to a higher rate of water flow through the
repository, stating that the "non-linear" nature of the disposal system
would be more sensitive to a dynamic, cyclical representation of
climate. This is not necessarily true, as the effects on the disposal
system would be highly affected by the timing of waste package failures
(e.g., whether they fail during a wetter or drier cycle). Some comments
cite recent climate research suggesting that anthropogenic climate
influences will postpone the next glacial cycle by roughly 500,000
years, or that today's climate at Yucca Mountain will actually be more
representative of future climates than would the wetter conditions known
to have occurred in the past. We believe that our final rule's approach
to climate change provides a reasonable approach to address a point of
fundamental uncertainty regarding long-term climate change and its role
in the performance assessments, an uncertainty that cannot be removed by
additional research into past climate cycles or modeling of present or
future climate behavior. We refer to NAS on this point: "Although the
typical nature of past climate changes is well known, it is obviously
*impossible to predict in detail either the nature or the timing of
future climate change*." (NAS Report p. 77, emphasis added) Although
continuing research will provide better understanding of past climate
fluctuations, we believe that predicting with high confidence the timing
and extent of climate fluctuations into the far future will remain an
unrealistic goal. We believe that the understanding of past climate
fluctuations and their potential effects on the Yucca Mountain
hydrologic system is valuable information and should be applied to
define the climate-related parameter values. As noted above, NRC has
used such information to propose climate-related parameter values, which
DOE will use to project the behavior of hydrologic processes at the
site. We believe that this approach to treatment of a "residual,
unquantifiable uncertainty" by the application of "informed judgment" is
consistent with NAS guidance (NAS Report[,]{.mark} p.80).

**IV. Statutory and Executive Order Reviews**

[A. Number range CHAPTERExecutive Order 12866: Regulatory Planning and
Review\
]{.underline}

Under Executive Order 12866 (58 Federal Register 51735, October 4,
1993), this action is a \"significant regulatory action\" because Number
range CHAPTERit raises novel legal or policy issues arising out of the
specific legal mandate of Section 801 of the Energy Policy Act of 1992.
Accordingly, EPA submitted this action to the Office of Management and
Budget for review under Executive Order 12866 and any changes made in
response to OMB recommendations have been documented in the docket for
this action.

[B. Number range CHAPTERPaperwork Reduction Act]{.underline}

This action does not impose an information collection burden under the
provisions of the Paperwork Reduction Act, 44 U.S.C. 3501
[et]{.underline} [seq]{.underline}. Number range CHAPTERWe have
determined that this rule contains no information collection
requirements within the scope of the Paperwork Reduction Act.

Burden means the total time, effort, or financial resources expended by
persons to generate, maintain, retain, or disclose or provide
information to or for a Federal agency. This includes the time needed to
review instructions; develop, acquire, install, and utilize technology
and systems for the purposes of collecting, validating, and verifying
information, processing and maintaining information, and disclosing and
providing information; adjust the existing ways to comply with any
previously applicable instructions and requirements; train personnel to
be able to respond to a collection of information; search data sources;
complete and review the collection of information; and transmit or
otherwise disclose the information.

An agency may not conduct or sponsor, and a person is not required to
respond to a collection of information unless it displays a currently
valid OMB control number. The OMB control numbers for EPA\'s regulations
in 40 CFR are listed in 40 CFR part 9.

Number range CHAPTER [C. Regulatory Flexibility Act]{.underline}

The Regulatory Flexibility Act (RFA) generally requires an agency to
prepare a regulatory flexibility analysis of any rule subject to notice
and comment rulemaking requirements under the Administrative Procedure
Act or any other statute unless the agency certifies that the rule will
not have a significant economic impact on a substantial number of small
entities. Small entities include small businesses, small organizations,
and small governmental jurisdictions.

Number range CHAPTERFor purposes of assessing the impacts of today\'s
rule on small entities, small entity is defined as: (1) a small business
as defined by the Small Business Administration's (SBA) regulations at
13 CFR 121.201; (2) a small governmental jurisdiction that is a
government of a city, county, town, school district or special district
with a population of less than 50,000; and (3) a small organization that
is any not-for-profit enterprise which is independently owned and
operated and is not dominant in its field.

After considering the economic impacts of today's final rule on small
entities, I certify that this action will not have a significant
economic impact upon a substantial number of small entities. This final
rule will not impose any requirements on small entities. This final rule
establishes requirements that apply only to DOE.

[D. Number range CHAPTERUnfunded Mandates Reform Act]{.underline}\
\
Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law
104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with \"Federal mandates\" that
may result in expenditures to State, local, and tribal governments, in
the aggregate, or to the private sector, of \$100 million or more in any
one year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and adopt
the least costly, most cost-effective or least burdensome alternative
that achieves the objectives of the rule. The provisions of section 205
do not apply when they are inconsistent with applicable law. Moreover,
section 205 allows EPA to adopt an alternative other than the least
costly, most cost-effective or least burdensome alternative if the
Administrator publishes with the final rule an explanation why that
alternative was not adopted. Before EPA establishes any regulatory
requirements that may significantly or uniquely affect small
governments, including tribal governments, it must have developed under
section 203 of the UMRA a small government agency plan. The plan must
provide for notifying potentially affected small governments, enabling
officials of affected small governments to have meaningful and timely
input in the development of EPA regulatory proposals with significant
Federal intergovernmental mandates, and informing, educating, and
advising small governments on compliance with the regulatory
requirements.

Number range CHAPTER Today\'s final rule contains no Federal mandates
(under the regulatory provisions of Title II of UMRA) for State, local,
or tribal governments or the private sector. This final rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and establishes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. The rule imposes no enforceable
duty on any State, local or tribal governments or the private sector.
Thus, today\'s rule is not subject to the requirements of sections 202
and 205 of UMRA.

EPA has determined that this rule contains no regulatory requirements
that might significantly or uniquely affect small governments. This
final rule implements requirements specifically set forth by the
Congress in section 801 of the EnPA and establishes radiological
protection standards applicable solely and exclusively to the Department
of Energy's potential storage and disposal facility at Yucca Mountain.
The rule imposes no enforceable duty on any small governments. Thus,
today\'s rule is not subject to the requirements of section 203 of UMRA.

[E. Number range CHAPTERExecutive Order 13132: Federalism]{.underline}

Executive Order 13132, entitled "Federalism" (64 FR 43255, August 10,
1999), requires EPA to develop an accountable process to ensure
"meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications."
"Policies that have federalism implications" is defined in the Executive
Order to include regulations that have "substantial direct effects on
the States, on the relationship between the national government and the
States, or on the distribution of power and responsibilities among the
various levels of government."

This final rule does not have federalism implications. It will not have
substantial direct effects on the States, on the relationship between
the national government and the States, or on the distribution of power
and responsibilities among the various levels of government, as
specified in Executive Order 13132. This final rule implements
requirements specifically set forth by the Congress in section 801 of
the EnPA and establishes radiological protection standards applicable
solely and exclusively to the Department of Energy's potential storage
and disposal facility at Yucca Mountain. Thus, Executive Order 13132
does not apply to this rule. Number range CHAPTERIn the spirit of
Executive Order 13132, and consistent with EPA policy to promote
communications between EPA and State and local governments, EPA
specifically solicited comment on the proposed rule from State and local
officials.

[Number range CHAPTERF. Executive Order 13175: Consultation and
Coordination with Indian Tribal Governments]{.underline}

Executive Order 13175, entitled "Consultation and Coordination with
Indian Tribal Governments" (65 FR 67249, November 9, 2000), requires EPA
to develop an accountable process to ensure "meaningful and timely input
by tribal officials in the development of regulatory policies that have
tribal implications." This final rule does not have tribal implications,
as specified in Executive Order 13175. This final rule will regulate
only DOE on land owned by the Federal government. The rule does not have
substantial direct effects on one or more Indian tribes, on the
relationship between the Federal Government and Indian tribes, or on the
distribution of power and responsibilities between the Federal
Government and Indian tribes. Thus, Executive Order 13175 does not apply
to this rule. Although Executive Order 13175 does not apply to this
rule, Number range CHAPTEREPA specifically solicited additional comment
on this proposed rule from tribal officials and consulted with tribal
officials in developing this rule. EPA directly contacted more than 20
tribal governments and conducted three conference calls with members of
tribal governments. In recognition of the importance of
government-to-government consultation with tribes and the significance
of tribal governments as sovereign nations, EPA extended the public
comment period for tribal governments to December 31, 2005. Comments
related to tribal issues, and our responses to them, may be found in
Section 13 of the Response to Comments document associated with this
final rule (docket ref).

[G. Executive Order 13045: Protection of Children from Environmental
Health & Safety Risks]{.underline}

Number range CHAPTERExecutive Order 13045: "Protection of Children from
Environmental Health Risks and Safety Risks" (62 FR 19885, April 23,
1997) applies to any rule that: (1) is determined to be \"economically
significant\" as defined under Executive Order 12866, and (2) concerns
an environmental health or safety risk that EPA has reason to believe
may have a disproportionate effect on children. If the regulatory action
meets both criteria, the Agency must evaluate the environmental health
or safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the Agency.

This final rule is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866, and
because the Agency does not have reason to believe the environmental
health risks or safety risks addressed by this action present a
disproportionate risk to children. Number range CHAPTER As discussed in
our 2001 rulemaking (66 FR 32080-32081 and 32085-32086), the primary
risk factor considered in our risk assessment is incidence of fatal
cancer. We have derived a risk value for the onset of fatal cancer that
considers children, since it is an overall average risk value (see
Chapter 6 of the BID for more details) that includes all ages from birth
onward, all exposure pathways, both genders, and most radionuclides. We
do note that the risk factor does not include the fetus. However, we
believe that the risk of fatal cancer per unit dose incurred by the
unborn is similar to that for those who have been born, but the exposure
period is very short compared to the rest of the individual's average
lifetime, so the risk of fatal cancer to the unborn is proportionately
lower and does not have a significant impact upon the overall risk of
fatal cancer incurred by an individual over a lifetime. (See Chapter 6
of the BID for more discussion of the risk of fatal cancer resulting
from in utero exposure.)

[H. Executive Order 13211: Actions that Significantly Affect Energy
Supply, Distribution, or Use]{.underline}

Number range CHAPTERThis rule is not a "significant energy action" as
defined in Executive Order 13211, "Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use" (66 FR 28355
(May 22, 2001)) because it is not likely to have a significant adverse
effect on the supply, distribution, or use of energy. This final rule
will apply only to DOE. Construction, operation, and closure of the
repository at Yucca Mountain would fulfill the Federal government's
commitment to manage the final disposition of spent nuclear fuel from
commercial power reactors. However, there is no direct link between
operation of the repository and an increased use of nuclear power. Other
economic, technical, and policy factors will influence the extent to
which nuclear energy is utilized.

[I. National Technology Transfer and Advancement Act]{.underline}

As noted in the proposed rule, Number range CHAPTERsection 12(d) of the
National Technology Transfer and Advancement Act of 1995 ("NTTAA"),
Public Law No. 104-113, 12(d) (15 U.S.C. 272 note) directs EPA to use
voluntary consensus standards in its regulatory activities unless to do
so would be inconsistent with applicable law or otherwise impractical.
Voluntary consensus standards are technical standards (e.g., materials
specifications, test methods, sampling procedures, and business
practices) that are developed or adopted by voluntary consensus
standards bodies. The NTTAA directs EPA to provide Congress, through
OMB, explanations when the Agency decides not to use available and
applicable voluntary consensus standards.

Number range CHAPTER This rulemaking involves technical standards.
Therefore, the Agency conducted a search to identify potentially
applicable voluntary consensus standards. In our original proposal (64
FR 46976, August 27, 1999), we requested public comment on potentially
applicable voluntary consensus standards that would be appropriate for
inclusion in the Yucca Mountain rule. However, we identified no such
standards, and none were brought to our attention in comments.
Therefore, the standards promulgated in 2001 and today's final revisions
are site-specific and developed solely for application to the Yucca
Mountain disposal facility.

[J. Congressional Review Act]{.underline}

The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the
Small Business Regulatory Enforcement Fairness Act of 1996, generally
provides that before a rule may take effect, the agency promulgating the
rule must submit a rule report, which includes a copy of the rule, to
each House of the Congress and to the Comptroller General of the United
States. Section 804 exempts from section 801 the following types of
rules: (1) rules of particular applicability; (2) rules relating to
agency management or personnel; and (3) rules of agency organization,
procedure, or practice that do not substantially affect the rights or
obligations of non-agency parties. 5 U.S.C. 804(3). EPA is not required
to submit a rule report regarding today's action under section 801
because this is a rule of particular applicability. This final rule will
apply only to DOE, and is issued by EPA in response to direction from
Congress in the EnPA.

**List of Subjects in 40 CFR Part 19**7

Number range CHAPTER Environmental protection, Nuclear energy, Radiation
protection, Radionuclides, Uranium, Waste treatment and disposal, Spent
nuclear fuel, High-level radioactive waste.

Dated:

Stephen L. Johnson,

Administrator.

**PART 197---PUBLIC HEALTH AND ENVIRONMENTAL RADIATION PROTECTION
STANDARDS FOR YUCCA MOUNTAIN, NEVADA**

1\. The authority citation for part 197 continues to read as follows:

**Authority:** Sec. 801, Pub. L. 102--486, 106 Stat. 2921, 42 U.S.C.
10141n.

**Subpart A---Public Health and Environmental Standards for Storage**

2\. Section 197.2 is amended by revising the definition of ''Effective
dose equivalent'' to read as follows:

**§ 197.2 What definitions apply in subpart A?**

\* \* \* \* \*

*Effective dose equivalent* means the sum of the products of the dose
equivalent received by specified tissues following an exposure of, or an
intake of radionuclides into, specified tissues of the body, multiplied
by appropriate weighting factors. Annual committed effective dose
equivalents shall be calculated using weighting factors in appendix A of
this part, unless otherwise directed by NRC in accordance with the
introduction to appendix A of this part.

\* \* \* \* \*

**Subpart B---Public Health and Environmental Standards for Disposal**

3\. Section 197.12 is amended by revising paragraph (1) of the
definition of ''Performance assessment'' and the definition of ''Period
of geologic stability'' to read as follows:

**§ 197.12 What definitions apply in subpart B?**

\* \* \* \* \*

*Performance assessment* means an analysis that:

\(1\) Identifies the features, events, processes, (except human
intrusion), and sequences of events and processes (except human
intrusion) that might affect the Yucca Mountain disposal system and
their probabilities of occurring;

\* \* \* \* \*

*Period of geologic stability* means the time during which the
variability of geologic characteristics and their future behavior in and
around the Yucca Mountain site can be bounded, that is, they can be
projected within a reasonable range of possibilities. This period is
defined to end at 1 million years after disposal.

\* \* \* \* \*

4\. Section 197.13 is revised to read as follows:

**§ 197.13 How is subpart B implemented?**

The NRC implements this subpart B. The DOE must demonstrate to NRC that
there is a reasonable expectation of compliance with this subpart before
NRC may issue a license.

\(a\) The NRC will determine compliance, based upon the arithmetic mean
of the projected doses from DOE's performance assessments for the period
within 10,000 years after disposal, with:

\(1\) §197.20(a)(1) of this subpart; and

\(2\) §§197.25(b)(1) and 197.30 of this subpart, if performance
assessment is used to demonstrate compliance with either or both of
these sections.

\(b\) NRC will determine compliance, based upon the arithmetic mean of
the projected doses from DOE's performance assessments for the period
after 10,000 years of disposal and through the period of geologic
stability. However, if the arithmetic mean exceeds the 75^th^ percentile
of the projected doses occurring at the time of peak mean dose during
the period of geologic stability after 10,000 years, NRC will use the
75^th^ percentile of the projected doses at the time of peak mean dose
to determine compliance with:

\(1\) § 197.20(a)(2) of this subpart; and

\(2\) § 197.25(b)(2), if a performance assessment is used to demonstrate
compliance.

5\. Section 197.15 is revised to read as follows:

**§ 197.15 How must DOE take into account the changes that will occur
during the period of geologic stability?**

The DOE should not project changes in society, the biosphere (other than
climate), human biology, or increases or decreases of human knowledge or
technology. In all analyses done to demonstrate compliance with this
part, DOE must assume that all of those factors remain constant as they
are at the time of license application submission to NRC. However, DOE
must vary factors related to the geology, hydrology, and climate based
upon cautious, but reasonable assumptions of the changes in these
factors that could affect the Yucca Mountain disposal system during the
period of geologic stability, consistent with the requirements for
performance assessments specified at § 197.36.

6\. Section 197.20 is revised to read as follows:

**§ 197.20 What standard must DOE meet?**

\(a\) The DOE must demonstrate, using performance assessment, that there
is a reasonable expectation that the reasonably maximally exposed
individual receives no more than the following annual committed
effective dose equivalent from releases from the undisturbed Yucca
Mountain disposal system:

\(1\) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and

\(2\) 3.5 millisieverts (350 millirems) after 10,000 years, but within
the period of geologic stability.

\(b\) The DOE's performance assessment must include all potential
pathways of radionuclide transport and exposure.

7\. Section 197.25 is revised to read as follows:

**§ 197.25 What standard must DOE meet?**

\(a\) The DOE must determine the earliest time after disposal that the
waste package would degrade sufficiently that a human intrusion (see
§197.26) could occur without recognition by the drillers.

\(b\) The DOE must demonstrate that there is a reasonable expectation
that the reasonably maximally exposed individual will receive an annual
committed effective dose equivalent, as a result of the human intrusion,
of no more than:

\(1\) 150 microsieverts (15 millirems) for 10,000 years following
disposal; and

\(2\) 3.5 millisieverts (350 millirems) after 10,000 years, but within
the period of geologic stability.

\(c\) The analysis must include all potential environmental pathways of
radionuclide transport and exposure.

1.  

8\. Section 197.35 is removed and reserved.

**§ 197.35 \[Removed and Reserved\]**

9\. Section 197.36 is revised to read as follows:

**§ 197.36 Are there limits on what DOE must consider in the performance
assessments?**

\(a\) Yes, there are limits on what DOE must consider in the performance
assessments.

\(1\) The DOE\'s performance assessments conducted to show compliance
with §§197.20(a)(1), 197.25(b)(1), and 197.30 shall not include
consideration of very unlikely features, events, or processes, i.e.,
those that are estimated to have less than one chance in 100,000,000 per
year of occurring. Features, events, and processes with a higher chance
of occurring shall be considered for use in performance assessments
conducted to show compliance with §§197.20(a)(1), 197.25(b)(1), and
197.30, except as stipulated in paragraph (b) of this section. In
addition, unless otherwise specified in these standards or NRC
regulations, DOE\'s performance assessments need not evaluate the
impacts resulting from features, events, and processes or sequences of
events and processes with a higher chance of occurring if the results of
the performance assessments would not be changed significantly in the
initial 10,000-year period after disposal.

\(2\) The same features, events, and processes identified in paragraph
(a)(1) of this section shall be used in performance assessments
conducted to show compliance with §§197.20(a)(2) and 197.25(b)(2), with
additional considerations as stipulated in paragraph (c) of this
section.

\(b\) For performance assessments conducted to show compliance with
§§197.25(b) and 197.30, DOE's performance assessments shall exclude
unlikely features, events, or processes, or sequences of events and
processes. The DOE should use the specific probability of the unlikely
features, events, and processes as specified by NRC.

\(c\) For performance assessments conducted to show compliance with
§§197.20(a)(2) and 197.25(b)(2), DOE's performance assessments shall
project the continued effects of the features, events, and processes
included in paragraph (a) of this section beyond the 10,000-year
post-disposal period through the period of geologic stability. The DOE
must evaluate all of the features, events, or processes included in
paragraph (a) of this section, and also:

\(1\) The DOE must assess the effects of seismic and igneous scenarios,
subject to the probability limits in paragraph (a) of this section for
very unlikely features, events, and processes. Performance assessments
conducted to show compliance with §197.25(b)(2) are also subject to the
probability limits for unlikely features, events, and processes as
specified by NRC.

\(i\) The seismic analysis may be limited to the effects caused by
damage to the drifts in the repository, failure of the waste packages,
and changes in the elevation of the water table under Yucca Mountain.
The magnitude of the elevation of the water table rise and its
significance on the results of the performance assessment will be
determined by NRC. If NRC determines that the increased elevation of the
water table does not significantly affect the results of the performance
assessment, NRC may choose to not require its consideration in the
performance assessment.

\(ii\) The igneous analysis may be limited to the effects of a volcanic
event directly intersecting the repository. The igneous event may be
limited to that causing damage to the waste packages directly, causing
releases of radionuclides to the biosphere, atmosphere, or ground water.

\(2\) The DOE must assess the effects of climate change. The climate
change analysis may be limited to the effects of increased water flow
through the repository as a result of climate change, and the resulting
transport and release of radionuclides to the accessible environment.
The nature and degree of climate change may be represented by constant
climate conditions. The analysis may commence at 10,000 years after
disposal and shall extend to the period of geologic stability. The NRC
shall specify in regulation the values to be used to represent climate
change, such as temperature, precipitation, or infiltration rate of
water.

\(3\) The DOE must assess the effects of general corrosion on engineered
barriers. The DOE may use a constant representative corrosion rate
throughout the period of geologic stability or a distribution of
corrosion rates correlated to other repository parameters.

1.  

10\. Appendix A to part 197 is added to read as follows:

**Appendix A to Part 197---Calculation of Annual Committed Effective
Dose Equivalent**

Unless otherwise directed by NRC, DOE shall use the radiation weighting
factors and tissue weighting factors in this Appendix to calculate
committed effective dose equivalent for compliance with §§197.20 and
197.25 of this part. NRC may allow DOE to use updated factors issued
after the effective date of this regulation. Any such factors shall have
been issued by consensus scientific organizations and incorporated by
EPA into Federal radiation guidance in order to be considered generally
accepted and eligible for this use. Further, they must be compatible
with the effective dose equivalent dose calculation methodology
established in ICRP 26 and 30, and continued in ICRP 60 and 72, and
incorporated in this appendix.

**I. Equivalent Dose**

The calculation of the committed effective dose equivalent (CEDE) begins
with the determination of the equivalent dose, *H*~T~ , to a tissue or
organ, T, listed in Table A.2 below by using the equation:

*H*~T~ =  *D*~T,R~  *w*~R~

R

where *D*~T,R~ is the absorbed dose in rads (one gray, an SI unit,
equals 100 rads) averaged over the tissue or organ, T, due to radiation
type, R, and *w*~R~ is the radiation weighting factor which is given in
Table A.1 below. The unit of equivalent dose is the rem (sievert, in SI
units).

Table A.1 \-- Radiation weighting factors, *w*~R~ ^1^

Radiation type and energy range ^2^ *w~R~* value

Photons, all energies 1

Electrons and muons, all energies 1

Neutrons, energy \< 10 keV 5

10 keV to 100 keV 10

\> 100 keV to 2 MeV 20

\>2 MeV to 20 MeV 10

\> 20 MeV 5

Protons, other than recoil protons, \> 2 MeV 5

Alpha particles, fission fragments, heavy nuclei 20

^1^All values relate to the radiation incident on the body or, for
internal sources, emitted from the source.

^2^See paragraph A14 in ICRP Publication 60 for the choice of values for
other radiation types and energies not in the table.

**II. Effective Dose Equivalent**

The next step is the calculation of the *effective dose equivalent,* E.
The probability of occurrence of a stochastic effect in a tissue or
organ is assumed to be proportional to the equivalent dose in the tissue
or organ. The constant of proportionality differs for the various
tissues of the body, but in assessing health detriment the total risk is
required. This is taken into account using the tissue weighting factors,
*w*~T~ in Table A.2, which represent the proportion of the stochastic
risk resulting from irradiation of the tissue or organ to the total risk
when the whole body is irradiated uniformly and *H*~T~ is the equivalent
dose in the tissue or organ, T, in the equation:

E =  *w*~T~  *H*~T.~

Table A.2 \-- Tissue weighting factors, *w*~T~

  ----------------------------------- -----------------------------------
  Tissue or organ                     W~T~ value

  Gonads                              0.20

  Bone marrow (red)                   0.12

  Colon                               0.12

  Lung                                0.12

  Stomach                             0.12

  Bladder                             0.05

  Breast                              0.05

  Liver                               0.05

  Esophagus                           0.05

  Thyroid                             0.05

  Skin                                0.01

  Bone surface                        0.01

  Remainder                           0.05^a,b^
  ----------------------------------- -----------------------------------

^a^Remainder is composed of the following tissues: adrenals, brain,
extrathoracic airways, small intestine, kidneys, muscle, pancreas,
spleen, thymus, and uterus.

b The value 0.05 is applied to the mass-weighted average dose to the
Remainder tissues group, except when the following "splitting rule"
applies: If a tissue of Remainder receives a dose in excess of that
received by any of the 12 tissues for which weighting factors are
specified, a weighting factor of 0.025 (half of Remainder) is applied to
that tissue or organ and 0.025 to the mass-averaged committed equivalent
dose equivalent in the rest of the Remainder tissues.

**III. Annual Committed Tissue or Organ Equivalent Dose**

For internal irradiation from incorporated radionuclides, the total
absorbed dose will be spread out in time, being gradually delivered as
the radionuclide decays. The time distribution of the absorbed dose rate
will vary with the radionuclide, its form, the mode of intake and the
tissue within which it is incorporated. To take account of this
distribution the quantity *committed equivalent dose,* H~T~(τ) where τ
is the integration time in years following an intake over any particular
year, is used and is the integral over time of the equivalent dose rate
in a particular tissue or organ that will be received by an individual
following an intake of radioactive material into the body:

t~0~ + τ

H~T~(τ) =  H~T~(t) dt

*t*~0~

for a single intake of activity at time t~0~ where H~T~(τ) is the
relevant equivalent-dose rate in a tissue or organ at time t. For the
purposes of this rule, the previously mentioned single intake may be
considered to be an annual intake.

**IV. Annual Committed Effective Dose Equivalent**

If the annual committed equivalent doses to the individual tissues or
organs resulting from an annual intake are multiplied by the appropriate
weighting factors, w~T~, from table A.2, and then summed, the result
will be the [annual committed effective dose equivalent]{.underline}
E(τ):

E(τ) =  w~T~ · H~T~(τ).

T

[^1]: ^1^ ^1^ These laws include the Atomic Energy Act of 1954, as
    amended (42 U.S.C. 2011-2296) and Reorganization Plan No. 3 of 1970
    (5 U.S.C. Appendix 1).

[^2]: EnPA, Public Law No. 102-486, 102 Stat. 2776, 42 U.S.C. 10141 n.
    (1994)

[^3]: 5 U.S.C. 553

[^4]: For example, in general guidance documents, the IAEA has stated
    that "little credibility can be attached to assessments beyond 10^6^
    years." ("Safety Indicators in Different Time Frames for the Safety
    Assessment of Underground Radioactive Waste Repositories,"
    IAEA-TECDOC-767, p. 19, 1994, Docket No. EPA-HQ-OAR-2005-0083-0044)
    In its final 2006 Safety Requirements for Geological Disposal of
    Radioactive Waste, IAEA also states "Care needs to be exercised in
    using the criteria beyond the time where the uncertainties become so
    large that the criteria may no longer serve as a reasonable basis
    for decision making." (Docket No.
    EPA-HQ-OAR-2005-0083-038[x]{.mark}, page 11, paragraph 2.12) As a
    country-specific example, final guidelines from the Swedish
    Radiation Protection Authority state that "the risk analysis should
    be extended in time as long as it provides important information
    about the possibility of improving the protective capability of the
    repository, although at the longest for a time period of one million
    years." (Docket No. EPA-HQ-OAR-2005-0083-[xxxx]{.mark}) Also, in an
    example where the official guidelines specify a risk target that is
    of undefined duration, the United Kingdom's National Radiological
    Protection Board has stated that "\[o\]ne million years is...the
    timescale over which stable geological formations can be expected to
    remain relatively unchanged," while concluding that the scientific
    basis for risk calculations past one million years is "highly
    questionable.' ("Board Statement on Radiological Protection
    Objectives for the Land-based Disposal of Solid Radioactive Wastes,"
    1982 Documents of the NRPB, Volume 3, No. 3, p. 15, Docket No.
    EPA-HQ-OAR-2005-0083-[xxxx]{.mark})