[Federal Register Volume 64, Number 181 (Monday, September 20, 1999)]
[Proposed Rules]
[Pages 50778-50781]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 99-24254]
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NUCLEAR REGULATORY COMMISSION
10 CFR Part 61
Proposed Compatibility Designation Change and Draft Emplacement
Criticality Guidance for Low-Level Waste
AGENCY: Nuclear Regulatory Commission.
ACTION: Request for comment.
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SUMMARY: The U.S. Nuclear Regulatory Commission (NRC) is requesting
public comment as to whether the compatibility designation of 10 CFR
61.16(b)(2) should be changed. The compatibility designation relates to
the extent which an Agreement State's regulations must be compatible
with NRC requirements. The section of the Commission's regulations
under consideration requires low-level waste (LLW) disposal facility
licensees who receive and possess special nuclear material (SNM) to
describe proposed procedures to avoid accidental criticality for
storage of SNM waste prior to disposal and after disposal in the
ground. In addition, NRC also is requesting comment on draft guidance
on emplacement criticality at LLW disposal facilities.
DATES: Submit comments by October 20, 1999. Comments received after
this date will be considered, if it is practical to do so, but
assurance of consideration can only be given to comments received on or
before this date.
ADDRESSES: Submit comments to David L. Meyer, Chief, Rules and
Directives Branch, Division of Administrative Services, Office of
Administration, U.S. Nuclear Regulatory Commission, Washington, DC
20555. Hand deliver comments to 11545 Rockville Pike, Rockville, MD
between 5:15 am and 4:30 pm on Federal workdays.
You may also provide comments via the NRC's interactive rulemaking
website through the NRC home page (http://www.nrc.gov). From the home
page, select ``Rulemaking'' from the tool bar. The interactive
rulemaking website can then be accessed by selecting ``New Rulemaking
Website.'' This site provides the ability to upload comments as files
(any format), if your web browser supports that function. For
information about the interactive rulemaking website, contact Ms. Carol
Gallagher, (301) 415-5905; e-mail [email protected].
A copy of the draft guidance (NUREG/CR-6626, Emplacement Guidance
for Criticality Safety in Low-Level Waste Disposal) can be obtained
from the Internet at ``http://ruleforum.llnl.gov,'' or contact Mr. Tim
Harris (see FOR FURTHER INFORMATION CONTACT).
FOR FURTHER INFORMATION CONTACT: Tim Harris, Office of Nuclear Material
Safety and Safeguards, U.S. Nuclear Regulatory Commission, Washington
DC, 20555, telephone (301) 415-6613, or e-mail at [email protected].
Background
Section 274 of the Atomic Energy Act of 1954 (AEA), as amended,
provides a statutory basis for discontinuance by the NRC, and the
assumption by the State, of regulatory authority for byproduct
material, source material, and SNM in quantities not sufficient to form
a critical mass. As stated in the Commission's Policy Statement on
Adequacy and Compatibility of Agreement State Programs (62FR46517,
September 3,1997), NRC and Agreement States have the responsibility to
ensure that there is adequate protection of public health and safety
and that radiation control programs are administered consistent and
compatible with NRC's program.
Quantities of SNM not sufficient to form a critical mass are
defined in 10 CFR 150.11 as enriched uranium not exceeding 350 grams,
uranium-233 not exceeding 200 grams, plutonium not
[[Page 50779]]
exceeding 200 grams, or mixtures where the sum of the fractions is less
than unity. These quantities of SNM can be regulated by the Agreement
States. In both Agreement States and non-Agreement States, an NRC
license is required for persons who possess quantities of SNM in excess
of the Sec. 150.11 limits. As it pertains to disposal facilities, the
possession limits apply to material above-ground. Once the SNM waste is
disposed of (i.e., placed in the disposal trench), SNM waste is not
restricted by the Sec. 150.11 limits.
Currently 10 CFR 61.16 is not a regulation required for Agreement
State adoption; therefore, there is no equivalent Agreement State
regulatory requirement for Agreement State licensees of existing or
future LLW facilities to follow the equivalent of Sec. 61.16(b)(2) and
to evaluate emplacement criticality safety. This section of 10 CFR Part
61 requires LLW disposal facility licensees who receive and possess SNM
waste to describe proposed procedures to avoid accidental criticality
for storage of SNM waste prior to disposal and after disposal in the
ground. Although the SNM mass limits in Part 150 restrict above-ground
possession and ensure criticality safety above-ground (during receipt
and storage), there is no equivalent mass restriction or other controls
which limit the amount of SNM waste that can be placed in a disposal
trench; and therefore, the question of criticality safety below-ground
after disposal is left open. A technical basis for NRC's concern
regarding emplacement criticality safety is presented in the DISCUSSION
section of this document.
LLW containing SNM is currently disposed of at three facilities:
Barnwell, South Carolina; Hanford, Washington; and Clive, Utah. All of
these facilities are licensed by Agreement States. From the 1970's to
1997, NRC licensed the Barnwell and Hanford facilities under 10 CFR
Part 70, to receive, possess, store, and dispose of kilogram quantities
of SNM waste. In 1997, these facilities requested that the SNM
possession limits be reduced to the Section 150.11 limits, and that NRC
licenses be transferred to the respective Agreement States. These
actions have been taken for both (Barnwell and Hanford).
The State of Washington incorporated NRC criticality controls for
emplaced waste in license conditions in its Hanford license. Although
not in the license, the State of South Carolina has required the
licensee to implement the SNM waste emplacement procedures that address
criticality safety. These procedures cannot be changed by the operator
without State approval. NRC recently issued an Order to Envirocare that
exempts Envirocare from the licensing requirements in 10 CFR Part 70
for possession of SNM waste at concentration limits in the Order, which
ensures criticality safety. The conditions of the Order have been
incorporated into the State of Utah license.
If NRC changes the compatibility designation for Sec. 61.16(b)(2),
then LLW disposal facility licensees would be required to develop
procedures for avoiding accidental criticality, during both storage of
SNM waste prior to disposal and after disposal in the ground. These
procedures would then be reviewed and approved by Agreement State
staffs. Given that licensees and Agreement State staffs may not have
experience in criticality safety, NRC has developed guidance that could
be used by licensees and Agreement State staffs to demonstrate
compliance with Sec. 61.16(b)(2). A summary of this draft guidance and
how the guidance is envisioned to be used are provided in the
DISCUSSION section of this document.
Discussion
This section presents a discussion of the following: (1) the
technical basis for requiring emplacement criticality controls; (2) NRC
staff's assessment of the compatibility designation for 10 CFR
61.16(b)(2); (3) a summary of the draft guidance; (4) the envisioned
implementation if the compatibility of Sec. 61.16(b)(2) is changed; and
(5) NRC staff's assessment of potential resource impacts on Agreement
States.
Technical Basis
Spontaneous nuclear fission occurs naturally in a very small
percent of radioactive decays in some elements. When fission occurs,
neutrons are emitted, along with fission fragments (e.g., cesium and
strontium). The neutrons that are produced may be absorbed by an atom
without causing a fission, may be absorbed by an SNM atom and cause a
fission, or may not collide with any atoms. SNM (i.e., uranium-235,
uranium-233, and plutonium) is unique from most materials in that a
fission, not associated with a radioactive decay, can occur when a
neutron collides with its nucleus. In natural materials such as soils
containing natural uranium, neutrons produced by spontaneous fission
are typically absorbed by uranium-238 atoms and do not collide with a
uranium-235 atom possibly resulting in fission. Criticality is a chain
reaction where large numbers of neutrons are produced, and can occur
when sufficient SNM is present.
For a criticality to occur, special conditions involving a number
of factors must occur. Important factors that affect the criticality
safety of a LLW disposal site are: (1) the isotope; (2) enrichment; (3)
mass; (4) concentration; and (5) presence of neutron moderating and
absorbing materials. Each of these is discussed below. (Following this
is a discussion of these factors relative to possible scenarios).
(1) Isotope: The SNM isotopes present in LLW are dependent on the
waste stream. The vast majority of SNM waste is generated from the
production of nuclear fuel for nuclear power plants and from LLW
generated by nuclear power plants. Of the SNM isotopes, uranium-235 is
the most common. Large quantities of plutonium and uranium-233 (the
other SNM isotopes) are not present in the commercial waste. However,
these materials are present in Department of Energy (DOE) facility
waste, and some DOE waste is being shipped to commercial LLW disposal
facilities.
(2) Enrichment: Enrichment is a ratio of the weight of uranium-235
to the weight of the total uranium and is commonly expressed as a
percent. Natural uranium, found in most soils, has an average
enrichment of 0.71 percent. In order to be used as nuclear fuel,
natural uranium must be enriched in uranium-235. Most nuclear fuel is
enriched to less than 6 percent, which is considered low-enriched
uranium; however, some nuclear fuel for special reactors such as those
in naval vessels is enriched to much higher values, which is considered
high enriched uranium. At enrichments less than about 0.96 percent,
criticality is not possible regardless of the mass or concentration. As
enrichment increases, criticality becomes a greater concern. Although
most of the SNM waste contains low-enriched uranium, some waste
contains high-enriched uranium.
(3) Mass: As discussed above, disposal facilities that are licensed
by Agreement States and do not have an NRC license are subject to the
SNM possession limits in Part 150 for above ground possession. These
limits are based on a fraction of the minimum mass required to achieve
a criticality. Under these limits, there is simply not enough SNM to
cause a criticality regardless of the enrichment or concentration.
However, these limits have been applied to above-ground possession, and
SNM waste that has been disposed of is no longer be subject to these
limits. Historic records at disposal sites indicate that some
[[Page 50780]]
disposal units (trenches) have a mass of uranium-235 in the hundreds of
kilogram range. Therefore, it is reasonable to assume that large masses
of SNM waste will be disposed of in disposal units in the future.
In some cases, the mass of SNM in individual packages is limited by
the requirements in Part 71 (Packaging and Transportation of
Radioactive Material). The majority of SNM waste shipped to a LLW
disposal facility is transported under 10 CFR 71.53 as ``fissile
exempt.'' This means it does not have to comply with the fissile
material package standards in Secs. 71.55 and 71.59. In order to be
``fissile exempt'', the quantity of unusual moderators (beryllium,
graphite, or deuterium) is limited as is either the mass per package,
the amount of moderator (water), concentration, enrichment, or mass per
consignment. For example, SNM waste can be shipped as fissile exempt,
if it contains no more than 15 grams of SNM per package. However, some
general licenses in Part 71 allow for SNM waste to be shipped at higher
masses per package. For example,10 CFR 71.22 allows up to 500 grams per
shipment, which could be in a single container, provided unusual
moderators are limited to 0.1 percent of the mass of the fissile
material. This general license does not restrict concentration or
enrichment. Therefore, mass cannot be eliminated as a factor of concern
based solely on packaging and transportation regulations. As mass
increases, criticality becomes a greater concern.
(4) Concentration: In some cases, the concentration of SNM received
by a LLW disposal facility is limited by the requirements in Part 71.
While significant quantities of SNM waste can be shipped under a number
of general licenses, the majority of SNM waste shipped to a LLW
disposal facility is transported as ``fissile exempt''. As noted above,
in order to be ``fissile exempt'', the quantity of unusual moderators
(beryllium, graphite, or deuterium) is limited, as is either the mass
per package, the amount of moderator (water), the concentration, the
enrichment, or the mass per consignment. For example, SNM waste can be
shipped as fissile exempt, if it contains no more than 5 grams of SNM
in any 10 liter volume. However, some general licenses in Part 71 allow
for SNM waste to be shipped at higher concentrations per package.
Therefore, concentration cannot be eliminated as a factor of concern
based solely on packaging and transportation regulations.
(5) Presence of neutron moderator and absorbers: Neutrons that are
produced during a fission have a relatively high energy and are termed
``fast'' neutrons. Moderators are materials that reduce the energy, or
slow neutrons. This is important because uranium-235 is much more
likely to be fissioned by slow neutrons than by fast neutrons.
Therefore, the presence of moderator materials can increase the
criticality concern. Elements such as hydrogen and carbon are
particularly good moderators. Because water is abundant and is a very
efficient moderator, assuming water is present is a common approach in
evaluating the criticality significance of situations. However, there
are certain materials such as beryllium, graphite, and deuterium that
are more efficient moderators than water. These material are commonly
termed ``unusual'' moderators.
Absorbers are materials that absorb or capture neutrons. Because
capturing neutrons prevents those neutrons from possibly causing a
fission, the presence of absorber materials will decrease the
criticality concern. Most materials act both as a moderator and an
absorber to varying degrees.
In some cases the presence of moderator material is limited by the
requirements in Part 71. However, this is not always the case. It is
reasonable to assume that moderators, such as water, will be present in
the waste. In analyzing the criticality hazard of waste at LLW disposal
facilities, it is conservative to assume that moderators will be
present in optimal amounts. The presence of absorber materials is not
limited by regulations. These materials, such as iron, calcium, etc.,
are present in LLW and in the waste containers. However, the amount and
distribution of absorbers cannot be assured, so they are typically
omitted in analyzing criticality hazards. For example, although a steel
drum acts as an absorber, the drum will corrode within tens of years
and can no longer be depended on to contain the waste and act as an
absorber.
Possible Scenarios
In order for a criticality to occur, several of the above factors
must be above certain values. For instance, a criticality cannot occur
if the mass of the SNM is below a certain value regardless of the
enrichment or concentration. A criticality cannot occur if the
concentration of the SNM is below a certain value regardless of the
enrichment or mass. A criticality cannot occur if the enrichment is
below a certain value regardless of the mass or concentration.
Considering what can be controlled by Parts 71 and 150, several
scenarios can be postulated. For waste shipped as ``fissile exempt'',
concentrations can be limited to 5 grams of SNM per 10 liters. This
translates to 104 grams of enriched uranium for a typical waste
container (i.e., 55-gallon drum). In addition, under the fissile
exemption unusual moderators are limited. Assuming a density of waste
of 68 pounds per cubic foot, this concentration (4.6E-4 gram of
uranium-235 per gram of waste) is smaller than the allowable
operational concentration limit in the draft guidance (NUREG/CR-6626,
Emplacement Guidance for Criticality Safety in Low-Level Waste
Disposal) and therefore is considered safe. The limits in the draft
guidance have been developed considering that absorbers are not present
and that moderation with water is optimal to maximize the possibility
of fissions.
For waste that does not meet the fissile exemption criteria,
concentration, enrichment, and mass are not controlled. Given that
disposal facilities licensed by Agreement States can only possess 350
grams, a package containing 350 grams of highly enriched uranium could
be shipped to a disposal facility. Using the example of waste shipped
in 55-gallon drums with a waste density of 68 pounds per cubic foot,
the uranium-235 concentration is 1.5E-3 gram of uranium-235 per gram of
waste. This concentration exceeds the limit for high enriched uranium
in the draft guidance (8.3E-4 gram U-235/gram of waste for a 10-foot
high disposal unit). While a single container would not represent a
criticality concern, an array of such drums could represent a
criticality concern.
Using the criticality calculations in NUREG/CR-6505 Volumes 1 and
2, ``The Potential for Criticality Following Disposal of Uranium at
Low-Level Waste Facilities,'' an array of low-enriched uranium (10
percent enrichment) drums stacked more than 15 feet high could pose a
criticality concern. An array of high-enriched uranium (100 percent
enrichment) drums stacked more than 11 feet high could pose a
criticality concern. Trenches at burial sites are deeper than 15 feet.
These calculations assume optimal water moderation and no absorbers.
Although there is significant uncertainty associated with a waste
facility receiving and disposing of numerous drums containing large
amounts of SNM, there are no regulatory limitations to preclude this
situation.
[[Page 50781]]
NRC Staff Assessment of Compatibility Designation
At the time the compatibility designations were originally selected
for Part 61 (1983), the NRC directly regulated SNM at LLW disposal
facilities. Becuase the NRC is responsible for SNM in greater than
critical mass quantities and regulated SNM at LLW disposal facilities,
there was no need for Agreement States to adopt these requirements.
These requirements were designated ``Not Required for Compatibility.''
As noted above, LLW disposal facilities reduced their SNM possession
limits to those provided in 10 CFR 150.11 (350 grams or less). This
authority was assumed by the respective Agreement State; thus, the NRC
no longer directly regulates SNM at LLW disposal facilities, including
the authority to administer waste emplacement criticality controls.
Therefore, the NRC is considering changing the compatibility
designation of Sec. 61.16(b)(2) to ensure these safety measures are
applied in the disposal of SNM.
NRC staff used the procedures outlined in Management Directive 5.9,
``Adequacy and Compatibility of Agreement State Programs,'' and
concluded that the compatibility designation for Sec. 61.16(b)(2)
should be revised from category ``Not Required for Compatibility'', to
category ``Health and Safety''. ``Health and Safety'' applies to
activities that could result directly in an exposure to an individual
in excess of basic radiation protection standards, if the essential
objectives of the provision were not adopted by an Agreement State. If
an inadvertent criticality were to occur at a LLW disposal facility,
workers could receive doses in excess of the 10 CFR Part 20 limits.
Under the ``Health and Safety'' category, Agreement States that have
currently operating LLW disposal facilities and those States which will
be establishing LLW disposal facilities in the future, would need to
adopt legally binding requirements that encompass the essential
objectives of 10 CFR 61.16(b)(2) within three years of the change of
designation in compatibility. This requirement would continue to be
designated as ``Not Required for Compatibility,'' for other Agreement
States.
Summary of Draft Emplacement Criticality Guidance
The draft guidance provides a general approach to emplacement
criticality safety. Five different SNM isotopic compositions were
studied: uranium-235 at 10 and 100 percent enrichment; uranium-233;
plutonium-239; and a mixture of plutonium-239, -240, and -241. Three
different graded approaches are presented. The first graded approach is
the most conservative, and can be used easily for facilities that
dispose of very low levels of SNM, or dispose of material with a low
average enrichment. This approach relies on the calculation of average
areal density, or grams of SNM per square foot, or on the average
enrichment of SNM. The area over which averaging may be performed also
is specified, but the emplacement depth and concentration are not
limited.
The second graded approach relies on limiting the average
concentration by weight of SNM in the waste, and on limiting the depth
of the emplacement. This method may be useful for facilities that
emplace somewhat higher areal densities of SNM, but which do not use
vaults or segmentation in the disposal emplacement.
The third graded approach relies on limiting the average
concentration by weight of SNM in the waste, and on the presence of
segmenting barriers, such as vaults, that will prevent movement of SNM
waste from one side of the barrier to the other. This method may be
useful for facilities that use concrete vaults in their disposal areas.
Envisioned Implementation of Guidance and Change in Compatibility
If the compatibility designation of 10 CFR 61.16(b)(2) were changed
from ``Not Required for Compatibility'' to ``Health and Safety'',
Agreement States would have three years to implement regulations or
other legally binding requirements compatible with Sec. 61.16(b)(2). As
noted earlier, the States of Washington and South Carolina currently
have emplacement criticality controls. The compatibility change will
assure that future LLW disposal facilities in Agreement States will
have criticality safety controls for emplaced SNM waste.
After these legally binding requirements have been implemented, the
Agreement State regulatory program would require their licensees
(disposal facility operators) to prepare and submit information
demonstrating compliance with their equivalent of 10 CFR 61.16(b)(2).
To assist the States and licensees, NRC has prepared emplacement
criticality safety guidance. Licensees would review the types of waste
and disposal operations and determine which of the graded approaches in
the guidance were appropriate for its facility. For each of the graded
approaches, the NRC draft guidance includes criticality safety limits
and a description of how to calculate the limits based on readily
available information. The draft guidance also indicates the type of
procedures that would need to be developed for each of the graded
approaches. This guidance would serve as a technical basis for
preparing the license amendment requests submitted to the Agreement
States.
The Agreement State regulator would then review this amendment
request and modify the license as appropriate. Again, the guidance
would serve as the technical basis for the State regulator.
NRC Staff Assessment of Potential Resource Impact on Agreement States
NRC staff has estimated the potential resource impacts on Agreement
States to implement a change in the compatibility of 10 CFR
61.16(b)(2). As indicated above, the first step would be to modify its
regulations or other legally binding requirements to be compatible with
Sec. 61.16(b)(2). We consider that only a minor modification would be
necessary to the existing Agreement State Part 61 equivalent
regulations, or that the compatibility change could be administered
through other legally binding requirements. We estimate that this will
take four to six-State staff weeks. The next step of an Agreement State
would be to review the licensee's amendment request and/or procedure
changes. We estimate that this will take two-State staff weeks. Some
additional effort would be required for inspection of the facility;
however, this effort is not estimated to be significant.
Dated at Rockville, Maryland this 9th day of September, 1999.
For the Nuclear Regulatory Commission.
Daniel M. Gillen,
Acting Chief, Uranium Recovery and Low-Level Waste Branch, Division of
Waste Management, Office of Nuclear Material Safety and Safeguards.
[FR Doc. 99-24254 Filed 9-17-99; 8:45 am]
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