Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 70475

present an undue risk to the public 

health and safety, and is consistent with 

the common defense and security. Also, 

special circumstances are present. 

Therefore, the Commission hereby 

grants FirstEnergy Nuclear Operating 

Company an amendment to the 

exemption from the requirements of 10 

CFR 50.46(a)(1)(ii) and 10 CFR Part 50, 

Appendix K, Section 1.D.1 for DavisBesse Nuclear Power Station. 

Pursuant to 10 CFR 51.32, the 

Commission has determined that the 

granting of this exemption will not have 

a significant effect on the quality of the 

human environment (69 FR 47469). 

This exemption is effective upon 

issuance and shall be implemented 

within 120 days.

Dated at Rockville, Maryland, this 29th day 

of November 2004.

For the Nuclear Regulatory Commission. 

Ledyard B. Marsh, 

Director, Division of Licensing Project 

Management, Office of Nuclear Reactor 

Regulation.

[FR Doc. 04–26692 Filed 12–3–04; 8:45 am] 

BILLING CODE 7590–01–P

NUCLEAR REGULATORY 

COMMISSION 

Notice of Availability of Interim Staff 

Guidance Documents for Fuel Cycle 

Facilities

AGENCY: Nuclear Regulatory 

Commission.

ACTION: Notice of availability.

FOR FURTHER INFORMATION CONTACT:

Wilkins Smith, Project manager, 

Technical Support Group, Division of 

Fuel Cycle Safety and Safeguards, Office 

of Nuclear Material Safety and 

Safeguards, U.S. Nuclear Regulatory 

Commission, Washington, DC 20005–

0001. Telephone: (301) 415–5788; fax 

number: (301) 415–5370; e-mail: 

wrs@nrc.gov.

SUPPLEMENTARY INFORMATION: 

I. Introduction 

The Nuclear Regulatory Commission 

(NRC) plans to issue Interim Staff 

Guidance (ISG) documents for fuel cycle 

facilities. These ISG documents provide 

clarifying guidance to the NRC staff 

when reviewing either a license 

application or a license amendment 

request for a fuel cycle facility under 10 

CFR part 70. The NRC is soliciting 

public comments on the ISG documents 

which will be considered in the final 

versions or subsequent revisions. 

II. Summary 

The purpose of this notice is to 

provide the public an opportunity to 

review and comment on a draft Interim 

Staff Guidance document for fuel cycle 

facilities. Interim Staff Guidance-10 

provides guidance to NRC staff relative 

to determining whether the minimum 

margin of subcriticality (MoS) is 

sufficient to provide an adequate 

assurance of subcriticality for safety to 

demonstrate compliance with the 

performance requirements of 10 CFR 

70.61(d). 

III. Further Information 

The document related to this action is 

available electronically at the NRC’s 

Electronic Reading Room at http://

www.nrc.gov/reading-rm/adams.html. 

From this site, you can access the NRC’s 

Agencywide Document Access and 

Management System (ADAMS), which 

provides text and image files of NRC’s 

public documents. The ADAMS 

ascension number for the document 

related to this notice is ML043290270. 

If you do not have access to ADAMS or 

if there are problems in accessing the 

document located in ADAMS, contact 

the NRC Public Document Room (PDR) 

Reference staff at 1–800–397–4209, 301–

415–4737, or by e-mail to pdr@nrc.gov. 

This document may also be viewed 

electronically on the public computers 

located at the NRC’s PDR, O 1 F21, One 

White Flint North, 11555 Rockville 

Pike, Rockville, MD 20852. The PDR 

reproduction contractor will copy 

documents for a fee. Comments and 

questions should be directed to the NRC 

contact listed above by January 5, 2005. 

Comments received after this date will 

be considered if it is practical to do so, 

but assurance of consideration cannot 

be given to comments received after this 

date.

Dated at Rockville, Maryland, this 24th day 

of November 2004.

For the Nuclear Regulatory Commission. 

Melanie A. Galloway, 

Chief, Technical Support Group, Division of 

Fuel Cycle Safety and Safeguards, Office of 

Nuclear Material Safety and Safeguards.

Draft—Division of Fuel Cycle Safety 

and Safeguards Interim Staff 

Guidance—10; Justification for 

Minimum Margin of Subcriticality for 

Safety Issue 

Technical justification for the 

selection of the minimum margin of 

subcriticality (MoS) for safety, as 

required by 10 CFR 70.61(d) 

Introduction 

10 CFR 70.61(d) requires, in part, that 

licensees demonstrate that ‘‘under 

normal and credible abnormal 

conditions, all nuclear processes are 

subcritical, including use of an 

approved margin of subcriticality for 

safety.’’ To demonstrate subcriticality, 

licensees perform validation studies in 

which critical experiments similar to 

actual or anticipated calculations are 

chosen and are then used to establish a 

mathematical criterion for subcriticality 

for all future calculations. This criterion 

is expressed in terms of a limit on the 

maximum value of the calculated keff, 

which will be referred to in this ISG as 

the upper subcritical limit (USL). The 

USL includes allowances for bias and 

bias uncertainty as well as an additional 

margin which will be referred to 

hereafter as the minimum margin of 

subcriticality (MoS). This MoS has been 

variously referred to within the nuclear 

industry as subcritical margin, arbitrary 

margin, and administrative margin. The 

term MoS will be used throughout this 

ISG for consistency, but these terms are 

frequently used interchangeably. This 

MoS is an allowance for any unknown 

errors in the calculational method that 

may bias the result of calculations, 

beyond those accounted for explicitly in 

the calculation of the bias and bias 

uncertainty. 

There is little guidance in the fuel 

facility Standard Review Plans (SRPs) as 

to what constitutes an acceptable MoS. 

NUREG–1520, Section 5.4.3.4.4, states 

that the MoS should be pre-approved by 

the NRC and that the MoS must 

‘‘include adequate allowance for 

uncertainty in the methodology, data, 

and bias to assure subcriticality.’’

However, there is little guidance on how 

to determine the amount of MoS that is 

appropriate. Partly due to the historical 

lack of guidance, there have been 

significantly different margins of 

subcriticality approved for different fuel 

cycle facilities over time. In addition, 

the different ways of defining the MoS 

and calculating keff limits significantly 

compound the potential for confusion. 

The MoS can have a significant effect on 

facility operations (e.g., storage capacity 

and throughput) and there has therefore 

been considerable recent interest in 

decreasing the margins of subcriticality 

below what has been accepted 

historically. These two factors—the lack 

of guidance and the increasing interest 

in reducing margins of subcriticality—

make clarification of what constitutes 

acceptable justification for the MoS 

necessary. In general, consistent with a 

risk-informed approach to regulation, 

smaller margins of subcriticality require 

more substantial technical justification. 

The purpose of this ISG therefore is to 

provide guidance on determining 

whether the MoS is sufficient to provide 

70476 Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 

1There are many different ways of computing 

bias as used in calculation of the USL. This may 

be an average bias, a least-squares fitted bias, a 

bounding bias, etc., as described in the applicant’s 

methodology.

2Not all licensees have a separate subcritical and 

operating limit. Use of administrative operating 

limits is optional, because the subcritical limit 

should conservatively take parametric tolerances 

into account.

an adequate assurance of subcriticality 

for safety, in accordance with 10 CFR 

70.61(d). 

Discussion 

The neutron multiplication factor of a 

fissile system (keff) depends, in general, 

on many different physical variables. 

The factors that can affect the calculated 

value of keff may be broadly divided into 

the following categories: (1) Geometric 

form; (2) material composition; and (3) 

neutron distribution. The geometric 

form and material composition of the 

system determine—together with the 

underlying nuclear data (e.g., v, X(E), 

and the set of cross section data)—the 

spatial and energy distribution of 

neutrons in the system (i.e., flux and 

energy spectrum). An error in the 

nuclear data or in the modeling of these 

systems can produce an error in the 

calculated value of keff. This difference 

between the calculated and true value of 

keff is referred to as the bias1. The bias 

is defined as the difference between the 

calculated and true values of keff, by the 

following equation: b = kcalc ¥ ktrue

The bias of a critical experiment may 

be known with a high degree of 

confidence because the true 

(experimental) value is known a priori 

(ktrue ≈ 1). Because both the 

experimental and the calculational 

uncertainty are known, there is a 

determinable uncertainty associated 

with the bias. The bias for a calculated 

system other than a critical experiment 

is not typically known with this same 

high degree of confidence, because ktrue

is not typically known. The MoS is 

therefore an allowance for any unknown 

errors that may affect the calculated 

value of keff, beyond those accounted for 

explicitly in the bias and bias 

uncertainty. An MoS is needed because 

the critical experiments chosen will, in 

general, exhibit somewhat different 

geometric forms, material compositions, 

and neutron spectra from those of actual 

system configurations, and the effect of 

these differences is difficult to quantify. 

Bias and bias uncertainty are estimated 

by calculating the keff of critical 

experiments with geometric forms, 

material compositions, and neutron 

spectra similar to those of actual or 

anticipated calculations. However, 

because of the many factors that can 

effect the bias, it must be recognized 

that this is only an estimate of the true 

bias of the system; it is not possible to 

guarantee that all sources of error have 

been accounted for during validation. 

Thus, use of a smaller MoS requires a 

greater level of assurance that all 

sources of uncertainty and bias have 

been taken into account and that the 

bias is known with a high degree of 

accuracy. The MoS should be large 

compared to known uncertainties in the 

nuclear data and limitations of the 

methodology (e.g., modeling 

approximations, convergence 

uncertainties). It should be noted that 

this MoS is only needed when 

subcritical limits are based on the use of 

calculational methods, including 

computer and hand calculations. The 

MoS is not needed when subcritical 

limits are based on other methods, such 

as experiment or published data (e.g., 

widely accepted handbooks or endorsed 

industry standards). 

Because the nuclear industry has 

employed widely different terminology 

regarding validation and margin, it is 

necessary to define the following terms 

as used in this ISG. These definitions 

are for clarity only and are not meant to 

prescribe any particular terminology. 

Bias: The difference between the 

calculated and true values of keff for a 

fissile system or set of systems. 

Bias Uncertainty: The calculated 

uncertainty in the bias as determined by 

a statistical method. 

Margin of subcriticality (MoS): Margin 

in keff applied in addition to bias and 

bias uncertainty to ensure subcriticality 

(also known as subcritical, arbitrary, or 

administrative margin). This term is 

shorthand for ‘‘minimum margin of 

subcriticality’’. 

Margin of safety: Margin in one or 

more system parameters that represents 

the difference between the value of the 

parameter at which it is controlled and 

the value at which the system becomes 

critical. (This represents an additional 

margin beyond the MoS.)

Upper Subcritical Limit: The 

maximum allowable keff value for a 

system. Generally, the USL is defined by 

the equation USL = 1¥bias¥bias 

uncertainty¥MoS. 

Subcritical Limit: The value of a 

system parameter at which it is 

controlled to ensure criticality safety, 

and at which keff does not exceed the 

USL (also known as safety limit). 

Operating Limit: The value of a 

system parameter at which it is 

administratively controlled to ensure 

that the system will not exceed the 

subcritical limit.2

If the USL is defined as described 

above, then the MoS represents the 

difference between the average 

calculated keff (including uncertainties) 

and the USL, thus: 

MoS = (1¥bias¥bias 

uncertainty)¥USL. 

There are many factors that can affect 

the code’s ability to accurately calculate 

keff and that can thus impact the 

analyst’s confidence in the estimation of 

the bias. Some of these factors are 

described in detail below. 

Benchmark Similarity 

Because the bias of calculations is 

estimated based on critical benchmarks 

with similar geometric form, material 

composition, and neutronic behavior to 

the systems being evaluated, the degree 

of similarity between benchmarks and 

actual or anticipated calculations is a 

key consideration in determining the 

appropriate MoS. The more closely the 

benchmarks represent the 

characteristics of systems being 

validated, the more confidence exists in 

the calculated bias and bias uncertainty. 

Allowing a comparison of the chosen 

benchmarks to actual or anticipated 

calculations requires that both the 

experiments and the calculations be 

described in sufficient detail to permit 

independent verification of results. This 

may be accomplished by submitting 

input decks for both benchmarks and 

calculations, or by providing detailed 

drawings, tables, or other such data to 

the NRC to permit a detailed 

comparison of system parameters. 

In evaluating benchmark similarity, 

some parameters are obviously more 

significant than others. The parameters 

that can have the greatest effect on the 

calculated keff of the system are those 

that are most significant. Historically, 

some parameters have been used as 

trending parameters because these are 

the parameters that are expected to have 

the greatest effect on the bias. They 

include the moderator-to-fuel ratio (e.g., 

H/U, H/X, vm/vf

), isotopic abundance 

(e.g., 235U, 239Pu, or overall Pu-content), 

and parameters characterizing the 

neutron spectrum (e.g., energy of 

average lethargy causing fission (EALF), 

or average energy group (AEG)). Other 

parameters, such as material density or 

overall geometric shape, are generally 

considered to be of less importance. 

Care should be taken that, when basing 

justification for a reduced MoS on the 

similarity of benchmarks to actual or 

anticipated calculations, all important 

system characteristics that can affect the 

bias have been taken into consideration. 

There are several ways to demonstrate 

that the chosen benchmarks are 

sufficiently similar to actual or 

anticipated calculations: 

Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 70477

1. NUREG/CR–6698, ‘‘Guide to 

Validation of Nuclear Criticality Safety 

Calculational Method,’’ Table 2.3, 

contains a set of screening criteria for 

determining benchmark applicability. 

As is stated in the NUREG, these criteria 

were arrived at by consensus among 

experienced NCS specialists and may be 

considered conservative. The NRC staff 

considers agreement on all screening 

criteria to be sufficient justification for 

demonstrating benchmark similarity. 

However, less conservative (i.e., 

broader) screening ranges may be used 

if appropriately justified. 

2. Use of an analytical method that 

systematically quantifies the degree of 

similarity between benchmarks and 

design applications, such as Oak Ridge 

National Laboratory’s TSUNAMI code 

in the SCALE 5 code package. 

TSUNAMI calculates a correlation 

coefficient indicating the degree of 

similarity between each benchmark and 

calculation in pair-wise fashion. The 

appropriate threshold value of the 

parameter indicating a sufficient degree 

of similarity is an unresolved issue with 

the use of this method. However, the 

NRC staff currently considers a 

correlation coefficient ck ≥ 0.95 to be 

indicative of a strong degree of 

similarity. Conversely, a correlation 

coefficient < 0.90 should not be used as 

demonstration of benchmark similarity 

without significant additional 

justification. These observations are 

tentative and are based on the staff’s 

observation that benchmarks and 

calculations having a correlation of at 

least 95% also appear to be very similar 

based on a traditional comparison of 

system parameters. TSUNAMI should 

not be used as a ‘‘black box,’’ but may 

be used to inform the benchmark 

selection process, due to the evolving 

nature of this tool. 

3. Sensitivity studies may be 

employed to demonstrate that the 

system keff is highly insensitive to a 

particular parameter. In such cases, a 

significant error in the parameter will 

have a small effect on the system bias. 

One example is when the number 

density of certain trace materials can be 

shown to have a negligible effect on keff. 

Another example is when the presence 

of a strong external absorber has only a 

slight effect on keff. In both cases, such 

a sensitivity study may be used to 

justify why agreement with regard to a 

given parameter is not important for 

demonstrating benchmark similarity. 

4. Physical arguments may be used to 

demonstrate benchmark similarity. For 

example, the fact that oxygen and 

fluorine are almost transparent to 

thermal neutrons (i.e., cross sections are 

very low) may be used as justification 

for why the differences in chemical 

form between UO2F2 and UO2 may be 

ignored. 

A combination of the above methods 

may also prove helpful in demonstrating 

benchmark similarity. For example, 

TSUNAMI may be used to identify the 

parameters to which keff is most 

sensitive, or a sensitivity study may be 

used to confirm TSUNAMI results or 

justify screening ranges. Care should be 

taken to ensure that all parameters 

which can measurably affect the bias are 

considered when comparing chosen 

benchmarks to calculations. For 

example, comparison should not be 

based solely on agreement in the 235U 

fission spectrum if 238U or 10B 

absorption or 1H scattering have a 

significant effect on the calculated keff. 

A method such as TSUNAMI that 

considers the complete set of reactions 

and nuclides present should be used 

rather than relying on a comparison of 

only the fission spectra. That all 

important parameters have been 

included can be determined based on a 

study of the keff sensitivity, as discussed 

in the next section. It is especially 

important that all materials present in 

calculations that can have more than a 

negligible effect on the bias are included 

in the chosen benchmarks. In addition, 

it is necessary that if the parameters 

associated with calculations are outside 

the range of the benchmark data, the 

effect of extrapolating the bias should be 

taken into account in setting the USL. 

This should be done by making use of 

trends in the bias. Both the trend and 

the uncertainty in the trend should be 

extrapolated using an established 

mathematical method. 

Some questions that should be asked 

in evaluating the chosen benchmarks 

include: 

• Are the critical experiments chosen 

all high-quality benchmarks from 

reliable (e.g., peer-reviewed and widelyaccepted) sources? 

• Are the benchmarks chosen taken 

from independent sources? 

• Do the most important benchmark 

parameters cover the entire range 

needed for actual or anticipated 

calculations? 

• Is the number of benchmarks 

sufficient to establish trends in the bias 

across the entire range? (The number 

depends on the specific statistical 

method employed.) 

• Are all important parameters that 

could affect the bias adequately 

represented in the chosen benchmarks? 

System Sensitivity 

Sensitivity of the calculated keff to 

changes in system parameters is a 

closely related concept to that of 

similarity. This is because those 

parameters to which keff is most 

sensitive should weigh more heavily in 

evaluating benchmark similarity. If keff

is highly sensitive to a given parameter, 

an error in the parameter could be 

expected to have a significant impact on 

the bias. Conversely, if keff is very 

insensitive to a given parameter, then an 

error would be expected to have a 

negligible impact on the bias. In the 

latter case, agreement with regard to that 

parameter is not important to 

establishing benchmark similarity.

Two major ways to determine the 

system’s keff sensitivity are: 

1. The TSUNAMI code in the SCALE 

5 code package can be used to calculate 

the sensitivity coefficients for each 

nuclide-reaction pair present in the 

problem. TSUNAMI calculates both an 

integral sensitivity coefficient (i.e., 

summed over all energy groups) and a 

sensitivity profile as a function of 

energy group. The sensitivity coefficient 

is defined as the fractional change in keff

for a 1% change in the nuclear cross 

section. It must be recognized that 

TSUNAMI only evaluates the keff

sensitivity to changes in the nuclear 

data, and not to other parameters that 

could affect the bias and should be 

considered. 

2. Direct sensitivity calculations can 

also be used to perturb the system and 

gauge the resulting effect on keff. 

Perturbation of the atomic number 

densities can also be used to confirm the 

integral sensitivity coefficients 

calculated by TSUNAMI (as when there 

is doubt as to convergence of the adjoint 

flux). 

The relationship between the keff

sensitivity and confidence in the bias is 

the reason that high-enriched uranium 

fuel facilities have historically required 

a greater MoS than low-enriched 

uranium facilities. High-enriched 

systems tend to be much more sensitive 

to changes in the underlying system 

parameters, and in such systems, the 

effect of any errors on the bias would be 

greatly magnified. For this same reason, 

systems involving weapons-grade 

plutonium would also be more 

susceptible to undetected errors than 

low-assay mixed oxide (i.e., a few 

percent Pu). The appropriate amount of 

MoS should therefore be commensurate 

with the sensitivity of the system to 

changes in the underlying parameters. 

Some questions that should be asked 

in evaluating the keff sensitivity include: 

• How sensitive is keff to changes in 

the underlying nuclear data (e.g., cross 

sections)? 

• How sensitive is keff to changes in 

the geometric form and material 

composition? 

70478 Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 

• Is the MoS large compared to the 

expected magnitude of changes in keff

resulting from errors in the underlying 

system parameters? 

Neutron Physics of the System 

Another consideration that may affect 

the appropriate MoS is the extent to 

which the physical behavior of the 

system is known. Fissile systems which 

are known to be subcritical with a high 

degree of confidence do not require as 

much MoS as systems where 

subcriticality is less certain. An example 

of a system known to be subcritical 

would be a finished fuel assembly. 

These systems typically can only be 

made critical when highly thermalized, 

and due to extensive analysis and 

reactor experience, the flooded case is 

known to be subcritical in isolation. In 

addition, the thermal neutron cross 

sections for materials in finished reactor 

fuel have been measured with an 

exceptionally high degree of accuracy 

(as opposed to the unresolved resonance 

region). Other examples may include 

systems consisting of very simple 

geometry or other idealized situations, 

in which there is strong evidence that 

the system is subcritical based on 

comparisons with highly similar 

systems in published references such as 

handbooks or standards. In these cases, 

the amount of MoS needed may be 

significantly reduced. 

An important factor in determining 

that the neutron physics of the system 

is well-known is ensuring that the 

configuration of the system is fixed. For 

example, a finished fuel assembly is 

subject to tight quality assurance checks 

and has a form that is well-characterized 

and highly stable. A solution or powder 

process with a complex geometric 

arrangement would be much more 

susceptible to having its configuration 

change to one whose neutron physics is 

not well-understood. Experience with 

similar processes may also be credited. 

Some questions that should be asked 

in evaluating the neutron physics of the 

system include: 

• Is the geometric form and material 

composition of the system rigid and 

unchanging? 

• Is the geometric form and material 

composition of the system subject to 

strict quality assurance? 

• Are there other reasons besides 

criticality calculations to conclude that 

the system will be subcritical (e.g., 

handbooks, standards, reactor fuel 

studies)? 

• How well-known are the cross 

sections in the energy range of interest? 

Rigor of Validation Methodology 

Having a high degree of confidence in 

the estimated bias and bias uncertainty 

requires both that there be a sufficient 

quantity of well-behaved benchmarks 

and that there be a sufficiently rigorous 

validation methodology. If either the 

data or the methodology is not adequate, 

a high degree of confidence in the 

results cannot be attained. The 

validation methodology must also be 

suitable for the data analyzed. For 

example, a statistical methodology 

relying on the data being normally 

distributed about the mean keff would 

not be appropriate to analyze data that 

are not normally distributed. A linear 

regression fit to data that has a nonlinear bias trend would similarly not be 

appropriate.

Having a sufficient quantity of wellbehaved benchmarks means that: (1) 

There are enough (applicable) 

benchmarks to make a statistically 

meaningful calculation of the bias and 

bias uncertainty; (2) the benchmarks 

span the entire range of all important 

parameters, without gaps requiring 

extrapolation or wide interpolation; and 

(3) the benchmarks do not display any 

apparent anomalies. Most of the 

statistical methods used rely on the 

benchmarks being normally distributed. 

To test for normality, there must be a 

statistically significant number of 

benchmarks (which may vary 

depending on the test employed). If 

there is insufficient data to verify 

normality to at least the 95% confidence 

level, then a non-parametric technique 

should be used to analyze the data. In 

addition, the benchmarks should 

provide a continuum of data across the 

entire validated range so that any 

variation in the bias as a function of 

important system parameters may be 

observed. Anomalies that may cast 

doubt on the results of the validation 

may include the presence of discrete 

clusters of experiments having a lower 

calculated keff than the set of 

benchmarks as a whole, an excessive 

fluctuation in keff values (e.g., having a 

X 2/N ® 1), or discarding an unusually 

high number of benchmarks as outliers 

(i.e., more than 1–2%). 

Having a sufficiently rigorous 

validation methodology means having a 

methodology that is appropriate for the 

number and distribution of benchmark 

experiments, that calculates the bias and 

bias uncertainty using an established 

statistical methodology, that accounts 

for any trends in the bias, and that 

accounts for all apparent sources of 

uncertainty in the bias (e.g., the increase 

in uncertainty due to extrapolating the 

bias beyond the range covered by the 

benchmark data). 

In addition, confidence that the code’s 

performance is well-understood means 

the bias should be relatively small (i.e., 

bias  ̈ 2%), or else the reason for the 

bias should be known, and no credit 

must be taken for positive bias. If the 

absolute value of the bias is very large 

(especially if the reason for the large 

bias is unknown), this may indicate that 

the calculational method is not very 

accurate, and a larger MoS may be 

appropriate. 

Some questions that should be asked 

in evaluating the data and the 

methodology include: 

• Is the methodology consistent with 

the distribution of the data (e.g., 

normal)? 

• Are there enough benchmarks to 

determine the behavior of the bias 

across the entire area of applicability? 

• Does the assumed functional form 

of the bias represent a good fit to the 

benchmark data? 

• Are there discrete clusters of 

benchmarks for which the overall bias 

appears to be non-conservative 

(especially consisting of the most 

applicable benchmarks)? 

• Has additional margin been applied 

to account for extrapolation or wide 

interpolation? 

• Have all apparent bias trends been 

taken into account? 

• Has an excessive number of 

benchmarks been discarded as statistical 

outliers? 

Performance of an adequate code 

validation alone is not sufficient 

justification for any specific MoS. The 

reason for this is that determination of 

the bias and bias uncertainty is separate 

from selection of an appropriate MoS. 

Therefore, performing an adequate code 

validation is not alone sufficient 

demonstration that an appropriate MoS 

has been chosen. 

Margin in System Parameters

The MoS is a reflection of the degree 

of confidence in the results of the 

validation analysis; the MoS is a margin 

in keff to provide a high degree of 

assurance that fissile systems calculated 

to be subcritical are in fact subcritical. 

However, there are other types of 

margin that can provide additional 

assurance of subcriticality; these 

margins are frequently expressed in 

terms of the system parameters rather 

than keff. It is generally acknowledged 

that the margin to criticality in system 

parameters (termed the margin of safety) 

is a better indication of the inherent 

safety of the system than margin in keff. 

In addition to establishing subcritical 

limits on controlled system parameters, 

Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 70479

licensees frequently establish operating 

limits to ensure that subcritical limits 

are not exceeded. The difference 

between the subcritical limit and the 

operating limit (if used) of a system 

parameter represents one type of margin 

that may be credited in justifying a 

lower MoS than would be otherwise 

acceptable. This difference between the 

subcritical limit and the operating limit 

should not be confused with the MoS. 

Confusion often arises, however, 

because systems in which keff is highly 

sensitive to changes in process 

parameters may require both: (1) A large 

margin between subcritical and 

operating limits, and (2) a large MoS. 

This is because systems in which keff is 

highly sensitive to changes in process 

parameters are highly sensitive to 

normal process variations and to any 

potential errors. Both the MoS and the 

margin between the subcritical and 

operating limits are thus dependent on 

the keff sensitivity of the system. 

In addition to the margin between the 

subcritical and operating limits, there is 

also usually a significant amount of 

conservatism in the facility’s technical 

practices with regard to modeling. In 

criticality calculations, controlled 

parameters are typically analyzed at 

their subcritical limits, whereas 

uncontrolled parameters are analyzed at 

their worst-case credible condition. In 

addition, tolerances must be 

conservatively taken into account. 

These technical practices generally 

result in conservatism of at least several 

percent in keff. Examples of this 

conservatism may include assuming 

optimum concentration in solution 

processes, neglect of neutron absorbers 

in structural materials, or requiring at 

least a 1-inch, tight-fitting reflector 

around process equipment. The margin 

due to this conservatism may be 

credited in justifying a smaller MoS 

than would otherwise be found 

acceptable. However, in order to take 

credit for this as part of the basis for the 

MoS, it should be demonstrated that the 

technical practices committed to in the 

license application will result in a 

predictable and consistent amount of 

conservatism in keff. If this modeling 

conservatism will not always be present, 

it should not be used as justification for 

the MoS. 

Some questions that should be asked 

in evaluating the margin in system 

parameters include: 

• How much margin in keff is present 

due to conservatism in the modeling 

practices? 

• Will this margin be present for all 

normal and credible abnormal condition 

calculations? 

Normal vs. Abnormal Conditions 

Historically, several licensees have 

distinguished between normal and 

abnormal condition keff limits, in that 

they have a higher keff limit for 

abnormal conditions. Separate limits for 

normal and abnormal condition keff

values are permissible but are not 

required. 

There is a certain likelihood 

associated with the MoS that processes 

calculated to be subcritical will in fact 

be critical. A somewhat higher 

likelihood is permissible for abnormal 

than for normal condition calculations. 

This is because the abnormal condition 

should be at least unlikely to occur, in 

accordance with the double contingency 

principle. That is, achieving the 

abnormal condition requires at least one 

contingency to have occurred and is 

likely to be promptly corrected upon 

detection. In addition, there is often 

additional conservatism present in the 

abnormal condition because 

uncontrolled parameters are analyzed at 

their worst-case credible conditions. 

As stated in NUREG–1718, the fact 

that abnormal conditions meet the 

standard of being at least unlikely from 

the standpoint of the double 

contingency principle may be used to 

justify having a lower MoS than would 

be permissible for normal conditions. In 

addition, the increased risk associated 

with the less conservative MoS should 

be commensurate with and offset by the 

unlikelihood of achieving the abnormal 

condition. That is, the likelihood that a 

process calculated to be subcritical will 

be critical increases when going from a 

normal to a higher abnormal condition 

keff limit. If the normal condition keff

limit is acceptable, then the abnormal 

limit will also be acceptable provided 

this increased likelihood is offset by the 

unlikelihood of going to the abnormal 

condition because of the controls that 

have been established. If a single keff

limit is used (i.e., no credit for 

unlikelihood of the abnormal 

condition), then it must be determined 

to be acceptable to cover both normal 

and credible abnormal conditions. 

Statistical Arguments 

Historically, the argument has been 

used that the MoS can be estimated 

based on comparing the results of two 

statistical methods. In the USLSTATS 

code issued with the SCALE code 

package there are two methods for 

calculating the USL: (1) The Confidence 

Band with Administrative Margin 

Approach, which calculates USL–1, and 

(2) the Lower Tolerance Band 

Approach, which calculates USL–2. The 

MoS is an input parameter to the 

Confidence Band Approach but is not 

included explicitly in the Lower 

Tolerance Band Approach. Justification 

that the MoS chosen in the Confidence 

Band Approach is adequate has been 

based on a comparison of USL–1 and 

USL–2 (i.e., the condition that USL–1, 

including the chosen MoS, is less than 

USL–2). However, this justification is 

not sufficient. 

The condition that USL–1 < USL–2 is 

necessary, but not sufficient, to show 

that an adequate MoS has been selected. 

These methods are two different 

statistical treatments of the data, and a 

comparison between them can only 

demonstrate whether the MoS is 

sufficient to bound statistical 

uncertainties included in the Lower 

Tolerance Band Approach but not 

included in the Confidence Band 

Approach. There may be other statistical 

or non-statistical errors in the 

calculation of keff that are not handled 

in the statistical treatments. Therefore, 

the NRC does not consider this an 

acceptable justification for selection of 

the MoS. 

Regulatory Basis 

In addition to complying with 

paragraphs (b) and (c) of this section, 

the risk of nuclear criticality accidents 

must be limited by assuring that under 

normal and credible abnormal 

conditions, all nuclear processes are 

subcritical, including use of an 

approved margin of subcriticality for 

safety. [10 CFR 70.61(d)] 

Technical Review Guidance 

Determination of an adequate MoS is 

strongly dependent upon the specific 

processes and conditions at the facility 

being licensed, which is largely the 

reason that different facilities have been 

licensed with different limits. 

Judgement and experience must be 

employed in evaluating the adequacy of 

the proposed MoS. Historically, 

however, an MoS of 0.05 in keff has 

generally been found acceptable for a 

typical low-enriched fuel fabrication 

facility. This will generally be the case 

provided there is a sufficient quantity of 

well-behaved benchmarks and a 

sufficiently rigorous validation 

methodology has been employed. For 

systems involving high-enriched 

uranium or plutonium, additional MoS 

may be appropriate to account for the 

increased sensitivity of keff to changes in 

system parameters. There is no 

consistent precedent for such facilities, 

but the amount of increased MoS should 

be commensurate with the increased keff

sensitivity of these systems. Therefore, 

an MoS of 0.05 in keff for low-enriched 

fuel facilities or an MoS of 0.1 for highVerDate jul<14>2003 15:43 Dec 03, 2004 Jkt 205001 PO 00000 Frm 00053 Fmt 4703 Sfmt 4703 E:\FR\FM\06DEN1.SGM 06DEN1
70480 Federal Register / Vol. 69, No. 233 / Monday, December 6, 2004 / Notices 

3NUREG–1718, Section 6.4.3.3.4, states that the 

applicant should submit justification for the MoS, 

but then states that an MoS of 0.05 is ‘‘generally 

considered to be acceptable without additional 

justification when both the bias and its uncertainty 

are determined to be negligible.’’ These statements 

are inconsistent. The statement about 0.05 being 

generally acceptable without additional justification 

is in error and should be removed from the next 

revision to the SRP.

1 17 CFR 240.10a–1.

2 ‘‘Short sale’’ is defined in Rule 200 of Regulation 

SHO, 17 CFR 242.200.

3Securities Exchange Act Release No. 50104 (July 

28, 2004), 69 FR 48032 (August 6, 2004). 

Specifically, the Pilot Order suspended price tests 

for the following: (1) Short sales in the securities 

identified in Appendix A to the Pilot Order; (2) 

short sales in the securities included in the Russell 

1000 index effected between 4:15 p.m. EST and the 

open of the effective transaction reporting plan of 

the Consolidated Tape Association (‘‘consolidated 

enriched or plutonium fuel facilities 

must be justified but will generally be 

found acceptable, with the caveats 

discussed above3.

For facility processes involving 

unusual materials or new process 

conditions, the validation should be 

reviewed in detail to ensure that there 

are no anomalies associated with unique 

system characteristics. 

In any case, the MoS should not be 

reduced below a minimum of 0.02. 

Reducing the MoS below 0.05 for lowenriched processes or 0.1 for highenriched or plutonium processes 

requires substantial additional 

justification, which may include: 

1. An unusually high degree of 

similarity between the chosen 

benchmarks and anticipated normal and 

credible abnormal conditions being 

validated.

2. Demonstration that the system keff

is highly insensitive to changes in 

underlying system parameters, such that 

the worst credible modeling or cross 

section errors would have a negligible 

effect on the bias. 

3. Demonstration that the system 

being modeled is known to be 

subcritical with a high degree of 

confidence. This requires that there be 

other strong evidence in addition to the 

calculations that the system is 

subcritical (such as comparison with 

highly similar systems in published 

references such as handbooks or 

standards). 

4. Demonstration that the validation 

methodology is exceptionally rigorous, 

so that any potential sources of error 

have been accounted for in calculating 

the USL. 

5. Demonstration that there is a 

dependable and consistent amount of 

conservatism in keff due to the 

conservatism in modeling practices. 

In addition, justification of the MoS 

for abnormal conditions may include: 

6. Demonstration that the increased 

likelihood of a process calculated as 

subcritical being critical is offset by the 

unlikelihood of achieving the abnormal 

condition. 

This list is not all-inclusive; other 

technical justification demonstrating 

that there is a high degree of confidence 

in the calculation of keff may be used. 

Recommendation 

The guidance in this ISG should 

supplement the current guidance in the 

NCS chapters of the fuel facility SRPs 

(NUREG–1520 and –1718). In addition, 

NUREG–1718, Section 6.4.3.3.4, should 

be revised to remove the following 

sentence: ‘‘A minimum subcritical 

margin of 0.05 is generally considered to 

be acceptable without additional 

justification when both the bias and its 

uncertainty are determined to be 

negligible.’’

References 

NUREG–1520, ‘‘Standard Review Plan for the 

Review of a License Application for a Fuel 

Cycle Facility’’

NUREG–1718, ‘‘Standard Review Plan for the 

Review of an Application for a Mixed 

Oxide (MOX) Fuel Fabrication Facility’’

NUREG/CR–6698, ‘‘Guide for Validation of 

Nuclear Criticality Safety Calculational 

Methodology’’

NUREG/CR–6361, ‘‘Criticality Benchmark 

Guide for Light-Water-Reactor Fuel in 

Transportation and Storage Packages’’

Approved: llllllllllll

Date: lllllllllllllll

Director, FCSS 

[FR Doc. 04–26688 Filed 12–3–04; 8:45 am] 

BILLING CODE 7590–01–P

OFFICE OF PERSONNEL 

MANAGEMENT 

Submission for OMB Review; 

Comment Request for Review of a 

Revised Information Collection: Form 

DPRS–2809

AGENCY: Office of Personnel 

Management.

ACTION: Notice.

SUMMARY: In accordance with the 

Paperwork Reduction Act of 1995 (Pub. 

L. 104–13, May 22, 1995), this notice 

announces that the Office of Personnel 

Management (OPM) has submitted to 

the Office of Management and Budget 

(OMB) a request for review of a revised 

information collection. DPRS–2809, 

Request to Change Federal Employees 

Health Benefits (FEHB) Enrollment or to 

Receive Plan Brochures, is used by 

former spouses, Temporary 

Continuation of Coverage enrollees, and 

direct pay annuitants to change health 

benefits enrollment or request plan 

brochures for plans they wish to 

consider for enrollment during open 

season. 

Approximately 27,000 DPRS–2809 

forms are completed annually. We 

estimate it takes approximately 45 

minutes to complete the form. The 

annual burden is 20,250 hours. 

For copies of this proposal, contact 

Mary Beth Smith-Toomey on (202) 606–

8358, FAX (202) 418–3251 or via E-mail 

to mbtoomey@opm.gov. Please include a 

mailing address with your request.

DATES: Comments on this proposal 

should be received within 30 calendar 

days from the date of this publication.

ADDRESSES: Send or deliver comments 

to—

Ellen Korchek, CEBS, Chief, Program 

Planning & Evaluation Group, 

Insurances Services Program, Center 

for Retirement and Insurance 

Services, U.S. Office of Personnel 

Management, 1900 E Street, NW., 

Room 3425, Washington, DC 20415–

3650

and 

Joseph F. Lackey, OPM Desk Officer, 

Office of Information and Regulatory 

Affairs, Office of Management & 

Budget, New Executive Office 

Building, NW., Room 10235, 

Washington, DC 20503.

For Information Regarding 

Administrative Coordination—Contact:

Cyrus S. Benson, Team Leader, 

Publications Team, Support Group, 

(202) 606–0623.

Office of Personnel Management. 

Kay Coles James, 

Director.

[FR Doc. 04–26729 Filed 12–3–04; 8:45 am] 

BILLING CODE 6325–38–P

SECURITIES AND EXCHANGE 

COMMISSION 

[Release No. 50747] 

Securities Exchange Act of 1934; 

Order Delaying Pilot Period for 

Suspension of the Operation of Short 

Sale Price Provisions 

November 29, 2004. 

On July 28, 2004, we issued an order 

(‘‘Pilot Order’’) establishing a one year 

Pilot (‘‘Pilot’’) suspending the 

provisions of Rule 10a-1(a) under the 

Securities Exchange Act of 1934 (the 

‘‘Act’’) 1 and any short sale price test of 

any exchange or national securities 

association for short sales 2 of certain 

securities.3 The Pilot Order provided