UNITED STATES 

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555

March 31, 2006 

NRC INFORMATION NOTICE 2006-07: INAPPROPRIATE USE OF A SINGLEPARAMETER LIMIT AS A NUCLEAR

CRITICALITY SAFETY LIMIT

ADDRESSEES

All licensees authorized to possess a critical mass of special nuclear material.

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert

addressees to a concern arising from the inappropriate use of single-parameter limits as

nuclear criticality safety limits at fuel cycle facilities. It is expected that recipients will review the

information for applicability to their facilities and consider actions, as appropriate, to avoid

similar problems. However, suggestions contained in this IN are not new NRC requirements;

therefore, no specific action nor written response is required.

DESCRIPTION OF CIRCUMSTANCES

Under 10 CFR Parts 70 and 76, certain licensees processing, storing, or handling critical

masses of fissile material are required to analyze accident scenarios leading to criticality and

provide safety limits that bound potential criticality events. A fuel cycle licensee relied on a

single-parameter limit from a national consensus standard as the safety limit for the

concentration of fissile material in a nonuniform aqueous solution stored in unsafe-geometry

tanks. The licensee took the single-parameter limit for concentration of uranium-235 in uranyl

nitrate solution from Table 1 of ANSI/ANS-8.1, “Nuclear Criticality Safety in Operations with

Fissionable Materials Outside Reactors,” and applied it as the concentration limit for a set of

large unsafe-geometry tanks used to store waste water. 

The value chosen is considered to be always subcritical for a uniform solution and was

designated in the consensus standard as applying to “uniform aqueous solutions.” The waste

water solution was mixed and uniform on discharge to storage, but settled into solids on arrival

in the storage tanks, in a verifiable manner such that the solution was nonuniform while actually

in storage. Although the licensee had referred to the concentration limit in parameter studies,

the limit was not discussed nor justified in criticality analyses for the systems and processes

involved.

Fuel cycle facilities licensed under Parts 70 and 76 have specific limits on subcritical margin

specified in their licenses or certificates. These limits on subcritical margin are typically

specified in terms of effective neutron multiplication or keff. The single-parameter limits in 

ML060580510
1Nuclear Science and Engineering: 81, 371 (1982), Subcritical Limits for Uranium-235

Systems, “...three [calculational] methods were used to compute limits...a margin as small as

0.01 was occasionally considered acceptable, even though with no method was the margin as

great as 0.02.”

IN 2006-07

Page 2 of 3

Table 1 of ANSI/ANS-8.1 were based on a subcritical margin of between 1 and 2% k

1

 as

noted in technical documentation of the standard. The minimum subcritical margin allowed by

the license was 2% )k, and the licensee did not otherwise specifically commit to this consensus

standard application in the facility license. As a result, the safety limit relied on by the licensee

for concentration control in this case exceeded the license limit on minimum subcritical margin

for normal and credible abnormal conditions. 

DISCUSSION

NRC Regulatory Guide 3.71, “Nuclear Criticality Safety Standards for Fuels and Material

Facilities,” describes methods that the NRC staff considers generally acceptable for complying 

with regulations. This regulatory guide endorses specific nuclear criticality safety standards

developed by the American Nuclear Society’s Standards Subcommittee 8 (ANS-8), “Operations

with Fissionable Materials Outside Reactors.” NRC endorsement of a consensus standard

applies to the use of information and methodology from the standard in licensing applications.

NRC endorsement of consensus standards does not supersede approved license conditions. 

The NRC is concerned that licensees clearly understand the relationship of consensus

standards to the approved facility license. The facility license in this situation allowed the use of

experimental data to establish safety limits. Data from ANSI/ANS-8 series nuclear criticality

safety standards are typically calculated by computer methods and should, in most cases, be

considered analytical data.

In addition, NRC is concerned about the application of limits from consensus standards without

regard to their applicability. In this case, the standard required aqueous solutions to be uniform

for the limits to be applicable and the underlying basis for the standard noted that subcritical

margin may be as low as 0.01. Inappropriate use of a single-parameter limit exposes fuel cycle

licensees to the possibility of implementing inadequate nuclear criticality safety controls which,

in this case, resulted in inadequate subcritical margin in an unfavorable geometry tank. 

NRC criticality safety inspections typically include the review of the adequacy and applicability

of single-parameter limits used as safety limits, including the completeness and adequacy of

nuclear criticality safety analyses, to ensure that controlled parameters and their associated

limits are adequate to maintain fissile material operations within subcritical limits. 
IN 2006-07

Page 3 of 3

This IN requires no specific action nor written response. If you have any questions about the

information in this notice, please contact the technical contact listed below.

/RA/

 Robert C. Pierson, Director

 Division of Fuel Cycle Safety

 and Safeguards

 Office of Nuclear Material Safety

 and Safeguards

Technical Contact: Tamara D. Powell, NMSS

301-415-5095

E-mail: tdp@nrc.gov 

Attachments: 

1. Referenced Codes and Standards 

2. “List of Recently Issued NMSS Generic Communications”
Attachment 1 

IN 2006-07

Page 1 of 1

Referenced Codes and Standards

American National Standards Institute/American Nuclear Society, ANSI/ANS 8.1-1998, “Nuclear

Criticality Safety in Operations With Fissionable Materials Outside Reactors,” ANS, LaGrange 

Park, Illinois 
 Attachment 2 

IN 2006-07

Page 1 of 1

Recently Issued NMSS Generic Communications

Date GC No. Subject Addressees

01/26/06 RIS-02-15,

Rev. 1

NRC Approval of

Commercial Data Encryption

Products For the Electronic

Transmission Of Safeguards

Information

All authorized recipients and

holders of sensitive unclassified

safeguards information (SGI).

01/24/06 RIS-06-01 Expiration Date for NRCApproved Spent Fuel

Transportation Routes

The U.S. Nuclear Regulatory

Commission (NRC) licensees who

transport, or deliver to a carrier for

transport, irradiated reactor fuel

(spent nuclear fuel (SNF)).

01/13/06 RIS-05-27,

Rev. 1

NRC Regulatory Issue

Summary 2005-27, Rev. 1,

NRC Timeliness Goals,

Prioritization of Incoming

License Applications and

Voluntary Submittal of

Schedule for Future Actions

for NRC Review

All 10 CFR Parts 71 and 72

licensees and certificate holders.

01/19/06 IN-06-02 Use of Galvanized Supports

and Cable Trays with

Meggitt Si 2400 StainlessSteel-jacketed Electrical

Cables

All holders of operating licenses

for nuclear reactors except those

who have permanently ceased

operations and have certified that

fuel has been permanently

removed from the reactor vessel;

and fuel cycle licensees and

certificate holders.

Note: NRC generic communications may be found on the NRC public website,

http://www.nrc.gov, under Electronic Reading Room/Document Collections.