UNITED STATES 

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555

May 17, 2005

NRC INFORMATION NOTICE 2005-13: POTENTIAL NON-CONSERVATIVE ERROR IN

MODELING GEOMETRIC REGIONS IN THE

KENO-V.A CRITICALITY CODE

ADDRESSEES

All licensees using the Keno-V.a criticality code module in Standardized Computer Analyses for

Licensing Evaluation (SCALE) software developed by Oak Ridge National Laboratory (ORNL).

PURPOSE

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

recipients of a potential non-conservative error in modeling geometric regions in the Keno-V.a

criticality code. It is expected that recipients will review the information for applicability to their

facilities and consider actions, as appropriate, to avoid non-conservative results. However,

suggestions contained in this IN are not new NRC requirements; therefore, no specific action

nor written response is required.

DESCRIPTION OF CIRCUMSTANCES

Keno-V.a is a Monte Carlo particle tracking code that solves the neutron transport equation in

arbitrary three-dimensional geometric configurations. Keno-V.a is part of the SCALE software

package.

An ORNL staff member identified a problem while performing a criticality safety benchmark

evaluation in which some cases produced 1 to 1.5 percent lower k-effective (k-eff) values. This

statistically significant difference was brought to the attention of the Keno-V.a code manager at

ORNL. ORNL’s review of the discrepancy determined that the observed difference in k-eff was

attributed to round-off error.

ORNL provided diagnostic guidance on its website for SCALE users to determine the impacts of

round-off errors on calculated k-eff results: 

http://www-rsicc.ornl.gov/rsic-cgi-bin/enote.pl?nb=scale5&action=view&page=56

ML051180228
IN 2005-13

Page 2 of 3

ORNL also provided preventive guidance, for SCALE users, to avoid modeling geometric

configurations that were potentially sensitive to numerical round-off errors. Subsequently, ORNL

issued a Keno-V.a patch to fix the numerical round-off error in SCALE Version 5. ORNL did not

issue a patch for earlier versions of SCALE. NRC informally notified fuel cycle licensees of the

existence of the Keno-V.a error.

NRC is continuing to evaluate the extent of the error and the adequacy of the patch. If relevant

new information is obtained, NRC will provide the information in a future communication.

DISCUSSION

Modeling geometric configurations using Keno-V.a is accomplished by defining regions

composed of simple geometric shapes. Holes are one type of geometric region that could be

used to construct complex geometric configurations. Holes may contain bodies or arrays

having any geometric shape. The numerical round-off error only occurs when using the hole

option to define geometric regions and the body in the hole is a cylinder or hemi-cylinder. A

similar round-off error was previously identified and corrected for holes containing other

geometric shapes. 

The reported condition arises when the end face of a hole, whose outer boundary is a cylinder

or a hemi-cylinder, shares a common flat boundary with another hole, region, or unit. Neutrons

crossing the common boundary are incorrectly tracked when the numerical round-off error

causes the code to misinterpret which region the neutron is entering. If the tracking failure

results in missing absorber material, calculated k-eff values could be overestimated. If the

tracking failure results in missing fuel, calculated k-eff values could be underestimated.

ORNL has recommended that geometric configurations that were modeled using

cylindrical/hemi-cylindrical holes be checked for shared boundaries. If shared boundaries exist,

ORNL recommends inserting a small gap, larger than potential round-off error [at least 1.0 x 10-5

cm (3.9 x 10-6 in.)], between the boundaries. ORNL has also issued the following patch, for

SCALE Version 5, that can be downloaded from the SCALE website: 

http://www.ornl.gov/sci/scale/download kenova.htm . This patch is intended to prevent tracking

errors in geometric configurations having boundaries. If the patch is installed, the modified

Keno-V.a code should be verified by re-running validation cases to ensure the code is working

correctly.
IN 2005-13

Page 3 of 3

CONTACT

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/ /RA/

 Patrick L. Hiland, Chief Robert C. Pierson, Director

 Reactor Operations Branch Division of Fuel Cycle Safety

 Division of Inspection Program Management and Safeguards

 Office of Nuclear Reactor Regulation Office of Nuclear Material Safety

 and Safeguards

Technical Contact: Lawrence Berg, NMSS

301-415-6215

E-mail: ljb2@nrc.gov

Attachment: “List of Recently Issued NMSS Generic Communications”

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

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

IN 2005 -13

Page 1 of 1

Recently Issued NMSS Generic Communications

Date GC No. Subject Addressees

04/18/05 RIS-05-06 Reporting Requirements for

Gauges Damaged at

Temporary Job Sites

All material licensees possessing

portable gauges, regulated under

10 CFR Part 30.

04/14/05 RIS-05-04 Guidance on the Protection

of Unattended Openings

that Intersect a Security

Boundary or Area

All holders of operating licenses

or construction permits for nuclear

power reactors, research and test

reactors, decommissioning

reactors with fuel on site,

Category 1 fuel cycle facilities,

critical mass facilities, uranium

conversion facility, independent

spent fuel storage installations,

gaseous diffusion plants, and

certain other material licensees.

02/28/05 RIS-05-003 10 CFR Part 40

Exemptions for Uranium

Contained in Aircraft

Counterweights - Storage

and Repair

All persons possessing aircraft

counterweights containing

uranium under the exemption in

10 CFR 40.13(c)(5).

05/17/05 IN-05-013 Potential Non-conservative

Error in Modeling

Geometric Regions in the

Keno-v.a Criticality Code

All licensees using the Keno-V.a

criticality code module in

Standardized Computer Analyses

for Licensing Evaluation (SCALE)

software developed by Oak Ridge

National Laboratory (ORNL)

05/17/05 IN-05-012 Excessively Large

Criticality Safety Limits Fail

to Provide Double

Contingency at Fuel Cycle

Facility

All licensees authorized to

possess a critical mass of special

nuclear material.

04/07/05 IN-05-010 Changes to 10 CFR Part 71

Packages

All 10 CFR Part 71 licensees and

certificate holders.
04/01/05 IN-05-007 Results of HEMYC

Electrical Raceway Fire

Barrier System Full Scale

Fire Testing

All holders of operating licenses

for nuclear power reactors, except

those who have permanently

ceased operations and have

certified that fuel has been

permanently removed from the

reactor vessel, and fuel facilities

licensees.

03/10/05 IN-05-005 Improving Material Control

and Accountability Interface

with Criticality Safety

Activities at Fuel Cycle

Facilities

All licensees authorized to

possess a critical mass of special

nuclear material.

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

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