UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, DC 20555

 May 17, 2005

NRC INFORMATION NOTICE 2005-12: EXCESSIVELY LARGE CRITICALITY SAFETY

LIMITS FAIL TO PROVIDE DOUBLE

CONTINGENCY AT FUEL CYCLE FACILITY

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 inform

addressees of a concern related to criticality safety at fuel fabrication and other facilities

processing, storing, or handling critical masses of fissile material. It is expected that licensees

will review this information 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

During a recent plant-wide review of criticality safety analysis at a fuel cycle licensee, a

licensee criticality safety engineer questioned the utility of a control scheme that relied on

mass and moderator controls for double contingency in a large uranium dioxide powder

hopper. Experience and process knowledge caused the engineer to believe that the controls

were too high and that exceeding one limit could lead to a critical configuration under credible

circumstances, without requiring the other limit to be exceeded.

Licensee investigation revealed that the system had previously relied on a level limit and that

during a revision of the safety basis, the controls had been changed to support new mass and

moderator limits. The licensee determined that the new mass and moderator limits had been

derived from the previous level limit (i.e., by determining how much mass was present at the

maximum allowed level and then determining how much moderator was required to make the

system critical in that configuration). There was no accompanying analysis to demonstrate that

the new values were actually limiting. The licensee demonstrated that, in fact, increasing the

amount of mass could make the system critical without exceeding the moderation limit.

ML051080466
IN 2005-12

Page 2 of 3

DISCUSSION

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,

establish subcritical limits, and provide reliable controls to provide an acceptable level of 

assurance that the subcritical limits are not exceeded. Dual, independent controls provide the

basis for double contingency protection against inadvertent criticality. Fuel cycle licensees

generally implement criticality safety controls on important process parameters that, when

upset, may create a critical system. 

NRC has several concerns about this issue. The first is that the practice of deconstructing an

existing criticality safety limit was apparently the licensee’s common practice, which many

licensee criticality safety engineers used for years, resulting in a number of questionable

control arrangements in the technical safety basis. This practice is associated with an

expansion of an established limit. The second concern is the failure to perform needed

analysis to assure that newly developed criticality safety limits would not lead to a critical

system by their individual failure and would ensure double contingency. No explicit modeling

was performed for this analysis, and such modeling is not normally performed for limits not

based on computer analysis due to conservatism normally employed. The final concern is the

failure of licensee technical reviewers and management to detect the excessively high limits

during routine review and approval. Review of these new limits did not include thorough

examination and questioning of existing technical analysis and assumptions.

Failure to establish and maintain fully independent nuclear criticality safety controls exposes

fuel cycle licensees to the possibility of failure of double contingency. Licensees should

consider actions, as appropriate, to identify and mitigate this vulnerability. Appropriate actions

may include: (1) identification of double contingency arrangements based on tabulated values,

hand calculations, or other related approximations, such as one-dimensional computer

analysis, that may result in the questionable limits being identified, (2) analysis to establish the

adequacy of the controls intended to support the limits identified in item (1), and (3)

incorporation of appropriate requirements into routine review and approval protocol to assure

implementation of suitably robust double contingency arrangements when new criticality safety

limits are derived from previous limits.
IN 2005-12

Page 3 of 3

CONTACT

This IN requires no specific action nor written response. Questions about this matter may be

referred to 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: Dennis Morey, NMSS

301-415-6107

e-mail: dcm@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 1

IN 2005 -12

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.