UNITED STATES 

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555

July 19, 2004

NRC INFORMATION NOTICE 2004-14: USE OF LESS THAN OPTIMAL BOUNDING

ASSUMPTIONS IN CRITICALITY SAFETY

ANALYSIS AT FUEL CYCLE FACILITIES

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 safety concern arising from the use of less than optimal bounding assumptions

in criticality safety analysis 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 all accident scenarios leading to criticality and

provide reliable controls to assure that inadvertent criticality events are highly unlikely. Typical

criticality analysis identifies credible accident sequences leading to criticality; identifies

reasonable bounding assumptions related to the processes, equipment, or material analyzed;

and establishes limits or boundaries of processes, equipment, or material within which bounding

assumptions are applicable. Criticality may be deemed not credible when inherent features of

the process, equipment, or material in a specific accident sequence leading to criticality can be

shown to constrain the reactivity of fissile material within subcritical limits. The safety concern

arises when accident scenarios leading to criticality are deemed not credible, based on

bounding assumptions that are less than optimal for the system involved. 

Recently, a licensee reported an event, to NRC, concerning operation of an incinerator outside

of the approved safety basis. The licensee had performed a criticality safety evaluation of an

incinerator approximately 8 years previously and concluded that criticality was not credible

outside of the primary combustion chamber. Licensee nuclear criticality safety (NCS) analysis

focused on accumulation of sufficient mass in the incinerator system to support criticality. 

ML041760122
1

 keff is the effective neutron multiplication factor for the system under consideration. 

The licensee had defined the critical point in its license as keff =0.98.

IN 2004-14

Page 2 of 3

Based on mass limits on the input waste stream, licensee NCS engineers determined that most

mass resulting from incineration would accumulate in the primary combustion chamber and that

ash resulting from incineration would never exceed a concentration of 21.6 weight percent

uranium (wt% U) which is always subcritical in infinite media at the optimal moderator ratio. 

Based on this conclusion, criticality safety limits and controls were developed and implemented

only for the primary combustion chamber, which was a small fraction of the incinerator system.

Licensee NCS engineers believed that very limited amounts of ash would carry over from the

incinerator primary combustion chamber to the remainder of the incinerator system and that

mass controls on the primary combustion chamber would limit uranium concentration in the ash

to less than 21.6 wt% throughout the incinerator system. This led the licensee’s NCS engineers

to conclude that criticality outside the primary combustion chamber was not credible.

On March 5, 2004, the licensee reported an event concerning the accumulation of significant

quantities of ash outside the primary combustion chamber at concentrations in excess of 21.6

wt% U. The licensee’s investigation revealed that ash deposits at various locations in the

incinerator routinely exceeded the 21.6 wt% uranium concentration assumed to be bounding for

ash and that the mass of ash deposited also exceeded expectations.

Discussion:

In the described event, the chosen uranium concentration was arbitrary and did not bound the

subject fissile system. 21.6 wt% U is not a natural limit on U concentration in incinerator ash

and is less than optimal because higher uranium concentrations produce a more reactive fissile

system. The value results from an infinite media calculation where optimum moderation

conditions are established with ash replaced by a uranium dioxide and water mixture. The

limiting concentration is related to the critical point, in this case keff = 0.98.1

 To complete

analysis of the incinerator, the licensee looked at data from selected parts of the incinerator

system and concluded that 21.6 wt% U far exceeded uranium concentrations typically expected

in the system. However, U concentration frequently exceeded 21.6 wt% in the primary and

secondary combustion chambers and flue. The licensee failure to recognize that the actual U

content of the ash was related, in part, to weak documentation of bounding assumptions and

poor definition of incinerator system boundaries to which the bounding assumptions applied. 

The accumulation of material outside the primary combustion chamber at the U concentrations

seen, along with the availability of water in the incinerator off-gas quench system, results in the

conclusion that criticality was actually credible in the incinerator secondary combustion

chamber.

Less than optimal bounding assumptions for criticality safety are most often seen in ventilation

and off-gas systems, waste-processing systems, and incinerators. Licensees should consider

actions, as appropriate, to mitigate this vulnerability. These actions could include reviewing all

accident sequences where less than optimal bounding assumptions were used to establish

NCS controls or determine that criticality is not credible. Actions could also include verifying 
IN 2004-14

Page 3 of 3

that bounding assumptions are actually bounding, for example, by reviewing available material 

composition data for evidence that attributes such as isotope concentration are actually within

expected values.

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/

William D. Beckner, 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

Attachments:

1. List of Recently Issued NMSS Information Notices

2. List of Recently Issued NRC Information Notices
____________________________________________________________________________________

OL = Operating License

CP = Construction Permit

Attachment 1

IN 2004-14

Page 1 of 1

LIST OF RECENTLY ISSUED

NMSS INFORMATION NOTICES

____________________________________________________________________________________

Information Date of 

Notice No. Subject Issuance Issued to

____________________________________________________________________________________

2004-13 Registration, Use, and Quality

Assurance Requirements for

NRC-Certified Transportation

Packages

06/30/2004 All materials and 

decommissioning reactor

licensees.

2004-03 Radiation Exposures to

Members of the Public in

Excess of Regulatory Limits

Caused by Failures to Perform

Appropriate Radiation Surveys

During Well-logging Operations

02/24/2004 All well-logging licensees.

2004-02 Strontium-90 Eye Applicators

New Calibration Values and

Use

02/05/2004 All U.S. Nuclear Regulatory

Commission (NRC) medicaluse licensees and NRC master

materials license medical-use

permittees.

2003-22 Heightened Awareness for

Patients Containing Detectable

Amounts of Radiation from

Medical Administrations

12/09/2003 All medical licensees and NRC

Master Materials License 

medical use permittees.

2003-21 High-Dose-Rate-RemoteAfterloader Equipment Failure11/24/2003 All medical licensees.

Note: NRC generic communications may be received in electronic format shortly after they are issued by

subscribing to the NRC listserver as follows:

To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following command in the

message portion:

subscribe gc-nrr firstname lastname
____________________________________________________________________________________

OL = Operating License

CP = Construction Permit

Attachment 2

IN 2004-14

Page 1 of 1

LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

____________________________________________________________________________________

Information Date of 

Notice No. Subject Issuance Issued to

____________________________________________________________________________________

2004-13 Registration, Use, and Quality

Assurance Requirements for

NRC-Certified Transportation

Packages

06/30/2004 All materials and

decommissioning reactor

licensees.

2004-12 Spent Fuel Rod Accountability 06/25/2004 All holders of operating licenses

for nuclear power reactors,

research and test reactors,

decommissioned sites storing

spent fuel in a pool, and wet

spent fuel storage sites.

2004-11 Cracking in Pressurizer Safety

and Relief Nozzles and in

Surge Line Nozzle

05/06/2004 All holders of operating licenses or

construction permits for nuclear

power reactors, except those that

have permanently ceased

operations and have certified that

fuel has been permanently

removed from the reactor.

2004-10 Loose Parts in Steam

Generators

05/04/2004 All holders of operating licenses

for pressurized-water reactors

(PWRs), except those who have

permanently ceased operations

and have certified that fuel has

been permanently removed from

the reactor.

2004-09 Corrosion of Steel

Containment and Containment

Liner

04/27/2004 All holders of operating licenses

for nuclear power reactors except

those who have permanently

ceased operation and have

certified that fuel has been

permanently removed from the

reactor vessel.

Note: NRC generic communications may be received in electronic format shortly after they are issued by

subscribing to the NRC listserver as follows:

To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following command in the

message portion:

subscribe gc-nrr firstname lastname