UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555

April 4, 2007

NRC INFORMATION NOTICE 2007-13: USE OF AS-FOUND CONDITIONS TO

EVALUATE CRITICALITY-RELATED PROCESS

UPSETS 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 inform

addressees of a concern regarding the use of as-found conditions to establish the efficacy of

double contingency protection during evaluation of criticality-related process upsets at fuel

cycle facilities. NRC expects that licensees will review this information and consider actions,

as appropriate, to avoid similar problems. Suggestions contained in this IN are not NRC

requirements; therefore, no specific action nor written response is required.

DESCRIPTION OF CIRCUMSTANCES

An NRC licensee transferred critical masses of low-enriched uranium in and out of unsafe

geometry vessels at a transfer facility on the licensee site. During processing operations at the

transfer facility, the fissile material vessels were weighed and briefly stored. A pit beneath the

scale in the facility was an unsafe geometry collection point for which the licensee had

implemented double contingency protection to prevent the accumulation of fissile material and

water. The licensee double contingency arrangement consisted of controls to assure the

structural integrity of fissile material-containing vessels and processing equipment along with

controls to limit the amount of water, allowed in the scale pit, to less than a safe slab height. 

Fissile material assay of 5.5 weight percent (wt%) 235U was considered a normal operating

condition for the transfer facility and was the analytical basis for the slab height limit in the

scale pit. The controls limiting the amount of water in the scale pit consisted of a moisture

indicator that notified operators with an alarm when the water level in the scale pit reached an

action limit of 2.5 inches and procedures to remove accumulating water before the established

criticality safety limit of 3.68 inches was reached. 

The licensee site recently experienced flooding in operational areas due to heavy rains. As a

result of the flooding, a drain system servicing the fissile material transfer facility overflowed,

allowing water to enter the scale pit. During the flooding event, no failure of structural integrity

or escape of fissile material was identified, but enough water entered the scale pit to exceed

the 3.68 inch slab height limit. Licensee investigation revealed that over 4.3 inches of water

had entered the scale pit during the flooding event, and that the moisture detector in the scale

pit had failed and had not notified operators to begin compensatory action to remove the

water.

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IN 2007-13

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The licensee had committed to report events involving the loss of double contingency to the

NRC, and it recognized that the criticality safety limit of 3.68 inches of water in the scale pit

had been violated due to failure of the planned and implemented criticality safety controls. 

Subsequent to the flooding event, the licensee determined that double contingency was not

violated by the accumulation of 4.3 inches of water in the scale pit due to the as-found

operating conditions at the time of the event. The licensee determined that the maximum

assay of fissile material in the transfer facility at the time of the event was 2.0 wt% 235U which

would have required more than 7.0 inches of water to create an unsafe slab in the scale pit. 

As a result of the conclusion that double contingency was maintained due to as-found

conditions during the event, the licensee did not report the failure of double contingency to the

NRC.

DISCUSSION

The specific commitment to double contingency at an NRC licensee is contained in the

individual facility license. In this case, the licensee had committed to a typical definition of

double contingency requiring that process designs should incorporate sufficient factors of

safety to require at least two unlikely, independent, and concurrent changes in process

conditions before a criticality accident is possible. Double contingency arrangements under

this and related definitions are developed from credible upsets that are based on bounding

normal case conditions. Planned double contingency arrangements are implemented by

establishing safety limits and then using controls to ensure that the limits are not exceeded

during operations. Double contingency protection is considered lost when control has been

lost over one of the controlled parameters, such that only one change in process conditions

would be needed to create a critical system. Exceeding a criticality safety limit on a process

parameter clearly demonstrates loss of control over that parameter. 

A double contingency arrangement is implemented with a specific accident sequence in mind.

In the case of the fissile material transfer facility, the accident sequence was the creation of a

critical system in the scale pit, due to the concurrent accumulation of fissile material and

intrusion of water. Fissile material in the facility is assumed to accumulate in the scale pit if it is

released from vessels or equipment; thus, the controlled parameter is the integrity of the

vessels or equipment. Fissile material in the scale pit accident sequence is assumed to mix

uniformly with water in the pit and is also assumed to be 5.5wt% 235U because 5.5wt% bounds

the uranium assay at the transfer facility, and there are no controls implemented at the facility

to limit assay to less than 5.5wt%. These assumptions are part of the analytical basis for the 

3.68 inch water level limit in the scale pit, resulting in controls designed to protect this particular

limit. The NRC considers that the control failure (i.e., failed moisture detector) and the safety

limit being exceeded during the flooding event constitute a failure of the planned and

implemented double contingency arrangement for the accident sequence of water intrusion

into the scale pit in the presence of a critical mass of low-enriched uranium.

The validity of a particular safety limit must be established before, not after, an upset has

occurred. If a licensee analysis of an accident sequence in an as-found condition results in a 

revised safety limit to bound a specific upset, this revision cannot alter what was relied on for

safety prior to the upset. In the transfer facility flooding event, on the basis of how the safety

limit was constructed, control was lost over the moderation parameter by water intrusion into 
 IN 2007-13

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the scale pit. The NRC is concerned that licensees understand that it is the loss of control

over a controlled parameter that is of concern in double contingency failure analysis. A safety

limit could theoretically be constructed to vary along with a specified process condition, if that

process condition varied predictably through a controlled range. For example, the limiting

height of a fissile slab could be tabulated based on a range of available fissile assay and the

tabulated values could be designated as the safety limit which would be applicable based on

the available assay at the time of an upset. Such an approach would require analysis in

advance, like any other safety limit. Without such analysis, it is not immediately clear that a

limiting slab height different from the one originally analyzed is bounding. In the transfer

facility flooding event accident sequence, variation in the safe slab height based in variation in

the facility fissile assay was not considered in advance.

Inappropriate consideration of as-found conditions, during event analysis, may result in the

failure to correctly characterize the event. Improper characterization of the event could, in turn,

lead to an inadequate response, such as a failure to make a required event report. NRC

criticality safety inspections typically include review of facility events to determine the

adequacy of licensee reportability determinations, including the licensee rationale for

conclusions regarding the efficacy of double contingency protection during the event.

CONTACT

This information notice does not require any specific action or written response. Please direct

any questions about this matter to the technical contact below.

 /RA/

 

 Robert C. Pierson, Director

 Division of Fuel Cycle Safety

 and Safeguards

Office of Nuclear Material Safety

 and Safeguards

Technical Contact: Dennis Morey, NMSS

301-415-6107

E-mail: dcm@nrc.gov

Enclosure: List of Recently Issued FSME/NMSS Generic Communications
Enclosure 

IN 2007-13

Page 1 of 1

Recently Issued FSME/NMSS Generic Communications

Date GC No. Subject Addressees

02/02/07 IN-07-03 Reportable Medical Events

Involving Patients Receiving

Dosages of Sodium

Iodide Iodine-131 less than the

Prescribed Dosage Because of

Capsules Remaining in Vials after

Administration

All U.S. Nuclear Regulatory Commission 

medical use licensees and NRC Master

Materials Licensees. All Agreement State

Radiation Control Program Directors and

State Liaison Officers.

02/28/07 IN-07-08 Potential Vulnerabilities of Timereliant Computer-based Systems

Due to Change in Daylight Saving

Time Dates

All U. S. Nuclear Regulatory Commission 

licensees and all Agreement State

Radiation Control Program Directors and

State Liaison Officers.

03/15/07 IN-07-10 Yttrium-90 Theraspheres® and

Sirspheres® Impurities

All U.S. Nuclear Regulatory Commission

(NRC) Medical Licensees and NRC

Master Materials Licensees. All

Agreement State Radiation Control

Program Directors and State Liaison

Officers.

03/01/07 RIS-07-03 Ionizing Radiation Warning

Symbol

All U.S. Nuclear Regulatory Commission

licensees and certificate holders. All

Radiation Control Program Directors and

State Liaison Officers

03/09/07 RIS-07-04 Personally Identifiable Information

Submitted to the U.S. Nuclear

Regulatory Commission

All holders of operating licenses for

nuclear power reactors and holders of

and applicants for certificates for reactor

designs. All licensees, certificate

holders, applicants, and other entities

subject to regulation by the U.S. Nuclear

Regulatory Commission (NRC) of the

use of source, byproduct, and special

nuclear material

03/20/07 RIS-07-05 Status and Plans for

Implementation of NRC

Regulatory Authority for Certain

Naturally-occurring and

Accelerator-produced Radioactive

Material

All NRC materials licensees, Radiation

Control Program Directors, State Liaison

Officers, and NRC’s Advisory Committee

on the Medical Uses of Isotopes

Note: NRC generic communications may be found on the NRC public website at http://www.nrc.gov, under

Electronic Reading Room/Document Collections.