UNITED STATES 

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555

July 29, 2005

NRC INFORMATION NOTICE 2005-22: INADEQUATE CRITICALITY SAFETY ANALYSIS

OF VENTILATION SYSTEMS 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 inadequate criticality safety analysis of ventilation

systems 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

Recently, two events occurred at NRC-licensed fuel cycle facilities involving the failure to

implement criticality safety controls on process off-gas or ventilation systems when minor

differences between otherwise similar systems, analyzed under a single broad criticality

analysis, were not recognized by criticality safety analysts. The first instance was noted

subsequent to a backflow event in an off-gas line from a uranium dissolver. The licensee used

a single criticality safety analysis for ventilation systems in the facility. The ventilation analysis

took credit for off-gas piping typically having either a siphon break and a drain, or two drains. 

However, a concern about off-gas accumulation in an enclosed area led to a design

modification for the off-gas line on the uranium dissolver such that only one drain was in the 

system. During preparation of the facility criticality safety analysis, criticality safety analysts 

failed to recognize that the design difference defeated the siphon break so that double

contingency was not established.

The second instance was noted when a fuel cycle licensee observed an accumulation of

uranium dioxide powder in a high-efficiency particulate air (HEPA) filter housing where no

uranium was expected. The licensee determined that what criticality safety analysts thought

was a breathing air-ventilation system was also connected to a process off-gas line from a hood

on a uranium oxidation furnace. The licensee identified a design difference in the system in

that ventilation and off-gas lines were connected differently, as they approached the HEPA

filter, than was customary in the remainder of the plant. The licensee had several broad

criticality safety analysis packages related to ventilation and process off-gas, grouping them as

ML051890406
IN 2005-22

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breathing air, dry off-gas, and wet off-gas. The criticality safety analyst’s failure to recognize

the design difference in duct connections in one part of the plant from other areas of the plant

led to the incorrect determination that the system was breathing air and criticality was not

credible. This incorrect determination resulted in the failure to implement criticality safety

controls typical for off-gas ventilation in the plant.

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 and

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

processes, systems, equipment, or procedures are repeated in a facility, licensees frequently

elect to combine similar processes, systems, equipment, or procedures into a single criticality

analysis. The safety concern arises when modifications resulting in minor design differences

between otherwise similar systems defeat the credited double-contingency arrangement or noncredibility determination.

In the two events described, the two licensees used a single criticality safety analysis to develop

controls for groups of ventilation and process off-gas systems that were similar in form and

function. While crafting the analyses, developing the criticality safety controls, and

implementing the credited controls, licensee criticality safety analysts failed to recognize design

differences between the systems that defeated some of the assumptions or credited controls

used in some portion of the facility.

In the first instance, a design change occurred, during construction of the system, that involved

placing an additional column into the system that effectively defeated the siphon break for the

uranium dissolver. The criticality safety review for this design change looked at the analysis for

the process, but did not consider the impact that the change would have on off-gas ventilation. 

In the second instance, contractors were constructing a new facility, and criticality safety

analysts did not recognize design differences in the ventilation system.

Minor design changes during construction of new processes or facilities are common at fuel

cycle licensees and may have a subtle effect on criticality controls. Licensees should consider

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

criticality safety analyses that group similar systems, to assure that all assumptions regarding

the forms and functions of the systems are valid for all applications. Actions could also include

verifying that the design change review process is adequate to trigger an in-depth criticality

safety review for changes arising during construction.

The Part 70 integrated safety analysis (ISA) and the Part 76 safety analysis report (SAR)

provide an integrated approach to assure that inter-relationships between accident scenarios

and their controls are appropriately evaluated during related design and change activities. 

Licensees should consider whether their ISA/SAR provides an adequate integrated review of

ventilation and related systems.
IN 2005-22

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: Dennis Morey, NMSS

301-415-6107

E-mail: dcm@nrc.gov

Attachment: “List of Recently Issued NMSS Generic Communications”
 Attachment 

IN 2005-22

Page 1 of 1

Recently Issued NMSS Generic Communications

Date GC No. Subject Addressees

07/13/05 RIS-05-13 NRC Incident Response and

the National Response Plan

All licensees and certificate

holders.

07/11/05 RIS-05-11 Requirements for Power

Reactor Licensees in

Possession of Devices

Subject to the General

License Requirements of 10

CFR 31.5

All holders of operating licenses

for nuclear power reactors and

generally licensed device

vendors.

06/10/05 RIS-05-10 Performance-Based

Approach for Associated

Equipment in 10 CFR 34.20

All industrial radiography

licensees and manufacturers and

distributors of industrial

radiography equipment.

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.

6/23/05 IN-05-17 Manual Brachytherapy

Source Jamming

All medical licensees authorized

to possess a Mick applicator.

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.

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

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