ML080650492 

April 16, 2008 

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS 

WASHINGTON, D.C. 20555 

NRC INFORMATION NOTICE 2008-08: MAINTENANCE PROCEDURES COMPROMISE 

DOUBLE-CONTINGENCY OF UO2 POWDERHANDLING EQUIPMENT 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 criticality safety concern about performance of maintenance on process 

equipment while the equipment contains fissile material. 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 operates a fuel cycle facility that processes low-enriched uranium into uranium 

dioxide (UO2) powder, using a dry-conversion process. The licensee dry-conversion process 

consists of exposing uranium hexafluoride (UF6) gas to steam and hydrogen in a reactor 

chamber and then passing the resulting uranyl fluoride (UO2F2) through a kiln, where it is 

converted to UO2 powder. The UO2 powder is collected in a powder outlet at the end of the kiln 

and then transferred out of the powder outlet, through two in-line hatch valves, to a Y junction, 

which passes the powder to one of two available nonfavorable-geometry powder coolinghoppers below the kiln (see Figure 1). The cooling-hoppers are covered with a nitrogen (N2)-

gas blanket that keeps moist air out of the hopper. 

A criticality related hazard of the dry-conversion process is the possibility that steam, used in the 

conversion process, could flow into the cooling-hoppers during normal operations. The licensee 

provides double-contingency protection to ensure that a criticality accident is prevented by 

precluding the intrusion of significant quantity of steam into the cooling-hoppers. The first 

double-contingency control consists of the hatch valves being configured so that both valves 

cannot be open at the same time, which limits the rate at which steam can enter the hopper 

under normal operating conditions. 
IN 2008-08 

Page 2 of 4 

The second double-contingency control consists of moisture detectors on the cooling-hopper 

nitrogen purge. The nitrogen purge lines from cooling-hoppers A and B flow past moisture 

sensors that, on detection of moisture above the set point in the nitrogen purge gas, trip an 

alarm. After an alarm is tripped for a period of 5 minutes, the sensors lock out the steam and 

UF6 supply to the reactor chamber. 

The licensee was performing maintenance consisting of rebuilding the conversion reactor/kiln 

on one of the dry-conversion lines. The maintenance included replacing the two in-line hatch 

valves leading to the cooling-hoppers. During the maintenance, one of the two hatch valves 

was incorrectly connected to its actuator, which resulted in the valve not being able to fully 

close. After completion of the maintenance, the licensee performed startup testing of the rebuilt 

equipment, which included sending steam to the reactor/kiln and cycling the hatch valves, to 

ensure that both hatch valves were properly seating. Hatch valve cycling consists of opening 

and closing one valve at a time and then pressure checking between the valves, while both are 

closed, to ensure that there is no direct path for steam to enter the hoppers. Pressure 

conditions between the two closed hatch valves could not be achieved and the licensee halted 

the start-up testing. To troubleshoot the condition, the licensee placed the equipment in hotmaintenance standby, which included shutting off the steam to the reactor/kiln. 

Figure 1 

Conversion Reactor/Kiln and Cooling-Hopper Equipment Arrangement 
IN 2008-08 

Page 3 of 4 

During the specific troubleshooting of the condition, licensee maintenance personnel secure 

nitrogen flow to the cooling-hoppers to reduce the likelihood of airborne contamination. 

Licensee personnel anticipated that moisture sensors would trip the appropriate alarms and 

interlocks because of the intrusion of ambient air when the nitrogen was shut off. Maintenance 

personnel informed the dry-conversion area control room of the nitrogen flow being secured. As 

the troubleshooting progressed, the moisture sensors alarmed and then interlocked, which 

triggered the steam/UF6 cutoff. Licensee operators acknowledged the moisture alarm, but failed 

to notice the interlock status. 

Dry-conversion process operators beginning a subsequent shift in the facility noticed the 

activated cooling-hopper moisture interlock and observed that cooling-hopper A contained 

approximately 36 kilograms of UO2 powder. Licensee management, unaware that the interlock 

had occurred during troubleshooting, decided that the moisture interlock possibly resulted from 

steam introduced during startup testing. Because the cooling-hoppers are a nonfavorable 

geometry and the amount of uranium involved appeared to be over the licensee’s 31 kilogram 

safety limit for uncontrolled powder, licensee staff initiated an alert. 

Subsequently, the powder in cooling-hopper A was determined to be dry. The licensee 

investigation concluded that the moisture detector tripped the interlock because of ambient air 

intrusion during troubleshooting, and that the maintenance staff had not passed on the 

information because they had not noticed the interlock status. 

DISCUSSION

Double-contingency may be compromised if maintenance affecting criticality controls is 

performed on nonfavorable-geometry equipment containing fissile material. The event revealed 

that, although the licensee maintenance procedure required fissile material to be absent from 

equipment that was being worked on, the procedure did not clearly specify boundaries. 

Licensee staff failed to recognize that the hatch valves were criticality safety controls for the 

nonfavorable-geometry powder cooling-hoppers. Thus, the hatch valves were replaced during 

the maintenance without first emptying the hoppers of UO2 powder thereby compromising the 

first double-contingency control against water intrusion into the cooling-hoppers. 

Because the licensee criticality safety staff thought that no fissile material would be present in 

equipment during maintenance, this staff did not normally review maintenance procedures. 

NRC expects licensees to provide for double-contingency protection during all phases of 

operations at their facilities, including maintenance. To this end, fuel cycle licensee criticality 

safety staff members are expected to be familiar with the conduct of process equipment 

maintenance, in particular, how maintenance procedures affect criticality-related controls. 

The failure to review or otherwise be familiar with in-plant maintenance procedures and the 

condition and status of equipment during all phases of operations and maintenance can result in 

the compromise of established criticality safety controls. NRC criticality safety inspections 

routinely review licensee facility operations, to ensure that credible accident sequences have 

been identified, analyzed, and controlled. NRC criticality safety inspections include review of 

licensee criticality analyses and related criticality controls, to ensure that analytical assumptions 

are adequate and not compromised after implementation. 
IN 2008-08 

Page 4 of 4 

CONTACT

This information notice does not require any specific action nor 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: Thomas Marenchin, NMSS 

 301-492-3209 

 e-mail: tjm7@nrc.gov
CONTACT

This information notice does not require any specific action nor 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: Thomas Marenchin, NMSS 

 301-492-3209 

 e-mail: tjm7@nrc.gov

DISTRIBUTION: 

FCSS r/f 

ADAMS Document Title: MAINTENANCE PROCEDURES COMPROMISE DOUBLECONTINGENCY OF UO2 POWDER- HANDLING EQUIPMENT AT 

FUEL CYCLE FACILITY 

ML080650492

OFC FCSS/TSB FCSS/TSB Tech ED FCSS FCSS/TSB FCSS FCSS 

NAME TMarenchin DMorey Ekraus: fax JSmith DJackson JGiitter RPierson 

DATE 3/18/08 3/18/08 2/26/08 4/10/08 4/3/08 4/14/08 4/16 /08 

OFFICIAL RECORD COPY