ML16252A171 

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS 

OFFICE OF NUCLEAR REACTOR REGULATION 

WASHINGTON, DC 20555 

September 28, 2016 

NRC INFORMATION NOTICE 2016-13: URANIUM ACCUMULATION IN FUEL 

CYCLE FACILITY VENTILATION AND 

SCRUBBER SYSTEMS 

ADDRESSEES

All holders of and applicants for a fuel facility license under Title 10 of the Code of Federal 

Regulations (10 CFR) Part 70, “Domestic Licensing of Special Nuclear Material” and 

10 CFR Part 70, Subpart H, “Additional Requirements for Certain Licensees Authorized To 

Possess a Critical Mass of Special Nuclear Material.” 

All holders of and applicants for a construction permit or operating license for a production 

facility, including facilities dedicated to the production of medical radioisotopes such as 

molybdenum-99, under 10 CFR Part 50, “Domestic Licensing of Production and Utilization 

Facilities,” except those who have permanently ceased operations. 

PURPOSE

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform 

addressees about the potential for uranium accumulation in off-gas ventilation and scrubber 

systems and some potential causal factors that could contribute to this type of event. Over time, 

uranium can build up in areas that are difficult to inspect and clean. As a result, a criticality 

safety evaluation (CSE) mass limit could be exceeded and challenge controls designed to meet 

the performance requirements of 10 CFR 70.61(b) and 10 CFR 70.61(d) and the double 

contingency principle1. 

The NRC requests recipients to review the information contained in this IN for applicability to 

their facilities and to consider actions, as appropriate, to avoid similar issues. Any suggestions 

contained in this IN are not NRC requirements; therefore, no specific action or written response 

is required. 

 

 

1 As described in 10 CFR 70.4, “Definitions”, the double contingency principle means 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. 
IN 2016-13 

Page 2 of 5 

DESCRIPTION OF CIRCUMSTANCES

During the most recent planned annual wet scrubber system cleanout at a low-enriched fuel 

fabrication facility, personnel noticed an abnormal amount of material buildup in the inlet 

transition region and associated ductwork (i.e., elbow). Over the course of the 2-day 

maintenance evolution, approximately 197 kilograms of material were removed from the 

scrubber transition region. The transition region is considered an unfavorable geometry from a 

criticality perspective. Because facility personnel assumed that this material had a low uranium 

concentration, operators attempted to break up and wash away the material to facilitate its 

removal. Facility personnel did not sample the material to confirm the uranium concentration 

before conducting any activities that could have disturbed the as-found condition. After the 

material was removed, grab samples of the material were taken to analyze for uranium 

concentration. 

The grab sample results indicated that the uranium concentrations ranged from 34 weight 

percent (wt %) – 55 wt% which corresponded to approximately 87 kilograms of uranium. As 

such, the CSE mass limit of 29 kilograms was exceeded by a factor of 3. After the cleanout 

activities were completed, the scrubber was restarted. The scrubber operated for 6 weeks and 

then facility personnel shut it down to perform another cleanout of the inlet transition region and 

elbow. Facility personnel removed about 24 kilograms of material, which corresponded to 

approximately 5 kilograms of uranium. The scrubber was restarted following the 6 week 

cleanout. Approximately 1 week later, while discussing extent of condition, the licensee decided 

to shut down the scrubber again and thoroughly inspect the entire scrubber to ensure that the 

scrubber was free of uranium accumulation. An additional 184 kilograms of material was 

removed from the scrubber body, and about 71 kilograms of material was removed from the 

packing material. The scrubber was shut down and the licensee commenced extent of 

condition and root cause evaluations and implemented several short-term corrective actions. 

BACKGROUND 

The scrubber in question was put into service in 2002. This scrubber combined two ventilation 

systems. In 2009, an additional feed stream was rerouted to the scrubber in question. This 

particular scrubber operates as a cross-flow horizontal packed-bed scrubber that uses a 

recirculating scrubbing liquid to absorb soluble gas molecules and knock down suspended 

solids, including uranium-bearing particles vented from several processes. The scrubber was 

originally designed to scrub mostly acidic off-gas; however, many of the current feed streams 

contain ammoniated off-gas. 

From 2002 through 2009, facility personnel removed and inspected the scrubber inlet transition 

region and elbow on three different occasions and noticed material buildup. Information on the 

volume, weight, and wt% of the material was not accurately and consistently recorded. For the 

next 7 years leading up to the event, the annual scrubber cleanout did not involve removing the 

inlet elbow and all the packing for inspection and cleaning. Instead, the elbow and transition 

region sections were periodically pressure-washed through a cleanout port. 

About 1 month before the most recent annual scrubber maintenance, the elbow and transition 

region were pressure-washed with a new sprayer that allowed cleaning of the upper surface of 

the scrubber. As described above, during the cleaning, operators observed that a large piece of 

accumulated material was dislodged from the upper surface of the transition region. During the 

annual scrubber maintenance, the inlet transition region and elbow were removed and cleaned. 

The material was weighed and sampled to reveal 87 kilograms of uranium, which exceeded the 
IN 2016-13 

Page 3 of 5 

CSE mass limit of 29 kilograms of uranium. As part of the extent of condition, facility personnel 

inspected scrubber and ventilation system components that had been permanently removed 

from service for years, and discovered some accumulation of uranium-bearing material.

DISCUSSION

Any event that involves exceeding a criticality parameter limit established by the CSE and 

results in not meeting the double contingency principle is a criticality safety concern. In this 

case, the mass limit was exceeded by a factor of 3; moderation was available from the scrubber 

spray nozzles and the pressure-washing; and the scrubber packing, elbow, and transition region 

sections are all unfavorable geometries. As a result, the safety margin available to preclude an 

inadvertent criticality was significantly degraded. 

The long-term accumulation of uranium in equipment with an unfavorable geometry, particularly 

in process ventilation and scrubber systems, has been a recurring issue throughout the nuclear 

fuel industry2. The amount of material that can be transported into process ventilation can be 

underestimated. Therefore, licensees are encouraged to verify the assumptions regarding the 

rate and mechanisms of accumulation. Furthermore, during process changes, licensees are 

encouraged to consider process conditions that can affect accumulation and the possible 

creation of chemical hazards when off-gas from different process areas is combined. Frequent 

inspection and cleanout may be necessary when the accumulation rate is poorly understood or 

controlled. The same rigor can be applied to the analysis and control of process areas even if 

they are considered auxiliary to the main process or are perceived to have low risk. Otherwise, 

areas perceived to be low risk may become safety-significant. 

Several causal factors appear to have contributed to the occurrence of the event described in 

this IN. The following are some of the contributing causes that the NRC staff considers 

important to understand in helping to prevent similar events from occurring in the future: 

• Administrative Items Relied On for Safety (IROFS). There are IROFS in certain criticality 

accident sequences that involve implementing a particular operating or maintenance 

procedure. It is important that these procedures provide the necessary details, clear 

instructions, and acceptance criteria to ensure that the intended function is reliable and 

available. Additionally, procedures implementing visual inspections are encouraged to 

contain specific pass/fail criteria, and the particular process equipment be designed so 

that personnel can perform an adequate inspection. In this event, the annual visual 

inspection and cleanout through the scrubber cleanout port was ineffective at identifying 

and removing the accumulated uranium-bearing material. 

• Configuration Management. A series of plant modifications to various systems, spread 

out over several years, can have a collective and unintended effect on the overall 

integrated system. Sufficient management measures need to be in place to ensure that 

the configuration of facility processes continues to be managed effectively. In this event, 

a series of modifications were made to several different systems that unintentionally 

resulted in accumulating more uranium-bearing material in the scrubber than expected. 

 

2 See IN 2004-14 (Agencywide Documents Access and Management System (ADAMS) Accession 

No. ML041760122), IN 2005-22 (ADAMS Accession No. ML051890406), and IN 2010-16 (ADAMS Accession 

No. ML100540070). 
IN 2016-13 

Page 4 of 5 

• Challenge Assumptions. Safety analyses and evaluations may include engineering and 

scientific assumptions. Incorrect assumptions can lead to non-conservatisms, 

inadequate evaluation of risks, and could improperly render certain events or accident 

sequences not credible. Licensees are encouraged to use information gained from 

system performance measurements and operating experience in order to verify and 

validate these assumptions. In this event, there was data and operating experience to 

suggest that the assumed low uranium concentration in the scrubber could have been 

challenged and its validity questioned during revisions and peer reviews of the CSEs. 

• Conservative Decisionmaking. After an abnormal or unexpected condition is identified, 

facility personnel are encouraged to ensure that the as-found condition and causes are 

sufficiently understood in responding to the event and before deciding to return to normal 

operations. In this event, a large amount of deposited material was removed. However, 

while the material was appropriately collected into safe-volume containers as though it 

had a high uranium content, facility personnel assumed that the uranium concentration 

was low, decided to wash the material away, and did not report the event. 

• Nuclear Safety Culture. Complex industrial facilities that process special nuclear 

material are confronted with criticality, chemical, and radiological hazards. In order to 

provide a safe environment for the workers and surrounding public stakeholders, facility 

personnel are encouraged to follow many guiding principles, including, but not limited to, 

maintaining a questioning attitude, avoiding complacency, and constantly examining 

engineering processes and procedures. In this event, some of the scrubber operators 

and process engineers were unaware of the uranium mass limits, and the criticality 

safety engineers were not adequately involved in the ventilation modifications, scrubber 

inspection and maintenance, and initial response to the discovery of unexpected 

material. 

 
IN 2016-13 

Page 5 of 5 

CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contact listed below.

/RA/ /RA/ 

Craig G. Erlanger, Director Louise Lund, Director 

Division of Fuel Cycle Safety, Safeguards, Division of Policy and Rulemaking 

 and Environmental Review Office of Nuclear Reactor Regulation 

Office of Nuclear Material Safety 

 and Safeguards 

Technical Contact Stephen Vaughn, NMSS/FCSE 

301-415-3640 

E-mail: Stephen.Vaughn@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, at 

http://www.nrc.gov, under NRC Library/Document Collections. 
IN 2016-13 

Page 5 of 5 

CONTACT 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contact listed below.

Craig G. Erlanger, Director Louise Lund, Director 

Division of Fuel Cycle Safety, Safeguards, Division of Policy and Rulemaking 

 and Environmental Review Office of Nuclear Reactor Regulation 

Office of Nuclear Material Safety 

 and Safeguards 

Technical Contact Stephen Vaughn, NMSS/FCSE 

301-415-3640 

E-mail: Stephen.Vaughn@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, at 

http://www.nrc.gov, under NRC Library/Document Collections. 

ADAMS Accession Number: ML16252A171 *concurred via e-mail

OFFICE NMSS/FCSE/PORS NMSS/FCSE/PORS * R-II/DFFI *NMSS/MSTR/MSEB * QTE

NAME SVaughn MKotzalas CEvans AMcIntosh CHsu 

DATE 09/14/2016 09/14/2016 09/15/2016 09/19/2016 09/19/2016 

OFFICE * NRR/DPR/PRLB *NRR/DPR/PGCB/BC *NRR/DPR/PGCB/LA NRR/DPR/D NMSS/FCSE/D

NAME AAdams SStuchell ELee LLund CErlanger 

DATE 09/22/2016 09/22/2016 09/23/2016 09/27/2016 09/28/2016 

OFFICIAL RECORD COPY