Information Notice 2016-13, Uranium Accumulation in Fuel Cycle Facility Ventilation and Scrubber Systems

Uranium Accumulation in Fuel Cycle Facility Ventilation and Scrubber Systems

ML16252A171
Person / Time
Issue date:	09/28/2016
From:	Craig Erlanger, Louise Lund Division of Fuel Cycle Safety, Safeguards, and Environmental Review, Division of Policy and Rulemaking
To:
Vaughn S
References
IN-16-013
Download: ML16252A171 (6)

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.

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 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). *

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.

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.

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