Information Notice 2016-13, 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) | |
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:
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