Information Notice 2018-05, Unanalyzed or Improperly Analyzed Conditions at Fuel Cycle Facilities
| ML18022A211 | |
| Person / Time | |
|---|---|
| Issue date: | 03/26/2018 |
| From: | Craig Erlanger, Chris Miller Division of Fuel Cycle Safety, Safeguards, and Environmental Review, Division of Inspection and Regional Support |
| To: | |
| Keene T | |
| References | |
| IN-18-005 | |
| Download: ML18022A211 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555 March 26, 2018 NRC INFORMATION NOTICE 2018-05: LONG-TERM FISSILE MATERIAL ACCUMULATION
DUE TO UNANALYZED OR IMPROPERLY
ANALYZED CONDITIONS AT FUEL CYCLE
FACILITIES
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 all holders
of, and applicants for, a construction permit or operating license for a production facility, including a radioisotope production facility, under 10 CFR Part 50, Domestic Licensing of
Production and Utilization Facilities.
Although they do not have hazards associated with inadvertent criticality as a result of the
presence of fissile material, this information may also be useful to holders of, and applicants for, a license under 10 CFR Part 40, Domestic Licensing of Source Material.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of recent operating experience involving unanticipated, long-term accumulation of
fissile material in uncontrolled geometry systems due to improper analysis of credible plant
conditions.
The NRC encourages recipients to review the information contained in this IN for applicability to
their facilities and to consider actions, as appropriate, to avoid similar issues. However, no
specific action is required.
DESCRIPTION OF CIRCUMSTANCES
At an NRC-licensed fuel cycle facility, fuel is produced through a series of operations in order to
support a variety of research and test reactor product lines. These operations take place in a
glovebox line, which is maintained in an argon atmosphere. This atmosphere is maintained by
an air purification system which removes oxygen and moisture from the process to address fire
safety concerns. For radiation protection purposes, the atmosphere also provides a negative
pressure inside the glovebox line. The system is purified using a desiccant material located
inside two desiccant filters. In July 2017, the licensee performed maintenance on the desiccant
filters to address decreased system performance. Upon removal of the filters from their location
within an equipment cabinet, the licensee discovered unexpected fissile material in both
desiccant filters. The licensee assumed that the system was non-fissile (i.e., non-uranium
ML18022A211 bearing), and therefore had no documented controls to prevent or monitor fissile material
accumulation. Additionally, the system was not routinely surveyed to detect or monitor the
accumulation of fissile material. Due to the unfavorable geometric properties of the filters, the
unknown moderation conditions inside the filters, and the presence of an unknown quantity of
fissile material mass, the licensee declared an alert1 emergency action level (EAL).
The licensee immediately physically separated the two filters to minimize neutronic interaction, isolated the area with personnel access restrictions, suspended work in other areas pending an
extent of condition review, performed non-destructive assay (NDA) measurements to estimate
the fissile mass present, and performed nuclear criticality safety (NCS) calculations to determine
the minimum fissile mass needed for criticality. Upon comparing the measurements to the
calculated minimum critical mass, the licensee determined criticality was not imminent and
terminated the alert EAL.
High-efficiency particulate arrestance (HEPA) filters that were installed between the gloveboxes
and desiccant filters as part of a separate ventilation system, were designed to ensure
subcriticality within the HEPA filters themselves. However, the possibility of fissile material
carryover beyond the HEPA filters and into the desiccant system was not considered in the
licensees integrated safety analysis (ISA). From its follow-up investigation, the licensee
identified that this carryover condition had occurred at some previous point in time. The
condition was discovered around 1986, after which the licensee developed an NCS analysis
that identified controls to prevent the unsafe accumulation of fissile material within the desiccant
filters. However, the controls were discontinued, and the accident sequence was not included
or considered during the development of the licensees ISA. The licensee determined that the
desiccant filters were last cleaned and the media was replaced in 1986, allowing material to
accumulate undetected for approximately 30 years.
The licensee identified that the HEPA filter design and installation were inadequate for the
referenced glovebox line. Both the crushing section and the blending section of the glovebox
line had significant gaps between the spacer and the gasket of the filter, allowing for fissile
material carryover beyond the HEPA filters. In addition, the licensee discovered that fissile
material could enter the desiccant system through the air purification systems argon purge lines
when the vacuum pump for the air purification system was operating. The licensee determined
that these issues resulted, in part, from the difficulty in achieving an effective seal during routine
HEPA filter change-outs. The licensee implemented corrective actions to ensure easier and
more consistent HEPA filter change-outs.
BACKGROUND
10 CFR 70.61(b) requires that the risk of each credible high-consequence event be limited such
that its likelihood of occurrence is highly unlikely. 10 CFR 70.61(d) requires that the risk of
nuclear criticality accidents be limited by assuring that all nuclear processes will be subcritical
under both normal and credible abnormal conditions. These requirements necessitate that, through the ISA, a licensee evaluates all credible pathways that could potentially lead to a
consequence of concern.
1 As defined in 10 CFR 30.4 and 10 CFR 70.4; Alert means events may occur, are in progress, or have occurred that
could lead to a release of radioactive material, but that the release is not expected to require a response by offsite
response organizations to protect persons offsite.
DISCUSSION
The chronic buildup of fissile material in uncontrolled geometry equipment and areas that are
either inaccessible or difficult to monitor has been a long-standing challenge in the nuclear fuel
cycle industry. Through the review of documented operating experience, the NRC has identified
instances where improper analysis, or in some cases no analysis, of credible events was
performed by certain licensees. Some licensees have based their analyses on incorrect
assumptions which have resulted in the unanticipated buildup of fissile material.
As discussed in IN 2015-082, approximately three ounces of fissile solution were discovered at
an NRC-licensed facility in a junction box during troubleshooting of a resistance temperature
detector in the uranium recovery area. The licensee determined that a thermowell had failed as
a result of pitting corrosion, allowing the solution to leak into the junction box. The electrical
conduit exiting the junction box provided a flow path that could have allowed the solution to
enter an unfavorable geometry electrical box had chronic leakage continued to occur
undetected. In performing the associated analysis, the licensee did not identify this potential
flow path or consider the electrical box.
As also discussed in IN 2015-08, accumulated solids in a catch tray beneath low-level
dissolvers were scraped together into several large piles at an NRC-licensed facility. The catch
tray typically collected small quantities of uranium. In evaluating this process, the licensee
assumed that the material allowed to accumulate in the catch tray was in the form of a solution, which was intended to represent a conservative bounding configuration. Based on this
assumption, the licensee implemented engineered controls to restrict the geometry of any
possible accumulation to a safe slab height. These controls, however, were ineffective at
restricting the geometry of solids to a safe slab height. This was identified after operators
scraped accumulated material in the catch tray into several large piles, which exceeded
established safe slab height limits, in preparation for moving the material into safe volume
containers. Additionally, upon discovery of the piles, the licensee assumed that the uranium
concentration would be low due to the nature of the low-level dissolver process. The licensee
did not have sufficient controls in place to ensure that the uranium concentration of chronic
accumulation was, in fact, low. Furthermore, there were not sufficient controls to monitor the
mass of the fissile material that was allowed to accumulate.
As discussed in IN 2016-133, a licensee discovered an accumulation of fissile material in excess
of established criticality safety limits in a scrubber designed to remove uranium and other
suspended solids. Although the accident sequence of accumulation in the scrubber and its inlet
transition was analyzed by the licensee, the criticality safety analysis was based on improper
technical assumptions, including an assumption that uranium carryover would be low.
Through the review of operating experience, the NRC has identified that similar contributing
factors (i.e., incorrect assumptions and improper analysis of credible events) have been present
when these events have occurred. In the most recent event involving the desiccant filters, the
licensee assumed that the material in the system was non-fissile and therefore no longer
2 Information Notice 2015-08, Criticality and Chemical Safety Event Involving Unanalyzed Conditions and
Unanticipated Unavailability of IROFS [items relied on for safety] at Fuel Cycle Facilities, dated September 2, 2015 (Agencywide Document Access and Management System (ADAMS) Accession No. ML15176A708).
3 Information Notice 2016-13, Uranium Accumulation in Fuel Cycle Facility Ventilation and Scrubber Systems, dated September 28, 2016 (ADAMS Accession No. ML16252A171). included the unfavorable geometry filters in the annual NDA survey. In this scenario, there were
neither controls to prevent, nor routine monitoring to detect, accumulation in the filters. In the
other referenced events, routine monitoring was either not performed or was ineffective at
detecting an unsafe buildup of fissile material because the respective licensees made incorrect
assumptions regarding expected fissile material concentrations.
The desiccant filter and scrubber events both involved the accumulation of fissile material within
process ventilation or air purification systems. However, the concern over fissile material
accumulation in uncontrolled geometry is not limited to material carryover in the gas phase into
ductwork, filter housings, and scrubbers. The thermowell event involved the carryover of
uranium in the form of a solution and the catch tray event involved material in the form of a
solid. Based on this operating experience, the NRC has identified instances where licensees
have underestimated the amount of entrained uranium carryover or distances such material can
travel in ventilation ductwork, even if siphon breaks or air gapswhich are very effective at
preventing backflow of the solution, but may be ineffective for uranium entrained in off-gasare
present. An important part of meeting the requirements of 10 CFR 70.61(b) and (d), involves
ensuring that all credible pathways for fissile material (i.e., either in the form of a solution or a
finely divided solid) to reach uncontrolled geometry are considered.
Following the issuance of IN 2016-13, several licensees performed extent-of-condition reviews
to assess whether their facilities were subject to the conditions described in the IN. At the
facility where unanticipated uranium accumulation was discovered in two desiccant filters, the
licensee had not evaluated the desiccant system because it was not considered a ventilation
system. Licensee extent-of-condition reviews that are too narrowly focused on a particular
system or operation rather than focusing on a holistic review of credible events, may result in
missed opportunities to identify similar issues in seemingly dissimilar processes.
In addition, the use of unverified process assumptions, including those relied on in determining
that an event sequence is not credible, can result in a licensee failing to identify process
characteristics that need to be controlled to ensure subcriticality under all credible abnormal
conditions. These events suggest that areas perceived as low risk, which have no controls
applied (e.g., ventilation and ancillary systems, low-concentration solutions, low-level waste),
may be subject to a higher degree of incorrect technical assumptions. In the most recent
desiccant filter event, the air purification system was considered non-fissile, and the potential for
uranium accumulation in the filters was not considered. The accumulated material in the catch
tray for the low-level dissolvers was assumed to be of low uranium concentration based on the
material type and quantity processed, and the uranium carryover to the scrubber was assumed
to be low without any verification. Narrowly assessing analytical assumptions can result in a
missed opportunity for licensees to detect latent safety issues. Those safety issues may include
the inadequate identification of all normal and credible abnormal conditions, incorrect
conclusions that actually credible conditions are not credible, incorrect assumptions with respect
to technical parameters associated with process operations, and inadequate application of
controls.
CONCLUSION
Recognizing the variability among processes and conditions throughout the fuel cycle industry, the NRC reviewed operating experience and identified that incorrect assumptions have led to
inadequate analysis of credible events and have been a contributing factor in the unanticipated
accumulation of fissile material. The NRC encourages addressees to consider the information in this IN for applicability to their
processes and facilities. The operating experience referenced in this IN suggests that there
may be credible events or conditions whose likelihood has either been underestimated or not
considered, especially those events or conditions involving the potential for long-term fissile
material accumulation. Additionally, recent operating experience suggests that areas perceived
as low risk, which have no controls applied, may be subject to a higher degree of improper
analysis due to unchallenged or unverified assumptions.
CONTACT
S
This IN requires no specific action or written response. Please direct any questions to the
technical contact listed below.
/RA/ /RA/
Craig G. Erlanger, Director Christopher G. Miller, Director
Division of Fuel Cycle Safety, Safeguards, Division of Inspection and Regional
and Environmental Review Support
Office of Nuclear Material Safety Office of Nuclear Reactor Regulation
and Safeguards
Technical Contact:
Jeremy Munson, NMSS/FCSE
Phone: 404-997-4732 E-mail: jeremy.munson@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.
ML18022A211; *concurred via email
OFFICE NMSS/FCSE/PORB QTE RII/DCO/IB4/BC NMSS/FCSE/PORB NRR/DIRS/IRGB/PM
NAME JMunson * JDougherty * RNease * MKotzalas* TKeene
DATE 01/30/ 2018 01/29/ 2018 02/08/2018 02/09/2018 02/12/2018 OFFICE NRR/DIRS/IRGB/LA NRR/DIRS/IRGB/BC NMSS/FCSE/D NRR/DIRS/D
NAME ELee HChernoff * CErlanger CMiller
DATE 02/12/2018 03/09/2018 03/15/2018 03/26/2018