LfE Database

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LFE ID Facility Type Description of Event Criticality Safety Parameters Affected Immediate Actions Taken Method of Resolution Apparent Cause(s) Root Cause Lessons Learnt Type of Operation Keyword(s)
001 Fuel Cycle Facility Uranium Accumulation in Casting Furnace Mass Material was cleaned out & whole set of Corrective Actions Incorrect Assumption Assumptions about uranium accumulations in the casting process did not align with operations in the field Need to understand process you are assessing - view it in action Operations Furnace, accumulation, hidden
002 Storage/Waste/Repository Storage of Materials Above Height of CAAS Analysis Assumptions (On Top of a Cabinet) CWS/CAAS/NIM Material immediately relocated Human Error Unknown - possibly lack of awareness of CAAS assumptions ? None given - but presumably lessons for (i) training / briefing, and (ii) investigate whether height restriction is required Emergency Planning/Recovery Storage, CAAS, stacking
003 Office based Parallel calculations were performed with two independent criticality safety codes. Agreement was generally good; however, discrepancies slightly exceeded those anticipated from statistical uncertainty. Review of one of the input decks identified a spacing definition inconsistency: H was modelled as a horizontal pitch and V as a vertical separation, contrary to the author's interpretation (in which H represented height and V the pitch in the triangular array). Interaction/Spacing The identified discrepancy was corrected and any affected calculations re-run. Team was briefed that parameter naming must be unambiguous. Human Error Insufficient detail provided in the code input 1) Define input parameters unambigiously. 2) Verify key input decks early. 3) Examine deviations beyond expected uncertainties, even if small. Criticality Safety Assessment (documents or analysis) Parallel Calculations, Discrepancies, Ambiguous Parameters
004 Fuel Cycle Facility Samples inputted on the wrong inventory system Mass Operations stopped to rectify issue Human Error Unknown - but believed to be different parts of the same Inventory sheet Unknown Operations Inventory, software
005 Fuel Cycle Facility Over pressurisation of domestic water supply resulting in multiple facility leaks Moderator Unknown Human Error Isolation activities in a different facility knocked onto main facility Many creative ways that water can leak from systems Operations Water, leak, over pressurisation
006 Storage/Waste/Repository Non-conservative assumption about concentration of product solution entering steam condensate isolation unit Concentration/Density Unit taken out of service & will not be operated until issue has been resolved Incorrect assumption Lack of understanding of process None given - but presume lessons regarding a full understanding of the process Operations Response, steam, dissolver
007 Storage/Waste/Repository Erroneous storage of contaminated exhaust pipes without spacing requirement Interaction/Spacing Erroneous items were removed and compliantly stored in a separate storage area Human Error Lack of understanding of requirements None given - but presume lessons for training Decommissioning Spacing, contamination
008 Storage/Waste/Repository Erroneous opening of transport container in a CSI array Concentration/Density None required as operation already completed - discovered during final moves Human Error Believed to be the right course of action on shielding grounds None given Operations Transport container, array, shielding
009 Fuel Cycle Facility Simultaneous loading of drop box with twice the permitted amount of material Mass Transferred material back to original location Human Error Simultaneous moves by two independent organisations None given Operations Mass, Drop Box, Simultaneous
010 Fuel Cycle Facility Transfer of erroneous concentrated liquor heel to a downstream process during flushing Concentration/Density Unknown Equipment Failure Level indication anomaly didn't reveal the erroneous heel which was subsequently transferred downstream during flushing None given Major Maintenance/Facility Modification/Shutdown Heel, Level Indication, Downstream transfer
011 Fuel Cycle Facility A portion of the ventilation system associated with a scrubber was found to have exceed the criticality safety mass limit Mass The operation was shutdown and the ventilation system was open and cleaned out; the extent of condition was evaluated for other scrubber systems in the facility The inspection and cleanout frequency was increased for this scrubber Incorrect Assumption Slow accumulations of material downstream of scrubers occur at different rates, the ventilation system had not had an extensive cleanout for many years. The accumulation rates of material should be checked on a regular frequency. The estimated accumulation rate for one location should not be applied to many locations. Ventilation systems will accumulate material over time and need to evaluated. Operations scrubbers, mass accumulations, ventilation system
012 Reactor A duty holder derived safety limits using random distributions of fuels rods in moderator. Ths came to light after the plant was running. Other The plant was shutdown until the dutyholder produced an adequate safety case. This safety case needed to address (a) the criticality code had not been validated (b) normal operations were assessed using a probabilistic approach. Human Error Use of unvalidated criticality safety assessment method. None given but lessons on use of a validated code and adequate safety cases. Operations Unvalidated code, probabilistic
013 Fuel Cycle Facility During an inspection, when asked about the fisile content of a Mobile Filtration Unit (MFU), the plant manager was not certain about the quantity. Mass A safety case was created to support the MFU. Although isolated and not in operational use, the organisation did not have a safety case for the MFU. As a result of the error a safety case was created. Human Error Uncertain Fisile content in Mobile Filtration Unit (MFU) None given but lessons on ensuring items on plant have a safety case Operations MFU, safety case, undertain mass
014 Fuel Cycle Facility HEPA filiter was found to have exceeded mass limits, while still having an acceptable delta pressure Mass Operation was shutdown; evaluate similar HEPA filiters and replace as needed; Increase monitoring of HEPA filiters Replace the HEPA design with one that did not need mass a control; develop a crititeria for using delta-P as a primary control Incorrect Assumption The delta-P limit was developed for UO2 powder with known particle size distribution; a new oxidation process produced U3O8 powders with different particle size distribution that did not correlate with the former uo2 delta-p behavior for primary HEPA filters. Staff should question the basis of a criticality limit on a regular basis. Accumulation locations should be monitored to verify compliance with the limits; new or changed operations should be monitored more frequently. Operations HEPA filiters, mass accumulations, ventilation system
015 Fuel Cycle Facility In a laboratory, the transfer of a sample between two units was performed without verifying compliance with the mass limit Mass Update of the mass record of the unit (1) Improvement of the ergonomics of the workstation(2) Addition of a lockout to prevent unverified transfer Human Error Misunderstanding of the transfer process (different from the other parts of the facility which are mostly automated) due to the lack of a documented procedure 1. Standardize transfer processes2. Establish detailed documentation and training for non-standardized transfers Operations transfer, overbatching
016 Research Inspection trolleys were used for the movement of fissile material, rather than the approved movement trolleys. The inspection trolleys had not been assessed by the criticality safety case and were not specifically designed to provide engineered spacing. Interaction/Spacing Use of inspection trolleys was embargoed until fully assessed. A criticality safety assessment of the inspection trolleys was conducted. Incorrect Assumption Unknown - possibly due to pressures to commence operations. 1. The need for auditable documentation of any proposed deviation from operations specified within the safety case; 2. The need for targeted criticality inspections to witness operations; 3. Encouraging a questioning attitude. Operations Movement trolleys, engineered spacing
017 Research Residue bottles were used to hold liquor at a higher concentration than stated in the criticality safety case. Plant personnel thought they were compliant because they considered the bottle to hold "product" rather than "residue". The safety case did not make such a distinction. Concentration/Density Advised plant personnel that the bottles in question must not be used to hold liquor at greater than the concentration limit irrespective of whether they consider the content of the bottles to be true “residue” material. The resolution will be the withdrawal of the concentration control. A review of the safety evaluation has concluded that other existing controls are adequate to ensure safety under normal and credible abnormal conditions. Incorrect Assumption Unknown - difficult to know how the difference in understanding of the definition of a “residue bottle” originated. 1. Ensure no unnecessary safety designations; 2. Greater use of diagrams/pictures could have helped understanding; 3. More plant visits/inspections may have identified this sooner. Test/Trial/Experiment Residue bottles, concentration
018 Fuel Cycle Facility Daily pressure reading was not collected due to an isolation valve being closed resulting in a nuclear criticality safety (NCS) violation. In this operation, pressure is a measure of mass. Mass While troubleshooting a different instrument problem, the closed isolation valves were identified; NCS was notified, and the isolation valves were opened. Recommended action was to ensure (i) analysis is documented and flowed into implementing procedures, (ii) controls are communicated, (iii) personnel are trained, and (iv) configuration control/human error countermeasures are implemented [e.g. position seals, caps on valves and descriptive placards]. Human Error Configuration control, communications, rigor, formality NCS controls need to be included in operating procedures with guidance on maintaining configuration control. Operations Configuration Control
019 Fuel Cycle Facility Over time, operations change, and equipment no longer needed is taken out of service. Equipment is often isolated when taken out of service and can leave behind a fissile material holdup. As part of a planned fissile accumulation review, several systems were identified with an accumulation greater than permitted in the current nuclear criticality safety (NCS) analysis. However, these accumulations were acceptable under legacy NCS analysis. Mass The identified systems had accumulations that were acceptable in legacy (and valid) NCS analysis. None Other Legacy systems are left in place in the event the process is needed again or because decommissioning and removal is not part of the core facility mission. Legacy equipment with a history of containing fissile material needs to be periodically evaluated and historical NCS documentation should be maintained. Decommissioning Non-Destructive Assay (NDA), Accumulation
020 Research While verifying the adequacy of a control set in a nuclear criticality safety (NCS) evaluation, an analyst determined that some of the previous assumptions may not be conservative,. More detailed modeling resulted in a calculated K-effective above the Upper Subcritical Limit (USL) for the material balance areas. Other A self-check and an independent peer review were performed; this prompted a pause in work for the affected material balance areas. While the work was paused, the impacted material balance areas were reevaluated using the corrected assumptions, and new limits were applied. Incorrect Assumption A reevaluation of the operations determined a new material distribution assumption was appropriate. The new assumption was not propagated into all the subsequent analysis When reviewing or revising a NCS evaluation, all assumptions and methodologies should be reviewed to ensure they are still current. Criticality Safety Assessment (documents or analysis) NCS Evaluation, Upper Subcritical Limit
021 Storage/Waste/Repository During preparations for operations with high fissile gram equivalent (FGE) containers, nuclear criticality safety (NCS) engineers were unable to locate the applicable documents from the prior contractor. After receiving the documents from the prior contractor, it was determined the documents did not have the required rigor for the operations being planned. A new NCS evaluation determined that the spacing control may not be adequate. Interaction/Spacing Reviewed the current inventory to be adequate under current NCS evaluation. Prohibited accepting new high FGE containers. A new set of control measures to increase the spacing required for high FGE containers. Other Contractor transition did not result in adequate transfer of knowledge. During a contractor transition it is important to ensure the adequate transfer of documentation. The new contractor needs to understand the basis for the transitioned documents and controls. Criticality Safety Assessment (documents or analysis) NCS Evaluation, Safety Basis, Contractor Transition
022 Fuel Cycle Facility Several batches of material were sampled, determined to be non-accountable and met the criteria to be disposed of as waste. The batches were then placed in a waste drum. When the waste drum was subsequently measured, it was determined that the fissile loading was higher than allowed. Mass Bulking of non-uranic organic waste was suspended. Procedures were modified in the plant to correctly distinguish steps between "non-accountable" and "non-fissile" material in different portions of the plant. The fissile mass was tracked as batches were added to a waste drum, instead of only after the drum was filled. Human Error Ambiguity in operations instruction, inadequate communications between workgroups. Each process needs to consider how fissile operations are impacted by identifying fissile material as "non-accountable". For operations when a small amount is being taken away, the reduction in procedural requirements is significant and appropriate. For operations that collect / bulk small amounts of material, additional procedural requirements are needed. Operations Waste Drums, Waste Accumulation
023 Fuel Cycle Facility A hydraulic leak occurred in a glovebox. As part of the remediation process, a nuclear criticality safety (NCS) review was needed that resulted in a Non-Destructive Assay (NDA) mass assessment being made of the lower part of the glovebox. The NDA evaluation determined that there was more fissile mass than expected and the existing NCS evaluation had not evaluated fissile material in the equipment in the lower portion of the glovebox. Mass All activities placed on hold. A temporary deviation was prepared to allow cleanout of the fissile material and determination of the total fissile mass involved. A new technical basis is needed before work resumes with reevaluation and fact finding of the occurrence. Equipment Failure Mass accumulated more rapidly than expected as a result of a a process change that introduced additional moderation. A NCS review for operations that are different than normal (e.g. remediation) is a good practice. Operations Non-Destructive Assay (NDA), Accumulation
024 Other During routine maintenance, water used to clean part of a nuclear processing area accidentally seeped into a nearby sealed glovebox containing fissile material residue, mixing with the contamination and forming a small liquid solution (with a very low fissile content). Moderator Work was stopped and the water lines were shut off, then the contaminated boxes were cleaned and sampled. An investigation traced the leak back to a faulty door seal. Checks were added for liquid buildup and installed drains, the door gaskets were fixed, the lighting was improved and procedures were updated to clean the glovebox every time it’s used. Equipment Failure Because no nuclear criticality safety (NCS) considerations were covered in the design review or operating procedures, the risk of liquid collecting in the glovebox was never assessed or inspected (allowing a leaking gasket, with poor visibility, to go undetected). Any equipment or process change requires a dedicated NCS review. Operating procedures must mandate inspections, cleaning, and regular testing of all components. Routine Maintenance Leaking Gasket, Seal
025 Fuel Cycle Facility Samples of raffinate are taken to verify low concentration before transferring to an unfavorable geometry vessel. Results from one sample contained a transcription error that indicated the concentration was above the transfer limit; however, the transfer was still authorized. This error was identified prior to the transfer during an unrelated event. Concentration/Density Stopped work due to the unrelated event Unknown Human Error The operator was asked to compare two essentially identical values before authorizing a transfer. The raffinate samples consistently came back with fissile concentrations below the allowed transfer limit. This is a transfer of liquid from an operation with multiple independent controls to an operation with essentially no controls. Setting up the approval criteria and procedure for accepting the transfer must take into account the human factors affecting operators approving the transfer. Operations Human factors, Waste management, Raffinate, Transcription error
026 Storage/Waste/Repository While restoring water pressure to the fire sprinkler system following a freeze-related event, a sprinkler discharged, flooding a vault containing fissile material. Moderator During the facility walkdown following restoration of the water pressure, water was discovered in the vault. Personnel immediately left the room, shut off the water supply, and contacted Nuclear Criticality Safety (NCS) staff. NCS staff entered the area with management approval and identified the conditions of most concern. NCS staff directed the separation of some touching mobile units by at least 12 inches, maintained the administrative boundary, and began developing guidance for safe drainage and handling of flooded sample bottles and materials in contact with floodwater. Equipment Failure Freeze protection required shutting off the water to the sprinkler system. Restoring the water was known to have the potential to cause leaks. Planning for upset conditions (potential leaks) minimized the extent of the flooding. Mobile unit design should consider that flooding is a possibility. The manipulation of storage configurations during high-pressure sprinkler releases should be considered in sprinkler areas. Routine Maintenance Sprinklers, Water lines, Flooding
027 Fuel Cycle Facility As part of an existing Nuclear Criticality Safety (NCS) activity to identify potentially uncharacterized items, a legacy waste drum containing solidified material and Raschig Rings was found during a walkdown. The nondestructive assay (NDA) results showed elevated fissile material content. Mass The drum was placed under additional NCS administrative controls, including being covered with plastic to prevent liquid entry, and routine walkrounds were initiated to monitor for potential leaks. Additional sampling and analysis to better characterize the drum's contents and hence inform the appropriate next step. Other Legacy waste fissile material often has minimal inventory records, since detailed characterization was not part of the mission. Raschig Rings (a neutron poison) were added to ensure that the waste would remain subcritical under unexpected conditions. This made it difficult to determine a disposal pathway that was consistent with current requirements. Legacy waste containers may need additional characterization to be handled and disposed of in compliance with current requirements for NCS and waste disposal. Operations Legacy waste containers, Raschig rings, NDA
028 Storage/Waste/Repository During nondestructive assay (NDA), a waste container loaded with contaminated metal pipes was identified with a fissile gram equivalent level greater than that allowed by the Nuclear Safety Analysis. During a subsequent NCS walkdown, it was found stored outside of a designated storage area. Mass Work in the affected area was paused, the area was isolated, and all material movements were stopped. An abnormal operating procedure for Nuclear Criticality Safety (NCS) infractions was initiated, and NCS staff conducted an area walkdown. NCS staff determined that the container and storage configuration were adequately controlled, and the stop-work was lifted. Incorrect Assumption The reliance on historical data that underestimated the fissile gram equivalent of the contaminated pipes left the Nuclear Safety Analysis limits insufficient to bound the actual waste contents. When performing operations with potentially inaccurate fissile mass estimates (e.g. legacy containers), it is important to have abnormal operating procedures in place to address potential infractions. Decommissioning Fissile Gram Equivalence, NDA, Historical data, Infractions
029 Fuel Cycle Facility During a revision of a Nuclear Criticality Safety Evaluation (NCSE), it was determined that the assay meter used to verify fissile material content lacked a sufficient technical basis. As a result, the assumed limit for how much fissile material could be mixed with other material and still be detected could not be validated. Mass When the issue was recognized, temporary controls were put in place to maintain safety until the NCSE could be corrected. Unknown Incorrect Assumption The NCSE depended upon a measurement tool that did not have sufficient technical basis to demonstrate it could verify fissile assay. It is important to question the technical basis for key elements of a NCSE when the evaluations are being reviewed or revised. Operations Technical Basis, Assay
030 Fuel Cycle Facility Facility operators were performing clean-up and waste preparation work when they placed six containers with fissile material on the floor, moving them from transfer units into a location designated for non-fissionable material operations. This was done without an approved Nuclear Criticality Safety Evaluation (NCSE), which was required because the mass exceeded permitted thresholds. The containers were moved from the transfer units due to egress issues, and the transfer units were then removed from the room. Mass Work was paused, access to the room was restricted, and Nuclear Criticality Safety (NCS) staff inspected the containers and confirmed that the situation was safe and stable. NCS staff determined that the items were safe to remain in place, access to the room was restricted, a fact finding exercise was conducted, and management required a recovery plan before further action. Human Error The operators focused on resolving a conventional safety concern with room congestion and emergency exit routes but did not consider NCS requirements when transferring the containers to the floor. Consider NCS aspects when addressing other operational concerns such as congestion or emergency egress. Operations Waste operations, Storage, Emergency exit routes
031 Fuel Cycle Facility During a procedure review, NCS staff identified that drum loading and unloading within storage arrays was being treated under broad “general handling” requirements instead of a process-specific Nuclear Criticality Safety Evaluation (NCSE). Because “general handling” was not clearly limited, it came to be interpreted as covering almost any movement or staging of material, even within areas governed by other process-specific NCSEs. This created confusion about the required controls. Other All drum activities were paused and placed on hold until procedures could be revised and proper controls established. The issue was investigated through fact-finding, causal analysis, and reviewing other drum loading activities to understand the extent of the condition. Additional NCS controls were established for affected locations, procedures and NCSEs were revised, and corrective actions were completed. Incorrect Assumption The definition and scope of “general handling” activities were vague and overly broad, leading to misinterpretation of how the requirements applied within process-specific operations. Care should be taken when relying on general handling requirements within areas covered by a process-specific NCSE. Overlapping boundaries between the scope of general handling activities and other process-specific activities can create confusion regarding the applicability of NCS controls. It is important to conduct procedure reviews with a questioning attitude and remain alert for potential scope creep. General operating procedures are particularly susceptible to scope creep. Operations Scope creep, Storage, Handling
032 Fuel Cycle Facility Discovery that the oil used in the equipment is more hydrogenated than water (moderation of the fissile material by this oil assessed in abnormal condition of an oil leak). Moderator None, existing margins were sufficient to not jeopardize the subcriticality of ongoing operations. Update of the Nuclear Criticality Safety (NCS) assessment. Incorrect Assumption The chemical composition of the oils used in equipments was not known by the operator (industrial secrecy of the manufacturer). An approximate formula (of the type CxHyOz) and a density value provided by the manufacturer were used in the NCS assessment, resulting in the moderation of these oils being bounded by water. Unfortunately, this approximation was incorrect. The origin of the raw formula, used for the design, was not documented. Control of the moderators composition is important and should be subject to requirements from operators to suppliers. During oil procurement the NCS requirement on oil composition (hydrogen in this case) shall be provided to the supplier. Operations Oil, Moderation, Composition
033 Fuel Cycle Facility The maintenance operations consisted of analyzing neutron-absorbing tubes in a tank filled with fissile sludge. Prior to the analysis, the operators checked that the height of the fissile sludge was below the criticality limit and proceeded with removing the tubes. For the purpose of the analysis, the tubes were cleaned before being removed from the sludge, which introduced water into the tank and resulted in an increase in the fissile height. In addition, the operators did not identify this situation as sensitive regarding criticality risk. Other Reintroduction of the neutron-absorbing tubes. N/A Human Error The operational documentation did not adequately describe the maintenance activities, in particular the situation associated with the removal of the neutron-absorbing tubes. Furthermore, shortcomings in the criticality safety culture were observed: the criticality requirement focused solely on the sludge height, while the impact of adding water was not considered in the operational documentation on the basis that it did not “increase the amount of sludge”. Maintenance operations have to be clearly described in the operational documentation, and credible abnormal situations — in this case, the exceedance of sludge height — must be explicitly addressed in the operational documentation. Major Maintenance/Facility Modification/Shutdown Absorbers, Moderation, Geometry
034 Fuel Cycle Facility Loss of the moderator (bitumen) required for the effectiveness of the cadmium neutron poison. Absorption Evaluation of the situation by Nuclear Criticality Safety (NCS) experts, consideration of the ongoing fissile material concentrations in order to demonstrate that a criticality accident was impossible in the absence of bitumen. Implementation of specific instructions to ensure subcriticality. Partial removal of the fissile material plus revised limits and instructions. Operation totally stopped a few months later. Equipment Failure Not fully known, but postulated that the radiolysis caused the bitumen to swell and be expelled (through breakage of a weld). Incorrect assessment of material behaviour under irradiation (ageing effect). Operations Absorbers, Ageing, Radiolysis
035 Fuel Cycle Facility Exceeding the permitted fissile mass in an unit due to misreading a fissile container barcode. Mass Return to compliant storage Instruction to verify visually the barcode automatically reported on the screen and, if it is not correct, enter manually the correct barcode seen on the container. Equipment Failure A fissile mass is associated with each container and a container is identified by a barcode. The barcode reading system had a predefined list of containers. Unfortunately, when the barcode did not exist in the list, the tool assigned another container reference to the container (leading to an incorrect accounting of the masses). Inadequate design of the tool, as it did not take into account the possibility of a barcode not being in the list (nor did it display an alert message in this case). Operations Identification, Barcode
036 Fuel Cycle Facility During the preparation of a mixture consisting of several batches of fissile powder and one batch of moderated product, an operator realized that he had forgotten to add the moderated product to the mixture. He attempted to add it afterwards, outside of the automatic sequence of the equipment. Believing he had successfully completed the addition, he drained the equipment. However, the moderated product had not actually been added to the mixture but remained trapped between two valves (due to the process). When the next batch was processed, the moderated product from the previous batch was introduced into the ongoing mixture. This led to an overloading of moderator. Moderator No immediate action was taken, as the error was only detected during analyses of the manufactured products. Training sessions were held with operators and management on compliance with the criticality safety requirement and on the importance of each person’s role. Signage was installed at the station where the moderated product is added. An assessment of the piece of equipment was carried out and did not identify the need to modify the piece of equipment to prevent this type of event. Human Error This was inappropriate behaviour from an operator (human factor), as the operator did not respond appropriately upon realizing his mistake. He should have informed his management. This event highlights the importance of operator training both in the strict requirement of rules and in the management of unexpected situations. Operations Moderation addition, Questioning attitude

Database

Developed by: M. Savage, G. Willock, B. Philpotts, D.A.Hill, M. Erlund, A. Till, A. Brown.

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