Lesson Learned Statement:During deactivation of the 247-F Facility at SRS, uranium hexafluoride (UF6) process lines needed to be removed. The project team evaluated the hazards and devised a plan to eliminate or mitigate each hazard; one process line was plugged with material and blanked over 10 years ago. Project personnel employed a "team hazards analysis" approach using the Automated Hazards Analysis (AHA) program. Team members from Industrial Hygiene, Engineering, Radiological Controls, and Project Management participated in briefings and conducted mockup training. This team AHA approach led to the successful removal of the UF6 and associated process lines.
Discussion:Plan for Managing Potential UF6 Hazards
Uranium hexafluoride, UF6, was processed in 247-F. During deactivation of the facility, two lines potentially containing uranium in the form of UF6 needed to be removed. One of the lines was still connected in the system, and one was abandoned in place with blind flanges installed on both ends. The system line did not indicate having uranium as holdup, but the blanked-off line was suspected to contain holdup equivalent to as much as 150 g of UF6. Upon exposure to moist air, this quantity of UF6 will generate approximately 34 g of hydrogen fluoride (HF) gas and 131 g of airborne UO2F2 particulate. Since these quantities could pose a significant health hazard to unprotected D&D workers, a simple system to flush and capture these materials (to avoid worker exposure) was developed and used.
In order to further reduce potential worker exposure, the filtered effluent from the pipe flushing system was discharged to the vicinity of a room air exhaust duct, where it was exhausted via a temporary building ventilation system. As additional protection, all workers involved in this activity were wearing an air-fed plastic suit, shoe covers, chemical resistant gloves, and appropriate safety equipment. To minimize personnel exposure, the number of workers performing the task was limited. Industrial Hygiene and Radiological Controls Operations (RCO) personnel monitored the job as it was performed.
Properties of UF6
Within a reasonable range of temperature and pressure, UF6 can be a solid, liquid, or gas. Solid UF6 is a white, dense, crystalline material that resembles rock salt. It is somewhat unusual in that at atmospheric pressure it transforms directly into gas without first going through the liquid state. At 75 degrees F, UF6 solid in a closed system generates a gas with pressure of 1 to 2 psia. If the closed system is breached, this gas will be released to the environment.
UF6 does not react with oxygen, nitrogen, carbon dioxide, or dry air, but it does react with water. When UF6 gas comes into contact with water, such as water vapor in the air, the UF6 and water react, forming corrosive HF and UO2F2. Consequently, release of UF6 to the environment could potentially affect the health of workers in the vicinity of the release. Exposure to HF could result in a range of health effects depending on the exposure level.
UO2F2 is a particulate that can be dispersed in air and inhaled. Once inhaled, UO2F2 is easily absorbed into the bloodstream because it is soluble. If large quantities are inhaled, kidney toxicity can result.
Properties of UF4
It is possible that some or the entire uranium holdup is in the form of uranium tetrafluoride (UF4), which is often called green-salt because of its characteristic color. UF4 is a solid composed of agglomerating particles with a texture similar to baking soda. It is non-volatile, non-hydroscopic, but only slightly soluble in water. After exposure to water, UF4 slowly dissolves and undergoes hydrolysis (reaction with water), forming several possible uranium compounds and HF. The time for hydrolysis can be significant. Since UF4 does not change directly from a solid to a gas, if the uranium holdup in this area is in the form of UF4, it will be much easier to contain. Therefore, it would be adequately addressed by the UF6 strategy.
Removal of Potential UF6 Lines
Lines potentially containing uranium were handled as if the uranium was in the form of UF6, since this form is more reactive than UF4.
To maintain containment during line removal, a line break within a RadCon glovebag was made at the flange where each line entered the facility process glovebox. Working within the RadCon glovebag, the line opening toward the glovebox was sealed in plastic. A second line break, within a separate RadCon glovebag, was made at the opposite end of the section to be removed. On the second line break, the end toward the first break from the glovebox was left open for flushing; the other broken end was sealed in plastic. The vacuum system was connected to one of the open ends of each pipe to be flushed. The vacuum was operated as air was allowed into the glovebag on the other end through as HEPA-filtered connection to establish a purge flow through the entire length of the pipe. Air was drawn through the pipe for 30 minutes to remove any potential UF6. Assuming 50% relative humidity, 60 degrees F dry bulb temperature, and 10 scfm flow, this time provides approximately four times the water required to completely convert all the UF6. At the completion of the flushing, the atmosphere at the pipe opening was checked using a Drager sample tube for the presence of HF (hydrofluoric Acid) to verify that the pipe has been adequately flushed.
The whole pipe was cut out in small pieces (~ 4 feet) beginning at the end opposite the vacuum system, while maintaining the pipe under a vacuum purge. The cuts were made in RadCon glovebags to contain any UF6 that was not removed during previous flushing.
The vacuum system contained an HF absorber filter packed with Purafil CHS-BX Select Media sorbent. This material is very effective in removing HF from air streams. The material has a bulk density of about 50 lb/ft3 and is supplied in 50-lb containers. Since the material can absorb up 10% of its weight in HF, a 50-lb container will absorb over 60 times the worst case release of HF expected, thus providing sufficient capacity.
The HEPA filter on the vacuum cleaner is designed to remove particles up to 0.3 microns in size (not visible to the naked eye). No information was found on the particle size of the UO2F2 formed during the UF6 reaction with water vapor. However, the UO2F2 produced by the reaction is known to be in the form of a white cloud of smoke, indicating that a significant fraction of the particles are large enough to be visible. Therefore, it is anticipated that the HEPA filter uranium removal efficiency will be high.
In addition to the localized monitoring for HF by Drager sample tube, a Zellweger Single Point Monitor capable of providing instantaneous digital readings and an alarm was used continuously to monitor the work area.
As the work was performed in an Airborne Radiation Area/Contamination Area continuous monitoring for airborne radioactively by CAMs and personal air monitoring pumps was conducted.
Analysis:Personnel took a Team Hazards Analysis approach. Personnel from all aspects of the job teamed together to perform an in-depth look at possible hazards before beginning the job.
This approach resulted in a successful job being completed.
Recommended Actions:WSRC-Specific Recommendations
- Challenge the status quo; look for new ideas to determine best practice. Integrate all hazards mitigation/elimination methods. Use most efficient that address multiple hazards.
- Perform a multi-discipline Automated Hazards Analysis (AHA) to determine all potential hazards.
- Evaluate methods to mitigate or eliminate each hazard. The methods should include; locally-proven, complex-wide and commercially-available methods.
- Develop mockups for testing and training. Conduct mockup training for the evolution with focus additionally on abnormalities.
- Enlist peer reviews of hazard mitigation or elimination methods by site experts.
- Utilize monitoring equipment during performance to identify hazard mitigation/elimination failure/ degradation.
Project Lessons Learned Coordinators should distribute this lessons learned to personnel involved in similar activities, including:
- D&D personnel
- work planners
- industrial hygiene
- project management
Personnel receiving this information should review this Best Practice and incorporate applicable aspects of it into work practices.
Originator:Westinghouse Savannah River Company
Contact:WSRC/Site D&D - Ron Boisvert (803)952-4463, email@example.com
Name Of Authorized Derivative Classifier:R.L. Shankle
Name Of Reviewing Official:STI
Priority Descriptor:Green / Good Work Practice
Keywords:Decommissioning, Hazard Analysis, Industrial Hygiene
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DOE Function / Work Categories:Decontamination & Decommissioning
ISM Category:Analyze Hazards
Develop / Implement Controls
Hazard:Personal Injury / Exposure - Other
Personal Injury / Exposure - Radiation / Contamination
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