Engineered controls are designed to isolate workers from the hazard or place a barrier between the worker and the hazard prior to the hazard coming into contact with the worker.
Hanford’s Double-Shell Tanks (DST) are actively ventilated with mechanical exhausters that ensure waste vapors are released well above the worker breathing zone. The inactive Single-Shell Tanks (SST) are each passively ventilated to the atmosphere via filtered ventilation risers at rates that vary with local meteorological conditions. Portable exhausters are added to SSTs before and during waste retrieval.
HEPA filters, which are part of the exhauster system, allow the tank to breathe with the outside air. This is necessary to prevent damage to the tank as outside air pressure changes with the weather. Although they do not prevent vapors from getting in the breathing space, they filter out 99.9% of particulates that the exhauster pulls from the tank headspace.
WRPS has made several modifications to tank exhaust systems to reduce tank vapor hazards and protect tank farm workers:
- Extended the height of the ventilation stacks, moving vapors farther away from workers’ breathing space
- Increased the speed of the exhaust gases through the stacks to disperse the vapors away from the work areas
Sealed Vapor Pathways
Another engineering control used by WRPS is to seal vapor pathways. Fugitive vapors are chemical vapors that migrate from the tank headspace into the work areas through openings in the tank dome. These vapors are controlled by sealing the pathways with foam. Foaming pits helps ensure that chemical vapors are not emitted from other unexpected sources, such as pit drains into the tanks.
Technologies to reduce Hanford Tank Farm vapor concentrations were evaluated, and the results were published in SRNL-STI-2016-00484, Hanford Tank Farms Vapors Abatement Technology and Vendor Proposals Assessment. The evaluation included vendor proposals and literature reviews. Proposals were ranked based on safety, technical feasibility, design features, operational considerations, secondary waste generation, and cost and schedule. The technologies considered included combinations of absorbents (e.g., activated carbon), absorption (e.g., water scrubbing), oxidation (e.g., thermal or catalytic), biofiltration, and dilution/dispersion (e.g., increased active ventilation and taller stacks). The highest ranked proposal was installing exhaust boosters that dilute, mix, and lift tank farm vapors. An exhaust booster demonstration project was funded for AW Farm and installed in 2018.