Determination of the internal exposure hazard from plutonium work in an open front hood
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Work with hazardous substances, such as radioactive material, can be done safely when engineered controls are used to maintain the worker effective dose below the International Commission on Radiological Protection ICRP 60 recommendation of 0.02 Sv/year and reduce the worker exposure to material to as low as reasonably achievable (ALARA). A primary engineered control used at a Los Alamos National Laboratory facility is the open-front hood. An open-front hood, also known as an open-front box, is a laboratory containment box that is fully enclosed except for a 15-cm opening along the front of the box. This research involved collection of the aerosol escaping an open-front hood while PuO2 sample digestion was simulated. Sodium chloride was used as a surrogate to mimic the behavior of PuO2. The NaCl aerosol was binned as a function of median aerodynamic diameter using a Micro-orifice Uniform Deposit Impactor (MOUDI, MSP Corporation, Shoreview, MN) cascade impactor. Using neutron activation analysis (NAA) to measure the mass of material in each of the nine bins of the MOUDI, the mass median diameter of the escaping aerosol was determined. Using the mass median diameter and the total mass of the particle distribution, dose was calculated using ICRP 60 methodology. Experimental conditions mimicked a stationary worker and a worker moving her hands in and out of the open front hood. Measurements were also done in the hood for comparison. The effect of the hands moving in and out of the box was modeled. Information necessary for Computational Fluid Dynamics (CFD) modeling is given, such as volumetric flow rates out of the open front hood and into the experimental room, detailed sketches of the experimental set-up, and energy provided by the hot plate and worker. This research is unique as it measures particle size distribution from routine working conditions. Current research uses tracer gases or describes non-routine conditions. It is important to have results that mimic routine conditions to allow for quantitative measurement of worker exposure and determination of the adequacy of the open front hood for this type of work. This work is important as it quantifies the effectiveness of the open front hood for controlling inhalation hazards. This information is crucial for managing the risk to workers. The mass median diameter of particles escaping the hood when a stationary worker sits in front of the hood is 0.54 ± 3.7 μm. The mass median diameter of particles escaping the hood when a worker performs work in the hood is 0.35 ± 5.1 μm. These particle sizes are in the range of those seen in the published liturature. (Raabe, et al., 1978; Dorrian and Bailey, 1995; and Cheng, et al., 2004) The effective dose from digestion of PuO2 in an open-front hood while a worker is moving her hands in and out of the hood was estimated to be 5 mSv. Based on the experimental error, this value could be low by a factor of 4. There was little difference between the dose calculated for a worker in motion and a stationary worker. The calculated dose while work was being performed is 5% higher. Comparison of these results to measured worker doses and continuous air monitoring results showed the experimental results may be somewhat higher. The lower limit of detection for urine bioassay is 0.002 Sv (Inkret, et al., 1999). Workers performing the activity mimicked in this experiment are routinely monitored and do not have measurable internal doses. The most likely reason for the high experimental results is the placement of the sample digestion apparatus. For this experiment, the material was placed 10 cm from the hood opening. In practice, the material is typically further back in the hood; placing the material further back in the hood likely decreases the amount of material escaping the hood. The cost-benefit analysis showed the use of the open-front hood as a reasonable protective measure. Although worker exposure may approach the ICRP limit, the cost of previously observed ergonomic injuries caused by work in a glove box is five thousand times greater than the dose received by the worker. Protective measures such as respiratory protection should be evaluated on a case by case basis to keep worker exposure as low as reasonably achievable.