Estimation of Effects of Filtration and Ventilation on Worker Inhalation Dose from Aerosols During Nuclear Dismantlement

During the decommissioning of nuclear power plants, radioactive contaminants may be released into the work environment in the form of aerosols, which can expose workers through inhalation, ingestion, and submersion pathways. Workers often perform dismantlement work in confined spaces and sealed-off environments. Typical engineering controls to reduce concentrations include air exchange as well as air filtration, which captures aerosols at their source. The dose reduction from these engineering controls is generally not well understood. Given that there exist a variety of filtration methods of varying efficiencies and throughputs, a method of estimating dose reduction for a variety of work scenarios is desirable. 

Methods: This work presents a model of radioactive aerosol concentration. It is used to estimate worker committed effective dose. The model considers dismantlement work parameters such as work time, aerosol source rate, air exchange along with air filtration and air filtration efficiency. The concentration over time is compared to a worst-case aerosol buildup based on no filtration nor air exchange.  

The committed effective dose (CED) due to inhalation from the aerosols is calculated applying the approach of ICRP 119: the dose is proportional to the inhaled activity [1]. The rate of activity inhalation is proportional to the concentration of aerosols in the air. By integrating the concentration over time for an entire shift, the dose to a worker can be estimated. These calculations involve integrations of elementary functions, which allows for scalability of the model and calculations that are easily done on spreadsheets. 

Results:Aerosol concentration over time is not dictated exclusively by the filter flow rate, the filter’s capture efficiency, and the air exchange rate of the room. Instead, it is dictated by a combination of all these factors, referred to as the cleaning period 𝜏. The cleaning period identifies the trade-offs that can be made between air exchange and filtration. Further, this is an engineering parameter which applies to any workspace size. 

By comparing to the worst-case of no ventilation or filtration, the reduction of worker dose over a 10-hour shift can be determined. The dose reduction is exclusively a function of the cleaning period 𝜏 and dismantlement shift structure, i.e., when work is and is not being performed. The dose reduction is independent of scenario-specific parameters such as workspace volume, aerosol source rate, PPE, etc. and so can be used to model aerosol dose reduction in different dismantlement scenarios. 

This paper produces two useful charts for filter selection. The first chart outlines the required cleaning period 𝜏 for a desired dose reduction for the modelled 10-hour worker shift. The second chart outlines the necessary combination of filter efficiency and flow rate for a single filter to achieve the cleaning period for such a space. These types of charts are a tool for engineers and health physicists to consider when building work packages, selecting engineering controls for dismantlement work, and so on. 

Conclusion: This paper presents a mathematical model of the evolution of aerosol concentrations over time during dismantlement work using various dismantlement work parameters, such as aerosol source rates, workspace size, air exchange and filtration, and so on. 

The engineering parameter referred to as a cleaning period 𝜏, which emerges from the model, determines the growth or reduction in aerosol concentrations over time, and ultimately the worker dose. 

For a desired dose reduction, charts to select for cleaning period and associated filtration parameters can be generated for work package design. The models suggest that filtration systems, which often tout their capture efficiencies, should also have their specified flow rates considered.