PNNL

Abstract

A radiation accident dose analysis was performed to support the preparation of the Environmental Impact Assessment for the New Safe Confinement (NSC) being designed for the Object Shelter (OS) at Chernobyl Unit 4. This paper describes the atmospheric models used to estimate the dispersion and deposition of material that might become airborne in the event of collapse of the Chernobyl OS and the resultant inhalation and submersion doses and fallout dose rates. Three cases are considered: collapse of the OS before the NSC is in place, collapse of the NSC on the OS, and collapse of the OS after the NSC is in place. The hypothetical source term in each case is an initial cloud of dust containing several kilograms of micron-sized fuel particles. The dispersion models are based on the Gaussian dispersion account for the initial dimensions of the cloud and depletion of the cloud by deposition on the ground. Deposition is calculated using deposition velocities that vary with particle size, density, and wind speed. A source depletion model is used to account for reduction in the airborne mass that results from deposition. If the OS collapses after the NSC is in place, dust raised by the energy of the collapse will initially be dispersed within the NSC. As time passes, the airborne material some of the airborne material will be released to the environment and the remainder will settle out on the surface within the NSC. In this case, evaluating the impacts on the environment is a two-part process. The initial part is estimation of the material that will enter the environment and its characteristics. A box model is used for this purpose. The second part is estimation of the dispersion and deposition of the material that is released to the environment. This evaluation is done using a Gaussian model similar, but not identical, to the model used in evaluating consequences of the collapse of the OS prior to or during placement of the NSC. Inhalation doses dominate the potential risk; doses to workers near the facility could be as high as 400 mSv, and to residents as close as 7 km of as high as 50 mSv if the OS collapses before the NSC is in place. After the NSC is in place doses are greatly reduced, with the reduction dependent on the assumed air exchange rate from the NSC to the environment. Doses for 1 air exchange per day range from about 90 mSv to a nearby worker to less than 1 mSv to nearby residents.