Plume depletion following postulated plutonium dioxide releases from mixed-oxide fuel--fabrication plants

An accidental atmospheric release of PuO2 particles from a nuclear facility may result in deposition of a major fraction of the particles within a few kilometers downwind. Estimates of plume depletion as a function of distance were computed using our atmospheric-diffusion particle-in-cell (ADPIC) code. This code is capable of computing the atmospheric transport, diffusion, gravitational settling, and dry deposition of the PuO2 particles within a three-dimensional grid under conditions of boundary layer, stratified shear flow. The calculations show the effect on plume depletion due to varying the source height, the particle size, and type of vegetation. Pasquill F stability was chosen for applicability to nuclear facility safety analyses. The fraction of activity remaining in the plume at a given distance increases with source height and is inversely affected by surface roughness. For submicrometer particles emitted at a height of 10 m, the fraction remaining airborne at 30 km downwind is about 0.5 over agricultural land covered with low growing, densely planted leafy vegetables, and only 0.2 over brushland. Because of their large gravitational settling velocities, essentially all particles greater than 5 μm emitted at a 10 m height are deposited within 5 km of the facility. As the source height is increased, the effect of varying the type of vegetation becomes minimal. Hence, for a 100-m source height, the fraction of submicrometer-size particles remaining airborne is roughly 0.9 at 30 km over agricultural as well as brushland and about 0.25 of the 5-μm particles are still airborne at 30 km. (U.S.)