Atmospheric plume progression as a function of time and distance from the release point for radioactive isotopes

• Atmospheric plume dilutions for short duration releases have a power-law relationship with distance from the release point.
• Coefficients for the power law depend on sample collection duration and radioactive decay.
• Half-height plume durations are mostly less than 12 h, even for transport distances of 3000 km.
• Distance-based rules of thumb for plume concentrations and durations are useful for developing monitoring networks.

The radionuclide network of the International Monitoring System comprises up to 80 stations around the world that have aerosol and xenon monitoring systems designed to detect releases of radioactive materials to the atmosphere from nuclear explosions. A rule of thumb description of plume concentration and duration versus time and distance from the release point is useful when designing and deploying new sample collection systems. This paper uses plume development from atmospheric transport modeling to provide a power-law rule describing atmospheric dilution factors as a function of distance from the release point. Consider the plume center-line concentration seen by a ground-level sampler as a function of time based on a short-duration ground-level release of a nondepositing radioactive tracer. The concentration C (Bq m−3) near the ground varies with distance from the source with the relationship C=R×AD,C×e−λ(−1.552+0.0405×D)×5.37×10−8×D−2.35C=R×AD,C×e−λ(−1.552+0.0405×D)×5.37×10−8×D−2.35 where R is the release magnitude (Bq), D is the separation distance (km) from the ground level release to the measurement location, λ is the decay constant (h−1) for the radionuclide of interest and AD,C is an attenuation factor that depends on the length of the sample collection period. This relationship is based on the median concentration for 10 release locations with different geographic characteristics and 365 days of releases at each location, and it has an R2 of 0.99 for 32 distances from 100 to 3000 km. In addition, 90 percent of the modeled plumes fall within approximately one order of magnitude of this curve for all distances.