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Radioactivity Dynamics in forests


What is the mechanism of radioactive cesium migration in the depth direction? 【Example of analysis】

The depth distribution of radioactive cesium exhibits an exponential curve in the early stage of fallout. After that, radioactive cesium either slowly migrates to the deep region while maintaining the exponential distribution or extends the tail of the distribution curve to the deep region.
Although existing models were unable to represent such tendencies, we were able to obtain results in good agreement with measured values by using a model incorporating the sorption/desorption kinetics.

● Development of a migration model that considers the reversible or irreversible sorption/desorption kinetics

● Successful reproduction of the exponential distribution and the tailing phenomenon, which the existing models were unable to reproduce

Furthermore, we performed various sensitivity analyses to elucidate the following mechanisms.

  • Exponential distribution is determined by the ratio between the speed of migration by diffusion and sorption kinetics. This forms the exponential distribution in the early stage of fallout.
  • The tailing into deep regions is caused because the desorption rate is lower than the sorption rate, leading to the slow progress of desorption in shallow regions, migration to deep regions, and slow sorption in deep regions.
  • The migration of radioactive cesium in the soil depth direction slows as cesium sorption to irreversible sites progresses, and then the profile no longer changes.