Air kerma rate from radionuclides distributed in forest ecosystem

This study evaluates the air kerma rate in radioactively contaminated forests. The air kerma rates created by plane sources of monoenergetic photons in the energy range 0.02-3 MeV located at different depths in soil up to 50 g cm^-2 and at different heights in the forest medium from 0.05 to 50 m were calculated using numeric solution of the transport (Boltzmann) equation. To simplify the practical use of the results obtained by solving the Boltzmann equation, the study additionally includes approximation formulae for calculating air kerma rate separately from contaminated soil, crowns and trunks of trees in a forest ecosystem for 20 radionuclides - fission products that significantly contribute to the external dose. Biomaterial of trunks and crowns was modeled as uniformly distributed in corresponding layers and homogeneously mixed with air. Different distributions of radionuclides in soil were considered including plane source located at different depths, exponential and uniform distribution. Based on the results, the effect of forest biomass presence as an absorbing and scattering medium on the air kerma rate at 1 m above soil was evaluated. The estimated relative difference in air kerma rate at 1 m above soil in the forest medium and in free air for monoenergetic photon sources with energies 0.1 MeV, 0.66 MeV and 3 MeV did not correlate significantly with the energy of photons. Its maximum value in forest medium with biomass density of 5 kg m^-3 was 15-20% for the source at soil depth ∼0.3 g cm^-2, decreasing to less than 5% when it is at soil depth greater than 7 g cm^-2. An example calculation of the air kerma rate dynamics is presented for the initial period after radioactive fallout considering weathering processes (rainfall and wind action) that contribute to the transfer of activity from the canopies to the forest floor. The differences in air kerma rate values, as an integral characteristic of the gamma radiation field from a radioactive cloud in the forest and in the open area, were evaluated.