Evaluation of dose equivalent rate reduction as a function of vertical migration of Cs-137 in contaminated soils

Following a major nuclear accident it is of great importance to apply rehab ilitation actions for the recovery of vast contaminated territories. In ord er to achieve this objective, it is essential to know the medium- and long -term evolution of the contamination and to evaluate how the dose equivalen t rate (DER) of the gamma-radiation changes with the migration of the radio nuclides in the soil. The aim of this paper is to define the modification of the DEF: as a result of Cs-137 penetration into the soil. Information is used which is related to the character of the Cs-137 distributions in different soil types and to Cs-137 vertical migration velocity (cm y(-1)) as a function of the types o f soils. Indeed, since 1986 when the Chernobyl accident occurred, soil prof iles were studied in undisturbed land in the region of Gomel, Belarus. The Cs-137 content of soil was assayed in the laboratory by gamma-spectrometry and the results obtained are classified into groups according to the type o f soil and the 137Cs migration velocity. The average DER was calculated as a function of Cs-137 vertical migration f or the above groups and for different levels of contamination ranging from 185 to 7400 kBq m(-2). In order to validate the theoretical computations, a comparison between measured in situ and calculated DERs was carried out. A good correlation was found between measured and calculated DERs. The data obtained show that, ten years after the accident in the studied te rritory, a significant reduction of the DER occurred and it ranges-relative to 1986 when 100% of the Cs-137 was distributed on the soil surface-from 1 .7.5% in the less contaminated soil with low Cs-137 migration velocity (< 0 .25 cm y(-1)) to 45% in the most contaminated soils with high Cs-137 migrat ion velocity (> 1.2 cm y(-1)). (C) 1999 Elsevier Science Ltd. All rights re served.