Gamma dose rate in air due to natural and anthropogenic radionuclides in soil

Abstract

Radioactive isotopes located in soil, both Naturally Occurring Radionuclides(NORM) and anthropogenic, contribute significantly to the total received dose in the outdoor environment. Among the geogenic sources (uranium, thorium and potassium) 40K is by far the major contribution to this dose. On average, radiopotassium is responsible for most than 95% of the gamma emission of all radionuclides present in the soil. The anthopogenic fission product, mainly isotopes of cesium, does also contribute to environmental radioactivity. Both 134Cs and 137Cs were introduced to terrestrial environments during nuclear aerial weapon tests (late 1950s-early 1960s) as much as from Chernobyl (1986) and Fukushima Dai-ichi (2011) Nuclear Power Plant accidents.For a particular region of the Earth the dose contribution due to NORM remainsapproximately constant. Instead, for radionuclides located in soil due to the fallout radioactive, the dose rate values are attenuated over time due to the radioactive decay and the soil downward migration.The assessment of the air irradiation originated by radionuclides in soil constitutes an extremely complex subject due to the large number of environmental factors which affect the gamma photon flux from the source until the receptor. Analytical and simulation model have been proposed in order to assess the dose rate in air as function of the source concentration and its position in the soil. This chapter is focused on the following issues:i) the assessment of the air dose rate attributable both to 134Cs and 137Cs soil activity concentration, ii) the dose rate temporal attenuation due to the radiocesium physics decay and its downward transport in soil, and iii) the comparison between the expected results from the theoretical approaches with dose rate measures in two places affected respectively by the Chernobyl and Fukushima Dai-ichi Nuclear Power Plants accidents.