Model of External Exposure of Population Living in the Areas Contaminated After the Chernobyl Accident and its Validation

INTRODUCTION Models of external exposure for the population living in contaminated areas developed before the Chernobyl accident (1) were improved by taking into account post-Chernobyl data in Russia (2) and in Ukraine (3). Later, within the framework of the joint international EU-FSU research project JSP-5 «Pathway analysis and dose distributions», a general model (JSP-5 model) applicable to the external exposure of the population in Russia, Ukraine and Belarus was developed (4). The developed model is assumed to be applicable from the year 1990 to end of the lifetime of people being born before the reactor accident of Chernobyl. In this time period Cs and Cs almost completely determine the field of gamma radiation. However, in the first weeks after the accident the gamma radiation field was determined by short-lived radionuclides. So, about 80% of the absorbed dose in air during the first month after the accident was determined by gamma radiation of radionuclides Te + I, I and Ba + La. Only since August 1986, 80% were attributed to gamma radiation of long-lived Cs and Cs, and beginning from 1987 this contribution was already more than 95%. Thus, the detailed radionuclide composition of depositions was a major parameter defining the external exposure of the population in an early period of time after an accident. Another relevant parameter was the dependence of location factors on time due to migration processes of radionuclides during this period. In the present article, the JSP-5 model was extended to earlier times after the accident, beginning at the end of radioactive depositions. Distributions of individual external doses were modelled with the same equations as in the deterministic JSP-5 model. Parameters of the model were varied to allow receiving distributions of individual doses due to external exposure. Validation of model predictions is one of the most important stages of the model development. The term "model validation" refers to the process of testing model predictions against data that are independent from those used to develop the model. As such data, we used the results of measurements of kerma rates in air (4) and individual external doses of inhabitants obtained with the TL dosimetry method (5).