An approach to reduction of uncertainties in internal doses reconstructed for the Techa River population

A methodology was developed for reduction of uncertainties in estimates of internal dose for residents of the Techa Riverside communities, who were exposed as a result of releases of radionuclides from the Mayak plutonium production facility in 1949-56. The 'Techa River Dosimetry System' (TRDS) was specifically elaborated for reconstruction of doses. A preliminary analysis of uncertainty for doses estimated using the current version of the TRDS showed large ranges in the uncertainty of internal absorbed dose and led to suggestions of methods to reduce uncertainties. The new methodological approaches described in this paper will allow for significant reduction of uncertainties of 90Sr-dose. The major sources of reduction are: making use of individual measured values of 90Sr and through development of a Household Registry to associate unmeasured persons with measured persons living in the same household(s).     

In Situ Methods for Quantifying Specific Radionuclides

Field spectrometry methods utilizing high resolution detectors can be used to quantify the concentration of radionuclides in soil, and the resulting external exposure rate. Advantages of the method compared to soil sampling followed by laboratory analysis are greater speed of analysis, integration of inhomogeneities of radionuclide areal distribution, and the immediate availability of data to guide further field studies. A disadvantage is that the calculation of results depends upon some knowledge or assumption about the distribution of radionuclides with depth in soil. The procedures necessary to calibrate and use a field spectrometer are discussed, and a practical guide is developed. Examples of uses of field spectrometry are the quantitation of complex mixtures of radionuclides in soil, the calculation of the resulting external gamma exposure rate apportioned by radionuclide, geochemical studies, preoperational and continuing studies at nuclear reactor sites, and the identification of short-lived, noble gas radionuclides in reactor plumes.     

The Significance of Tritium Releases to the Environment

Tritium is produced naturally and was present in low concentrations in precipitation and natural bodies of water before atmospheric testing of nuclear weapons. Other sources of tritium are now present from which tritium is released to the environment. Nuclear reactor tritium production, according to recent estimates, will equal natural tritium production before the year 2000. Predicted increases of tritium in the environment will take place first on a local ecological level and then appear on a biospheric level. Tritium introduced into the environment as THO will move through ecological systems in the same manner as stable water. Tritium will enter the hydrologic cycle either via evapo-transpiration or the surface bodies of water. Ecological experiments have been conducted to determine the movement of tritium in the environment. Field-grown plants were exposed to liquid and vapor THO for periods of one-half and one hours. Tritium concentrations were determined in leaf samples collected after exposure for periods of time up to 45 days. Tritium decays rapidly in the plant species studied and exhibited a three component half-life when plants were exposed to THO vapor. The length of exposure, and sources of THO in the soil affect the half-time of tritium in the plant tissues. Data produced in ecological experiments on tritium movement are used in a theoretical consideration of acute and chronic vapor releases of tritium in an agricultural environment.     

Chernobyl accident: reconstruction of thyroid dose for inhabitants of the Republic of Belarus

The Chernobyl accident in April 1986 resulted in widespread contamination of the environment with radioactive materials, including (131)I and other radioiodines. This environmental contamination led to substantial radiation doses in the thyroids of many inhabitants of the Republic of Belarus. The reconstruction of thyroid doses received by Belarussians is based primarily on exposure rates measured against the neck of more than 200,000 people in the more contaminated territories; these measurements were carried out within a few weeks after the accident and before the decay of (131)I to negligible levels. Preliminary estimates of thyroid dose have been divided into 3 classes: Class 1 ("measured" doses), Class 2 (doses "derived by affinity"), and Class 3 ("empirically-derived" doses). Class 1 doses are estimated directly from the measured thyroidal (131)I content of the person considered, plus information on lifestyle and dietary habits. Such estimates are available for about 130,000 individuals from the contaminated areas of the Gomel and Mogilev Oblasts and from the city of Minsk. Maximum individual doses are estimated to range up to about 60 Gy. For every village with a sufficient number of residents with Class 1 doses, individual thyroid dose distributions are determined for several age groups and levels of milk consumption. These data are used to derive Class 2 thyroid dose estimates for unmeasured inhabitants of these villages. For any village where the number of residents with Class 1 thyroid doses is small or equal to zero, individual thyroid doses of Class 3 are derived from the relationship obtained between the mean adult thyroid dose and the deposition density of (131)I or 137Cs in villages with Class 2 thyroid doses presenting characteristics similar to those of the village considered. In order to improve the reliability of the Class 3 thyroid doses, an extensive program of measurement of (129)I in soils is envisaged.