Answer to Question #5487 Submitted to "Ask the Experts"

Q

I'm looking for radiation levels in Belarus today and radiation levels that existed in 1986. I want to see how they have risen or decreased over the years since the accident.

A

This question is more complex than it might seem, because Belarus is a large country. The radiation levels at the time of the accident varied all the way from life threatening to trivial. A recent report from the Chernobyl Forum, "Environmental Consequences of the Chernobyl Accident and Their Remediation: Twenty Years of Experience," addresses the issues of concern here.

Fig. 3.6 on page 25 of this report shows the varying levels of ¹³⁷Cs deposition density for much of Belarus. The higher levels were within the 30-km zone from the Chernobyl Nuclear Power Plant and in the Gomel and Mogilev Oblasts (an oblast is a political/administrative unit similar to a state in the United States). The deposition of radionuclides in the capital city of Minsk was very small. Persons within the 30-km zone were evacuated and other persons in nearby areas were relocated at later times. If persons had not been evacuated or relocated, some of them might have received effective doses in excess of 500 mSv (50 rem).

Following the accident, the external dose rate experienced by the remaining population decreased rapidly (within days to weeks), due to the decay of short-lived radionuclides. This decrease with time is noted in Figs. 5.3 to 5.6 on page 104 of the report mentioned above. Much of the long-term concern has been with the radionuclides ¹³⁴Cs (half-life of about two years) and ¹³⁷Cs (half-life of about 30 years). Twenty years after the accident, the ¹³⁴Cs is essentially gone, and the remaining concern is with ¹³⁷Cs. The long-term decrease in external dose rate due to ¹³⁷Cs is shown in Fig. 5.7 on page 105. Note that this dose rate is decreasing faster than due to radioactive decay because the ¹³⁷Cs is weathering into the soil, and soil offers shielding for the gamma ray emitted following the decay of ¹³⁷Cs.There has also been long-term concern with the radiation dose from the ingestion of ¹³⁷Cs.

There has also been long-term concern with the radiation dose from the ingestion of ¹³⁷Cs with foods (milk, meat, etc.). There has been significant variation in the uptake of cesium by plants due to varying soil types. In general about 80% of the dose to be delivered via this pathway (soil to plants to animals to humans) was delivered during the first 20 years. Thus, about 20% of the ultimate dose is yet to be received. The decrease with time of ¹³⁷Cs in various foods is shown in Figs. 3.24 and 3.25.

There are still some areas in Belarus, Ukraine, and Russia where the annual dose exceeds 1 mSv (100 mrem).

The clearly demonstrated health effects following the accident are an increase in the diagnosis of thyroid cancer in persons who were children at the time of the accident. This increase is presumed to be due to the ingestion of ¹³¹I with food (mainly milk) within weeks following the accident. Because ¹³¹I has a half-life of only eight days, exposure to the thyroid from ¹³¹I was complete within a few weeks.

Much more information is contained in the Chernobyl Forum report. I encourage you to download this report for information that is considered to be a consensus of many international and local (Belarus, Ukraine, and Russia) experts. Data are included on additional radionuclides, but radioiodine and radiocesiums were the more important.

Lynn Anspaugh