3. RADIOACTIVITY IN THE DNIEPER RIVER BASIN

Radioactivity is the property of unstable atoms (called ‘radionuclides’) that spontaneously disintegrate with emission of radiation. Everyone is exposed to radiation from radioactivity in the natural environment. In addition, human activities involving the use of radiation and radioactive substances cause radiation exposure. Some of these activities, such as the mining of radioactive ores and the burning of coal containing radioactive substances, enhance exposure to natural radiation. Nuclear power plants and other nuclear installations release radioactivity into the environment and produce radioactive waste, which is a potential source of radiation exposure. Another source is the use of radiation and radioisotopes in medicine, industry and research. The medical use of radiation is the largest human-made source of radiation exposure [3.1].

Exposure to ionizing radiation can damage living organisms and cause health effects in humans, including leukaemia and other cancers. The effects of radiation on human health are discussed in Section 7.

This report deals only with the assessment of those sources of radiation and radioactivity that are of special concern in the Dnieper River basin. The first task of the project team was to identify the main sources (actual and potential) of radiation exposure meeting this criterion. The identified sources are:

(a)Areas affected by the Chernobyl nuclear accident;

(b)Nuclear power plants;

(c)Uranium mining and ore processing;

(d)Radioactive waste storage and disposal sites;

(e)Non-power sources (e.g. from the use of radiation and radioisotopes in medicine, industry and research).

Figure 3.1 shows the locations of the most important sources. Each is assessed in detail in separate sections of this report. The following sections give a brief introduction to each of these sources.

3.1.AREAS AFFECTED BY THE CHERNOBYL NUCLEAR ACCIDENT

The Chernobyl nuclear power plant is located alongside the Pripyat River in northern Ukraine, about 130 km north-east of Kiev. It is 12 km from the border with Belarus and 140 km from the border with the Russian Federation (see Fig. 3.1). On 26 April 1986 the worst ever nuclear accident occurred at unit 4 of the plant. Following a criticality excursion, two major steam explosions destroyed the reactor and badly damaged the reactor building and other structures (see Fig. 3.2). A major release of radioactivity occurred as a result of the explosions. Subsequent burning of the graphite moderator resulted in continued release of radioactivity over a period of ten days. Overall, about 50% of the 131I and 30% of the 137Cs in the reactor core were released [3.1, 3.2].

The fallout of this radioactivity was dependent on the vagaries of the wind direction and rainfall over the period of the releases. The most serious consequences of the Chernobyl accident to the public were caused by exposure to short lived radionuclides, especially 131I, which resulted in many thyroid cancers [3.1]. Other health effects are expected in the future from the exposures received by some individuals during the accident phase. A large number of studies have been carried out on the health effects arising from exposure of reactor personnel, emergency workers and the general public during the immediate period after the accident [3.1, 3.3–3.8].

This assessment is concerned mainly with current and future exposures to radiation. Of those radionuclides still remaining from the Chernobyl accident, 137Cs (half-life 30 years) and 90Sr (half-life 29.1 years) are the most important from an environmental and public health perspective. Caesium-137, with its short lived daughter, 137mBa, emits beta and gamma radiation; 90Sr, with its short lived daughter, 90Y, emits beta radiation.

Caesium is volatile at the high temperatures that were experienced during the Chernobyl accident. Consequently, it tended to travel substantial distances before being deposited. Both in the environment and in the human body caesium radionuclides behave like potassium. However, strontium is not particularly volatile and was mainly

associated with fuel particles deposited much closer to the release point. In the environment and in the human body, strontium radionuclides behave like calcium (hence strontium is a ‘bone seeker’).

The area in the immediate vicinity of the Chernobyl nuclear power plant was the most contaminated. In 1986 the 30 km Chernobyl exclusion zone (CEZ) was established around the Chernobyl nuclear power plant and the public was evacuated from the area. Within the CEZ are a number of important sources:

(a)The damaged nuclear reactor. In May 1986 a decision was taken to enclose the area around unit 4 to prevent the further spread of radioactivity into the environment and to reduce the exposure of personnel working on the Chernobyl nuclear power plant site. An

enclosing building, known as the shelter or ‘sarcophagus’, was completed in November 1986 (see Fig. 3.3)It was erected under difficult circumstances in very high radiation fields using remote construction methods. Unfortunately, as a result of construction difficulties, there are now openings and breaks in the walls and roof of the shelter, which is estimated to total about 1200 m2 in area.

(b)The Chernobyl cooling pond. The cooling pond is an artificial lake built to provide the cooling water for the condensers of the four Chernobyl reactor units. The pond covers an area of approximately 23 km2 and contains approximately 149 × 106 m3 of water. The cooling pond is less than 1 km from the Chernobyl nuclear power plant; a dam separates the pond from the Pripyat River.

FIG. 3.2. The damaged unit 4 of the Chernobyl nuclear power plant (from Ref. [3.2]).

FIG. 3.3. Construction of the Chernobyl shelter (from Ref. [3.2]).

The pond was heavily contaminated during the Chernobyl accident and by subsequent dumping of radioactive liquid waste into it.

(c)Waste burial sites. After the Chernobyl accident, contaminated material, including debris, structures, equipment, dead trees and contaminated soil, were buried within the

CEZ in trenches and under mounds. The purpose was to reduce radiation levels near the Chernobyl nuclear power plant and to prevent dispersion of radioactivity. Leaching into groundwater and migration from these sites is a potential source of contamination of waterways.

(d)Contaminated floodplain. The floodplain along the Pripyat River is highly contaminated from the Chernobyl accident, especially with

90Sr; concentrations exceed 4000 kBq/m2 over large areas. This area is regularly inundated, especially during spring floods. Moreover, some of the waste burial sites are located within the floodplain. Engineering works have been undertaken to mitigate the flooding but have not been completed due to financial problems. Radioactivity, especially 90Sr, is washed off the floodplain during times of high flood and transported via the Pripyat River to the Dnieper system.

Although the deposition of radioactivity was highest in the CEZ, significant fallout occurred throughout much of Europe. However, deposition was greatest in the three countries (Belarus, Russian Federation and Ukraine) that lie within the Dnieper River basin. Figure 3.1 shows the areas of highest contamination. On the territory of Belarus the worst affected areas are the Gomel and Mogilev regions. Within the Russian Federation the southwest part of the Bryansk region is the most affected. In Ukraine contamination is particularly high in the Kiev, Zhytomyr and Chernihiv regions. In total, about 85 000 km2 of the Dnieper River basin received a surface contamination of 137Cs above 37 kBq/m2 (1 Ci/km2)1. Section 4 gives detailed information on the deposition of radioactivity.

One of the more serious long term ecological effects of the Chernobyl fallout was the secondary runoff (wash-off) of radionuclides from the initially contaminated areas through the river networks of Belarus, the Russian Federation and Ukraine into the Dnieper reservoir system, thereby expanding the spatial scale of the accident and exposing

1 Radioactivity is measured in units of becquerel (Bq), which is one disintegration per second. An older unit called the curie (Ci), equivalent to 3.7 × 1010 Bq, is still in common use. The becquerel is a very small unit and hence large units are commonly used, for example 1 kBq = 103 Bq, 1 MBq = 106 Bq, 1 GBq = 109 Bq, 1 TBq = 1012 Bq and 1 PBq = 1015 Bq.

millions of people who use the downstream resources of the Dnieper River. Section 4 discusses the water-borne dispersal of radionuclides in detail.

3.2. NUCLEAR POWER PLANTS

There are 17 operating nuclear power reactors in the Dnieper River basin (Fig. 3.1), ten in Ukraine at three sites (Zaporozhe, Rovno and Khmelnitski) and seven in the Russian Federation at two sites (Kursk, Smolensk). In addition, there are three nuclear power reactors (South Ukraine nuclear power plant) in the Bug River basin. The Zaporozhe nuclear power plant, comprising six 1000 MW(e) units, is one of the largest in the world (see Fig. 3.4). Currently, one 1000 MW(e) nuclear power reactor is being commissioned at Rovno, while another is under construction at Khmelnitski.

Nuclear power plants can release radioactivity to the environment in a number of ways:

(a)By routine releases to air and water;

(b)By releases from spent nuclear fuel storage facilities;

(c)By releases from radioactive waste storage facilities;

(d)By transport accidents;

(e)By major accidents affecting the nuclear core, where releases are difficult to control.

Under normal conditions, nuclear power plants release small amounts of radioactivity into the air and sometimes into cooling water systems. However, such releases do not result in significant

FIG. 3.4. Zaporozhe nuclear power plant and the cooling pond and Kakhovka reservoir on the Dnieper River.

radiation exposures of the general public. Data on normal releases from nuclear reactors in the Dnieper River basin are presented in Section 5.

Radioactive material resulting from reactor operations (such as spent nuclear fuel and radioactive waste) is stored in specially designated facilities that are subject to regular inspection by regulatory authorities. Release of radioactivity is possible, especially from liquid waste storage facilities, but would normally require a breakdown in a number of barriers. Monitoring systems are designed to detect any release of radioactivity at an early stage.

Accidents can occur during transport, but transport containers are designed to withstand the most serious credible accidents. Worldwide, the nuclear industry has a very safe record in the transport of nuclear material.

A major reactor accident (such as occurred at Chernobyl) can have very serious consequences. The adequacy of ongoing measures taken to prevent nuclear accidents in the Dnieper River basin is assessed in Section 5, while Section 9 reports on the consequences of a hypothetical accident affecting the Dnieper River basin and the Black Sea.

3.3. URANIUM MINING AND PROCESSING

The only uranium mining and ore processing in the Dnieper River basin is in Ukraine. Uranium exploration started in 1944 and led to the discovery of 21 deposits. Many of the deposits are within the watershed of the Dnieper River basin, while some are within the basins of the southern Bug and Seversky Donets Rivers. Figure 3.1 shows the locations of the deposits and the ore processing operations. The effects of uranium mining and processing are localized and only affect the Dnieper River basin in southern Ukraine.

The first uranium processing plant in Ukraine was the Prydniprovsky chemical plant, which started up in 1948 using ores shipped from countries in central Europe. It is situated a few kilometres from the Dnieper River in the city of Dniprodzerzhinsk and ceased operations in 1991.

The Zhovti Vody hydrometallurgical plant commenced production in January 1959 to process ores from the region. Current production is about 1000 t U/a. Most of the current production comes from the Ingulsky mine developed on the Michurynske deposit.