Radioactivity in Soils (Cesium-134 and Cesium-137) 1991
The reactor accident at Chernobyl on 26 April 1986 dramatically demonstrated the dangers involved in man-made radioactivity. The Chernobyl catastrophe was incomparably larger than the reactor accident in Harrisburg, USA in 1979. Chernobyl released enormous amounts of radioactive substances. Wind and weather dynamics transported these substances and deposited them onto the earth as radioactive dust and rain (fallout, washout).
Sources of Natural Radiation
Natural sources of radiation are cosmic radiation and terrestrial radiation arising from the decay of naturally occurring radioactive substances. Two hundred years of industrialization has produced and redistributed increasing amounts of radioactive matter. Industrialization releases additional radiation through mining (especially uranium), coal combustion, cement production, street construction and other human activities. Radioactive elements defined by the number of protons in their nuclei are differentiated from their isotopes. An isotope is formed by the penetration of sub-atomic particles such as neutrons into the nucleus, resulting in a new, usually unstable, nucleus. These natural radionuclides are background loads, distributed over great areas. They enter the human body through foodstuffs, drinking water and air. This background level in Germany varies as much as 20%, depending on the geological nature of soils and the altitude of residency. Figure 1 presents some naturally occurring radioactive isotopes.
Sources of Man-made Radiation
The atmospheric nuclear weapons tests of the 50’s and 60’s were the primary causes of high levels of global fallout. Radioactive materials were diffused depending on the extent and form of the emissions, and are found even in areas otherwise uninfluenced by human activity.
Man-made radioactivity is further differentiated between medical diagnosis and therapy and nuclear technology facilities. Nuclear fission in a nuclear reactor produces a range of radioactive matter, the fission products. The exact nature and composition of these often long-lived products depends on the type of reactor, its operational life, and its rate of utilization. Knowledge of these factors enable clear statements to be made about the emissions from the Chernobyl accident. The primary emissions were radioactive iodine and cesium. Other emissions were strontium, molybdenum, barium, and a less familiar element, ruthenium. The cesium-137 isotope is the most important nuclide, both because it is so long-lived and because of its percentual proportion in the Chernobyl spectrum of emissions (cf. Tab. 1).
The effects on health caused by ionizing radiation are of particular interest. These complex interrelations are still not completely clear today. In simplified form, the interrelations can be described as follows: radioactive decay transforms instable nuclides into stable isotopes by either emitting particles (electrons, positrons, neutrons) and/or electromagnetic radiation (photons). The energy of radiation is of great importance for dangers to health. After the emission of particles, new nuclides are often formed, which can also decay.
The activity of a radioactive substance is measured in Becquerel (Bq) units. It expresses the number of nuclear disintegrations per second. 1 Bq signifies one disintegration per second. Half-life is the time required for the decay process to reduce activity by one-half. Half-life thus quantifies the time-period of higher activity and accumulation of that element in the environment. Activity is not the only physical factor that must be known in order to judge the effects of ionizing radiation (load charge transmission) on the human body. The fundamental danger of ionizing radiation is that changes in the cells can cause cancer or genetic damage. In order to judge these dangers in individual cases, other physical qualities must be taken into consideration. The effective equivalent dose, measured in Sievert (Sv), has a special significance. The Sievert attempts, by means of conversion factors, to quantitatively express the different biological effects of different kinds of radiation on individual organs of the human body – and in respect to their differing susceptibility to radiation. This is used as a basis in radiation protection regulations for determining limit values, assuming the average foodstuff consumption habits of a healthy adult. The determination of limit values also differentiates between persons exposed to radiation in their professions, and the general population.
Development of Contamination by Man-made Radioactivity
Investigations have deepened knowledge of the global spread and distribution of radionuclides in various areas of the biosphere ever since nuclear weapons tests began releasing man-made radionuclides into the atmosphere.
A measurement series to determine average radioactive contamination in the air in Berlin was conducted for many years. Average man-made radiation values amount to less than 10% of natural levels (5-7 Bq/m3) even in periods of high contamination. Peak values for individual days, such as those reached after the reactor accident in Chernobyl, however, can be considerably above these values (cf. Fig. 4). Figure 2 shows a clear link between airborne contamination and atmospheric nuclear tests. Values in Berlin are to be interpreted as delayed effects, appearing in Berlin about one year after the nuclear tests. The length of time nuclides remained in higher levels of the atmosphere is an important factor. There were particularly high contamination levels 1963, caused by the nuclear weapon tests of 1961/62, when high-yield hydrogen bombs were detonated in the atmosphere.
Radioactivity measured in the 70’s was due to tests by China and France, carried out in spite of the test ban. There was a drop-off to a very low level in the 80’s. The level of man-made radiation in 1963 was a thousand times greater than from 1982 to 1985. Contamination values were driven up again only with the Chernobyl incident in 1986. Within a few days, a single event caused contamination levels over a large portion of Europe that are comparable only to the effects of the nuclear tests of 1961/62. Washout by rain and settling (sedimentation) led to a reduction of airborne contamination to “pre-Chernobyl-levels” by 1986.
Course of the Chernobyl Reactor Incident
At Chernobyl on 26 April 1986 at 1:23 a.m. local time, a sudden loss of performance occurred in one reactor core of a block. A fire in the reactor and the ensuing high temperatures carried released nuclides to an altitude of 1,500 m. The altitude of emissions, wind direction and speed resulted in the airborne transport and deposition of radioactive materials over several thousand kilometers, including the Berlin area. Emission was finally stopped on 13 May 1986. The airborne radioactivity reached Berlin on 30 April 1986 and produced the first outstanding peak level of contamination.
Figure 3 shows the diffusion of particles emitted on 29 April. The first two days of prevailing east winds carried emissions into Hungary and Austria. On 2 May the direction changed and south winds transported contaminants over large portions of southern Germany. Radionuclides in this air mass reached Berlin on 4 May and produced a second peak level in measurements of the air (cf. Fig. 4).
The release and deposit of radioactive substances from the burning reactor initially endangered the population through inhalation of contaminated air. But the long-term processes – such as through deposition, are also significant. They are to be interpreted in relation to the environmental media of soil, ground and surface water, and foodstuffs. Soil is a special factor because it is the most important medium of departure for all storage transfer processes.