Massive contamination of agricultural lands by radionuclides occurs, as a rule, due to large accidents at the enterprises of nuclear industry or nuclear power stations. Such accidents occurred at Windscale (England) and South Ural (Russia). The largest accident happened in April 1986 at the Chernobyl nuclear power station. The latter entailed intensive contamination of soils over several thousands of square kilometres.
The major factors contributing to radiation effects in agricultural areas are as follows:
- whether radiation is from a single or a long-term exposure
- total quantity of radioactive substances entering the environment
- ratio of radionuclides in the fallout
- distance from the source of radiation to agricultural lands and settlements
- hydrogeological and soil characteristics of agricultural lands and the purpose of their use
- peculiarities of work of the rural population; diet, water supply
- time since the radiological accident.
As a result of the Chernobyl accident more than 50 million Curies (Ci) of mostly volatile radionuclides entered the environment. At the first stage, which covered 2.5 months (the “iodine period”), iodine-131 produced the greatest biological hazard, with significant doses of high-energy gamma radiation.
Work on agricultural lands during the iodine period should be strictly regulated. Iodine-131 accumulates in the thyroid gland and damages it. After the Chernobyl accident, a zone of very high radiation intensity, where no one was permitted to live or work, was defined by a 30 km radius around the station.
Outside this prohibited zone, four zones with various rates of gamma radiation on the soils were distinguished according to which types of agricultural work could be performed; during the iodine period, the four zones had the following radiation levels measured in roentgen (R):
- zone 1—less than 0.1 mR/h
- zone 2—0.1 to 1 mR/h
- zone 3—1.0 to 5 mR/h
- zone 4—5 mR/h and more.
Actually, due to the “spot” contamination by radionuclides over the iodine period, agricultural work in these zones was performed at levels of gamma irradiation from 0.2 to 25 mR/h. Apart from uneven contamination, variation in gamma radiation levels was caused by different concentrations of radionuclides in different crops. Forage crops in particular are exposed to high levels of gamma emitters during harvesting, transportation, ensilage and when they are used as fodder.
After the decay of iodine-131, the major hazard for agricultural workers is presented by the long-lived nuclides caesium-137 and strontium-90. Caesium-137, a gamma emitter, is a chemical analogue of potassium; its intake by humans or animals results in uniform distribution throughout the body and it is relatively quickly excreted with urine and faeces. Thus, the manure in the contaminated areas is an additional source of radiation and it must be removed as quickly as possible from stock farms and stored in special sites.
Strontium-90, a beta emitter, is a chemical analogue of calcium; it is deposited in bone marrow in humans and animals. Strontium-90 and caesium-137 can enter the human body through contaminated milk, meat or vegetables.
The division of agricultural lands into zones after the decay of short-lived radionuclides is carried out according to a different principle. Here, it is not the level of gamma radiation, but the amount of soil contamination by caesium-137, strontium-90 and plutonium-239 that are taken into account.
In the case of particularly severe contamination, the population is evacuated from such areas and farm work is performed on a 2-week rotation schedule. The criteria for zone demarcation in the contaminated areas are given in table 1.
Table 1. Criteria for contamination zones
|
Contamination zones |
Soil contamination limits |
Dosage limits |
Type of action |
|
1. 30 km zone |
– |
– |
Residing of |
|
2. Unconditional |
15 (Ci)/km2 |
0.5 cSv/year |
Agricultural work is performed with 2-week rotation schedule under strict radiological control. |
|
3. Voluntary |
5–15 Ci/km2 |
0.01–0.5 |
Measures are undertaken to reduce |
|
4. Radio- ecological |
1–5 Ci/km2 |
0.01 cSv/year |
Agricultural work is |
When people work on agricultural lands contaminated by radionuclides, the intake of radionuclides by the body through respiration and contact with soil and vegetable dusts may occur. Here, both beta emitters (strontium-90) and alpha emitters are extremely dangerous.
As a result of accidents at nuclear power stations, part of radioactive materials entering the environment are low-dispersed, highly active particles of the reactor fuel—“hot particles”.
Considerable amounts of dust containing hot particles are generated during agricultural work and in windy periods. This was confirmed by the results of investigations of tractor air filters taken from machines which were operated on the contaminated lands.
The assessment of dose loads on the lungs of agricultural workers exposed to hot particles revealed that outside the 30 km zone the doses amounted to several millisieverts (Loshchilov et al. 1993).
According to the data of Bruk et al. (1989) the total activity of caesium-137 and caesium-134 in the inspired dust in machine operators amounted to 0.005 to 1.5 nCi/m3. According to their calculations, over the total period of field work the effective dose to lungs ranged from 2 to
70 cSv.
The relation between the amount of soil contamination by caesium-137 and radioactivity of work zone air was established. According to the data of the Kiev Institute for Occupational Health it was found that when the soil contamination by caesium-137 amounted to 7.0 to 30.0 Ci/km2 the radioactivity of the breathing zone air reached 13.0 Bq/m3. In the control area, where the density of contamination amounted to 0.23 to 0.61 Ci/km3, the radioactivity of work zone air ranged from 0.1 to 1.0 Bq/m3 (Krasnyuk, Chernyuk and Stezhka 1993).
The medical examinations of agricultural machine operators in the “clear” and contaminated zones revealed an increase in cardiovascular diseases in workers in the contaminated zones, in the form of ischaemic heart disease and neurocirculatory dystonia. Among other disorders dysplasia of the thyroid gland and an increased level of monocytes in the blood were registered more frequently.
Hygienic Requirements
Work schedules
After large accidents at nuclear power stations, temporary regulations for the population are usually adopted. After the Chernobyl accident temporary regulations for a period of one year were adopted, with the TLV of 10 cSv. It is assumed that workers receive 50% of their dose due to external radiation during work. Here, the threshold of intensity of radiation dose over the eight-hour work day should not exceed 2.1 mR/h.
During agricultural work, the radiation levels at workplaces can fluctuate significantly, depending on the concentrations of radioactive substances in soils and plants; they also fluctuate during technological processing (siloing, preparation of dry fodder and so on). In order to reduce dosages to workers, regulations of time limits for agricultural work are introduced. Figure 1 shows regulations which were introduced after the Chernobyl accident.
Figure 1. Time limits for agricultural work depending on intensity of gamma-ray radiation at workplaces.
Agrotechnologies
When carrying out agricultural work in conditions of high contamination of soils and plants, it is necessary to strictly observe measures directed at prevention of dust contamination. The loading and unloading of dry and dusty substances should be mechanized; the neck of the conveyer tube should be covered with fabric. Measures directed at the decrease of dust release must be undertaken for all types of field work.
Work using agricultural machinery should be carried out taking due account of cabin pressurization and the choice of the proper direction of operation, with the wind at the side being preferable. If possible it is desirable to first water the areas being cultivated. The wide use of industrial technologies is recommended so as to eliminate manual work on the fields as much as possible.
It is appropriate to apply substances to the soils which can promote absorption and fixation of radionuclides, changing them into insoluble compounds and thus preventing the transfer of radionuclides into plants.
Agricultural machinery
One of the major hazards for the workers is agricultural machinery contaminated by radionuclides. The allowable work time on the machines depends on the intensity of gamma radiation emitted from the cabin surfaces. Not only is the thorough pressurization of cabins required, but due control over ventilation and air conditioning systems as well. After work, wet cleaning of cabins and replacement of filters should be carried out.
When maintaining and repairing the machines after decontamination procedures, the intensity of gamma radiation at the outer surfaces should not exceed 0.3 mR/h.
Buildings
Routine wet cleaning should be done inside and outside buildings. Buildings should be equipped with showers. When preparing fodder which contains dust components, it is necessary to adhere to procedures aimed at prevention of dust intake by the workers, as well as to keep the dust off the floor, equipment and so on.
Pressurization of the equipment should be under control. Workplaces should be equipped with effective general ventilation.
Use of pesticides and mineral fertilizers
The application of dust and granular pesticides and mineral fertilizers, as well as spraying from aeroplanes, should be restricted. Machine spraying and application of granular chemicals as well as liquid mixed fertilizers are preferable. The dust mineral fertilizers should be stored and transported only in tightly closed containers.
Loading and unloading work, preparation of pesticide solutions and other activities should be performed using maximum individual protective equipment (overalls, helmets, goggles, respirators, rubber gauntlets and boots).
Water supply and diet
There should be special closed premises or motor vans without draughts where workers can take their meals. Before taking meals workers should clean their clothes and thoroughly wash their hands and faces with soap and running water. During summer periods field workers should be supplied with drinking water. The water should be kept in closed containers. Dust must not enter containers when filling them with water.
Preventive medical examinations of workers
Periodic medical examinations should be carried out by a physician; laboratory analysis of blood, ECG and tests of respiratory function are compulsory. Where radiation levels do not exceed permissible limits, the frequency of medical examinations should be not less than once every 12 months. Where there are higher levels of ionizing radiation the examinations should be carried out more frequently (after sowing, harvesting and so on) with due account of radiation intensity at workplaces and the total absorbed dose.
Organization of Radiological Control over Agricultural Areas
The major indices characterizing the radiological situation after fallout are gamma radiation intensity in the area, contamination of agricultural lands by the selected radionuclides and content of radionuclides in agricultural products.
The determination of gamma radiation levels in the areas allows the drawing of the borders of severely contaminated areas, estimation of doses of external radiation to people engaged in agricultural work and the establishing of corresponding schedules providing for radiological safety.
The functions of radiological monitoring in agriculture are usually the responsibility of radiological laboratories of the sanitary service as well as veterinary and agrochemical radiological laboratories. The training and education of the personnel engaged in dosimetric control and consultations for the rural population are carried out by these laboratories.