(90) Sr, (137) Cs In Cereals, Vegetables And Other Plant Interception And Translocation In Experimental Research

The air-borne radionuclide deposits on plants, and then into human food-chain through interception and translocation which is very important to make contribution to the public dose. This pathway is generally considered in the dose models for nuclear environmental assessment.When materials deposit on plants from the atmosphere, whether by wet or dry deposition processes, some fraction of the materials are intercepted by vegetation, with the remainder reaching the ground. Radionuclide deposited on crop surfaces (e.g. leaves), which is discharged from nuclear facilities, both in normal and in accidental operating conditions, will be intercepted by vegetation and transferred into other plant organs (e.g. fruit) during further growth. This process is described by interception coefficient f and translocation coefficient TLFa (or TLFy). Interception coefficient f is referred to as the fraction of deposited activity intercepted and initially retained by vegetation. Translocation coefficient TLFa based on area (m²/m²) (or TLFy based on yield, m²/kg) is defined as the ratio of the activity in the edible parts in 1 m2 soil area (or activity concentration in edible parts, Bq/kg) at harvest time and the activity initially retained on foliage of the same area (Bq/m²).When Chernobyl nuclear accident happened in the former Soviet Union in 1986, tens of eastern countries were polluted by dispersion and fallout of radioactive particles and the diameter of the polluted particles were mainly in micrometers which could be transferred to locations thousands of kilometers away. Soon after the accident, IAEA and CEC co-organized the Research Program on "The Validation of Models for the Transfer of Radionuclide in Terrestrial, Urban and Aquatic Environments and the Acquisition of Data for that Purpose" (the short title is "VAMP", 1988～1994). Then different dynamic food-chain models of radionuclide transfer in environments are established in different countries, including RODOS of European Economic Community, ECOSYS of Germany, PATHWAY of the United States, and FARMLAND of the England. Transfer factors related to radionuclide interception and translocation in plants, such as interception coefficient f, translocation coefficient TLFa and TLFy, are all inputs in the models. Then the behaviors of radioactive particles in dry or wet deposition have been drawing attention internationally. The IAEA Handbook (IAEA TRS-364, 1994) provides expected TLFy values with the time from deposition to harvest. The elements involved are Sr, Cs, I, Co,Fe, Sb, etc. There is no data from China in that Handbook.Related researches in China are still in preliminary stage. In the practice of radiation environmental impact assessment of nuclear facilities in China, for lack of Chinese experimental data in radionuclide interception and translocation on vegetation, related coefficient datum from foreign literatures have been adopted.The determination of translocation coefficient are associated with some uncertainty: the characteristics of climatic during the experimental period, plant species, plant diseases and insect pests, bio-availability of radionuclide, initial retention of deposited activity on vegetation, diameter of deposited particles, etc. Even tiny changes in plant development during deposition experiments may have effect on interception coefficient.China is in the Asia. There are differences in food chain, agricultural cultivation, crop varieties, climatic characteristics and soil types between China, European countries and the Americans. Thus, it's important to study the interception and translocation of radionuclide on plants in Chinese conditions..In this experiment, by using the aerosol simulation facility, the behavior of dry deposition and translocation of ⁹⁰Sr, ¹³⁷Cs aerosol with the diameter of micron meters on plants were studied in China for the first time. The plants involved were rice, winter wheat, spring wheat, radish, cabbage and tomato. The interception coefficent f (and f/B), translocation coefficient TLFa (and TLFy), and LAI varying with time were presented respectively. The interrelationship of plant's growth stage, biomass B, plant leaf area index (LAI) and f, TLFa were discussed. Following conclusion are drawn from this experiment:I. For cereals, the interception coefficient f of ⁹⁰Sr for rice and winter wheat are higher in mature period and jointing stage than that in seedling stage, and f is in direct ratio with plant's biomass B and the plant leaf area index LAI (with an exception in the mature period of rice and winter wheat, when LAI was not increased with higher f, as the leaves were dry and old in the period). The f of ¹³⁷Cs for cereals including rice, winter wheat and spring wheat are the highest in jointing stage and the lowest in seedling stage, and f is in direct ratio with plant's biomass B and the LAI. II. For vegetables, the f of ⁹⁰Sr for tomato and radish are higher in seedling stage than that in other growth stages. For cabbage the f of ⁹⁰Sr is higher in mature period than that in seedling stage. The f of ¹³⁷Cs for radish and cabbage are higher in mature period and lower in seedling stage, and the f of ¹³⁷Cs for cabbage is in direct ratio with biomass B and LAI at deposition time. The f of ¹³⁷Cs for tomato is higher in fruit stage than that in other period (for the results from the same year).III. The values of f/B are also presented in this paper. The results show that f/B for ⁹⁰Sr are the highest in seedling stage for cereals including rice, winter wheat and spring wheat, in the florescence for tomato, and in the seedling stage for radish and Chinese cabbage. The f/B for ¹³⁷Cs are the highest in seedling stage for cereals including rice, winter wheat and spring, in the florescence for tomato, and in the seedling stage for radish and cabbage. The results also showed that in the early growth stage of plant (e.g. seedling stage and florescence) when biomass is low, the values of f/B is high.IV. For cereals, the interception coefficient TLFa of ⁹⁰Sr for the grains of winter wheat and spring wheat are the highest in jointing stage, and the highest in seedling stage for the grains of rice. The results salso indicate that ⁹⁰Sr pollution in the soil have obvious effect on the rice's TLFa. The values of TLFa of ¹³⁷Cs for the grains of rice, winter wheat and spring wheat are the highest in the jointing stage. For cereals are growing abundantly in jointing stage, the values of TLFy are relatively higher too. Comparing to the experimental results of ⁹⁰Sr, the ¹³⁷Cs pollution in the soil has no obvious effect on TLFa values of rice.V. For vegetables, the TLFa of ⁹⁰Sr for tomato are higher both in florescence and in fruit period. The TLFa for radish and cabbage are higher in the seedling stage (for the results in the same year). The TLFa of ⁹⁰Sr for radish leaves is usually higher than that of radish root. Cabbage leave samples were washed at sampling time. Therefore the values of TLFa for cabbage responded the true uptake of ⁹⁰Sr in cabbage tissue. For ¹³⁷Cs, the TLFa values are the highest in the in florescence and fruit period for tomato, and in the seedling stage for radish and cabbage. The TLFa of ¹³⁷Cs for radish leaves are 7-84 times higher than that of radish root.VI. The least square method was applied to the linear fitting of the experimental datum. The results show that the interception coefficient f of ⁹⁰Sr and ¹³⁷Cs for all experimental plants (including cereals and vegetables) are in direct ratio with the biomass of vegetation B and the leaf area index LAI at deposition time, and there is direct correlation between f and B, LAI. The f/B of ⁹⁰Sr and ¹³⁷Cs for all experimental plants are in direct ratio with LAI at deposition time, and there is inverse correlation between f/B and LAI. For single crops, there are some exceptions (e.g., there is inverse correlation between the f of ⁹⁰Sr for spring wheat and radish and B, LAI at deposition time), which may be caused by differences in plant growth and experimental conduction during different years.Further research works should be focused on the influence of physiochemical properties of radionuclide, different deposition techniques of aerosol particles (such as simulated aerosol of nuclear accidental release, deposition of radioactive dust in nuclear sites, etc.), plant species, and meteorological factors (wind, rains, and so on) on interception and translocation of radionuclides on plants.