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Content archived on 2024-04-15

TIME-DEPENDENT TRANSFER OF RADIONUCLIDES FROM ATMOSPHERE AND SOIL TO CROPS, FOLLOWING SIMULATED REACTOR ACCIDENTS

Objective

ESTIMATES OF COLLECTIVE DOSE TO MAN VIA FOOD-CHAINS FOLLOWING ACCIDENTAL RELEASES FROM NUCLEAR REACTORS HAVE LARGELY BEEN BASED ON PREDICTIONS FROM STATIC MODELS WHICH DESCRIBE ENVIRONMENTAL TRANSFER UNDER EQUILIBRIUM SITUATIONS. SUCH ESTIMATES ARE OFTEN OVER-RESTRICTIVE AND CONSEQUENTLY THERE IS AN URGENT NEED FOR MORE REALISTIC VALUES BASED ON TIME-DEPENDENT TRANSFERS OF RADIONUCLIDES UNDER FIELD CONDITIONS. THESE WILL PROVIDE DATA TO ENABLE MORE PRECISE COST-BENEFIT ANALYSES TO BE PERFORMED ON SUCH CRITERIA AS LAND RE-UTILISATION, CROP USAGE AND MOVEMENT OF THE PUBLIC.
RESEARCH IN RECENT YEARS AT IMPERIAL COLLEGE HAS BEEN AIMED AT DETERMINING TIME-DEPENDENT TRANSFER FACTORS BETWEEN SOILS AND CROPS UNDER FIELD CONDITIONS. IN PARTICULAR, CEC SUPPORT UNDER THE 3RD RADIATION PROTECTION PROGRAMME HAS ENABLED THE EFFECT ON UPTAKE OF CS-137, RU-106, CE-144 AND SB-125 INTO WHEAT OVER THE COURSE OF CONSECUTIVE GROWING SEASONS TO BE DETERMINED WITH RESPECT TO VARIATIONS IN SOIL TYPE, CLIMATE, AGRICULTURAL PRACTICE AND DIETARY HABIT WITHIN THE MAIN WHEAT GROWING REGIONS OF THE CEC. THIS WORK IS ESSENTIALLY OF A LONG-TERM NATURE, USING LARGE LYSIMETERS SUBJECTED TO A SINGLE SURFACE APPLICATION OF THE RADIONUCLIDES. CURRENT DEVELOPMENTS INCLUDE THE CONSTRUCTION OF A LARGE WIND-TUNNEL, IN WHICH CROPS WILL BE CONTAMINATED DIRECTLY FROM THE AIR WITH RADIONUCLIDE AEROSOLS, WITH PARTICLE SIZES WITHIN THE RANGE ANTICIPATED FROM ACCIDENTAL RELEASES, AND UNDER AERODYNAMIC CONDITIONS CHARACTERISTIC OF THE FIELD.
THIS PROPOSAL IS AIMED AT FURTHERING THE UNDERSTANDING OF THE FACTORS AFFECTING UPTAKE AND SUBSEQUENT RETENTION OF RADIOACTIVE AEROSOLS BY AERIAL PLANT PARTS, AND ELUCIDATING THE RELATIVE IMPORTANCE OF SOIL- PLANT AND AIR - PLANT PATHWAYS FOR SELECTED RADIONUCLIDES ON THE ULTIMATE COLLECTIVE DOSE TO MAN.
Following simulated reactor accidents, the time dependent transfer of radionuclides from the atmosphere and the soil to crops has been studied.

Continuation of lysimeter studies with cabbage, with sequential harvests up to maturity. Estimation of changes in the exchangeable; total Cs ratio in experimental soils from all harvests throughout the programme. Measurement of contribution of soil resuspension to contamination of aerial plant parts, using Ti as a tracer. Dual-isotope experiment with cabbage in the wind-tunnel followed by assessment of the fate of the foliar and soil applied Cs up to maturity.
A RANGE OF MAJOR CROP SPECIES, INCLUDING WHEAT, GRASS SWARDS, LEAF VEGETABLES, ROOT VEGETABLES AND FRUITS WILL BE CONTAMINATED IN THE WIND-TUNNEL WITH CS-137, RU-106, CE-144 AND SB-125 AT DIFFERENT STAGES OF GROWTH, AND TRANSFERRED TO THE FIELD, WHERE THE SUBSEQUENT FATE OF THE RADIONUCLIDES WILL BE FOLLOWED WITH RESPECT TO UPTAKE, TRANSLOCATION, AND FIELD-LOSS. THE INFLUENCE OF CLIMATIC VARIABLES, PARTICULARLY WIND AND SURFACE WETNESS, ON FOLIAR DEPOSITION WILL ALSO BE STUDIED IN THE WIND-TUNNEL EXPERIMENTS. ON SELECTED SUITABLE CROPS, THE RELATIVE CONTRIBUTIONS OF AERIAL DEPOSITION AND UPTAKE VIA THE SOIL WILL BE DETERMINED USING DIFFERENT ISOTOPES OF THE SAME ELEMENTS (CS-134/CS-137; RU-103/RU-106; CE-141/CE-144; SB-122/SB-125) FOR THE TWO PATHWAYS: ONE ISOTOPE WILL BE APPLIED TO THE SOIL, WITH THE AERIAL PART OF THE PLANT PROTECTED FROM CONTAMINATION, FOLLOWED BY A SECOND AERIAL APPLICATION WITH THE OTHER ISOTOPE, WITH THE SOIL SURFACE BUT NOT THE PLANT PROTECTED; THE PLANTS WILL THEN BE TRANSFERRED TO THE FIELD AND THE SUBSEQUENT FATE OF THE TWO ISOTOPES DETERMINED. THE STUDIES WILL ALSO TAKE INTO ACCOUNT THE EFFECT OF DIFFERENT CHEMICAL FORMS OF THE RADIONUCLIDES ON DEPOSITION AND RETENTION BY CROPS.
WHILE THE WIND-TUNNEL HAS BEEN DESIGNED TO STIMULATE AERODYNAMIC CONDITIONS IN THE FIELD AS CLOSELY AS POSSIBLE, IT CANNOT, HOWEVER, PROVIDE FULLY REALISTIC FIELD CONDITIONS. THUS IT IS DESIRABLE TO CONTAMINATE CROPS IN THE FIELD, AS WELL AS IN THE WIND-TUNNEL IN ORDER TO DETERMINE THE DEGREE TO WHICH THE LATTER SIMULATES REALISTIC CONDITIONS. HOWEVER, SAFETY CONSIDERATIONS PRECLUDE THE USE OF RADIONUCLIDES IN QUANTITY IN THE FIELD. THUS IT IS PROPOSED TO COMPARE THE UPTAKE BY CROPS AND FATE OF THE RADIONUCLIDES, WITH THAT OF THEIR STABLE ANALOGUES, IN A SERIES OF WIND-TUNNEL EXPERIMENTS. IF THE STABLE ANALOGUES PROVE TO BE SUITABLE AS A SURROGATE FOR THE RADIONUCLIDES, THEN A SERIES OF FIELD EXPERIMENTS WILL BE CARRIED OUT, WITH AERIAL APPLICATION, FOLLOWED BY NEUTRON ACTIVATION ANALYSIS.
FOLIAR CONTAMINATION IS ONLY OF IMPORTANCE IN THE SHORT-TERM, FOLLOWING AN ACCIDENT. THUS IT IS PROPOSED TO DETERMINE THE MEDIUM/LONG-TERM FATE OF RADIONUCLIDES IN SOIL/CROP SYSTEMS BY CONTINUING THE EXISTING LYSIMETER EXPERIMENTS, BUT EXTENDED TO THE CROPS USED IN THE WIND-TUNNEL EXPERIMENTS. RESUSPENSION OF CONTAMINATED SOIL PARTICLES ONTO AERIAL PARTS OF THE PLANTS WILL BE MEASURED, USING TI AND SC SOIL TRACERS; THIS TECHNIQUE WILL BE ALSO USED IN THE WIND-TUNNEL EXPERIMENTS, AS APPROPRIATE. APPLICATION OF A SECOND RADIONUCLIDE OF THE SAME ELEMENT TO SOILS IN THE LYSIMETERS WILL ENABLE A COMPARISON TO BE MADE OF SHORT-TERM UPTAKE INTO CROPS WITH THAT RESULTING FROM SEVERAL YEARS EQUILIBRATION IN THE SOIL. BOTH TOTAL AND EXCHANGEABLE RADIONUCLIDE CONTENTS OF THE SOIL, AS WELL AS THEIR MIGRATION WITHIN THE PROFILE, WILL BE DETERMINED OVER THE 5-YEAR PERIOD, PROVIDING INFORMATION ON LONG-TERM CHANGES IN THEIR AVAILABILITY TO PLANTS.

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Coordinator

Imperial College of Science, Technology And Medicine
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SW7 2AZ London
United Kingdom

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