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TRANSFER OF RADIONUCLIDES IN THE FOOD CHAIN

Ziel


An experimental wind tunnel and soil column arrangement to simulate tritium reemission from soil under controlled environmental conditions was completed. The 2-phase time behaviour of the tritiated water (HTO) reemission rate under constant environmental conditions indicated that as long as the top soil layers were moist, HTO resupply occurred predominantly by liquid transport in soil. With proceeding reemission of HTO and water (H20) the tope soil dried out, and much slower resupply by diffusion prevailed, reducing the reemission rate.

Specific activity ratios greater than unity were also found with HTO deposition processes, and were in agreement with the understanding of molecular transports. This is not an isotopic effort, but the result of the individual driving forces for HTO and H20 movement. Prediction of HTO reemission rates on the basis of evaporation data via specific tritium activity of soil water would considerably underestimate dose consequences.
Fusion test reactors and associated tritium handling facilities as well as first generation fusion plants will contain large inventories of tritium fuel, mainly in highly mobile forms (eg HT and HTO). The safety analysis and licensing of these facilities require the availability of dynamic models to predict the environmental behaviour and impact of hypothetical tritium releases. Especially in the case of accidental short term releases to the atmosphere, it is important to know the time history of the resulting local dose to evaluate risks and to prepare countermeasures. Furthermore and most important, in the design phase of a new technology like fusion, thorough knowledge on its environmental consequences provides the opportunity to incorporate passive safety into the practical realization, before the first tritium fueled reactor starts operation.

The evaluation of the recent experimental field releases in France and Canada has identified tritium remission from soil and vegetation after deposition from a primary plume of HT or HTO as a key process in tritium behaviour of higher relevancy dose. The remission rate determines the early dose from inhalation and skin absorption, the formation of HTO pools in the soil and plant system and the tritium clearance from the local environment. However no model exists, as yet, to predict remission rates on the basis of general or site specific parameters.

Research in this project is focusing on the investigation and establishment of key relationships between the remission rate and soil physical as well as meteorological parameters like soil texture, moisture content, temperature, wind speed and air HTO/H2O contents. This is done with help of a wind tunnel and soil column arrangement equipped with a unit to control defined steady or dynamic conditions. The relevant relationships are evaluated from selected single parameter studies.

The experimental results obtained so far, under steady meteorological conditions with homogeneously labelled soils, show that the remission rate is much higher immediately after deposition and decreases faster during the first few hours than derived from low time resolution measurements during the field releases. Furthermore, HTO remission is principally not coupled to H2O evaporation, though evaporation conditions may also be favourable for tritium remission because of energy reasons. This independence of HTO and H2O transports across the air and soil interface confirms the findings of the previous study on HTO deposition from air to soil.

After the establishment of principal relationships it is planned to proceed towards more realistic conditions with respect to HTO soil profiles resulting from actual deposition processes and with respect to dynamic meteorological sequences.

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INST. RADIOVKOLOGIE NIEDERSACHSEN
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