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Build-up of Organically-Bound Tritium in Crops

#2526


Studying the Build-up of Organically-Bound Tritium in Agricultural Crops

Tech Area / Field

  • CHE-RAD/Photo and Radiation Chemistry/Chemistry
  • ENV-EHS/Environmental Health and Safety/Environment

Status
8 Project completed

Registration date
30.05.2002

Completion date
25.10.2011

Senior Project Manager
Visser H

Leading Institute
VNIIEF, Russia, N. Novgorod reg., Sarov

Collaborators

  • CEA / DAM, France, Paris\nLawrence Livermore National Laboratory / Operations and Regulatory Affairs Division / Environmental Protection Department, USA, CA, Livermore\nCEA, France, Paris

Project summary

The Project is devoted to studying behavior of organically bound tritium (OBT) in biosphere, specifically, the build-up of OBT in agricultural crops.

Project objective is to determine the transfer rate of tritium oxide (HTO) from air to the crops and the following build-up of OBT in the crop.

Primary attention during the course of the Project will be drawn to experimental investigation of the process dynamics of the OBT build-up in agricultural crops and to develop a mathematical model to describe this process.

During the course of the Project, experimental data on OBT accumulation and loss in crops will be obtained and mathematical models describing these processes will be developed.

The data obtained will be used to predict consequences of environmental tritium emission.

Tritium, along with carbon-14, plays an essential role in radioactive contamination of the environment and creates a specific problem in nuclear technologies as against majority of radionuclides. The essence of this problem is that elementary tritium (HT) interacting with soil, plants and atmospheric air becomes an integral part of water (НТО) and organic matter of biological entities. HT in the form of these compounds gets into the human body due to inhalation and via food chains. The hazard of tritium being the source of internal irradiation is defined by its ability to be present in any tissues of human body, the genetic material of a cell included. Behavior of tritium in the form of HT and HTO has been studied in essential details, while behavior of tritium in the form of OBT was studied insufficiently. Therefore, studying the behavior of OBT in the environment is a pressing problem.

Vegetable food constitutes an essential part of diet of a population and respectively OBT can essentially contribute to human dose taken in due to internal irradiation. By now, insufficient experimental data on OBT build-up in crops has been gathered. Besides, no guidelines concerning estimation of OBT contribution into the internal irradiation dose rate are available. Insufficient experimental data lead to the fact that the issues of OBT ingress into the cell substance of biological entities and possible radiation damages of cell’s genetic material are intensively discussed in the community. All these facts enhance the value of studying OBT behavior in biosphere.

Dose assessments with tritium models for accidental release scenarios have indicated that the ingestion pathways dominates the dose assuming local production and consumption habits – as was the case after the Chernobyl accident. The ingestion dose however, is dominated by the OBT as this remains for a long time in the edible part of the crop. HTO on the other hand, being the highest very shortly after the accident is reemitted rapidly into the atmosphere again.

Tritium models describing the behavior of tritium after an potential accident can be relatively complex such as UFOTRI [Raskob, 1992], however, the experimental data base to determine the transfer parameters is sparse in some areas. In particular the knowledge on animal transfer rates and the build-up of OBT is limited to some species. For OBT in crops, experimental work was performed mainly for winter wheat. Transfer processes for other crops were mainly developed based on crop physiological dependencies. Therefore, there is a need to validate transfer parameter for other crops experimentally and thus increase the confidence in the dose predictions of these models.

The role of crops as a source of OBT uptake by the human body is the following

First, crops represent the major portion of human’s diet; annual consumption of crops can constitute up to several hundred kilograms per a person.

Second, crops are widely used for producing meat and milk products, fodder.

Third, chemical conversion of tritium forms available in air, soil and moisture takes place in crops.

Studying methodology

The Project objective is to study experimentally the build-up of OBT in such plants as potatoes, grain crops, fruit trees and to develop a mathematical model of OBT build-up. During the experiments, the processes responsible for OBT build-up in crops and parameters of these processes will be identified. The experimental results will underlie the development of OBT build-up model in crops.

OBT build-up in crops due to tritium uptake through leaves will be studied. Exposure of crops is to be implemented in a green-houses with a fumigator at specified HTO specific activities in green house atmosphere. During the exposure, primary parameters are to be measured, they are: ambient temperature, air humidity, solar radiation, HTO activity in the air, exposure duration. Exposure duration and stage of the crop development will be determined based on the crop’s physiology. The plan of the experiments involving agricultural crops is presented in the table.

Studying OBT build-up in crops




Grain crops

Root vegetables

Fruit trees

Exposure in the HTO containing greenhouse

+

+

+

HTO ingress from soil moisture

+

+

-

OBT measuring will be performed in the following way:

- Preparing plant samples

- Extraction of tissue free water by the thermal vacuum desorption

- Removal of exchangeable tritium by distilled water flushing

- Extraction of free water by the thermal vacuum desorption

- Oxidation of organic matter with vanadium oxide and extraction of water generated

- Enrichment of the solution under investigation

- Measuring tritium in the extracted water by using the scintillation technique

Research program

The following activities are to be performed under the project:

  1. Analysis of the available information on OBT build-up in crops
  2. Developing a model for the process of OBT build-up in crops
  3. Developing a layout and techniques for conducting experiments
  4. Preparing greenhouses and crops
  5. Conducting experiments
  6. Crops sampling. Measurement of activity of HTO, exchangeable and non-exchangeable OBT
  7. Modelling of OBT build-up in crops
  8. Intercomparison modelling results and experiments. Models updating.

The following scientists/organizations expressed their interest in the Project proposed studies:

Ring Peterson, Environmental Monitoring Department, LLNL, USA

Yves Belot, CEA, Environmental Consultant, France

Fabien Pointurier, CEA/DAM/DASE, France

Ghislaine Guinois, CEA/DAM/DASE

Wolfgang Raskob, IKET, FZK, Germany

Siegfried Strack, FZK, Germany

The Project proposal has been discussed in details on the working meetings on October 15-19 2002 in Paris (France) and Karlsruhe (Germany) as well as through extensive correspondence. Description of results of meeting with CEA repesentatives is attached to the text of proposal ( Section 3 , point 13 “Supporting information” )

Brief description of the raised comments, which are to be included in the Project Work plan is presented below:

  1. To validate the OBT measurement technique and define an OBT reference material to be able to perform intercomparison in analytical laboratories.
  2. To reduce the number of plant species and to perform measurements in a greater detail.
  3. In addition to HTO exposure, there is a need to assess the impact of organic tritiated contaminants. An experiment can be done with tritiated organic molecules.
  4. Structure of the green house: plastic greenhouse of 36 m3(3*6*2m). Soil can be protected by 5cm of additional soil layer, which should be removed after the exposure Atmospheric moisture would be about 80% inside the green house. Temperature could be a little higher than the ambient temperature: about 25°C.
  5. Method of exposure: As a continuous flow or as discrete entries of contaminants. As the duration of the experiment is short (a few hours), the contaminant is introduced in the beginning of the experiment. A fan ensures that the atmospheric activity inside the green house is homogeneous.
  6. Time of exposure: for daytime experiments: 9-11; for nighttime experiments: 23-2.
  7. Measured parameters: The measurement of shortwave solar irradiance, air humidity and temperature is mandatory, and should be performed not only in the exposure chamber, but also continuously in a small weather station within 100 m of the experimental site. The measurement of CO2 inside the chamber can be made using an infrared gas analyzer or using a simple chemical method.
  8. Sampling: it is important to sample the leaves very quickly to avoid losses of HTO
  9. It is important to observe the growth rate of the plant organs considered. A plot of similarly cultivated plants can be reserved to determine the growth of the plant organs by taking samples during the course of the plant development.
  10. Analysis: TFWT in leaves after the exposure, OBT in leaves after the exposure, OBT in leaves and storage organs of crops at harvest; OBT in leaves, branches and roots of tree seedlings late in September

It seems better to concentrate efforts on the most important species and arrive at solid conclusions rather than to disperse efforts over many species that are secondary in terms of food supply or earth coverage.


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