Last update on April 2015
FORTRESS project (Foliar transfers of radionuclides in agricultural ecosystems) is an IRSN project conducted in partnership with ANDRA, and which ran from 2007 to 2011. It was designed to quantify, under realistic conditions of full-field crop cultivation, the factors of translocation to the consumable parts of four cultivated crops (wheat, radish, potato, green bean) for three elements (chlorine, selenium and iodine) sprayed onto the leaves of the plants.
Context and objectives
As well as conducting critical analyses of safety demonstrations for nuclear installations and storage sites, the IRSN works on acquiring know-how so as to understand and limit as far as possible the consequences of any malfunctions there, however low the probability of occurrence. In a scenario where the water used for irrigation might contain radionuclides emanating from a radioactive waste storage site, what would be the risk of contamination for agricultural crops? One of the main paths of contamination for crops would be foliar transfer, in particular via the water sprayed on for irrigation. The contaminants migrate from the surface of the leaves to all the organs of the plant, including the consumable parts (root, seeds, tuber, fruit). By consuming these plants, humans are exposed to a dose of ionizing radiation that it must be possible to assess in the context of surveillance or incident management.
In the event of contamination, the quantity of radionuclides retained by the foliage of the plant depends on several factors: the stage of development of the plant (developed leaf area) and the quantity of water adsorbed on the leaves (plant species effect), on the one hand; and the concentration and chemical form of the radionuclides in the irrigation water on the other (contamination vector effect). All these factors determine the interception of the contaminant. Part of the contamination is then internalized via foliar absorption, and translocation is what we call the transfer of the contaminant from the leaves to a target organ that has not intercepted the initial contamination.
The translocation factor (ftr) corresponds to the ratio of the transferred contamination measured on harvesting in the target organ to the contamination intercepted by the leaves as measured during contamination. In natural conditions, the plant is subject to rainfall leaching (rainfall during the crop cycle), and the leaching activity into the ground then participates in the root transfer. By definition, the ftr measures the intensity (or efficiency) of the transfer in planta from the leaf to the organ stricto sensu. It is specific to each organ-radionuclide pairing and can be expressed as a percentage.
The main radionuclides to be studied in the context described above are chlorine 36, selenium 79 and iodine 129, long-lived radioactive isotopes of chemical elements whose mobility in the environment, including incorporation into the human food chain, is in principle greater than that of cesium or strontium. A bibliographical review had brought to light that although there is abundant experimental data for the transfer of the latter two elements to leaves, no such data existed for chlorine 36, selenium 79 and iodine 129, whether for one-time or for chronic contamination situations throughout a crop's development cycle.
The objective therefore of FORTRESS project Fortress was to obtain experimentally in situ, with a high degree of precision (10%), the translocation factors for chlorine 36, iodine 129 and selenium 79 for the main categories of crop consumed, subsequent to strictly foliar contamination via spray irrigation.
Project procedure and results
The study was carried out in the "vegetable garden" situated in the exclusion zone close to the damaged Chernobyl nuclear reactor, on uncontaminated soil taken from outside the exclusion zone. This vegetable garden is maintained by the Ukrainian Institute of Agricultural Radiology (UIAR), which provided logistical support to the IRSN throughout the project. In order to limit the radiological impact for the iodine and selenium, and in light of similar chemical behavior for the isotopes of any given element, short-lived isotopes were chosen (125I, 75Se). For the chlorine, it was demonstrated that the quantities of 36Cl used generated no significant radiological impact.
Protection systems : on the left, from rains ; on the right, from contamination by soils © IRSN
In order to obtain translocation values strictly reflecting foliar contamination, the soil was protected from all contamination and the plants protected from rainfall with the purpose of avoiding interference from root transfers and rainfall leaching, and maximizing translocation.
Four plant species representative of consumption groups defined by the IAEA (IAEA, 2009) were selected: cereals (wheat), fruiting vegetables (beans), root vegetables (radish) and tubers (potatoes). Their leaves were all sprayed in order to simulate a spray irrigation scenario, over the course of two full crop cycles (May to August, in 2009 and 2010), at several key stages of development: early and advanced development phase for leaves and stems; flowering; development of grain, fruit or tubers; maturation. Contamination was carried out according to two methods: one-time contamination (at a single stage of development) or chronic contamination (at all stages of development successively).
Wheat contamination by spraying © IRSN
The results obtained demonstrated that the variability of the climatic parameters over the year exerts a strong influence on the intensity of the measured transfers. The translocation factors of the three radionuclides observed were established for each plant species. The iodine exhibited low translocation (0.1% to 2.6% depending on conditions), fixing principally in the leaves. The chlorine 36 demonstrated high translocation capacity (0.5% to 31.5%), and spread throughout the plant, particularly in the potato. Lastly, the selenium presented very high translocation (1.6% to 32.6%), particularly at two of the most sensitive key stages of development: flowering, and the early development stage ("young leaves"). The latter two elements therefore appear to be at least as mobile as, if not more mobile than, cesium, which notwithstanding is considered to be extremely mobile in planta (chemical analog of potassium).
Furthermore, the comparison between one-time and chronic contamination modes showed that, in the absence of data for the latter mode, the highest translocation factor calculated for one-time contamination constitutes the best estimation of a radionuclide's mobility.
FORTRESS project has made it possible to build up a unique experimental database on the translocation of 36Cl, 79Se and 129I. The translocation factors obtained in this way are maximum values, which will make it possible in particular to characterize more accurately the intensity of transfers to humans via the ingestion of contaminated agricultural produce for these radionuclides.