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Long-term investigations of radiocaesium activity concentrations in carps in the Republic of Croatia are presented. The radiocaesium levels in carps decreased exponentially and the effective ecological half-life of 137Cs in carps was estimated to be about 1 year for 1987-2002 period and 5 years for 1993-2005 period. The observed 134Cs:137Cs activity ratio in carps has been found to be similar to the ratio that has been observed in other environmental samples. Concentration factor for carps (wet weight) was roughly estimated to be 128 +/- 74 Lkg-1, which is in reasonable agreement with model prediction based on K+ concentrations in water. Estimated annual effective doses received by 134Cs and 137Cs intake due to consumption of carps for an adult member of Croatian population are small, per caput dose for the 1987 - 2005 estimated to be 0.5 +/- 0.2 microSv. Due to minor freshwater fish consumption in Croatia and low radiocaesium activity concentrations in carps, it can be concluded that carps consumption was not a critical pathway for the transfer of radiocaesium from fallout to humans after the Chernobyl accident., 21 pages, including 5 figures; final version; accepted by Journal of Environmental Radioactivity

Systematic investigations of 137Cs and 134Cs activity concentrations in potatoes (Solanum tuberosum) for the post-Chernobyl period (1986-2005) in the Republic of Croatia are summarized. The correlation between 137Cs activity concentrations in fallout and potatoes, has been found to be very good, the correlation coefficient being r2=0.88 with P(t) < 0.001 for 18 degrees of freedom. As the radiocaesium levels in potatoes decreased exponentially, the mean residence time of 137Cs in potatoes was estimated by fitting the measured activity concentrations to the exponential curve. The mean residence time was found to be 6.8 +/- 1.1 years, the standard deviation being estimated by the Monte Carlo simulations. The initial observed 134Cs:137Cs activity ratio in potatoes has been found to be quite variable, but slightly lesser than theoretically predicted value of 0.5, calculated by applying the known inventory of these radionuclides in the Chernobyl reactor to the equation for the differential radioactive decay. This can be explained by presence of the pre-Chernobyl 137Cs in soil that originated from nuclear fallout. The annual effective doses received by 134Cs and 137Cs intake due to consumption of potatoes estimated for an adult member of Croatian population were found to be very small, as the per caput dose for the entire 1986 - 2005 period was calculated to be about 2.9 mSv, 134Cs accounting approximately for 1/3 of the entire dose. Therefore, after the Chernobyl accident consumption of potatoes was not the critical pathway for human intake of radiocaesium from the environment in Croatia., 19 pages, including 4 figures; final version; accepted by Journal of Environmental Science and Health, Part B

The Arctic Ocean expedition in 1991 with the Swedish icebreaker M/S Oden was focused on oceanography and geology. The aim of our project was exploring the activity concentrations in surface air of 7Be, 210Pb. and 210Po in the surface air, radioactive isotopes of Caesium (134Cs, 137Cs) and plutonium (239+240Pu) in seawater. During the cruise in the Arctic Ocean during 1991-07-28 to 1991-10-04 the average activity concentrations in surface air of 7Be was 0.6±0.4 mBq.m-3 , 210Pb 46±34 microBq.m-3 and 210Po 37±23 microBq.m-3 The activity concentration of 137Cs in the surface of the Arctic Ocean was in the range of 8-12 Bq.m-3. When crossing the Nansen basin the activity concentration of 137Cs increased to about 18 Bq.m-3 at 88 °N 80 °E, and there was an accumulation of 137Cs in an area around at 88 °N and 80-100 °E and locally increased activity at 83 °N 10 °E. The 134Cs/137Cs activity ratios was about 0.02 due to the contribution mainly from Sellafield and a few percent contribution from Chernobyl. The 134Cs/137Cs activity ratio decreased to about 0.002-0.005 in areas of high 137Cs activity concentration which exclude contribution of 134Cs of nuclear reactor fuel. The activity concentration of 239+240Pu in the surface of the Arctic Ocean was in the range of 6 - 8 mBq.m-3. But locally the activity concentration of 239+240Pu was found to be increased to 11 mBq.m-3 at 86°N 48-53°E, and to 16 mBq.m-3 at 83°N 10°E.

Une étude concernant le transfert du césium radioactif dans une chaîne trophique simplifiée d'eau douce, a été réalisée dans le cadre des Contrats (CCE) n° BI6-B-198-P et n° BI6-0245-P. Les objectifs étaient la connaissance de la radioécologie du fleuve Tejo (Tage) et le comportement du césium radioactif dans un écosystème naturel. Dans le présent document, sont présentées l'accumulation par voies différentes et la rétention du 134Cs par Daphnia magna Straus (Crustacea, Cladocera).L'accumulation directe à partir de l'eau du fleuve contaminée avec du 134Cs, a été effectuée avec une concentration initiale, approximative de 10 Bq ml-1. Le facteur de concentration (FC), rapporté au poids frais des daphnies, a évolué selon la fonction : FC(t) = 57,4 (1 - e-0,275t), avec le temps (t) en jours.L'élimination du radionucléide est exprimée par la fonction de rétention : R(t) = 8,5e-12,211 t + 91,5e-0.441t, qui indique donc deux périodes biologiques, Tb1 = 0,06 jour et Tb2 =1,6 jours.Quand la contamination de D. magna se réalise à partir de la nourriture (Selenastrum capricornutum) contaminée au 134Cs,le facteur de transfert trophique suit la fonction : F T (t) = 5, 1 (1 - e-0,245t).Dans ce cas, le processus de rétention est représenté par une fonction exponentielle à un seul terme : R(t) . 100 e-0,410t et correspond donc à une seule période biologique, Tb = 1,7 jour.La contamination simultanée par les deux vecteurs semble montrer que la voie de contamination la plus importante est le transfert à partir de l'eau. Etant donné que la concentration cellulaire et la concentration en 134Cs de la microalgue S. capricorrrutum étaient approximativement stabilisées, la nourriture n'a été responsable que de 28,5 ± 8,3 % de la concentration du radionucléide dans D. magna.En ce qui concerne l'étude de la rétention, on observe que, pour la contamination par l'eau, la période biologique la plus courte doit correspondre à une désorption au niveau des surfaces, tandis que la période la plus longue doit correspondre à l'élimination du 134Cs assimilé. La rétention consécutive à la fixation par l'ingestion de nourriture contaminé, conduit à une seule période biologique, qui ressemble beaucoup à la période longue du premier cas., Considering the potential radiological risk, due to the normal operation of the spanish nuclear power plants (Jose CABRERA, TRILLO and ALMARAZ) sited in the Tejo (Tagus) River watershed, the radioecological study of the river is of the utmost interest.Aiming for a better understanding of radionuclides behaviour in the Fratel dam, a particular freshwater ecosystem in the Tejo River located a few kilometers from the border, several studies have been carried out.Besides the field follow up, an experimental study concerning the radioactive cesium transfer in a simplified freshwater trophic chain was developed, under the Contracts (CEC) n° BI6-B-1 98-P and n° BI6-0245-P, whose objectives were respectively the knowledge of Tejo River radioecology and the radiocesium behaviour in a natural ecosystem.In a radiological assessment, the use of site specific data is always recommendable, instead of using default values, what is another perspective of the present work.The importance of cesium behaviour in the environment, is related to its chemical similarity with potassium, which is a wide-spread element in the living tissues. Following a hypotetical ingestion of radiocesium contaminated foodstuff or water by man, the contamination may then be extended to the whale body, mainly the muscles. Recently, the accident in the ucranian Chernobyl power plant, has alerted the international scientific community to the problem of the environmental dispersion of this radionuclide.134Cs, a gamma radiation emitter, was the isotope selected for the experiments. Its usual concentration in the radioactive liquid effluents released by PWR plants, is similar to the one of 137Cs, an isotope with a half-life of 30.15 years. 134Cs has a half-life of only 2.07 years, which is an advantage regarding the production of radioactive wastes resulting from the experiments.Our objective is to understand the mechanisms of the 134Cs transfer along the trophic chain, and to determine the parameters that might integrate e radioecological model, namely the concentration factors between organisms and water, and the trophic transfer factors between consumers and nourishment.The simplified trophic chain concerned : a primary producer, the green microalgae Selenastrum capricernutum Printz (CORISCO and CARREIRO, 1990 a); a primary consumer, the planktonic crustacean Daphnia magna Straus; and a secondary consumer, the omnivorous fish Tinca tinta L. (CORISCO and CARREIRO, 1990 b).In this paper the 134Cs uptake and retention by Daphnia magna Straus, (Crustacea, Cladocera) via different pathways, are presented.D. magna is a filter-feeding organism very common in freshwater ecosystems and frequently used as a test organism in toxicology. Several authors have been using it, as well, in the field of experimental radioecology. These small crustaceans, hardly exceeding a 3 mm mesh size, reproduce predominantly by parthenogenesis and, in laboratorial conditions, this process may be permanent if optimum temperature, abundant feeding and water quality are maintained.The main chemical characteristics of Fratel water, which was used in this study, are presented. The average stable cesium concentration, determined by instrumental neutron activation analysis, is 6 x 10-5 ppm.During the experiments, the temperature was kept at 20 ± 2 °C and artificial light was maintained for 15 h a day, using daylight fluorescent tubes (36 watt).The radiocesium used was a 134Cs chloride in a 0.1 M solution, with a total cesium content of 1 µg ml-1.The radioactive measurements were performed in e well type 1 1/4" diameter and 2 1/2" deep sodium iodine (thalium activated) detector of 4" x 4", connected to monochannel analyser.The direct uptake from 134Cs Iabelled river water was carried out using an initial radionuclide concentration in the water near to 10 Bq ml-1. The concentration factor (CF), referred to wet weight of daphnids, evolved according to the function : CF (t)=57.4 (1 - e-0.275t) , with time (t) referred to days.When the contamination of D. magna was carried out through the nourishment, 134Cs Iabelled Selenastrum capricornutum, the trophic transfer factor (TF) followed the exponential function : TF(t)=5.1 (1 - e-0.245t).Combining both pathways simultaneously, water appears to be the main contamination source. The number of cells and 134Cs concentration in microalgae S. capricornutum being approximately stabilized, nourishment was responsable for 28.5 ± 8.3 % of the radionuclide concentration in D. magna.However, using the functions issued from the separated pathways experiments, the assessed contamination of D. magna suggests that the trophic pathway would be predominant.The evaluation of the relative importance of the contamination pathways, would very likely be dependent on the experimental conditions. Therefore, experimental results must be carefully taken in consideration, and an approach as best as possible to the environmental conditions should be attempted.The elimination of the radionuclide , after the uptake from water, is expressed by the retention function : R (t)=8.5e-12.211t + 91.5e-0.441t, meaning two biological retention compartments with half-lives Tb1 = 0.06 day and Tb2 = 1.6 days. Following the uptake from the food pathway, retention is expressed by an exponential with only one term : R(t)=100 e-0.494t and, therefore, only one hall-life, Tb = 1.7 days.From the retention study after 134Cs uptake from water, it appears that the shorter hall-life might correspond to the surface desorption, whereas the longer one might correspond to the loss of assimilated 134Cs. Following uptake from ingestion of labelled food, retention study lead to a single hall-life, very similar to the longer one in the previous case.These results would suggest that water and food pathways act differently when tested together or separately, and assimilation must be the main concentration mechanism of 134Cs for the crustacean Daphnia magna.Considering the importance of these planktonic crustaceans as a link in the freshwater trophic chains, the concentration factor at equilibrium (57 ± 3) and mainly the trophic transfer factor et equilibrium (5.0 ± 0.6), a significative contamination concerning the 134Cs, could be reached.

Investigations in the post-Chernobyl period (1986-2009) of radiocaesium activity concentrations in Adriatic pilchards are presented. Compared with pre-Chernobyl period, the Chernobyl nuclear accident caused increase of (137)Cs activity concentrations in pilchards. By fitting the measured (137)Cs activity concentrations to the theoretical curve was estimated to be 1.5±0.4 y for 1986-90 and 5.8±0.4 y for 1991-2009 and the bimodal behaviour for the ecological half-life of (137)Cs in pilchards has been observed. Estimated annual effective doses received by (134)Cs and (137)Cs intake due to consumption for an adult member of Croatian population are small. Collective dose for the 1986-2009 period was 4.9+0.3 person-Sv. The observed (134)Cs/(137)Cs activity ratio in pilchards was similar to the ratio that has been found in other environmental samples. The concentration factor for pilchards was roughly estimated to be 93.7±39.2 l kg(-1), which is consistent with the values observed elsewhere.

OBJECTIVE: This study aims to estimate the ecological exposure of adult residents of Fukushima Prefecture to (134)cesium (Cs) and (137)Cs through ingestion and inhalation between July 2 and July 8, 2011. METHODS: Fifty-five sets of meals with tap water, each representing one person's daily intake, were purchased in local towns in Fukushima Prefecture. Locally produced cow's milk (21 samples) and vegetables (43 samples) were also purchased. In parallel, air sampling was conducted at 12 different sites using a high-volume sampler. Nineteen sets of control meals were collected in Kyoto in July 2011. (134)Cs and (137)Cs levels in the samples were measured using a germanium detector. RESULTS: Radioactivity was detected in 36 of the 55 sample meals from Fukushima, compared with one of 19 controls from Kyoto. The median estimated dose level (μSv/year) was 3.0, ranging from not detectable to 83.1. None of the cow's milk (21) or vegetable (49) samples showed levels of contamination above the current recommended limits (Bq/kg) of 200 for milk and 500 for vegetables. The total effective dose levels by inhalation were estimated to be, 福島県成人住民の放射性セシウムへの経口、吸入曝露の先行評価を実施. 京都大学プレスリリース. 2011-11-14.

The influence of arbuscular mycorrhizal fungus on 134Cs uptake by Helianthus annuus was studied in a pilot study under growth chamber conditions. Mycorrhizal plants took up five times more 134Cs (up to 250,000 Bq kg-1 dry weight) than non mycorrhizal plants. Silver and titanium nanoparticles, supplied into the surface soil layer decreased both the mycorrhizal colonization and Cs uptake by mycorrhizal plants. The application of activated carbon attenuated the effect of nanoparticles and increased 134Cs uptake in the presence of mycorrhizal fungi (up to 400,000 Bq kg-1 dry weight). The results underline the possible application of phytoremediation techniques based on mycorrhiza assisted plants in decontamination of both radionuclides and nanoparticles., Se estudió mediante una prueba piloto en una cámara de crecimiento la influencia de hongos formadores de micorrizas arbusculares en la captación de 134Cs por Helianthus annuus. Las plantas micorrizadas incrementaron hasta cinco veces la captación de 134Cs (hasta 250.000 Bq kg-1 peso seco). La adición de nanopartículas de plata y titanio en la superficie del suelo disminuyó tanto la colonización del hongo como la captación de 134Cs en las plantas micorrizadas. La aplicación de carbón activado atenuó el efecto de las nanopartículas e incrementó la captación de 134Cs en presencia de hongos micorrízicos (hasta 400.000 Bq kg-1 peso seco). Los resultados sugieren una posible aplicación de plantas micorrizadas en técnicas de fitorremediación para la recuperación de suelos contaminados con radionucleotidos y nanopartículas.

The long-term behaviour of (137)Cs activity concentrations in air and fallout has been studied in the city of Zagreb for the post-Chernobyl period (1986-2006) as a part of an extended monitoring program of radioactive contamination of human environment in Croatia. Annual mean (137)Cs activity concentrations in air and annual total deposition fluxes (wet plus dry) decreased from 2.8 x 10(-4) Bq m(-3) in September 1986 to 3.0 x 10(-6) Bq m(-3) in last quarter of 2006 and from 6,410 Bq m(-2) year(-1) in 1986 to 2 Bq m(-2) year(-1) in 2006 respectively. By fitting the measured (137)Cs activity concentrations to the theoretical curve the ecological half-lives of (137)Cs in air and fallout were estimated with respective values of 0.46 and 0.54 years for immediate post-Chernobyl period, increasing to 5.52 and 3.97 years afterwards. Using the data on (137)Cs activity concentrations in air and fallout total caesium deposition velocity of (3.34 +/- 3.13) x 10(-2) ms(-1) was estimated with median value being 2.13 x 10(-2) ms(-1). Such relatively high (137)Cs deposition velocities compared with pre-Chernobyl ones, are characteristic for the post-Chernobyl period and, according to Stokes' settling law, indicate that the diameters of aerosol particles associated with (137)Cs originated from the Chernobyl accident are pretty large, i.e.1 microm. (134)Cs/(137)Cs activity ratio in fallout and in air has been found to be similar to the theoretically predicted values, initial value being about 0.5 and decreasing according to differential radioactive decay. The similar ratio has been observed in most of the other environmental samples.

Pot experiments with different triticale and oats varieties spread in Bulgaria were carried out. Three varieties of triticale (Rakita, TC-210, AD-7291) and three varieties of oats (Obrazcov chifl ik, W-!6 and W-17) were analized. The plants were grown on soil type Dystric planosol /FAO. The soil was contaminated with radionuclide 134Cs. It was established that the radiocesium is unevenly accumulated in the different parts of the plants. The highest 134Cs concentration was found in the leaves and the lowest in the grains. Variety differences of uptake of 134Cs in investigated crops were established. The highest level of accumulation of the radiocesium has been detected in triticale variety Rakita, and in oats variety W-17. It has been determined that the uptake of the radionuclide by plants of the triticale is more intensive then that of by the oats plants., Проведени бяха съдови опити с различни сортове тритикале и овес, разпространени в България. Анализирани бяха 3 сорта тритикале: (Ракита, ТС-210, АД-7291) и 3 сорта овес: (Образцов чифлик 4, W-16 и W-17). Растенията бяха отгледани върху канелено подзолиста почва, контаминирана с радионуклида цезий-134. Констатирано бе, че радиоцезия се натрупва неравномерно в различните органи на растенията. Най-висока концентрация бе установена в листата, а най-ниска в зърното на изследваните култури. Установени бяха сортови различия в усвояването на 134Cs от изследваните култури. Най-висока степен на натрупване на радиоцезия бе отбелязано при тритикале сорт Ракита, а при овеса в сорт W-17. Наблюдавани са видови различия при натрупването на радионуклида в изследваните култури. Тритикалето има подчертана способност да извлича от почвата и акумулира радиоцезия в надземните органи по-силно от овеса.

In the paper are summarized the results of systematic investigations of 137Cs and 134Cs activity concentrations in potatoes (Solanum tuberosum) for the post-Chernobyl period in the Republic of Croatia. Potatoes are very important foodstuff in Croatia, the average annual consumption being about 40 kg per person. Due to a comparatively high contribution of the ingestion doses to the total dose received by population after the exposure to nuclear fallout, a reliable prognosis of the expected ingestion doses is of utmost importance. The ingestion dose strongly depends on the consumption of various types of foodstuffs, and related activity concentrations of respective radionuclides in those foodstuffs, which themselves usually depend upon the transfer from fallout. In addition, a reliable prediction of the expected ingestion dose received by consumption of a particular foodstuff requires the detailed knowledge of decreasing behaviour of activity concentrations in the environment and respective foodstuffs. The correlation between 137Cs activity concentrations in fallout and potatoes, has been found to be very good, the correlation coefficient being r2=0.88 with P(t) < 0.001 for 17 degrees of freedom. As the radiocaesium levels in potatoes decreased exponentially, the mean residence time of 137Cs in potatoes was estimated by fitting the measured activity concentrations to the exponential curve. The mean residence time was found to be 6.3 ± 0.8 years, the standard deviation being estimated by the Monte Carlo simulations. The initial observed 134Cs:137Cs activity ratio in potatoes has been found to be quite variable, but slightly lesser than theoretically predicted value of 0.5, calculated by applying the known inventory of these radionuclides in the Chernobyl reactor to the equation for the differential radioactive decay. This can be explained by presence of the pre-Chernobyl 137Cs in soil that originated from nuclear fallout. As in other environmental samples, 134Cs relatively quickly disappeared from potatoes and its activity concentrations were in 1990 under the detection limit of the instrument. The annual effective doses received by 134Cs and 137Cs intake due to consumption of potatoes estimated for an adult member of Croatian population were found to be very small, as the per caput dose for the entire 1986 – 2004 period was calculated to be about 2.5 μ Sv, 134Cs accounting approximately for 1/3 of the entire dose. Consequently, it can be argued that after the Chernobyl accident consumption of potatoes was not the critical pathway for human intake of radiocaesium from the environment in Croatia.

Results of systematic, long-term measurements of 137Cs activities in wheat are summarized. 137Cs fallout activity affected wheat activity the coefficient of correlation being 0.91. A model used by UNSCEAR was used to describe l37Cs transfer from fallout deposition to wheat. The transfer coefficient was calculated to be 6.1 x 10-3 Bq yr kg-1/(Bq m-2). The dose incuned by wheat consumption was estimated for the Croatian population, the annual collective equivalent dose being approximately 550 man Sv for the 1965-1992 period. After the Chernobyl nuclear accident the 134Cs: 137Cs activity ratio in wheat was - 0.5, and did not differ from that for other environmental samples., Prikazani su rezultati dugogodišnjih sustavnih mjerenja aktivnosti 137Cs u pšenici. Aktvnost 137Cs u radioaktivnim oborinama utječe na aktivnost pšenice, uz koeficijent korelacije 0,91. Transfer 137Cs od radioaktivnih oborina do pšenice najbolje opisuje model UNSCEAR-a. Transferni koeficijent procijenjen je na 6.1 x 10-3 Bq god kg-1/(Bq m-2). Kolektivna ekvivalentna doza uslijed konzumacije pšenice za hrvatsku populaciju procijenjena je na oko 550 čovjek Sv za period od 1965. do 1992. godine. Poslije nuklearne nesreće u Černobilju omjer aktivnosti 134Cs:137Cs u pšenici bio je - 0,5 i nije se razlikovao od vrijednosti omjera u ostalim uzorcima iz okoliša.

The influence of arbuscular mycorrhizal fungus on 134Cs uptake by Helianthus annuus was studied in a pilot study under growth chamber conditions. Mycorrhizal plants took up five times more 134Cs (up to 250,000 Bq kg-1 dry weight) than non mycorrhizal plants. Silver and titanium nanoparticles, supplied into the surface soil layer decreased both the mycorrhizal colonization and Cs uptake by mycorrhizal plants. The application of activated carbon attenuated the effect of nanoparticles and increased 134Cs uptake in the presence of mycorrhizal fungi (up to 400,000 Bq kg-1 dry weight). The results underline the possible application of phytoremediation techniques based on mycorrhiza assisted plants in decontamination of both radionuclides and nanoparticles. Se estudió mediante una prueba piloto en una cámara de crecimiento la influencia de hongos formadores de micorrizas arbusculares en la captación de 134Cs por Helianthus annuus. Las plantas micorrizadas incrementaron hasta cinco veces la captación de 134Cs (hasta 250.000 Bq kg-1 peso seco). La adición de nanopartículas de plata y titanio en la superficie del suelo disminuyó tanto la colonización del hongo como la captación de 134Cs en las plantas micorrizadas. La aplicación de carbón activado atenuó el efecto de las nanopartículas e incrementó la captación de 134Cs en presencia de hongos micorrízicos (hasta 400.000 Bq kg-1 peso seco). Los resultados sugieren una posible aplicación de plantas micorrizadas en técnicas de fitorremediación para la recuperación de suelos contaminados con radionucleotidos y nanopartículas.

Avec l'objectif de mieux connaître le comportement du césium radioactif et son transfert dans un écosystème naturel d'eau douce, une étude concernant une chaîne trophique du fleuve Tejo a été conduite dans le cadre du contrat (CCE) n° B16-0245-P et aussi du contrat (CCE) n° B16-B-198-P. Ce document présente l'étude de l'interaction eau-producteur primaire, Selenastrum capricomufum Printz, algue planctonique unicellulaire, en présence du 134Cs.Les essais de contamination de la microalgue ont été conduits en utilisant des cultures en milieu confiné et en phase de stabilisation de la croissance. Quand la composition chimique de l'eau de Fratel n'est pas modifiée, sauf en ce qui est de l'addition du radioélément, le facteur de concentration évalué est de (1,6 ±0,2)103 (rapporté au poids sec). Par ailleurs, quand il y a un enrichissement en plusieurs cations par l'addition d'un milieu nutritif, le facteur de concentration baisse à (1,5 ± 0,2)102. Dans tous les cas l'équilibre est atteint dans les premières 24 h.L'étude de la rétention a été réalisée en repiquant des microalgues, préalablement contaminées, dans l'eau du fleuve non radioactive et en les maintenant soit en milieu confiné, soit en milieu renouvelé une seule fois au bout d'un jour, ou plusieurs fois au cours de l'expérience. Comme les résultats obtenus dans ces trois conditions sont semblables, un modèle général exprimant la variation du pourcentage de rétention a pu étre établi :R (t) = 76,7 e-45,0 t + 20,1 e-1,28 t (t exprimé en jours)ce modèle met en évidence l'existence de deux périodes biologiques, respectivement Tb1 = 0,015 jours et Tb2 = 0,54 jours.Compte tenu de la dilution biologique due à la prolifération des algues unicellulaires au cours des expériences, la cinétique de désorption permet de faire l'hypothèse que le 134Cs est d'une part adsorbé sur les surfaces et d'autre part absorbé dans les cellules., For a better understanding of the radiocesium transfer in a freshwater ecosystem, a study concerning a simplified trophic chain from the Fratel dam, in the River Tejo, is being developed under the CEC contract n° B16-0245-P and also the CEC contract n° B16-B-198-P. In this paper is presented the study of the interaction water - primary producer (Selenastrum tapricornutum Printz, an unicellular planktonic algae) in the presence of 134Cs.The accumulation experiments were carried out in confined medium, during stabilization of the culture development. When the microalgae were cultivated in labelled river water, without any chemical change except for the addition of 134Cs, the evaluated concentration factor was CF = (1,6 ± 0,2)103, referred to dry weight. Otherwise, when the water was enriched by several cations by the addition of nutrients, the concentration factor was lower, CF = (1,6 ± 0,2)103. In every case, equilibrium was reached within the first 24 h.The retention was also carried out in river water and was studied through three parallel experiments : the labelled microalgae were maintained respectively in a confined medium, in a renewed medium with one single renewal made after 24 h's experiment, and in a renewed medium with renewals made at each sampling time. In every case, the 134Cs loss obeyed a two-differential equation and the results obtained were similar, namely concerning the biological half-lives (Tb1 = 0.015 days in every case and Tb2 respectively 0.53, 0.42 and 0.54 days); therefore a total model could be deduced :R (t) = 76,7 e-45,0 t + 20,1 e-128 t (t in days)showing two biological half-lives, which were Tb1 = 0.015 and Tb2 = 0.54 days.Taking into account the biological dilution due to the biomass increase during the experiments, the elimination kinetics suggests that 134Cs is on the one hand adsorbed on the cell surface and on the other band is absorbed intracellularly.