Plutonium in the environment: key factors related to impact assessment in case of an accidental atmospheric release

International audienceThis paper deals with plutonium and key factors related to impact assessment. It is based on recent work performed by CEA which summarize the main features of plutonium behaviour from sources inside installations to the environment and man, and to report current knowledge on the different parameters used in models for environmental and radiological impact assessment. These key factors are illustrated through a case study based on an accidental atmospheric release of Pu in a nuclear facility

Optimization of exposure and countryside waste management for different accidental radioactive release

International audienceSince the Fukushima accident, Japanese scientists have been intensively monitoring ambient radiations in the highly contaminated territories situated within 80 km of the nuclear site. The surveys that were conducted through mainly carborne, airborne and in situ gamma-ray measurement devices, enabled to efficiently characterize the spatial distribution and temporal evolution of air dose rates induced by Caesium-134 and Caesium-137 in the terrestrial systems. These measurements revealed that radiation levels decreased at rates greater than expected from physical decay in 2011-2012 (up to a factor of 2), and dependent on the type of environment (i.e. urban, agricultural or forest). Unlike carborne measurements that may have been strongly influenced by the depuration of road surfaces, no obvious reason can be invoked for airborne measurements, especially above forests that are known to efficiently retain and recycle radiocaesium.The purpose of our research project is to develop a comprehensive understanding of the data acquired by Japanese, and identify the environmental mechanisms or factors that may explain such decays. The methodology relies on the use of a process-based and spatially-distributed dynamic model that predicts radiocaesium transfer and associated air dose rates inside/above a terrestrial environment (e.g., forests, croplands, meadows, bare soils and urban areas).Despite the lack of site-specific data, our numerical study predicts decrease rates that are globally consistent with both aerial and in situ observations. The simulation at a flying altitude of 200 m indicated that ambient radiation levels decreased over the first 12 months by about 45% over dense urban areas, 15% above evergreen coniferous forests and between 2 and 12% above agricultural lands, owing to environmental processes that are identified and discussed. In particular, we demonstrate that the decrease over evergreen coniferous regions might be due the combined effects of canopy depuration (through biological and physical mechanisms) and the shielding of gamma rays emitted from the forest floor by vegetation. Our study finally suggests that airborne surveys might have not reflected dose rates at ground level in forest systems, which were predicted to slightly increase by 5 to 10% during the same period of time

Les deux radioactivités, entre réglementation et réalité

Sachant que tout est radioactif, et qu’il existe un bruit de fond naturel de rayonnement sur la planète induisant une exposition externe mesurée dans des habitations allant jusqu’à 135 mSv/an

Extraction de l'eau libre d'une matrice et mesures de l'activité tritium

National audienceEtant un isotope de l'hydrogène, le tritium est ubiquiste et peut être intégré dans la plupart des molécules constitutives du vivant, notamment dans l'eau et les molécules organiques. Que ce soit pour la surveillance de l'environnement ou dans la recherche, la mesure du tritium dans des matrices biologiques se fait généralement dans leur eau libre ainsi que dans leur matière sèche et nécessite ainsi une séparation systématique de ces deux compartiments. Les principales techniques d'extraction d'eau utilisées sont la lyophilisation et la distillation (azéotropique ou non, sous pression réduite ou atmosphérique). Il est montré que l'extraction de l'eau libre est susceptible d'induire des biais sensibles et systématiques dans les mesures du tritium de l'eau libre et du tritium lié de ces matrices. Afin de quantifier l'effet de chacune des sources de biais et d'essayer de comprendre les mécanismes en jeu, différentes hypothèses ont été testées en supposant : - que la technique d'extraction d'eau ait un effet sur les activités mesurées, - que la masse de l'échantillon traité ait une influence sur les résultats, - que le taux d'eau extrait influe sur les activités tritium mesurées, - que la nature de la matrice traitée puisse influer sur ces résultats. Les résultats valident chacune des hypothèses et quantifient les biais induits : - les techniques de distillations donnent des résultats proches alors que les activités mesurées sur l'eau extraite par lyophilisation ont une plus grande dispersion et activité moyenne nettement plus élevées, - la dispersion des activités mesurées sur les échantillons apparaît comme inversement proportionnelle à la masse de l'échantillon dans la plage testée, - le taux d'extraction de l'eau libre influe sur la pertinence de la mesure du fait d'une discrimination isotopique marquée qui conduit à l'enrichissement de l'eau restant dans la matière sèche et donc à l'appauvrissement de l'eau extraite jusqu'à l'assèchement total de l'échantillon, - outre la facilité à extraire l'eau de la matrice, la nature de l'échantillon influence également le " profil " de la discrimination isotopique au cours de l'avancement de la déshydratation. Ces résultats permettent ainsi de mettre en évidence les précautions à prendre pour limiter les erreurs ou pouvoir, le cas échéant, les estimer de manière fine. Ils mettent également en évidence des " profils " originaux de discrimination isotopique, spécifiques des matrices considérées. Cela pose la question de l'enrichissement artificiel du tritium organique échangeable au cours de l'extraction d'eau et cela soulève également des questions sur les techniques d'évaluation du tritium organique non-échangeable dans une matrice. : est-ce que la déduction d'un ratio théorique " tritium organique échangeable " sur " tritium organique total " permet de s'affranchir, dans des conditions satisfaisantes, de l'échange labile pour l'estimation/ du tritium organique non-échangeable

Isotopic fractionation of tritium in biological systems

International audienceIsotopic fractionation of tritium is a highly relevant issue in radiation protection and requires certain radioecological considerations. Sound evaluation of this factor is indeed necessary to determine whether environmental compartments are enriched/depleted in tritium or if tritium is, on the contrary, isotopically well-distributed in a given system. The ubiquity of tritium and the standard analytical methods used to assay it may induce biases in both the measurement and the signification that is accorded to the so-called fractionation: based on an exhaustive review of the literature, we show how, sometimes large deviations may appear. It is shown that when comparing the non-exchangeable fraction of organically bound tritium (neOBT) to another fraction of tritium (e.g. tritiated water) the preparation of samples and the measurement of neOBT reported frequently led to underestimation of the ratio of tritium to hydrogen (T/H) in the non-exchangeable compartment by a factor of 5 % to 50 %. In the present study, corrections are proposed for most of the biological matrices studied so far. Nevertheless, the values of isotopic fractionation reported in the literature remain difficult to compare with each other, especially since the physical quantities and units often vary between authors. Some improvements are proposed to better define what should encompass the concepts of exchangeable and non-exchangeable fractions

Tritium levels in milk in the vicinity of chronic tritium releases

Tritium is the radioactive isotope of hydrogen. It can be integrated into most biological molecules. Eventhough its radiotoxicity is weak, the effects of tritium can be increased following concentration in criticalcompartments of living organisms. For a better understanding of tritium circulation in the environmentand to highlight transfer constants between compartments, we studied the tritiation of different agriculturalmatrices chronically exposed to tritium.Milk is one of the most frequently monitored foodstuffs in the vicinity of points known for chronicrelease of radionuclides firstly because dairy products find their way into most homes but also because itintegrates deposition over large areas at a local scale. It is a food which contains all the main nutrients,especially proteins, carbohydrates and lipids. We thus studied the tritium levels of milk in chronicexposure conditions by comparing the tritiation of the main hydrogenated components of milk, first,component by component, then, sample by sample. Significant correlations were found between thespecific activities of drinking water and free water of milk as well as between the tritium levels of cattlefeed dry matter and of the main organic components of milk. Our findings stress the importance of themetabolism on the distribution of tritium in the different compartments.Overall, dilution of hydrogen in the environmental compartments was found to play an important roledimming possible isotopic effects even in a food chain chronically exposed to tritium

Measurement of tritium in the free water of milk : spotting and quantifying some biases and proposing ways of improvement

International audience11 4 : CEA/HC 12 13 Corresponding author: 14 [email protected] -+333 80 23 40 00 -+333 80 23 52 09 15 16 Abstract 17 As one of the three natural isotopes of hydrogen, tritium is ubiquitous and might potentially 18 be present in any water or organic molecule that constitutes a biological matrix. Milk is one of 19 the most frequently monitored foodstuffs in the vicinity of chronic release of radionuclides, as 20 it is a very common product and also because it integrates deposition on large areas of grass at 21 a local scale. Different parameters have been studied to assess their impact on the reliability 22 of tritium measurements. The volume of the sample, the technique used to extract the water 23 and the level of dehydration modulate the results but in different ways: dispersion of results, 24 under-or overestimation of the tritium activity. The influence of sample storage and 25 preparation has also been investigated. Methodological improvements of tritium 26 measurements in the free water of milk are proposed. 27 2