Impact de la grande oie des neiges sur les marais à scirpe de l'estuaire du Saint-Laurent, Québec

La Grande Oie des Neiges (GON) est un oiseau migrateur qui fait halte dans les marais à scirpe américain (Schoenoplectus americanus) de l'estuaire du St-Laurent au printemps et à l'automne. Les oies s'y nourrissent principalement des rhizomes du scirpe. Depuis les années 80, la population connaît une croissance et des études réalisées dans les années 90 avaient montré que leur utilisation des marais dépassait la capacité de support du milieu. Afin d'évaluer si la situation s'était aggravée et pour bien caractériser l'effet du broutement des oies, un protocole expérimental, incluant l'établissement de parcelles permanentes et la pose d'exclos, a été mis en place dans quatre marais de l'estuaire: Cap Tourmente et Montmagny en 2004, l'Île-aux-Grues et Cap St-Ignace en 2005. Ces cages qui empêchaient les oies de brouter étaient installées avant leur arrivée au printemps et enlevées après leur départ à l'automne. À chaque été, un inventaire de la végétation a été fait dans les exclos et dans les parcelles témoins broutées. En utilisant des équations allométriques reliant la masse et la hauteur des tiges, la biomasse aérienne a pu être calculée pour chaque parcelle de façon non destructive. Ces équations allométriques ont été développées en 2007 et validées à l'aide d'un échantillonnage destructif. Suite à cette validation, des facteurs de correction ont été établis puisque l'échantillonnage non destructif surévaluait les biomasses aériennes réelles. Certains facteurs biophysiques ont également été étudiés au cours de 2007 dans tous les marais afin d'évaluer leur impact sur la biomasse: la sédimentation, le type de substrat, le temps de submersion et pour Cap Tourmente, les éléments nutritifs contenus dans les sédiments. La biomasse aérienne a augmenté significativement dans les exclos de tous les marais au cours de l'étude, tandis qu'elle n'a pas varié dans les parcelles témoins. Cette différence était attribuable surtout à l'augmentation de la densité des tiges et non à la biomasse des tiges individuelles dont la hauteur a peu varié dans le temps. Après quatre ans d'exclusion des oies, la biomasse aérienne était 104 et 128% plus élevée dans les exclos que dans les témoins à Cap Tourmente et Montmagny, respectivement. À Cap St-Ignace et à l'Île-aux-\ud Grues, les différences étaient de 29 et 66%, respectivement, après trois ans d'exclusion. Il y avait une relation significative entre la biomasse aérienne et souterraine du scirpe ce qui indique que l'effet du broutement observé grâce à l'échantillonnage de la végétation aérienne a aussi un effet sur la biomasse souterraine qui contribue à l'alimentation des oies et à la production du scirpe. Finalement, les analyses de régression multiple ont montré qu'à Cap Tourmente, le phosphore organique et l'effet du broutement des oies étaient les deux variables explicatives de la variation de la biomasse aérienne du scirpe américain. Lorsque tous les marais étaient considérés, seul l'effet du broutement des oies expliquait la biomasse aérienne du scirpe. En conclusion, les résultats obtenus entre 2004 et 2007 confirment les différences observées au cours des années antérieures et montrent que les oies ont encore un impact sur les marais à scirpe. Elles maintiennent la productivité des marais à scirpe à un niveau faible par rapport à leur potentiel mais encore stable, comme observé par Giroux et Bédard (1987a) il y a plus de vingt ans. ______________________________________________________________________________ MOTS-CLÉS DE L’AUTEUR : Grande Oies des Neiges, Scirpe américain, Sagittaire, Zizanie aquatique, marais, biomasse, productivité

The CIRENE loop: a tool to study ACP deposits and to validate the PACTOLE code

International audienceThe understanding of Activated Corrosion Product (ACP) transfers in the primary circuit is of great importance for the PWR industry. For this purpose, the CEA, in collaboration with EDF and AREVA NP, has launched an R&D program for many years involving specific experiments related to the development and validation of the PACTOLE code. Among them, the CIRENE out-of-pile loop is dedicated to the study of ACP deposits on fuel rods and on steam generator tubes. The first results were presented at the Jeju Island International Conference. This paper presents the experimental results of a new CIRENE test and compares the numerical simulations obtained with the PACTOLE V3.2 code. This test has been carried out with thermal-hydraulic parameters corresponding to a subcooled boiling regime at the outlet of the core section, and chemical conditions representative of a French PWR primary circuit. A radioactive tracing methodology have been applied, using gamma spectrometry measurements with the injection of specific radiotracers, 58 Co and 59 Fe: in-situ gamma spectrometry measurements are performed at the outlet of the core section and the SG tubes, while frequent radiochemical analyses of the primary fluid are carried out for checking the instantaneous mass balances. The comparison between the experimental results and simulations with the PACTOLE V3.2 code has led to the main following comments: • deposited activity variations in operation : for the core section as well as for the SG tube section, the calculated variations reproduce the experimental ones; • residual deposited activities after shutdown : the calculated activities are coherent with the experimental measurements. Consequently, the modelling developed in the PACTOLE code is globally validated for the CIRENE loop, and the differences between simulation and experimental results point out some remaining issues in the modelling of ACP behaviour in PWRs

Proceedings of IEEE Computer Society Annual Symposium on VLSI (ISVLSI 2015)

International audienc

Simulations of corrosion product transfer with the OSCAR v1.2 code

International audienceActivated Corrosion Products (ACPs) generate a radiation field in PWRs, which is the major contributor to the dose absorbed by nuclear power plant staff working during shutdown operations and maintenance. Therefore, a thorough understanding of the mechanisms that control the corrosion product transfer is of the highest importance. Since the 1970's, the R&D strategy in France has been based on experiments in test loops representative of PWR conditions, on in-situ gamma spectrometry measurements of the PWR primary system contamination and on simulation code development. The simulation of corrosion product transfers in PWR primary circuits is a major challenge since it involves many physical and chemical phenomena including: corrosion, dissolution, precipitation, erosion, deposition, convection, activation… In addition to the intrinsic difficulty of multi-physics modelling, the primary systems present severe operating conditions (300 °C, 150 bar, neutron flux, fluid velocity up to 15 m.s-1 and very low corrosion product concentrations). The purpose of the OSCAR code, developed by the CEA in cooperation with EDF and AREVA NP, is to predict the PWR primary system contamination by corrosion and fission products. The OSCAR code is considered to be not only a tool for numerical simulations and predictions (operational practices improvements and new-built PWRs design) but also one that might combine and organise all new knowledge useful to progress on contamination. The OSCAR code for Products of Corrosion, OSCAR PC, allows researchers to analyse the corrosion product behaviour and to calculate the ACP volume and surface activities of the primary and auxiliary systems. In the new version, OSCAR PC V1.2, the corrosion product transfer in the particulate form is enhanced and a new feature is the possibility to simulate cold shutdowns. In order to validate this version, the contamination transfer has been simulated in 5 French PWRs with different operating and design characteristics. After a description of the models of the main transfer mechanisms, the paper presents the calculated ACP surface and volume activities, the calculated concentrations of metallic elements and their comparisons with on-site measurements for one of the 5 validation cases. The simulations of a steam generator replacement and a cold shutdown are also presented. There is a good agreement between the OSCAR PC V1.2 results and the measured values during power operation and cold shutdown as well. Furthermore, the variations with operating cycle of the surface activities are correctly reproduced. Compared to the previous versions, these improvements are mainly due to the improvement of the thermodynamic database of the OSCAR chemistry module, PHREEQCEA, and to the enhancement of the corrosion product transfer in the particulate form

The OSCAR code package : A unique tool for simulating PWR contamination

International audienceUnderstanding the PWR primary circuit contamination by corrosion products, fission productsand actinides is a crucial issue for reactor operation and design. The main challenges aredecreasing the impact on personnel exposure to radiation, optimizing the plant operation,limiting the activity of the wastes produced during the reactor lifetime and preparingdecommissioning.In cooperation with EDF and AREVA NP, CEA has developed the OSCAR code package, aunique tool for simulating PWR contamination. The OSCAR package results from the mergingof two codes, which simulate PWR contamination by fission products and actinides (PROFIPcode) and by activated corrosion products (PACTOLE code).These two codes have been validated separately against an extensive set of data obtained over 40years from in-situ gamma spectrometry measurements, sampling and analysing campaigns ofprimary coolant, as well as experiments in test loops or experimental reactors, which arerepresentative of PWR conditions.In this paper, a new step is presented with the OSCAR code package, combining the features ofthe two codes and motivated by the fact that, wherever they originate from, the contaminationproducts are subject to the same severe conditions (300 °C, 150 bar, neutron flux, water velocityup to 15 m.s-1) and follow the same transport mechanisms in the primary circuit. The main processes involved are erosion/deposition, dissolution/precipitation, adsorption/desorption,convection, purification, neutron activation, radioactive decrease.The V1.1 version of the OSCAR package is qualified for fission products (Xe, Kr, I, Sr),actinides (U, Np, Pu, Am, Cm) and corrosion products (Ni, Fe, Co, Cr).This paper presents the different release modes (defective fuel rod release, fissile materialdissemination, material corrosion and release), then the processes which govern contaminationtransfer, and finally, we give examples of the comparison of the OSCAR package results withmeasurements in French PWR primary circuit obtained for representative radioisotopes : $^{133}$Xe,$^{90}$Sr, $^{58}$Co, $^{60}$Co. In particular, we focus on the main upgrades in the OSCAR simulations compared to thePROFIP and PACTOLE codes : adaptation of the MARGARET module to assess fission productrelease out of fuel pellets in a defective rod, adsorption/desorption model development forstrontium behaviour, multi-criteria calibration of input data which are not well known forcorrosion product simulation