A new spatially resolved model for defects and fission gas bubbles interaction at the mesoscale.

International audienceFission gases, especially Xenon, produced during irradiation in a nuclear fuel, have very low solubility in the $UO_{2}$ fuel matrix and precipitate into bubbles. These gas bubbles interact with point defects of the fuel (vacancies, self-interstitials, etc.) causing significant microstructural evolution which may eventually affect the overall performance of the fuel. Spatially resolved models are developed to predict and model the microstructural change at the mesoscale. We present a new model, which focuses on modeling the interaction between point defects and xenon gas bubbles. This new model overcomes the limitation of the existing cluster dynamics models as it can account for spatialization as well as the limitation of the spatially resolved phase-field models as it can also account for very small defect clusters, even below the individual grid spacing. The modeling of the phenomena of coalescence of two bubbles in a vacancy supersaturation and the vanishing of a small bubble in the presence of a larger bubble (Ostwald ripening) prove the credibility of the new model. 2-D analysis of a case depicting the movement and growth of bubbles in a vacancy concentration gradient is presented and is in good agreement with the associated physics

What can be learned from the study of the hydrological evolution of the Ounianga lake region (NE Chad) over recent Holocene?

International audienceA hydrological view to the Ounianga lake region (NE Chad) evolution during the recent Holocene period is proposed. It relies upon multidisciplinary inputs combining hydrological characterization and modeling, results from climate modeling and from the recent sedimentary core analysis by Kroepelin et al. 2008 from lake Yoa. The latter shows that 4 major phases in drying have been observed in this Saharan region over the Holocene, including, (1) disappearance of tropical and altitude pollen species by 4300 cal yr BP, (2) evolution of the lake from a freshwater habitat to the present day hyper-saline oasis (around 3900 cal yr BP), (3) establishment of today's terrestrial desert ecosystem as the result of continuous vegetation succession (5600 to 2700 cal yr BP), (4) evolution in regional wind regime with onsets of major dust mobilization at roughly 4300 cal yr BP and establishment of modern, near continuous northeasterly winds by 2700 cal yr BP. The overall project strategy consists in studying the drying of the region and identifying the controlling parameters from a drastic climate change evolution, from humid (traces of tropical pollen types) to present day hyper arid environment. The interplay between climate change, vegetation, hydrological inputs from distant mountains and a regional aquifer makes this zone a complex and interesting system to study the impact of past climate changes. In a first step of the hydrological study, analysis of the topography of the zone (ArcGis with SRTM data and radar images) shows that the water catchment area of lake Yoa is large (67000 km², including the Erdi plateaux and part of the Tibesti mountain range, ie 18600 km²). A connection with the Tibesti mountains accounts for the altitude pollen source present in the core before 4300 cal yr BP. Globally, the lake is situated at a very favorable position that probably benefited from relatively large and perennial water inputs associated with distant recharge from the Tibesti massif and groundwater from the Nubian Sandstone Aquifer. In a second step, a groundwater model (Cast3M code) is developed for the SW part of the Nubian Sandstone Aquifer to quantify the slow discharge to the lake from a humid period (African Humid Period, 5000 BP roughly) until now. It is forced by precipitation and evaporation issued from climate simulations. Resulting piezometric head fields are compared with present piezometric levels. Lake levels are simulated as resulting from groundwater inputs and routing of surface water for the precipitation time history considered. The reconstruction is confronted to the evolution of salinity issued from the analysis by Kroepelin et al. 2008. Addressed herein are the issues of the level of confidence / uncertainty associated with the reconstruction of the drying phase, the identification of thresholds in the hydrological system influencing the change in the vegetation, the impact of future climate change on this system, lessons learned from the study of the paleo system

Coupled modeling of irradiated fuel thermochemistry and gas diffusion during severe accidents

In this paper, a novel approach where irradiated fuel thermochemistry and gas release are coupled is presented in details and illustrated by the simulations of some tests of the VERCORS program characterized by increasing temperatures and varying gas composition in the furnace (oxidizing or reducing conditions). At each step of the tests, the oxidation/reduction of the nuclear fuel and the fission product chemical speciation are precisely assessed thanks to a thermochemical equilibrium calculation relying on the OpenCalphad thermochemical solver and on a built-in thermochemical database derived from the SGTE database and completed by a solid solution model for the U-O-fission product system. Fission product releases are estimated from the chemically reactive gases that form in the fuel (according to the thermochemical calculation) and from a gas diffusion model based on the equivalent sphere model. The gas diffusion model takes into account not only the noble gases available in the fuel prior to the test but also the chemically reactive gases that form during the test. It is shown that the proposed coupled approach provides a consistent estimation of fission product release (I, Te, Cs, Mo, Ba) during the VERCORS tests in spite of the simple gas diffusion mechanism considered in the simulations (no distinction between the fission products). The proposed coupled approach is used to test some thermochemical hypotheses to improve the calculated release of some fission products (Ba, Mo)

Modeling of fission product release during severe accidents with the fuel performance code ALCYONE

This paper presents simulations of four tests performed on medium to high burnup fuel during the VERCORS and VERDON experimental programs. The tests are representative of a Severe Accident (SA) sequence with a temperature increase up to fuel-clad melting and oxidizing/reducing conditions within the furnace. The simulations are performed with the fuel performance code ALCYONE where irradiated fuel thermochemistry and fission gas release are coupled. In this paper, the impact of the radial burnup and Fission Product (FP) profiles within the fuel pellet on the FP release from the sample during the SA sequence is studied. Simulations of the fuel sample behavior during nominal irradiation in commercial reactors are first performed to assess the initial state of the fuel. The simulations of the SA sequences include a burnup dependent fission gas release model. The simulated release curves of various volatile and semi-volatile FPs (Xe, I, Te, Cs, Mo and Ba) are compared successfully to online measurements. The impact of the burnup and FP radial profiles on both the thermochemical equilibria within the pellet and the FP release kinetics is discussed. It is shown that the FP release from the fuel pellets is not significantly increased by the consideration of the burnup and FP radial profiles. This conclusion is due to the limited radial extension of the peaked burnup radial profile in the fuel pellet and to the uniform temperature of the fuel samples

Modeling high burnup fuel thermochemistry, fission product release and fuel melting during the VERDON 1 and RT6 tests

This paper presents simulations of the VERDON 1 and RT6 tests (temperature increase up to fuel-clad melting, oxidizing and/or reducing conditions within the furnace) performed with high burnup UO 2 fuel (i.e., up to 72 GWd/tU) and considering a coupling between irradiated fuel thermochemistry and a fission gas release model. The thermochemical calculations rely on the Thermodynamics of Advanced Fuels-International Database (TAF-ID) for the description of the phases likely to form from the 15 fission product considered in the fuel (Ba