Oxide phase characterization in simulated high burn-up UO2 fuels in the early stages of a nuclear severe accident

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Electrochemical characterisation of the Ce(IV) limiting carbonate complex

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3d thermo-chemical-mechanical simulation of power ramps with alcyone fuel code

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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

3D simulation of a power ramp including oxygen thermo-diffusion and its impact on thermochemistry

International audienceThis paper presents the coupling of the thermochemical solver ANGE (Advanced Gibbs Energy Minimizer) with an oxygen thermo-diffusion model. The coupling is implemented within the fuel performance code ALCYONE co-developed by CEA, EDF and FRAMATOME within the PLEIADES environment. An application to a 3D simulation of a power ramp on a Cr-doped UO 2 fuel is developed. Post-ramp EPMA measurements of chromia doped fuel show reduction of chromium and molybdenum oxides in the central part of the pellet, indicative of thermo-diffusion of oxygen. These phenomena are well reproduced by the coupled thermo-chemical-mechanical simulations. Impact of oxygen redistribution on speciation of fission products is then studied. Chemical state of caesium, iodine and tellurium is important as regard PCI, as they can form gaseous species (CsI$_{(g),}$ I $_{(g),}$ I$_{(2g),}$ TeI$_{(2g)}$) that can react with the cladding and induce SCC. Release of gaseous species and concentration of chemically reactive iodine compounds near the cladding are calculated in order to investigate I-SCC

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

Experimental Evidence of Oxygen Thermo-Migration in PWR UO2_2 Fuels during Power Ramps using In-situ Oxido-Reduction Indicators

International audienceThe present study describes the in-situ electrochemical modifications which affect irradiated PWR UO$_2$ fuels in the course of a power ramp, by means of in-situ oxido-reduction indicators such as chromium or neo-formed chemical phases. It is shown that irradiated fuels (of nominal stoichiometry close to 2.000) under temperature gradient such as that occurring during high power transients are submitted to strong oxido-reduction perturbations, owing to radial migration of oxygen from the hot center to the cold periphery of the pellet. The oxygen redistribution, similar to that encountered in Sodium Fast Reactors fuels, induces a massive reduction/precipitation of the fission products Mo, Ru, Tc and Cr (if present) in the high temperature pellet section and the formation of highly oxidized neo-formed grey phases of U$_4$O$_9$ type in its cold section, of lower temperature.The parameters governing the oxidation states of UO$_2$ fuels under power ramps are finally debated from a cross-analysis of our results and other published information. The potential chemical benefits brought by oxido-reductive additives in UO$_2$ fuel such as chromium oxide, in connection with their oxygen buffering properties, are discussed

Intercomparison between TRIO-EF and IMPACT codes with reference to experimental strontium migration data.

International audienceThis work presents an intercomparison exercise between two geochemical migration codes, TRIO-EF (an object-oriented finite element code) and IMPACT (a chemical engineering code using mixing cells in series). The predictions of the two codes are compared with the reference experimental results obtained in a previous study of strontium transport in soil columns. This simulated geochemical system is well documented, and includes ion exchange and dissolution-precipitation reactions. The solution transport is simulated by a one-dimensional advection-dispersion model. The predictions of TRIO-EF and IMPACT are both in good agreement with the experimental results. However, slight differences can be observed between the two codes, especially when concentration discontinuities are involved, such as precipitation fronts or changes in boundary conditions. These discrepancies between the two codes can mainly be attributed to the different discretisation approaches

3D simulation of a power ramp including fuel thermochemistry and oxygen thermodiffusion

cited By 0This paper presents 3D simulations of power ramps in pressurized water reactors with the fuel performance code ALCYONE, which is part of the computing environment PLEIADES. The code has been upgraded to couple the description of irradiated fuel thermochemistry already available with oxygen transport taking into account thermodiffusion. The impact of oxygen redistribution during a power transient on irradiated fuel thermochemistry in the fuel and on chemically reactive gas release from the fuel (I ( g ) , I 2 ( g ) , CsI ( g ) , TeI 2 ( g ) , Cs ( g ) , Cs 2 ( g ) ) is studied. The simulations show that oxygen redistribution, even if moderate in magnitude, leads to the reduction of metallic oxides (molybdenum dioxide, cesium molybdates, chromium oxide) at the fuel pellet center and consequently to the release of a much greater quantity of gaseous cesium. Pellet-Cladding Interaction failure propensity is shown to decrease in consequence of the thermodynamically favourable reaction of iodine with released cesium in the free volume of the rod. © 2019 Elsevier B.V