Interaction between caesium iodide particles and gaseous boric acid in a flowing system through a thermal gradient tube (1030 K–450 K) and analysis with ASTEC/SOPHAEROS

International audienceThe present work aimed at studying the interaction between caesium iodide particles and gaseous boric acidthrough a Thermal Gradient Tube (TGT) from 1023 K to 453 K under Ar/H2O. Particles size range of transportedparticles was measured by ELPI and the fraction of gaseous compounds by ICP-MS and UV–visible spectroscopy.Reaction between the two compounds was deduced by measuring a significant fraction of gaseous iodine at theoutlet of the facility, representing more than 80% of the total iodine sampled at the outlet. The reaction rate wasshown to be lower when the flow rate inside the facility was increased. Analysis with SOPHAEROS module ofASTEC code was performed. The ASTEC fission products models allowed performing the evaluation of theexperimentally observed results for the analysis of the transport of pure compounds. However, the heterogeneousinteraction between the caesium iodide particles and the gaseous boric acid was not reproduced, as the modelsare not taken into account in the version v2.1_1_6 of the ASTEC/SOPHAEROS module. The next step would be toidentify the mechanism of the reaction by comparing the results with other studies and to determine the reactionrates. Then, a first development in SOPHAEROS would be to implement such phenomena

Corium experimental thermodynamics: A review and some perspectives

International audienceMore than thirty years ago a specialist meeting was held at Joint Research Center Ispra (Italy) from 15 to 17th January 1990 to review the current understanding of chemistry during severe accidents in Light Water Reactors (LWR). Let consider that, at the end of the 1980s, thermodynamics introduced in the severe accident codes was really poor, only some equilibrium constants for few simple reactions between stoichiometric compounds were used or some simple correlations giving estimates of solidus and liquidus temperatures. In the same time, the CALPHAD method was developed and was full of promise to approximate the thermodynamic properties of a complex thermochemical system by the way of a critical assessment of experimental data, a definition of a simple physical model and an optimization procedure to define the values of the model parameters. It was evident that a nuclear thermodynamic database had to be developed with that new technique to obtain quite rapidly a prominent progress in the knowledge of thermochemistry in the severe accident research area Discussions focused on the important chemical phenomena that could occur across the wide range of conditions of a damaged nuclear plant. The most pressing need for improved chemical models is identified with condensed phase mixtures to model the corium progression. This paper reviews more than 30 years of experimental data production related to corium thermodynamics. This work has been conducted through multiple international programs (EURATOM, ISTC, OECD) as well as through more specific studies conducted at the national scale. This research has been capitalized in specific databases such as NUCLEA and TAF-ID, databases developed at IRSN and at CEA respectively, and are now used in degradation models in severe accident simulation codes. In the conclusion, we outline the research perspectives that need to be considered in order to address today's and tomorrow's issues

Phenomenology of BWR fuel assembly degradation

International audienceSevere accidents occurred at the Fukushima-Daiichi Nuclear Power Station (FDNPS) which required an immediate re-examination of fuel degradation phenomenology. The present paper reviews the updated knowledge on the phenomenology of the fuel degradation, focusing mainly on the BWR fuel assembly degradation at the macroscopic scale and that of the individual interactions at the meso-scale. Oxidation of boron carbide (B4C) control rods potentially generates far larger amounts of heat and hydrogen under BWR accident conditions. All integral tests with B4C control rods or control blades have shown early failure, liquefaction, relocation and oxidation of B4C starting at temperatures around 1250 °C, well below the significant interaction temperatures of UO2-Zry. These interactions or reactions potentially influence the progress of fuel degradation in the early phase. The steam-starved conditions, which are being discussed as a likely scenario at the FDNPS accident, highly influence the individual interactions and potentially lead the fuel degradation in non-prototypical directions. The detailed phenomenology of individual interactions and their influence on the transient and on the late phase of the severe accidents are also discussed. © 2017 Elsevier B.V

Chemical thermodynamics of the Ag-Zr system

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Chemical thermodynamics of the Ag-Zr system

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Volatilization and trapping of ruthenium under a loss of cooling accident on high level liquid waste (HLLW) storage tanks in reprocessing plants

International audienceThe reprocessing of spent nuclear fuel produces high level liquid waste (HLLW). Due to the decay heat, these concentrated nitric solutions containing fission products are stored in cooled tanks to prevent the solution from boiling, evaporating and drying out. In case of a total loss of cooling, potential large releases of radioactive materials into the environment, especially volatile species derived from ruthenium, can happen. The loss-of-cooling accident on HLLW storage tanks is one of the accident scenarios identified as a very dreaded situation. Besides, an extensive literature review performed at IRSN confirms the lack of reliable data on the behaviour of ruthenium in nitric acid solutions and concerning mechanism of releases. It highlights that research works on this topic can be classified in several categories: ruthenium chemistry in a nitric medium characterized by the formation of nitrosyl ruthenium ion RuNO3+; behaviour of volatile forms of ruthenium in presence of steam, nitric acid vapour and nitrogen oxides (recombination, decomposition, etc.); transfer phenomena and stability of the different gaseous species containing ruthenium through the ventilation network. Subsequently, the efficiency and the performance of various trapping systems that can be used for mitigation of ruthenium release (gas/liquid absorbers/traps, steel filters, porous media such as zeolites etc.), or even various means of preventing its volatilization (recombination, addition of reducing agents in situ, etc.) have been investigated by different authors. Previous experimental work performed at IRSN on severe accident scenarios in nuclear facilities allowed to characterize usual filtration devices such as active charcoals or metallic filters, with respect to gaseous RuO4. It showed that these latter do not trap efficiently RuO4(g).From these findings, IRSN started a research program aiming at improving the knowledge on this topic. A specific test bench has been developed in order to study the volatilization of a nitric acid solution containing Ru nitrosyl, simulating a real HLLW in terms of acidity and ruthenium concentration, and to investigate the possible inhibition of Ru volatilization by addition of specific reducing compounds (nitrogen oxides, sucrose, etc). A first series of tests showed that the quantities of released ruthenium obtained for different temperature levels are consistent with the literature, before testing inhibitors. The experimental setup mentioned before dedicated to gaseous RuO4 is also used to study trapping of RuO4(g) by different porous materials: zeolites, rare earth oxides, etc. Decontamination factors (DF) and RuO4(g) retention capacity have been determined for several of these compounds

About the active role played by the UO2 oxidation on irradiated fuel collapse temperature : towards an alternative explanation

International audienceThe analysis of the PHEBUS FPT0 and FPT1 tests and some VERCORS tests performed under oxidising atmospheres with non irradiated (FPT0) and irradiated UO2 fuel evidenced that the fuel collapse temperature was well below the standard melting temperature of UO2 (3120 K) and above all lower than the 'eutectic' temperature of the UO2-ZrO2 pseudo-binary phase diagram (2820 K). Different assumptions as the role played by the fission products on the fuel degradation mechanism or the interaction between the structural materials and the fuel rods in the PHEBUS tests were advanced to explain a such behaviour. The fuel oxidation was also considered but on the basis of the old experimental data of Latta et al. , it was not retained as a dominant process . Liquidus and solidus temperatures were recently re-measured in the UO2+x composition domain by Manara . The main difference with the Latta's data is that the Manara's transition temperatures were accurately determined using a self-crucible technique while the former data were obtained in a W crucible and then suspected of crucible contamination. According to this recent data, a new thermodynamic modelling of U-O phase diagram is presented and introduced in the European NUCLEA thermodynamic database for Corium Applications . At high temperatures (T> 2000 K), the available information is not very abundant. Two invariant reactions may occur, e.g. (1) U3O8 ó Gas + UO2+x and (2) Gas + UO2+x ó Liquid. The temperature of the second reaction which is of great interest for the fuel collapse temperature in oxidising conditions is linked on the one hand, to the values of the oxygen potentials in the hyperstoichiometric solid solution region, which are extrapolated from the low temperature data and on the other hand, to the experimental data concerning the (Liquid + UO2+x) diphasic equilibrium. These latter values are properly defined if the liquidus shape is precisely known in this composition field. The new set of parameters fitting the Manara's data is then presented and it shows that the temperature of the reaction (2) is located around 2700 K at atmospheric pressure. An important consequence of this new optimisation for safety applications is that a liquid phase may appear in the O-UO2-ZrO2 composition domain of the U-O-Zr phase diagram at 2600 K at atmospheric pressure (this temperature decreasing with increase of pressure, about 2500 K at 2 atm.). This temperature should be still decreased by 100 K, depending of the physical model considered for the Gibbs energy description of the (U,Zr)O2+x fluorite structure. These temperatures can be associated with the temperature at which the fuel assembly could lose its integrity in oxidising conditions and then with what was observed in some of the VERCORS tests where fuel collapse was detected in the temperature range of 2400-2600 K or in the PHEBUS tests where indications of early fuel collapse at 2500-2600 K were identified

Energy landscape of hydrogen in the vicinity of monovacancy in beryllium

International audienceThe present work is devoted to fill the lack of data related to the hydrogen behavior in the vicinity of monovacancy in the hexagonal compact structure of beryllium. These data are of importance to feed coupled reaction–diffusion models dedicated to the simulation of hydrogen behavior in beryllium in order to analyze its thermal release as function of concentration and temperature. This work aims to evaluate the trapping and detrapping energies for several pathways surrounding a vacancy as well as the impact of multiple trapping of hydrogen on these data. In addition, the data to compute the detrapping rates for a single hydrogen are given. The determination of the energy landscape in the vicinity of the monovacancy has shown that the trapping energy varies from 0.39 eV (equal to the activation energy for diffusion in non-defective bulk) down to 0.12 eV while the activation energy for detrapping mechanism rises to 1.08 eV up to 1.48 eV. The lowest detrapping energy, 0.86 eV, is obtained from hydrogen filled vacancy whereas the multi-trapping does not significantly affect the trapping energy

About the enthalpy of formation of the UZr2 compound

International audienceThe U-Zr phase diagram has been the subject of many recent studies, particularly for the development of metallic fuels for future fast reactors. The so-called -UZr2 phase is part of this renewed interest as it is likely to be formed during irradiation of U-Zr10wt% fuel. Although this phase has been the subject of several structural studies, there is a single experimental determination of its enthalpy of formation by dissolution calorimetry. Most of the Calphad and DFT models of this phase rely on this value to support their validation. In this paper, we propose a reinterpretation of this measurement which tends to show that the original interpretation of the authors was probably wrong. However, our reinterpretation leads to a new endothermic estimate of the standard enthalpy of formation of -UZr2 difficult to reconcile with the phase diagram. The need for new heat capacity and formation enthalpy measurements is highlighted. The present work does not invalidate the most recent CALPHAD descriptions

Chemical thermodynamics of the Ag-Zr system

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