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Modern thermodynamic assessment studies of multicomponent systems typically employ the Compound Energy Formalism (CEF) to describe phases of variable composition, which may show effects of chemical ordering. In CEF, the ordering is described by splitting a crystallographic site in which several components can substitute for each other into subsites (sublattices) and by allowing fractional occupancies of these subsites to vary. This type of component redistribution over subsites (sublattices) is classified in solid solution theories as long-range order (LRO). A well-known shortcoming of CEF is the absence of a direct description of the effects of short-range order (SRO), which do not involve a redistribution of components between sublattices, but reveal themselves in various modes of clustering. Here we show that SRO can be taken into account within CEF as an additional (fictive) LRO effect driven by an extra negative excess enthalpy term. We argue that the mathematical form of this term should be constrained such that it ensures its vanishing both in low- and high-temperature limits. Including such a term into CEF allows a significant improvement in the description of phase relations in the system of calcite-magnesite, CaCO3-MgCO3. The modified CEF model with SRO emulation is made available in the GEM-Selektor software.

The release of fission product and salt compounds from a molten salt reactor fuel under accident conditions was investigated with coupled computer simulations. The thermodynamic modeling of the salt and fission product mixture was performed in The Gibbs Energy Minimization Software GEMS and the obtained compound vapor pressures were exchanged with the severe accident code MELCOR, where the evaporation from a salt surface located at the bottom of a confinement building was simulated. The fuel salt considered in the simulations was LiF-ThF4-UF4 with fission products Cs and I. The composition of the fuel salt material was obtained from an equilibrium fuel cycle simulation of the salt using the EQL0D routine coupled to the Serpent 2 code. The results were compared to simulations using pure compound vapor pressures in the evaporation simulations. It was observed that by modeling the salt mixing the release of fission products and salt materials was reduced when compared to the pure compound simulations. The mixing effects in the salt, when compared to the pure compound simulation also affected evaporation temperatures and therefore the timing of the release of compounds. In an additional simulation in which the depressurization of the confinement was considered, the total evaporated mass of compounds increased due to increased mass transfer at the salt surface. The simulation process described in this paper can be used for a more comprehensive accident analysis of molten salt reactors once the detailed description of the reactor confinement and accident sequences are available and more fission product elements have been added to the analysis.

The enhancement of safety, sustainability, and social acceptance is the main requirements for a nuclear renaissance. Two Gen IV concepts, the molten salt reactor and the high-temperature reactor, have a potential for the paradigm shift in nuclear safety, and Switzerland is involved in the international research efforts dedicated to both of these reactors. Due to the liquid fuel state, especially molten salt reactor (MSR) can act as a safe, sustainable, and acceptable reactor for the future. Main aim of the MSR research program in Switzerland is the safety. Nonetheless, possible MSR designs and so the fuel cycles are also evaluated. At the same time, tools are being developed for MSR transient analysis and thermochemical simulations.

The treatment of chemistry and thermodynamics in the integral severe accident codes is typically limited. A more accurate treatment of the chemistry during the severe accident modelling is, therefore, of great interest. For this purpose, the work is focused on the development of coupling of the MELCOR code with chemical thermodynamic calculations using GEMS codes and HERACLES database. Developed coupling between the two codes, called cGEMS, allowed for the improved thermodynamic description of the fission product release from the nuclear fuel under severe accident conditions. VERDON-1 test was selected as an experimental reference for the simulations. Experimental release behaviour of Mo, Cs and Ba observed in VERDON-1 experiment was reproduced by the developed coupled code. Performed simulations provided detailed information about the fission product speciation at different redox conditions. The obtained information and the developed code provides a more accurate description of the fission product behaviour and release during severe accidents.

Classical Molecular Dynamics (MD) was used to investigate the effect of nanometric size pores on the thermal conductivity of irradiated UO 2 . The Green–Kubo approach was used for the thermal conductivity calculation. The effects of pores size, porosity and pores separation were simulated. A comparison with existing theoretical models is presented and an analytical model adapted to irradiated fuel is obtained. The results demonstrate that, for realistic bubbles size and concentrations, the impact on the fuel thermal conductivity is higher than predicted by the correlations used to quantify the impact of porosity: the impact of 0.3 vol.% of nanometric pores is of the same order of magnitude as that of 4.5 vol.% of micrometric pores.

There is still no clear understanding of the plutonium content influence on the thermal conductivity behaviour of the (U,Pu) O2 MOX fuels. In this work Classical Molecular Dynamics (MD) was used to investigate the (U,Pu) O2 thermal conductivity in the whole concentration range and in the temperature range from 400 K to 1600 K. The Green–Kubo approach was used for the thermal conductivity calculation and an algorithm was proposed to improve the accuracy of the calculation. The obtained results are in good agreement with the literature experimental data and results of modelling of other authors. On the basis of the obtained results we give recommendations for the MOX thermal conductivity evaluation in the concentration range from pure UO2 up to pure PuO2.

In this study, we present new experimental heat capacity data of solid and liquid phase of CsF covering the temperature range from 5 K to 1400 K. The low temperature data were obtained by adiabatic calorimetry and compared with the theoretical heat capacity computed from ab initio harmonic crystal approximation. The absolute entropy at T = 298.15 K determined based on the presented results is 93.60 J · K - 1 · mol - 1 . The high temperature heat capacity was measured using a drop calorimeter and the data for the solid phase revealed significant excess properties. The data for the liquid phase were correlated with the molecular dynamic simulation obtaining a good agreement. The recommended molar heat capacity of the CsF solid phase is: C p , m ( J · K - 1 · mol - 1 ) = 24.291 + 6.4607 · 10 - 2 · ( T / K ) + 589981 · ( T / K ) - 2 J · K - 1 · mol - 1 and the one of the liquid phase was determined as: C p , m = 70.56 J · K - 1 · mol - 1 . From the calorimetric results the enthalpy of fusion of CsF was determined as: △ fus H = 22.1 ± 2.0 kJ · mol - 1 .

In the present work, calculations of the thermal conductivity of UO 2 were carried out applying classical Molecular Dynamics for the isothermal-isobaric (NPT) statistical ensemble, using the Green–Kubo approach. The thermal conductivity calculated for perfect stoichiometric UO 2 is in good agreement with the literature data over the temperature range corresponding to heat transfer by phonons (up to 1700 K). The effect of non-stoichiometry on the thermal conductivity was calculated taking into account the presence of polarons. It was found that for the same value of the stoichiometry deviation, the effect of oxygen vacancies (hypo-stoichiometry) is more pronounced than the effect of oxygen interstitials (hyper-stoichiometry). Then the influence of the oxygen Frenkel pairs on the thermal conductivity was calculated. The simultaneous impact of non-stoichiometry and OFP on the thermal conductivity was investigated and it was shown that the two effects can be combined using the interpretation obtained with the classical phonons scattering theory. Finally, simplified correlations were deduced for the calculation of the thermal conductivity of UO 2 taking into account the effect of non-stoichiometry and of Frenkel pairs, these two effects being present during irradiation.

This paper reports new experimental data for the specific enthalpy of the solution of isobutane (refrigerant R600a, CH(CH3)2−CH3, CAS Registry No. 75-28-5) with mineral naphthenic compressor oil WF 15A. The experimental data were obtained in the temperature range from (273 to 355) K by a mixing method realized in an ice calorimeter. The behavior of the temperature and weight fraction dependencies of the enthalpy for the refrigerant−oil solution (ROS) of R600a and mineral compressor oil WF 15A have been examined.

This paper presents a theoretical evaluation of the influence of compressor oil admixtures on the thermodynamic performance of a vapor compression system using natural refrigerant R600a. The performance determination is based on the developed pressure–enthalpy diagrams (P–h) for the refrigerant oil solution (isobutane-mineral compressor oil Azmol). A method for calculating the enthalpy of refrigerant–oil solutions has been proposed and the influence of the compressor oil admixtures to isotherms the pressure–enthalpy diagrams has been analyzed. The change of enthalpy at the different oil concentrations in the working fluid in an evaporator has been investigated. An application of the developed refrigerant–oil P–h diagrams for the theoretical evaluation of the efficiency of the vapor compression systems is also demonstrated.

International audience; This article presents a synthesis of the investigations at the atomic scale of the transport properties of defects and fission gases in uranium dioxide, as well as of the transfer of results from the atomic scale to models at the mesoscopic scale, performed during the F-BRIDGE European project (2008-2012). We first present the mesoscale models used to investigate uranium oxide fuel under irradiation, and in particular the cluster dynamics and kinetic Monte Carlo methods employed to model the behaviour of defects and fission gases in UO$_2$ , as well as the parameters of these models. Second, we describe briefly the atomic scale methods employed, i.e. electronic structure calculations and empirical potential methods. Then, we show the results of the calculation of the data necessary for the mesoscale models using these atomic scale methods. Finally, we summarise the links built between the atomic and mesoscopic scale by listing the data calculated at the atomic scale which are to be used as input in mesoscale modelling. Despite specific difficulties in the description of fuel materials, the results obtained in F-BRIDGE show that atomic scale modelling methods are now mature enough to obtain precise data to feed higher scale models and help interpret experiments on nuclear fuels. These methods bring valuable insight, in particular the formation, binding and migration energies of point and extended defects, fission product local-ization, incorporation energies and migration pathways, elementary mechanisms of irradiation induced processes. These studies open the way for the investigation of other significant phenomena involved in fuel behaviour, in particular the thermochemical and thermomechanical properties and their evolution in-pile, complex microstructures, as well as of more complex fuels.