At the end of the 80s it clearly appeared that the development of a thermodynamic database devoted to the nuclear field was a necessity, and more precisely for the applications in safey and severe accident calculation codes in which the used data were simplistic. At the same time the emergence of the CALPHAD method allowed hope for important progress in the quality and reliability of thermodynamic data for complex thermochemical systems. More than 10 years were required to develop such a database because it was a large-scale task. Today NUCLEA, developed by THERMODATA/INPG/CNRS with the support of the French Institut de Radioprotection et de Sûreté Nucléaire, allows the thermodynamic properties and equilibrium determination in the chemical system with 18 elements Ag-Al-B-Ba-C-Ca-Cr-Fe-In-La-Mg-Ni-O-Ru-Si-Sr-U-Zr. These chemical elements include all the components which could interact in the case of an hypothetical accident in a nuclear plant, i.e. the fuel rod materials (UO2, Zr), the steel structures (Fe, Ni, Cr), the control rods (B, C, In, Ag), the assumed low volatile fission products (Ru, La, Ba, Sr) which could potentially contribute to the residual power and the concrete components (Al2O3, CaO, SiO2, MgO).
Today, the qualification of this large database is on the way. The matrix of validation is reported in this presentation. It contains experimental results obtained in multicomponent chemical systems (in particular U-O-Zr-Fe-Ni-Cr and SiO2-CaO-UO2-ZrO2) which allow to check the quality of the database in key composition domains for the safety analysis and the consistency of the CALPHAD approach to model very complex phase phenomena.
In the same time, the development of the database continues on the basis of new experimental data. The thermodynamic modelling of the U-O phase diagram is of a first importance in the development of a nuclear thermodynamic database. The experimental data concerning phase diagram and compound properties are very numerous and their compilation has been undertaken for many years and their modelling published in successive papers. Among these data, the oxygen potential values are of great importance for the modelling in order to fix the energy levels of the different phases. At high temperatures (T> 2000 K), the available information is not very abundant. Two invariant reactions may occur, e.g. (1) U3O8 <-> Gas + UO2+x and (2) Gas + UO2+x <-> Liquid. The temperature of the second reaction 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. Up to now the optimisation of the parameters of the model was based on the Latta’s data which was suspected of crucible contamination. Liquidus and solidus temperatures were recently and accurately re-measured in the UO2+x composition domain by Manara. A new set of parameters 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 of 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. This point will be illustrated by what was observed in some of the VERCORS tests where fuel collapse was detected in the temperature range of 2400-2600 K. Similar indications of early fuel collapse were identified in the PHEBUS integral tests performed under 2 atmospheres.
(1) : IRSN
(2) : THERMODATA/INPG/CNRS