On the Dynamic Phase Transition for Nb-Containing Cladding Alloys

The licensing of new materials requires modifications to fuel performance codes that are extensively used by industry and safety authorities to verify compliance with the fuel safety criteria. Nevertheless, two types of fuel performance codes are generally being applied in the licensing process, corresponding to the normal operation and the design basis accident (DBA) conditions respectively. In order to simplify the code management by limiting the number of programs and in order to take advantage of the hardware improvements, one should generate a single fuel performance code that can cope with the different conditions. On one hand, extending the application range of a fuel performance code originally developed for steady-state conditions to accident conditions requires modifications to the basic equations in the thermal-mechanical description of the fuel rod behaviour, stable numerical algorithms and a proper time-step control, in addition to the implementation of specific models dealing with the high temperature behaviour of cladding such as observed under loss of coolant (LOCA) conditions. On the other hand, for fuel performance codes developed to simulate some aspects of the nuclear fuel behaviour under accident conditions, such as TESPA, MFPR, or FRAPTRAN, either the thermo-mechanical behaviour of the fuel must be incorporated and/or the extension of models to normal operating conditions is necessary to consider burnup dependent phenomena such as thermal conductivity degradation, fission gas release and swelling as well as cladding corrosion. Such a posteriori modifications of the fuel performance code may entail difficulties in terms of convergence and calculation time. Thanks to a clearly defined mechanical and mathematical framework as well as a consistent modelling, the TRANSURANUS fuel performance code has been able to cope with normal, off-normal and accidental operating conditions right from the beginning. Despite the fact that the numerical solution techniques enable handling of non-steady-state conditions, some of the phenomena important for DBA were not incorporated. As a first step towards this goal, a project was launched in order to extend the TRANSURANUS code capabilities to LOCA conditions. This project focused on the simulation of the Zr1%Nb cladding performance under LOCA conditions. In parallel to this project, similar models for Western type PWRs have been implemented and tested as well. There is, however, a difference between the models implemented for Zry-4 and E110 with respect to the crystallographic phase transformation. More precisely, the model for the onset temperature for the ¿¿¿ phase transition in E110 relies on the thermodynamic equilibrium curve that is independent of the temperature rate, whereas the model for the Zry-4 accounts for the shift of the onset temperature as a function of the heating or cooling rates (dynamic phase transition). The main reason for this discrepancy results from a lack of accurate experimental data reported in the open literature for E110. In order to fill the gap of experimental data, a series of measurements was launched by means of a differential scanning calorimeter (DSC). The main objective of the present manuscript is to present these results together with a new model for the crystallographic phase transition in E110 material that can deal with both static and dynamic conditions, and to compare this with that published for other cladding materials. The second objective of the paper is to present preliminary DSC measurements obtained on M5 samples.

VAN UFFELEN Paul;  BENES Ondrej;  VAN DE LAAR Jacques;  GYOERI Cs.; 

2008-12-16

Forschungszentrum Karlsruhe

JRC49018