During the post-DNB (Departure from Nucleate Boiling) phase of a reactivity initiated accident (RIA), the fuel cladding temperature may increase very steeply (~1000 °C/s) up to temperatures of around 900°C while the clads are internally pressurized. All these conditions lead to the ballooning of the fuel rod until its potential burst. This work aims at characterizing the creep behavior of the Zircaloy-4 cladding under such conditions.
In a first part, the (a -> B) phase transformation of the material was studied upon fast heating rates. Dilatometry tests were performed with heating rates ranging from 50 to 2000 °C/s. Special care was taken to the thermal measurements, to achieve sufficient accuracy under fast thermal transients. The material microstructure was also analyzed on quenched specimens. All these results enabled a phase transformation model to be characterized, based on a Leblond’s formulation.
In a second part, the experimental device ELLIE was updated to reproduce simulated thermo-mechanical post-DNB conditions on fuel cladding sections. Creep balloon ingtests were performed with well controlled thermal transients of about 1200 °C/s. Around twenty tests were carried out in simulated post-DNB conditions with internal pressures of 7 and 11 bar and for temperatures of interest ranging from 840 to 1020°C. Kinematic and thermal full-fields were obtained on the sample surface by stereo-correlation and near infrared thermography, respectively. A method was developed to obtain these two kinds of fields using the same two cameras. The tests performed highlighted a complex creep behavior in the two-phase domain, which appears to be closely correlated to the phase fraction within the material. A strong impact of the heating rate on the creep properties was also observed, with much higher strain rates following a high thermal transient. The finite element model updating (FEMU) method was used to identify the creep behavior of the cladding. The finite element model is based on a 3D representation of the region of interest seen by the cameras. A Love-Kirchhoff kinematic was imposed through the clad section using the kinematic field obtain by stereo-correlation. The identification method was first validated on a virtual case, and then enabled a creep law to be characterized, coupled to the phase fraction within the material. The model thus identified well reproduced the creep rates during the first ten seconds of the tests. A final study proposes to include a grain growth contribution in the creep law.