Before the definitive shutdown of the prototype Phénix, a final set of experiments was performed to gather important data about the operation and safety of sodium-cooled fast reactors (SFRs).

Among the accident sequences that are to be taken into account, inadvertent withdrawal of a control rod is considered. During operation at nominal power, such a sequence induces a general power increase and local deformations of the power shape. Afterward, local fuel temperature increases can thereby lead to fuel melting and clad failure.

The quasi-static control rod withdrawal test was specially designed to gather local power data on fissile assemblies and to complete validation databases of neutronic codes. The maximal deformation of the power shape reached ±12% and was obtained when two control rods were shifted in opposite directions.

The test analysis was conducted with the neutronics code ERANOS-2.2. Comparisons between calculated and measured values were satisfying. Most of the discrepancies in power estimation can be explained by measurement problems (heat transfer, sodium mixing).

The association of ERANOS-2.2 and the nuclear library JEFF-3.1, presently used for the predesign phase of the ASTRID reactor, constitutes an acceptable predictive tool for local and integral parameter estimations in SFRs, specifically in the evaluation of the control rod withdrawal incident.