The Phenix final tests

After the final shutdown of the Phenix sodium cooled fast reactor and before decommissioning, a final set of tests were performed covering core physics (5 tests: measurement of individual subassembly reactivity worth, decay heat measurement, control rod offsetting, the reactivity effect of a gas bubble, and control rod worth measurement)), fuel behavior (1 test: MOX fuel pin melting) and thermal-hydraulics areas (2 tests: asymmetrical regime and natural convection). Detailed analysis of the tests results is ongoing. It will allow the extension of code validation for core physics, for thermal-hydraulics and for fuel behavior. In addition, the program included two tests related to the comprehension of the four negative reactivity transients (AURN in French acronym) experienced during the reactor operation in 1989 and 1990. The analysis of the AURN transients performed in the early nineties had concluded that the most likely mechanism for a fast decrease of the core reactivity was a radial core deformation named core flowering. The core flowering test was carried out by introducing a mechanical device to deform Phenix core. The purpose of the test was to measure the reactivity effect of such deformation at subcritical and critical zero power conditions. The mechanical behaviour of the core was close to what was expected. Very small changes on core radius give significant reactivity modifications. The standard instrumentation of the reactor was completed by specifically designed devises. The PHENIX final tests program was performed successfully within the planned time schedule and provided a significant amount of data for the development of sodium fast reactors