New investigations of the Phenix negative reactivity events

In 1989 and 1990, four very fast and high amplitude decreases of the measured neutron flux, known as 'AURN' events, occurred in the Phenix fast neutron reactor and were not well explained despite a significant investigation program carried out in the 90 s. The perspective of the Phenix end of life led the French Nuclear Energy Div. to reinitiate AURN studies in particular with dedicated Phenix tests. It comes out from simulations with improved fluid-structure interaction model of the core movement that an outward movement of the sub-assemblies can generate a power variation consistent with the AURN signals. 'AURN' Phenix tests confirm the abnormal thermal behavior of an experimental sub-assembly, a new A URN scenario has been proposed on this basis. (authors)