Evaluation of the Accident Scenario Initiated by a Total Instantaneous Blockage in a Phénix Subassembly

Upon request of the French Safety Authorities, the transient phase, initiated by a total instantaneous blockage (TIB) in a Phénix reactor fuel assembly, has been investigated. Based on an experimental program performed with the SCARABEE reactor, the phenomenology of the accident is first presented. The SIMMER code, which models fast reactor core disruptive accidents, is then used to analyze the transient phase in order to establish the chronology of the events occurring in the faulty assembly after its blockage. This study concludes that the time between the TIB and the onset of structure melting in the neighboring assemblies is sufficient to scram the reactor by control rod insertion before propagation of the accident to the rest of the core.