Abstract
Since a fast reactor core with uranium-plutonium fuel is not in its most reactive configuration under operating conditions, redistribution of the core materials (fuel, steel, sodium) during a core disruptive accident (CDA) may lead to recriticalities and as a consequence to severe nuclear power excursions. The prevention, or at least the mitigation, of core disruption is therefore of the utmost importance. In the current paper, we analyze an innovative fast reactor concept developed within the CP-ESFR European project, focusing on the phenomena affecting the initiation and the transition phases of an unprotected loss of flow (ULOF) accident. Key phenomena for the initiation phase are coolant boiling onset and further voiding of the core that lead to a reactivity increase in the case of a positive void reactivity effect. Therefore, the first level of optimization involves the reduction, by design, of the positive void effect in order to avoid entering a severe accident. If the core disruption cannot be avoided, the accident enters into the transition phase, characterized by the progression of core melting and recriticalities due to fuel compaction. Dedicated features that enhance and guarantee a sufficient and timely fuel discharge are considered for the optimization of this phase.
Highlights
For the long term nuclear energy sustainability, the transition to a fast reactor based fleet and the adoption of closed fuel cycles is envisaged, as indicated by several international studies [1,2,3]
Due to specific features of fast spectrum reactors, fuels containing a fraction of minor actinides (MAs), can be loaded into their cores and closed fuel cycles can be implemented providing an option for MAs transmutation [4,5,7,8]
For long term nuclear energy sustainability, the transition to a fast reactor based fleet and the use of closed fuel cycles is envisaged in many countries
Summary
For the long term nuclear energy sustainability, the transition to a fast reactor based fleet and the adoption of closed fuel cycles is envisaged, as indicated by several international studies [1,2,3]. In order to further reduce the void worth, other solutions have been investigated and introduced in the framework of the CP-ESFR project by [25]; e.g., adoption of pins of diluents or empty pins in each subassembly These attempts have a large impact on the other parameters (mainly power distribution) but they have been considered because some advantages (preferential path for corium relocation) are expected. For this reason in the paper, a special section is dedicated to the investigation of potential effects during the late phases of the accidents For this purpose, SIMMER-III analyses have been performed for a slightly different SFR model using 19 empty pins implemented into each fuel subassembly. The SIMMER–III (2D) and SIMMER–IV (3D) [31] multi-physics code systems have been developed primarily to analyze transients and accidents in fast reactors with liquid metal cooling (LMFRs) and are used as reference codes for severe accident simulation
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