Simulation with SAS-SFR of a ULOF transient on ASTRID-like core and analysis of molten clad relocation dynamics in heterogeneous subassemblies with SAS-SFR

摘要

In the framework of the ASTRID project, CEA, AREVA and EDF are working together to build a robust safety demonstration for innovative Sodium Fast Reactors (SFR). The CFV concept is a low sodium voiding effect core designed by the CEA to improve the reactor behavior in case of accidental transients. Up to now, no evaluation of this core during the primary phase of the accident was available as soon as the core degradation begins. Indeed some elements led to think that the dedicated code SAS-SFR would not be able to deal with the axial heterogeneity of this core. This paper aims at evaluating SAS-SFR ability to simulate an unprotected loss of flow (ULOF) transient applied to the CFV core and to assess the advantages of this core during such a transient. No specific problem has been identified which could invalidate the use of SAS-SFR in such conditions during the simulation. In particular, it has been checked that conditions to form two separate molten cavities in a pin were not met in this transient. The behavior of the core during the transient in terms of boiling phenomenon and its feedback with neutron physics is the same as what was expected according to the experience gained at EDF R&D on radially heterogeneous cores with equivalent neutronic properties. Namely, the core partial dry-out does not induce any power excursion since the reactivity feedback resulting from boiling remains negative. A focus on clad relocation in the axially heterogeneous sub-assemblies has shown that the cold fertile plate inserted in the inner core could trap molten clad in the middle of the core and thus could lower the reactivity insertion due to clad movement in the primary phase of the accident. However, a parametric study on the main parameters involved indicates that clad relocation kinetics and the resulting reactivity effects are very sensitive to vapor friction factor and to the axial power profile of the subassembly. Therefore, these parameters and their effect on transients will be further investigated in the frame of the ASTRID project in order to consolidate the preliminary conclusions on core behavior issues.