Coupled thermalhydraulic-neutronic stability extended criterion in a SFR core

Highlights: • A coupled thermalhydraulic-neutronic flow stability criterion is established for SFRs. • Pressure drop and power evolutions are considered in the criterion. • A linear stability analysis of 2 equations expressing flow and power evolution is done. • Reactivity feedback of enthalpy change on SA power is modelled. • The capability of the criterion to predict ULOF flow behavior is shown. - Abstract: A coupled thermalhydraulic-neutronic extended criterion developed in order to assess the conditions of static instability of the sodium flow in a SFR core during the beginning of an unprotected loss of flow (ULOF) is presented in the first part of this paper. This extended criterion takes into account the evolution of the pressure drop in a sub-assembly (SA) as well as the evolution of its power when the reactor is affected by a flow rate (or power) perturbation by starting from a steady-state. The considered steady states are typical of quasi-steady states reached by a SFR during a ULOF. The temporal evolution of the flow and the temporal evolution of the power can be represented by a system of two differential equations whose linear stability analysis has enabled to define the aforementioned extended criterion. Then, in the second part of the paper, a verification of this extended criterion is performed by simulating the behavior of a GEN IV SFR of 1500 MWth. Four various steady states of the reactor have been investigated with a simulation tool devoted to unprotected loss of flow simulations (MACARENa). First simulations have enabled to calculate the various physical parameters needed to check the extended criterion. Perturbations have been applied to these four states. The extended criterion, proposed in this paper, has predicted flow stabilization for 3 states whereas it has predicted a flow excursion (sudden flow rate decrease leading to SA dry-out) for the last state. In this latter case, the classical Ledinegg criterion, which relies only on pressure drop evolution at constant power, has predicted a stable configuration. Then, the extended criterion predictions have been verified with MACARENa simulations, by slightly and temporary reducing the driving pressure head of the flow rate in the studied SA from each of the four initial states. For stable states, when setting again the pressure head of the flow to the value of the initial state, the initial flow rate is recovered. For the last initial state (predicted as unstable by the extended criterion), after a convergence period carried-out by imposing the SA steady flow rate, the solving of the transient momentum equation have shown that a flow excursion occurs in the MACARENa simulation as expected from the extended criterion. Consequently, the extended criterion proposed in this paper has been verified and could enable to define stable and unstable domain of couple flow rate/power in order to design future cores.