Reactor dynamics of in-pin fuel motion in fast breeder reactors

Highlights: • Coupling of thermal hydraulics with reactor dynamics. • Molten fuel flow field benchmarking against CABRI-E9 (bis) test. • Neutronics impact of in-pin fuel motion on UTOPA. • Possibility of molten fuel vaporization during UTOPA. - Abstract: In-pin fuel motion is a multi-phase hydrodynamic movement of molten fuel inside the fuel pins of a fast reactor during a core disruptive accident. This study focuses on the potential consequences of this phenomenon on fast breeder reactor dynamics during an unprotected transient overpower (UTOP) accident. An in-house developed solver is benchmarked with the CABRI-E9bis test to addresses the complex interplay of hydrodynamic forces. It is deduced that in slow overpower, molten fuel hydrodynamics is dominated by gravity and solidification blockage. Next, the solver is integrated with an in-house reactor dynamics code 'PREDIS' to simulate UTOP under the effect of in-pin fuel motion. A 500 MWe mixed oxide fuel based fast reactor is chosen for analyses. Results show that in-pin fuel motion generates a negative reactivity feedback of the order of −0.45 $. This results in a reduced peak power (182%) and stabilized state power (175%). Damage to the reactor core is contained more effectively. It is found that in a beginning of equilibrium core (BOEC), high internal pressure of the fuel pin does not permit molten fuel vaporization at high temperatures. Higher reactivity insertion rates of up to 20 pcm/s are also countered with in-pin fuel motion. From this study, it is inferred that in-pin fuel motion feedback enhances the inherent safety features of fast reactors during UTOP. It scales in magnitude with the fuel Doppler, and is therefore an important parameter for future safety analyses of oxide fuelled fast reactors.