Fast spectrum minor actinide (MA) burner designs, with high minor actinide loads and consumptions, have been assessed. As reactivity and kinetic coefficients are poor in such cores (low delayed neutron fraction and Doppler feedback, high coolant void coefficient), special attention has been paid to their dynamic behavior during transient conditions. A dynamics code, MAT4 DYN, has been expressly developed to study loss-of-flow, reactivity insertion, and loss-of-coolant accidents. It takes into account two fuel geometries (cylindrical and spherical) and two thermal-hydraulics models for the coolant (incompressible for liquid metals and compressible for helium).

Three nitride-fuel configurations are analyzed according to their coolant: sodium and lead (both with pin fuel) and helium (with particle fuel). Dynamics calculations show that if the fuel nature is appropriately chosen, with sufficient margins during transients, then this can counterbalance the poor reactivity coefficients for liquid-metal-cooled cores, thus proving the interest of this kind of concept. On the other hand, the gas-cooled core dynamics is very badly affected by the high value of the helium void coefficient in a hard spectrum, this effect being amplified by the very low thermal inertia of the fuel particles. Hence, concepts other than a particle-bed fuel should be investigated for a helium-cooled fast-spectrum MA burner.