The GEFEST800 software-hardware system was developed for performing real-time neutron-physical calculations of the BN-800 sodium-cooled fast reactor, viz. calculations of the effective breeding ratio, maximum reactivity excess, efficiency of single and groups of control rods, total, specific, and linear density of power release in fuel assemblies, coefficients of nonuniformity of energy release in fuel assemblies and the reactor, damaging dose of irradiation of fuel assemblies, fuel burnup, reactivity coefficients (temperature, power, density) and void effect of reactivity, effective delayed-neutron fraction, characteristics of transient processes for normal operating regimes, residual energy release, activity of spent fuel assemblies, and others. The software-hardware system provides for calculations of normal operating regimes (stationary and nonstationary regimes at the lowest controllable power level and at power while controlling transient processes with rods, fuel burnup).