[en] The effect is studied of fuel particle size on the course of power excursion in a fast reactor fuelled with Th-Pu0₂ kernels by taking into account the Doppler feedback effect as the excursion terminating mechanism. Reactor power, energy release, temperatures of the fissile and fertile parts are studied as a function of time for different reactivity inputs and sizes of the fuel particles. For temperature calculations the finite difference method has been employed to solve the heat conduction equations in spherical geometry. Power calculations have been performed by solving the point kinetics equations using Cohn's method. It has been found that: (a) high thermal conductivity and lower specific heat of thorium metal results in lower peak temperatures of kernel during the excursion: (b) as the fuel burn-up proceeds, larger reactivity additions could also be tolerated; and (c) the numerical approach to solve the heat conduction equations is quite straightforward and can be used conveniently to solve such problems. (author)