PURPOSE: To realize a reactor of a pebble bed type which can efficiently convert fertile nuclides into fissionable ones by using burning fuel balls constituted substantially of fissionable nuclides and breeding fuel balls constituted substantially of fertile nuclides.

CONSTITUTION: Burning fuel ball and breeding fuel balls consisting substantially of fissionable nuclides (e.g. Pu239) and fertile nuclides (e.g. U238) are loaded in a core and a reactor is actuated. Since fissionable nuclides decrease in number with the progress of burning, fuel balls are taken out from the lower part of the core. The breeding fuel balls which have been taken out are reloaded in the core over and over again as long as the integrity of fuel is maintained. The burn-up of the breeding fuel balls which have been taken out is measured with a burn-up measuring instrument, and fuel balls which do not reach the prescribed burn-up are reloaded in the core. Burning fuel balls which reach the prescribed burn-up are removed, and burning fuel balls substantially of the same number as the removed ones are loaded from the upper part of the core. This enables breeding fuel balls to stay in a core for a long time and fertile nuclides to be converted into fissionable nuclides sufficiently.