“…The fission products fall into three groups which are important from a melt refining viewpoint, namely, (a) those which volatilize (volatile elements), (b) those which oxidize to form a dross which remains in the crucible as part of the skull after pouring (reactive eleinents), and (c) those which remain with the fuel and are not selectively removed (noble elements). (23,25) For some time after reactor startup, the fuel lost by burnup and in the melt refining skull will be replaced by fresh fuel material. Under this condition, the noble fission elements would build up with each successive reactor pass until an equilibrium would be reached wherein the amount produced per reactor pass would be equal to that removed with fuel in the metal part of the skull.…”