an apparatus for achieving increased fuel efficiency in a nuclear reactor wherein spectral shift is utilized to adjust for excess reactivity . this feature is achieved by stationary displacer rods within the fluid moderator of the reactor , with these stationary displacer rods decreasing ineffective volume during operation of the reactor whereby the effective volume of the fluid moderator increases as the nuclear fuel is burned . this decrease in effective volume is achieved by providing a sacrificial material in the displacer rods that is dissolved by the fluid moderator . the composition of the sacrificial material can be varied along the length of the rod so as to achieve the desired reduction of volume despite a temperature gradient along the rod .