Abstract: A method and 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 in effective 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 (or volatilized, etc.) by the fluid moderator. The compositon 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.