The Mark IIB oscillator rod enables measurement of reactor transfer functions by rotating an absorber material (boron carbide) toward and away from the center of the reactor to perturb reactivity. Its design has eliminated two operating problems encountered with previous EBR-II oscillator rods: bowing and consequent rubbing of the rod in its guide tube, and failure of the seals that prevent air from contacting the sodium coolant. The Mark IIB rod and its drive were installed in EBR-II in April 1971. Both haveperformed satisfactorily since then.