Relative Control Rod Worths of Some Rare Earth Oxides

In order to determine the relative merits of the various rare earths as reactor control materials, a series of relative worth measurements was made in the Dresden Critical Assembly at Vallecitos Atomic Laboratory. Combinations of the oxides of five rare earths with highest thermal cross sections (dysprosium, erbium, europium, gadolinium, and samarium) were compared with cadmium and 2 per cent boron steel. Dy₂O₃, Gd₂O₃, and Sm₂O₃, separately and in combination, were found to be roughly equivalent in worth. Eu₂O₃ was found to be the strongest absorber. Er₂O₃ was found to be much less effective than the other materials studied.