Irradiation performance of sodium-bonded control rod for the fast breeder reactor

The lifetime of control rods is limited by the absorber (B4C pellets)-cladding mechanical interaction (ACMI). Therefore, sodium (Na)-bonded control rods were developed for long-life control rods. Na-bonded control rods have been irradiated in the experimental fast breeder reactor, JOYO MK-III, and the diametrical changes of the Na-bonded absorber pins after the irradiation were measured in detail. In this paper, these detailed measurements were compared with the results obtained in helium (He)-bonded control rods with and without the shroud tube in a wide burn-up range. From the comparison, it was found that the diametric changes were smaller in the Na-bonded absorber pins than in the He-bonded ones. It was concluded that the Na-bonded absorber pins are very effective for achieving long-life control rods.