A seismic analysis has been performed showing that the seismic interaction between the UIS and the FHM can be avoided eliminating the internal clearance of bearing at the FHM arm joint. An angular contact ball bearing is suitable for the new FHM since it can eliminate gaps by preload pressure. A major problem of the FHM bearings is lubrication since the contact pressure between the steel rings and ball of the ball bearing is larger than that of the roller bearing used in the previous design. Additionally, FHM operating temperature is about 200deg-C and normal grease is not applicable under argon gas with sodium vapor to prevent contamination of grease in the primary sodium coolant. An endurance test with 1/10 scale bearings in the air has been performed to show applicability of angular contact ball bearings to the FHM arm joint. The results with 20,000cycle showed that bearings with combination of MoS2 coating steel rings and ceramics balls can be tolerable as the FHM operating condition. A full-scale bearing test in argon gas with sodium vapor has also been demonstrated to reveal bearing size and sodium vapor effects.

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