Vertical thimbles extend from the top of the calandria up through the reactivity mechanism deck. They enclose the moderator and its cover gas while being immersed in the vault shield water. Metal bellows are welded to the deck penetrations to maintain a seal between the vault shield water and the reactivity mechanism deck. The bellows also permit free vertical movement to accommodate thermal expansion.
Horizontal thimbles extend from the shield wall through the vault wall to the calandria nozzles. These, too, are sealed by metallic bellows.