Uranium dioxide pellets

Purpose: To relax stress concentration to fuel cans thereby preventing the rod rupture. Constitution: In uranium dioxide pellets, a surface layer with the oxygen/uranium atomic ratio from 2.2 to 2.4 is formed to the outer circumferential surface of the core with the oxygen/uranium atomic ratio from 1.999 to 2.03. That is, a uranium dioxide pellet with the oxygen/uranium atomic ratio from 1.99 to 2.03 is sintered at first by heating in a reducing atmosphere, for example, in hydrogen for three hours at about 1700 deg C. Then, the sintered uranium dioxide pellet is fabricated into a circular or square cylindrical shape. When the uranium dioxide pellet is heated in air, it is gradually oxidized usually depending on the heating temperature and the time. In view of the above, the uranium dioxide pellet with the oxygen/uranium atomic ratio from 1.99 to 2.03 is heated from the outside of the circumferential surface in an oxidizing atmosphere to form a surface layer with the oxygen/uranium atomic ratio from 2.2 to 2.4 to a predetermined thickness. (Yoshihara, H.)