Fuel densification and swelling: Relationship between burn-up induced axial and radial fuel dimensional changes

By comparison of axial length change measurements, both in-pile and PIE (Post Irradiated Examination), with radial fuel dimensional changes, the following are obtained: (1) measurements of in-pile axial fuel stack length generally show a densification which often occurs at beginning-of-life; (2) recent PIE measurements of radial dimensional changes have shown, however, that axial fuel stack length changes do not reflect properly the radial swelling which is taking place. The reason is because the swelling occurs primarily in the hot center portion of the fuel pellets, thus the pellet shoulders, which determine the axial dimensions, do not experience the swelling. (orig.)