Model of the mechanical behaviour of a PWR fuel element

Irradiated fuel in pressurized water reactors (PWRs) frequently displays rod bowing, due to two kinds of assymmetry. The first originates in the fabrication of the sheath, causing eccentricity, ovalization and thickness non-uniformity. The other comes from in-pile fuel element conditions such as off-line grids compressing the rods, circumferential thermal gradients on the sheath, and pellet-clad interactions. The MAC code was developed for parametric studies of some of these effects. It shows that: In the case of fuel rods undergoing compressive forces by the spacer grids the usual friction forces are unable to bow the rods significantly, except when the rods are blocked by the spacer grid springs. In some assembly configurations, the temperature difference between adjacent rods is able to bow them, requiring an increase in number of spacer grids. Localized pellet-clad interactions may cause significant bowing, particularly when they occur near the grids. (orig.)