In-Pile Dimensional Changes of Zircaloy-4 Tubing Having Low Hoop Stresses (Light Water Breeder Reactor Development Program)

Diameter and length changes have been measured for 0.25- to 0.30-in.-o.d. fuel rods having Zircaloy-4 cladding thicknesses from 17 to 22 mil. The external coolant pressure (2000 psi) yielded hoop stresses in the range of 11 000 to 15 000 psi. Short rods (6 to 11 in.) were irradiated in an experimental test reactor (ETR) loop [peak fast flux ≅ 1.5 x 10¹⁴ n/(cm² sec) while long rods (∼10 ft) were irradiated in a NRX loop [2.5 x 10¹³ n/(cm² sec)]. A method has been developed for separating components of the diameter and length changes caused by Zircaloy growth, external pressure, and fuel-clad interactions. It is concluded that the diameter shrinkages of the former rods (up to about 1 mil, 0.4%) were caused entirely by the external pressure. However, those of the latter rods (up to ∼2 mil, 0.8%) were strongly dependent on fuel-clad interactions, with large diameter shrinkages accompanied by large length increases (up to about 0.7%). A calculational method based on the CYGRO computer program simulates such behavior for long rods.