Radial hydride precipitation in stress relieved Zircaloy-4 fuel
claddings is studied using a new thermal–mechanical test. Two maximum
temperatures for radial hydride precipitation heat treatment are
studied, 350 and 450 °C with hydrogen contents ranging between 50 and 600 wppm.
The new test provides two main results of interest: the minimum hoop
stress required to precipitate radial hydrides and a maximum stress
above which, all hydrides precipitate in the radial direction. Based on
these two extreme stress conditions, a model is derived to determine the
stress level required to obtain a given fraction of radial hydrides
after high temperature thermal–mechanical heat treatment. The proposed
model is validated using metallographic observation data on pressurized
tubes cooled down under constant pressure. Most of the samples with
reoriented hydrides are further subjected to a ductility test. Using
finite element modeling, the test results are analyzed in terms of crack
nucleation within radial hydrides at the outer diameter and crack
growth through the thickness of the tubular samples. The combination of
test results shows that samples with hydrogen contents of about 100 wppm had the lowest ductility.