Out-of-pile test for yielding behavior of PWR fuel cladding material

Abstract

The confirmed integrity of nuclear fuel cladding materials is an important object during steady state and transient operations at nuclear power plant. In this context, clad material yielding behavior is especially important because of pellet-clad gap expansion. During the steep power excursion, the in-pile irradiation behavior differences between uranium-dioxide fuel pellet and zircaloy clad induce the contact pressure between them. If this pressure reaches the zircaloy clad yield pressure, the zircaloy clad will be plastically deformed. After the reactor power resumed to normal state, this plastic permanent expansion of clad tube give rise to the pellet-clad gap expansion. In this paper, the simple mandrel expansion test method which utilizes thermal expansion difference between copper mandrel and zircaloy tube was adopted to simulate these states. That is, copper mandrel which has approximately three times of thermal expansion coefficient of zircaloy-4 (PWR fuel cladding material) were used in this experiment at the temperature regions from $400^\circ C$ to $700^\circ C$. The measured plastic expansion of zircaloy outer radius and derived mathematical relations give the yield pressure, yield stress of zircaloy-4 clad at the various clad wall temperatures, the activation energy of zircaloy tube yielding, and pellet-clad gap expansion. The obtained yielding pressure and stresses of zircaloy-4 tube are about 41.5 Mpa and 325.5 Mpa at $400^\circ C$, 29.4 Mpa and 234.8 Mpa at $500^\circ C$, 17.4 Mpa and 140.5 Mpa at $600^\circ C$, and 12.1 Mpa and 98.1 Mpa at $700^\circ C$. The activation energy of yielding process of zircaloy-4 tube is about 10.75 KJ/mole for temperature range of 400-$474^\circ C$ and 26.06 KJ/mole for 474-$700^\circ C$. The calculated gap expansion between pellet and zircaloy-4 clad is about maximum 17\% at $700^\circ C$ and 0.9\% mandrel expansion beyond zircaloy inner radius. These results are in good agreement with previous experimental resul...