Hydrogen diffusion and precipitation in duplex zirconium nuclear fuel cladding quantified by high-resolution neutron imaging

Abstract

Hydrogen distribution in zirconium nuclear fuel claddings can often be non-uniform because of the high mobility of hydrogen interstitial atoms, raising the risk to the fuel rod integrity. Therefore, the assessment of hydrogen migration behavior is of great importance when considering its potential detrimental effects during spent fuel storage. Here, we present the quantitative results of hydrogen redistribution measured by high-resolution neutron imaging in conjunction with thermodynamic modeling. A duplex cladding, with a 150 μm external layer of low-tin Zircaloy-4, so-called liner, bonded to an inner standard Zircaloy-4 substrate, underwent various cooling rates subsequent to hydrogenation and solution heat treatment. The tube segments were measured using a state-of-the-art neutron imaging setup of PSI Neutron Microscope with an effective spatial resolution of 9.6 μm. The hydrogen concentration was quantified across the cladding wall thickness with a precision of 9 wppm, revealing the effect of cooling rate on hydrogen migration from the substrate into the liner. Depending on the cooling rate, hydrogen accumulation in the liner near the interface reached more than 1000 wppm, which consisted primarily of intragranular hydrides in the accumulation strip. The equilibrium concentration during solid solution treatment was identified to be lower in the liner than in the substrate. The thermodynamic calculations reveal that the hydrogen migration into the liner is attributed to the composition-induced difference in the reference chemical potential between the liner and the substrate, and the high precipitation rate promoted hydride accumulation near the interface instead of the liner interior.